Build a MOSFET Citation 12

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					                 Build a MOSFET Citation 12

                6~ NELSON P A S S
               art by K IRK R A D E R

    Audio Amateur has published a number of        THE HARMON KARDON                                              IG.   1 f
                                                                                                                          MOSFE T        TUBE           BIPOLAR
    projects modifying popular kits, usually       CITATION 12                                                                             I                    I
     Dynaco’s. Obviously it is much less ex-
     pensive to use the chassis and power          The Citation 12, introduced about 10
     supply, pots, connectors, switches, PC        years ago, is based on the circuitry
     boards, and heat sinks that can be had        found in the RCA transistor manuals
     for the price of a kit than to buy the com-   (for example, see RCA data sheet file
     ponents individually or have them             #647). This circuitry was the basis for
     made. Creating a one-off copy commer-         nearly all the quasi-complementary                               Fil. 1. Morft, tube, and bipolar drawing.
    cially typically costs as much as making                                                                 L-

     10 copies of the same item, because the                                                                                                                        _
     dominant costs are design and set-up           IG. * lO.OOA
     time which do not increase with quanti-
     ty. Small wonder then that most techni-
     cians and designers who write for this
     and similar magazines base their pro-                                                                                    /RF-I00
    jects on yesterday’s readily available                                       BIPOLAR                                      MOSFET
     kits, which, often as not, languish unap-
     preciated in closets and basements. This
     is one such project.
        We will, however, do two things here
     you may not have seen before. We’ll
     modify the Harmon Kardon Citation
     12, a popular device which has somehow
     escaped the kit modifier’s attention; and            l.OOA
     we’ll do so with power mosfet circuitry
     instead of the traditional bipolar power
     transistors. This project is for experienc-
     ed builders only, and Harmon Kardon
     want me to remind you that it will void
     any warranty coverage on the Citation
     12:                                                   .3OA

     After this article was prepared In-
     ternational Rectifier withdrew the
     IRF-100 from production and dis-
     tribution and offered an alter-                       .0/A
     native, the IRF-130 as a replace-                                                                                                  GRID 2 =
     ment.                                                                                                                              3oov
        Author Pass has tested these new
     devices in the prototype and re-
     ports that performance is as good
     or better than that using the                         .0_3A
     IRF-100’s. The change came too                                ov           /V    2v     3v         4v       5v      6V                        7v       8V
     late to alter the diagrams so the text                        B I P O L A R - MOSFET ‘ON ’ V O L T A G E - C U R R E N T

     and drawings have been left in                            -35v   -30v           -25v       -20v        -/sv              -/ov   -5v           ov
     their original form. In all cases                          7027A GRID         ! ‘CONTROL’ VOLTAGE VS CURRENT
     where IRF-100 is indicated, the
                                                           Fit. 2 Characteristics of Mosfts, tubes, and bipolars.
     IRF-130 should be used.-EDITOR.

6      THE AUDIO AMATEUR 2/1%1
                                                 =IG. 3
 designs in existence, as typified by the                 200
 large power amplifiers of the early ‘70’s.
                                                          so --
     In its time the Citation 12 was a truly                                                                                2N3055
 excellent amplifier, e m p l o y i n g a                                                                                   VCE’4
 number of concepts which have only
 recently become popular: dual split                      100 --

 power supplies, DC output coupling,                       70 --

 and no active current limiting. These            HFE 5 0 - -
 characteristics allowed it to sonically
 outperform the Dynaco 120 which was                       30 --
 its popular competitor. It could drive
 anything with high reliability, and in the               20    --

 time I spent as a repair technician I
 fixed a few Dynaco, Phase Linear,
 M c I n t o s h , Pioneer, a n d S a n s u i              10 -1
                                                             .2   .3    .5  .7    I     2   3 5 7 IO 2 0
 amplifiers, but I’ve never seen a broken                       DC COLLECTOR CURRENT (AMPS)
 Citation 12.                                             Fig. 3. Bipolar current gain cwuc.
    I have another, nostalgic, reason for
 choosing the Citation 12: it was the first
 amplifier I built myself. About eight           :IG.4      20A

 years ago, when I was an impecunious
 student working part-time for ESS Inc.                                                                                     vG5= 7 V
 (no, I did not design their electronics),
 the home-brew power amplifier built by
 a friend broke down, leaving me without
 music. Like most of you in a similar
                                                                                                                          80ySEC P U L S E              TEST
 position, I decided to do without some
 other necessity, and shortly my kitchen
 table was covered with solder blobs and
 the parts to the Citation 12.                                                                        1                                  6V

    I decided to fire up the completed unit                  i2A - -

 on a variac instead of merely plugging it
 into the wall: and was dismayed to
 discover that both channels exhibited ex-
 tremely large DC offset when I applied
 AC power. After some weeks of recheck-                        8A--
 ing my work and finding no errors, I                                                                                                    5v
 finally decided to connect it up and plug                                   /
 it into the wall anyway?* When I did,
the woofers on my speakers plunged ful-
ly forward as if attempting to escape the                      4A--
amplifier; but after a few seconds they
returned to normal and music appeared.                                                                                                   4 v
Thus I discovered one of the Citation’s
few faults: an enormous subsonic turn-
on thump.                                                                                                                                3 v
                                                                                                                      1                          I
    I kept the amplifier for several years                      0        -                I             I                                                       1
and ultimately sold it to a friend, only to                          0                   IOV          2 o v                                     4 o v          5OV
                                                                             D R A I N CURRET   VS   DRAIN    TO   SOURCE        O       T
end up buying it back. It sat in my closet
                                                                                                                             V       L       A G E

for a long time, a candidate for that                        Fig. 4. Characterzsttc of IRF 1001.
perfect project, until one day at
Threshold we discovered a vendor had            will treat the mosfet as a black box                           change in output current versus input
accidentally shipped us several power           possessing certain simple characteristics.                     voltage is greatest in the bipolar tran-
mosfets instead of diodes, and this arti-       In the power mosfet we have a gain                             sistor, a difference resulting from its be-
cle began falling into place. The power         device which combines the best proper-                         ing a current gain device, where the in-
mosfets, reasonable analogs of the              ties of tubes and bipolar transistors.                         put current is to be multiplied linearly
hornetaxial npn devices in the Citation         Seen in Fig. 1, the gate is analogous to                       by the gain. Figure 3 gives a better pic-
12, fit quite neatly into the original          the grid and the base, the source cor-                         ture of the bipolar device’s gain and its
layout and provide for a state-of-the-art       responds to the cathode and emitter,                           comparative linearity. As Fig. 2 shows,
super-position upon a classic piece of          and the drain corresponds to the plate                         power mosfets and tubes have similar
circuitry, improving the speed and              and collector.                                                 characteristics, but the mosfet has much
distortion characteristics in an even              In all cases the current through the                        higher transconductance and operates at
more simple topology than in the                device (terminal 1 to terminal 3) is a                         bipolar voltages. Considered overall, the
original.                                       function primarily of the voltage be-                          power mosfet is like a tube but retains
                                                tween terminals 1 and 3. As the voltage                        the high transconductance and current
                                                increases between 2 and 3 and/or be-                           of bipolar technology. Power mosfets
As is the case with most electronic com-        tween 1 and 3, so does the current                             can also be made complementary, with
ponents, an exact understanding of              through terminals 1 and 3. Figure 2                            the p and n charactertistics comparable
mosfet operation is a formidable goal,          shows a typical relationship of these                          to pnp and npn bipolar transistors,
and I won’t address it here, rather we          parameters for mosfets, tubes, and                             where the devices’ polarity makes them
                                                bipolar transistors.                                           mirror each other. a f e a t u r e n o t
*rctr>mmrn<lrd .‘““1 rrrommcnded                   In Fig. 2 we note that the rate of                          available in tubes.

                                                                                                                          THE AUDIO AMATEUR                    2/ 198 1   7
    ADVANTAGES OF                                 power mosfets may not necessarily be                        Fortunately, in actual use much of the
    POWER MOSFETS                                 paralleled without the use of source                     capacitance is associated with the gate-
    The power mosfets used in this project        resistances to equalize the current draw                 source, which sees only a small fraction
    are International Rectifier IRF 130’s, a      between devices. Unless the devices are                  of the gate-to-drain voltage swing. In
    150 watt, 100 volt, 12 amp device in a        matched, the designer cannot assume                      our project amplifier, .003A from the
    TO-3 style case. They are part of a           that one device will not take on more or                 front end circuitry will drive the positive
    relatively new line of transistors using      less than its share of the load. In more                 output transistor’s capacitance at about
    IR’s proprietary hexfet process, which        than one commercially available ampli-                   4OVI@ for an effective capacitance of
    combines a particularly high voltage          fier paralleled mosfets are accompanied                  some 100pF. This is about five times
    capability with very fast switching times     by source resistors, to ensure that their                better than the raw capacitance figure
    and low saturation losses. Except for the     characteristics are equalized.                           quoted for the device, indicating that
    steep price tags, this type of device has a      One more aspect affecting these                       most of the effective capacitance is gate
    number of advantages over regular bi-         device’s audio quality is their relatively               to source.
    polar transistors in audio power              high intrinsic capacitance, on the order                    I must mention one final and actual
    amplifier use.                                of 500-lOOOpF, which makes special                       drawback. Because the transconduc-
       First, their simple low current cir-       demands on any circuitry which would                     tance is low compared to bipolars, there
    cuitry eliminates the need for driver         realize the high speed capability. For ex-               is significant loss in maximum power for
    transistors. Second, they are immune to       ample, the current required to slew                      a follower unless the source voltage can
    the second breakdown phenomenon               lOOV/$S i n t o 1OOOpF is .lA, a c o n -                 deliver several more volts than the sup-
    which robs bipolar transistors of their       siderably higher current than most front                 ply to the output transistor, resulting in
    power rating at higher voltages. This         end circuits are designed to source                      either lower efficiency, or the use of
    breakdown results from the positive           without an additional g>in stage.                        tiered power supplies to feed the
    temperature coefficient which en-
    courages local current hogging within a        :IG. 5

    area on the chip, so that at higher
    voltages one small part of the transistor                                                              R3
    tends to do most of the work, resulting                                                                                         Q6
                                                                          RI :
    in more probable failure. In a power
    mosfet the temperature coefficient is                                                                              Q4
    negative, and energy dissipates more
    evenly across the surface of the chip,
    allowing full power application at the
    highest rated voltage. This characteristic
    also avoids the bipolar design problem
    of thermal bias runaway, and eliminates
    costly compensations to maintain reli-
    able operation over a range of condi-
       Third, because mosfets are majority                                                                       (,
    carrier devices (bipolars are minority                                                                 Q 3                      Q 7
    carriers) their intrinsic speed is much
    higher. Rise and fall times are about                                 ,b       /                                         ,b

    150ns while similarly rugged bipolar
                                                                     R2                                                 R5
    devices have rise and fall times several
    factors larger.
                                                            Fig. 5. Citation 12 conceptual schematic.
    Basically, there aren’t any. Some ad-          ZIG. 6
    vantages popularly attributed to them
    are however, not necesarily true. For ex-                                                              P 3
    ample, the negative temperature coeffi-
                                                                                                                                   Q6     D
    cient does not in itself guarantee the                                RI X

    device is indestructible. While this cha-                                                              R 4
    racteristic eliminates the second break-
                                                                                                  C l E,
    down mechanism, it does not offer bet-                                                                       0
    ter reliability at lower voltage levels, so
    that in many well designed bipolar
    amplifiers (including the Citation 12)
    this effect does not provide for signili-
    cantly better reliability.
       In many applications the power
    mosfet will exhibit greater linearity than
    bipolar devices. An important exception
    to this is when the device is used as a
    follower driven by a low impedance
    source. In this case, the higher intrinsic
    transconductance allows for more ac-
    curate voltage following and experimen-
    tally, bipolars show about one-third the
    distortion of mosfets.                                  Fig. 6. Mozfet 12 conceptual schematic.
       Also contrary to popular conception,

8     THE AUDIO AMATEUR        2/1981
FIG. 7


               DUAL     DIFFERENTIAL

                                                                                                                                                4 OUTPUT

                                                                                              DUAL     DIODE

           Y   U S E D lN
               LATER V E R S I O N

         Fig. 7. Citation 12 schematic.

‘IG. 8

                                                                                                                      /RF- 100

/NPUT     RY

                                                                                                                                            .    OUTPUT

                                                                                                                                      210 % M Y L A R

                                          A L L   RESi5TORS   A RE   R L - 0 7   M E TA   L   F I LM   2 %, 1/4   w

                                          UNLESS     OTHERWISE       SPECIFIED

         Fig. 8. Mm&-t 12 schematic.

                                                                                                                          THE AUDIO AMATEUR 211981         9
    amplifier’s front end several more volts               equally to Q and Qs. The 1mA of cur-         load may produce. Re, RT, and C3 form
    than the output stage.                                 rent through Q then goes through RZ to       the feedback loop, with a low frequency
                                                           provide the approximately .65V bias be-      rolloff of .7Hz.
CIRCUITRY                                                  tween the base and emitter of Q. This           A word is appropriate here to explain
The IRF 130’s selected for this project                    DC current with an AC component              the operation of the bootstrapping cir-
are sufficiently similar to the original                   modulates the current through Q,             cuitry of RB, R+, and Ci. The idea is to
HK-12 devices that they lit into the                       which, passed through the high im-           cheaply achieve the effect of a constant
amplifier’s quasi-complementary                            pedance of R3 and R+ (bootstrapped by        current source load for Q, where the
topology with a minimum of modifica-                       C,) produces a large voltage gain at the     AC impedance of the load is very high,
tion, as seen in the simplified schematics                 collector of Q. This voltage gain is         giving maximal gain for that stage, and
of the HK-12 and the project circuit in                    followed by Q, and on the negative half      w h o s e D C i m p e d a n c e i s RS + Rh
Figs. 5 and 6.                                             of the circuit, by the Q,, Q combina-        (9.4kG). This provides about 3mA of
   In Fig. 5 we see the standard differen-                 tion. In this scheme of things, Rs’s ac-     DC class A bias current for Q without
tial input pair (Q, Qz) driving an npn                     tion is similar to Rz’s, and Rg limits Q’s   loading the gain stage.
voltage gain transistor (Q) where Ri                       gain.                                            This effect occurs because the point
and Rz are used to properly bias the                          The combination of Ri6, Ris, and Rio      between RS and Rd is bootstrapped by
three transistors. The output of Q sees                    damp out the resonances which would          C 1, which is connected to the amplifier’s
a bootstrapped current source (Rs, Rh,                     develop from the combinations of gain        low impedance output and causes the
and C,) and a bias voltage source for the                  devices and internal capacitances, and       node between RS and R+ to follow the
output stage. Q provides a level shifting                  prevent parasitic oscillation. CS is in      output. Because the output voltage is
device which elicits complementary ac-                     series with RI0 to ensure that this          nearly identical to the voltage at the col-
tion from Q, the negative output tran-                     resistor is not driven by audio frequen-     lector of Q, the voltage across Ra is
sistor, and Q+ provides the follower cur-                  cies, which would overheat and destroy       nearly constant, resulting in effectively
rent gain to drive Q, the positive output                  it. C*, a damping capacitor, enhances        constant current through R+, the action
transistor. The Fig. 6 circuit is basically                the circuit’s stability by providing a       of a constant current source. In this
the same; but Q and Q have become                          secondary high frequency loop, causing       way, Q can swing full output into an 80
power mosfets and Qt has disappeared                       the feedback loop to ignore the output’s     load with only about 10 percent varia-
because the additional current gain is                     high frequency operation in favor of the     tion about the bias current, as opposed
not required to drive Q.                                   front end’s low-order output at frequen-     to the f 100 percent variation that
                                                           cies above 800kHz. This allows the           would occur without the bootstrapping.
    THE ACTUAL SCHEMATIC                                   amplifier’s input stage to dominate the      This contributes greatly to the system’s
In Figs. 7 and 8 we see the actual                         high frequency response with a less than     gain and linearity and achieves the
schematics of the two power amplifiers;                    two-pole characteristic, providing for       highly desirable effect of the constant
for clarity the part numbers correspond                    high stability under transient condi-        current source with great simplicity and
to Figs. 5 and 6. In Fig. 8, Rii and CT                    tions.                                       little cost. The effect is good across the
filter the power supply line to prevent                       The bias network of Figs. 5 and 6 has     AC band, rolling off at about .14Hz on
ripple and transients from modulating                      been replaced by Qp, Ri7, Ris, and Cg,       the low end and at about 300kHz at high
the bias, causing noise and distortion.                    forming a constant-voltage source trim-      frequencies.
RI2 and C2 form a low pass filter to pre-                  mable by R17 and frequency-stabilized
vent spurious high frequency input                         by Cs. DS ensures symmetric clipping,        CONSTRUCTION
signals from being amplified.                              and D,, D2 provide a current path for        Start by removing the PC board and
   Resistor RI feeds 2mA of current                        the flyback energy which an inductive        stripping off all components except the

    FIG. 9

                                                                     RIGHT       CHANNEL                     T                    l   +42v
                                               3 A
                                                                                                            __z t_ 6 0 0 0
                                                                         4 :,
                                                               BLK/WHT     I                                --IOOV _-- SO”
                                                               I                                                                  9 COM
                                               BRN/   WH   T

                                                                     L E F T     iHANNEL                                  ~+42V

             Fi’. 9. Power supp(p schematic.
connectors. Use solder wick or a solder
sucker, and take great care not to            FIG. 11                             FIG. 12

separate the copper foil from the PC
board. You will also have to remove
many of the components from the
underside of the chassis (see Fig. 13), in-
cluding the heat sinks. While you may
elect to retain the original input and out-
put connectors, I replaced mine with
higher quality gold-plated types.
   You may also elect to retain the
original amplifier’s output breakers
and/or thermostats; however, I removed
them as unnecessary. Personally, I don’t
trust the quality of connection offered by
the output transistor sockets (especially
old ones) and I decided not to use them,
preferring solder connections to the
TO-3 pins. Remove and discard all the
capacitors on the underside of the
chassis except the four computer grade
electrolytic power supply capacitors.
   Figure 13 shows the primary AC wir-
ing; it lacks the thermostats but is other-
wise virtually identical to the original.
Mount the power mosfet output devices
on the heat sinks, using mica TO-3 in-
sulators, silicone grease, plastic shoulder
washers, 6-32 screws/washers/nuts, and
a solder lug for electrical connection to
the transistor case (drain).
   Figure 14 shows the output transistor
wiring with the lead wires soldered to
the pins. The wires from the outputs
feed down through the holes in the

             FIG. 10
     PARTS LIST (one channel)

Rs, Ris        1on
RN          3303
R7          4703
RB, Ru, RIS 68OQ
RI3         8203
Rs, R,I     lk
Rl              3.3k
R3, R1, RIG 4.7k
Rs, RIO, Rlr 10k
RI              18k
R20             1OQ 1W 5% carbon
R17            5k CTS vert. PC mount
All resistors are % W f 2% metal film
RL-07, or * 1% RN-55D or equiv.
unless otherwise specified.
c1    470/5OV Radial Electrolytic
      390pf f 5 % Dipped Silver Mica
c”:   20pF f 5% Dipped Silver Mica
      470/16V Radial Elect.
c”:   4.7125V Tant.
CS 47l5OV Radial Elect.
C7    .068/1OOV Mylar
Q, Q, Q MPS-L51 M o t o r o l a
             MPS-LO1 Motorola
Z:$          International Rectifier
D1, Dz, Ds lN4004
2 .01/1.4kV CAPS IN
                                                Fix. Il. Component side of PCB.     Fix. 12. Foil side of PCB
4 .47/1oov

                                                                                            THE AUDIO AMATEUR   2/1981   11
     chassis to the connector pins as shown in                    amplifier will run quite well without         the polarities of the components and
     Fig. 13. After mounting the output                           them. I also placed .OlpF capacitors          avoid damaging the components by
     devices to the heat sinks, check first for                   across the dioded bridges to suppress rf      overheating the leads when soldering.
     possible connection between the tran-                        emission by the diodes; these too are op-     As this project is for the more advanced
     sistor case or either of the two pins and                    tional.                                       home-brewer, I will not mention that
     the heat sink. Use an ohmmeter with                             Figures 11 and 12 display the compo-       the use of a solder gun is forbidden.
     one probe connected to an unanodized                         nent layout on both sides of the PC           Leave the TO-92 transistor leads as long
     portion of the heat sink, (possibly in the                   board and are self-explanatory. Please        as possible. Q and Q require press-fit
     tapped holes) and the other probe                            note the 18 gauge jumper wires on the         heat sinks.
     touching the various parts of the tran-                      back. Two replace the coil/resistor in
     sistor. The transistor must not connect                      series with the output of the original cir-   TURNING ON THE AMPLIFIER
     with the heat sink.                                          cuit (a standard feature on 99 percent of      When you first switch on the amplifier,
        When you have mounted and wired                           today’s solid state amplifiers), and to        turn the bias pots so as to exhibit max-
     the output transistors, reattach the heat                    reduce the distortion inherent in the          imum resistance. This minimizes the
     sinks to the chassis and solder the wires                    original PC board. The original PC art-        amplifier’s initial bias current when first
     to the PC board connector pins. Note in                      work, of which more than one version           fired up, a valuable safety precaution
     Fig. 13 that some of the pins are hard-                      apparently exists, takes the feedback         which should be checked with an ohm-
     wired together to achieve the best possi-                    from a point separated from the actual         meter. The pot’s wrong extreme setting
     ble connector contact by paralleling con-                    output node by a short length of copper        should read about 0 ohms and the right
     nections to the PC board. Also shown in                      which carries an asymmetrical portion          setting will read 5000 ohms, depending
     Fig. 13 are the .47pF/lOOV capacitors                        of the output current. The feedback thus      on the polarity of the probes (note the pn
     which I placed in parallel with the power                    operates off a voltage several milivolts      junction of the bias transistor).
     supply capacitors to achieve lower                           removed from the actual output,                   The following test is important, and
     dissipation factor at the highest frequen-                   generating even-ordered distortions.           should be carefully performed, first for
     cies. These are optional and the                                As always, take special care to orient                             Continued on page 14
      ZIG.   13                                                             FUSE              FUSE
                               INPUT                        OUTPUT           3A      P I L O T 3A          OUTPUT                     INPUT


                                                                                       t.   t-


                                      ‘~_.      _-      _   _..                                                 .-   ---.__._.__-./

                                                                                              LINE CORD

             Fig. 13. Underside of chassis.

12       THE AUDIO AMATEUR                    21198 1
                                                 floating with respect to the oscilloscope   only a variac, slowly raise the line voltage
BUILD A MOSFET CITATION 12                       (you can do this by using a 3-to-2 prong    to the channel while watching the out-
            Continued from page 12               cheater adaptor on the oscillator’s AC      put. The output voltage should exhibit
                                                 line if it has an earth connection). The    DC offset at first and then settle into a
one channel and then for the other.              oscilloscope itself should be earth          *2V sine wave; if it does, continue to
Remove the AC line fuse of the channel           grounded. Measure the output without        slowly raise the voltage until the full line
not being tested and install a 1A fast           a load, connecting it and its ground to     voltage is achieved. If you manage this
blow fuse in the channel being tested.           the oscilloscope input.                     without blowing the fuse, the next step is
Drive the channel at a low level (. IV) at          Set the scope vertical scale at 5V/div   to slowly increase the input voltage from
1kHz by an oscillator whose ground is            and the horizontal at .2mS/div. U s i n g                         Continued on page 16
                                                  FIG. 15
                  TABLE 1                           2608
    Distortion Figures of the Two Units

 STOCK HK 12       MOSFET HK 12
       Left  Right  Left  Right
  20H  ,025  ,027   ,010  ,010
 1OOH  ,024  ,023   ,009  ,009
 1kH   ,018  ,022   ,006  ,007
 5kH   ,023  ,024   ,016  ,019
 20kH ,045   ,060   ,050  ,064
 IM    ,012  ,015   ,020  ,024

  20H      ,025      ,028       ,008     ,008
 1OOH      ,022      .023       ,006     ,007
 1kH       ,015      ,020       ,005     ,006
 5kH       ,021      ,025       ,015     ,018      2 2 LIB --

 20kH      ,095      ,120       ,050     ,058
 IM        ,034      ,040       ,012     ,015

 20H       ,022      ,028       ,007     ,008
 1OOH      ,025      .028       ,006     ,007
  1kH      ,022      ,024       ,004     ,005
  5kH      .027      ,031       ,013     ,016
 20kH      ,190      ,240       ,044     ,052
 IM        ,060      ,065       ,007     ,009                                                                                             _
                                                  FIG. 16

  20H      ,075      ,050       ,007     .008
 1OOH      ,090      ,070       ,007     ,007
  1kH      ,085      .080       ,004     .004
  5kH      ,075      ,076       ,013     ,015
 20kH      ,390      ,540       ,041     ,055
 IM        ,140      ,130       ,007     ,007

 FIG. 14


   RED     LEAD



   SOURCE                                             - 3o”--
   BLACK    LEAD      i

                                                                         1/             1             I               1              I
  Fig. 14. Mosfet wiring diagram.                           I           /O            /co            ,K             IOK            IOOK
 FIG. 17
                                                                                                                               BUILD A MOSFET CITATION 12
                                                                                                                                           Continued from page 14
             /    --
                                                                                                                               the oscillator until the output reaches
                                                                                                              -60 W- RT        clipping (at about f 3OV), watching for
                                                                                                              -6OW- LT         oscillation or severe distortion.
                                                                                                                                  Having accomplished that, repeat the
           .3     --                                                                                      /
                                                                                                                               test for the other channel. When both
                                                                                                                                                    Continued on page 18
                          - - - - _                _ _ _
           ./     --    -
                        _--- --                    _              ---         _ i ,-/W-RT

           .03    --

           .0/    --

           ,003 - -
                          RESIDUAL DISTORTION
           ,002 ____________________--______-___---

           ,001              I          ,          11   4      I               1I       I             I         I1         i
                  IO       30        100          300        lK               3K                IOK            30K             Fig. 21. 20k THD wauc HK-12
           Fig. 17. HK-I.? distortion curues.

 FIG. 18

             I    --

           .3     --

           ./     --                                                           &&k                                    x:       Fig. 22. 20k square waue HK-12

           .03    --                                                     p                            /               z

                                                                                            /     /
                                                                                    /       y
           .Ol     --
                         =_z_-_~.=Y,_ _ _ _ _ _ ’ 2
                                              -1 - - - - - -            I ’
           ,003 - -

           .OOZ                  R _____~~_-_~~.
                   _ _ _ _ _ _ _ _ E SID U A L DIS T O R T IO N _._~~~.___

                                         /          I
           .oo/               I                     I        II                I
                                                                               I                  I
                                                                                                  I              II        I
                  IO         30         IGC       3CO       lK                3K                IOK            70
           Fig, 18. MF-12 distortion cumes.
                                                                                                                               Fi’. 23. 20k square ulaue MF-12

Fi,. 19 1kHz T H D uxue MF-12.                              Fig 20. 20k THD W~UE MF-12                                         Fig. 24. Current through outpuf stage MF-12.
                                                    the channel for 15 minutes by placing           Figure 25 shows how one amplifier
     BUILD A MOSFET CITATION 12                     your hand on the heat sink. Ideally the      channel (A) uses the oscillator’s signal to
                 Continued from page 16             channel should operate slightly warm at      source current into the other channel (B)
                                                    idle. If it becomes hotter than this, ad-    through an 83 resistor. At the output of
     channels operate successfully without a       just the bias current down. If it stays       channel B we will find that channel A’s
     load, repeat the test for each channel         cool, you may with care, slightly in-        voltage output has been divided by
     with a 4A fuse and an 89 load, still           crease the bias.                             channel B’s damping factor. In this
     watching for waveform problems or fuse           After 15 more minutes recheck each         amplifier the broadband output impe-
     blowing. You will see crossover distor-        channel’s temperature and distortion,        dance is .0640 (83/.0643 = 125 damp-
     tion at this point, which is normal for an     readjusting as necessary. If you are for-    ing factor), resulting in a condition
     unbiased amplifier.                            tunate enough to possess a current pro-      where channel A’s voltage output is
        When both channels can successfully        be, this entire procedure can be ac-          reduced to .8% at the channel B output,
     drive an 83 load at full power, it is time     complished by simply setting each chan-      plus the distortion of channel B caused
     to bias the amplifier’s output stage. Put      nel to draw lOOmA from the supply,           by sinking the current.
     most simply, we want to adjust the bias        again rechecking after 15 minutes.              At channel B’s output we therefore
     potentiometer so as to remove crossover       Alternatively, you may monitor the bias       see the channel’s notch and other distor-
     distortion, a condition which cor-             current by inserting 1Q in series with the   tions in response to channel A’s drive
     responds to about lOOmA of idle current        positive power supply lead and measur-       conditions, but much less obscured by
     through the output stage. We can do this       ing 1OOmV DC across it.                      the fundamental voltage. Because in this
     as follows.                                                                                 and most other amplifiers the distortions
        Using the test setup shown in Fig. 2.5     USING THE AMPLIFIER                           are dominated by current fluctuations (a
     (see following paragraphs) and with the       TO ANALYZE ITSELF                             concept addressed b y t h e S t a s i s
     drive channel at +8V into the 8h2 load,       It may surprise you to know that one          amplifiers) this serves as a useful and in-
     look at the notch distortion on the other     can quite easily perform cursory distor-      expensive bench technique and for the
     channel and adjust the bias poten-            tion analysis on this (and many) power        most part it is easy to see the distortion
     tiometer until the spike just disap-          amplifiers using an 83 power resistor,        added to a .8% fundamental. This
     pears--and not more. Be careful here: it is   an oscilloscope, oscillator, and the am-      procedure is useful in biasing the
     easy to set the bias too high in search of    plifier itself. The oscilloscope need not     amplifier as You can clearly see
     perfection. Disconnect the load and           be of particularly high quality and the       crossover notch and other effects.
     monitor the operating temperature of          oscillator need not have low distortion.                           Continued on page 46
      FIG. 25


           Fig. 25. Distortton hookup.

18        THE AUDIO AMATEUR               2/1981
                                                    * .225V sinewave) plus an equivalent of       B and AB amplifiers. This gives reduced
BUILD A MOSFET CITATION 12                         channel B’s distortion when operated at        crossover distortion and the quasi-Class
                      Continued from page 18       that level into 8ti. In this case a .28V       A operation at lower levels which other-
                                                   peak distortion spike would indicate a         wise we could only achieve in a bipolar
   This test will also reveal distortions          1% peak distortion (not uncommon in            circuit with dynamic biasing.
caused by poor connections between the             amplifier’s with .1 ‘$J average distortion).      The amplifier is sonically a significant
PC board and the connectors (some-                                                                improvement over the original, par-
thing I encountered in the actual unit             PERFORMANCE                                    ticularly in the high end where the Cita-
and which I cured by cleaning and                  Figures 15-24 document the performance         tion 12’s veiled characteristic is replaced
reseating the connectors). To evaluate             of the prototype amplifier, which was          by a detailed, somewhat sweet sound.
the percentage of distortion remember              built without selected components. Of          The imaging and midrange definition
that the voltages seen by the ‘scope               particular note are the distortion and         are also much improved; but the bass
divided by the output voltage of the               square wave comparisons against the            response (one of the Citation 12’s
other channel will equal the fraction of           original, showing a significant improve-       strongest points) remains much the
distortion. For example, if channel A’s            ment in distortion characteristic and          same-ideal for planar loudspeakers like
output is at + 28V (20VRMS) through                transient response. Figure 24 shows the        MG II’s, less so for acoustic suspension
the 8 Ohms into channel B, then chan-              current waveform through half the out-         woofers.
nel B’s output will show a signal com-             put stage. Note that the current through          In conclusion, I hope many of you
prised of channel A’s fundamental                  the output device idles down instead of        will try this one and enjoy yourselves.
divided by the damping factor (for                 shutting off abruptly as in bipolar Class      No amplifier sounds as good as the one

                                                   margin to the design and was not really        tions get more pronounced at higher
POWER MODIFICATIONS                                necessary. Watch the lead length on the        power levels, since the output im-
for the ST- 150-BJ- 1,                             transistors themselves if you swap,            pedance of the unregulated supply is
DH-200, and ST-400                                 however. The ST-150 heatsink is so             fairly high, and also strongly frequency
                     Continued from page 45        thick that the leads barely make it into       dependent.
                                                   the sockets, and some brands of tran-             The fact that we can control sine wave
signal across the power supply caps when           sistor make marginal contact. If in            steady-state distortion with regulation
driven hard; this signal is the combina-           doubt, use the old devices.                    almost certainly means that operating-
tion of both channels of load, and it                 Pat’s listening comments are more           point-dependent distortions which are
couples the two channels together,                 than supported by my own experience; I         the result of supply variations, such as
resulting in a variety of distortion pro-          think the combination of changes               dynamic compression and some types of
ducts. The stock regulator reduces this            covered in his article turned a rather         TIM, are being controlled as well. We
signal by 60dB (a l-volt signal is reduc-          lackluster, low/medium power amp into          have solid, measurable evidence that
ed to 1 millivolt at the regulator output);        an    open,      dynamic,       exciting       regulation of the supply can make a
Walt’s additions to the circuit reduce it          high/medium (140W/chan) amp which              significant contribution to overall sonic
by 80dB (1 volt is reduced to 100 micro-           is sonically the equal of anything I have      performance.
volts at the output). Either way you are           heard. The photos show the care and
going to see a substantial reduction in            craftsmanship Pat has applied to the           TEST RESULTS
problems from this source.                         changes, and demonstrate that the              We performed tests into an 8fI nonin-
                                                   ST-150 was indeed an ideal candidate           ductive load, both channels driven, after
IMPLEMENTING CHANGES                               both physically and electrically for the       a 1 hour warmup period at 25W and
Pat used the Old Colony kit for the Pass           addition of full regulation.                   1kHz signal. Unregulated readings are
 f 32V supply as the basis for his                                                                taken from Dyna supplied charts.
regulator; he wanted a 50 volt output, so          RESULTS AND SIGNIFICANCE
he substituted a 33V zener for the 15V             The test results show something even
zener supplied at Z2 (see Table 1). P a t          more interesting than low distortion or                         IM vs. POWER
changed the value of Rs to 0.333 to give           high power. The distortion of the
the regulator a 9.5A max current (see              regulated amplifier is nearly constant at         POWER REG. AMP UNREG.
my TAA l/80 article for details). He also          all power levels, and across the entire            (W)    IM %  AMP’ IM %
added a 2200pF cap to the output to flat-          audio bandwidth. Let me stress the im-               .1    .Ol     ,011
ten the impedance curve at a low fre-              portance of this finding.                                1             .Ol        ,018
quency. The Old Colony kits turned out                The normal pattern of transistor                     10             ,014       ,022
to be quite a bargain, as Pat ordered two          amplifier designs, and of the unmodified                50             ,016       ,040
kits and received two complete plus and            ST-150, is to have the THD measure-                     75             ,017       .050
minus regulators in each kit. The kits all         ments fluctuate by 50% of their 1 kHz                  100             ,018        n/a
contained LM340T regulators, however               values, or more, at various points in the              140             ,020        n/a
this is the result of my own drawing               frequency spectrum. Also, the varia-
error in the l/80 article which showed an
LM340 regulator on the negative sche-
matic, where an LM320T-15 should                                      THD vs. FREQUENCY, SEVERAL POWERS
have been. (Present kits include LM340T
and LM320T regulators.                 -ED./                                     Regulated Amp                         Unregulated Amp
   We swapped the ST-150 output tran-                        Power .75W         7.5w     75w       140w         .75w      7.5w    75w
sistors with the regulator transistors                       Freq.
(TR,) because the TCG180 and 181’s
supplied by Old Colony are 2OOW,                             20Hz  ,030         ,030      ,030     ,033         ,035       ,035   ,045
lOOV, 30A devices, and the amplifier                         1OOHz     ,030     ,030      ,030     ,033         ,035       ,035   ,045
output transistors will see larger power                     1OOOHz    .030     ,030      ,030     ,033         ,030       ,030   ,040
products t h a n t h e r e g u l a t o r p a s s             1OkHz     ,030     .030      ,030     ,033         .031       ,031   ,035
elements. This change just gives a little                    20kHz     ,030     ,030      ,030     ,032         .035       .035   ,040
 you built yourself”, and (
 commercial amplifier will give ydu a:
 much satisfaction or frustration. A:
 careful as the Editor and I have been,
                                              Ir;               Audio Amateur Publications                                                          Til
                                                        are pleased to announce their appointment
 there is probably an error or two; 1
 recommend that you cross-check thr                          as agent in the United States for
 parts list. schematic, and pc boarc
    My amplifier has performed well fo1
 six months, as of this writing, and I en-
 countered no exotic problems during its
 construction and testing. Nevertheless,
 inevitably some of you will need help,         The distinguished French publication             tridge; Defining and measuring the prin-
 which you may obtain by calling or             dedicated to a new approach to high              cipal characteristics of the high frequency
 writing me at Threshold Corporation,           quality audio: both construction and             speaker; Do Asians and Europeans hear
 1832 Tribute Road, Suite E, Sacramen-          sonic arts.                                      differently? Arts Sonores; Sound
 t o , C A 95815-(916) 927-5061.                   L’Audiophile is a 140 page bi-                Engineers’ View: Pierre Lavoix of Erato;
                                                monthly, 7x9% “, beautifully printed             Listening to the Onken-Mahul system; The
 Although I am pleased to help, those of
                                                and illustrated magazine which ex-               Saxhorns, a study of the tuba’s cousins;
 you who call should be reconciled to the                                                        The Concertgebouw of Amsterdam, study
 possibility of only a short conversation.      plores the engineering reasons for
                                                                                                 of a hall; Outstanding new discs; Impulse
    Good luck, and have fun.            0       high quality ‘in sound reproduction
                                                                                                 testing linear and non-linear systems.
                                               equipment. Published now for nearly
                                               four years by Editions Frequences                 The viewpoints expressed by L’Aud-
                                               and edited by Gerard Chretien and                 iophile’s editors are personal, opin-
                                               Jean Hiraga, the articles critically ex-          ionated, and are a refreshing depar-
                 Audio Aids                    amine all sorts of equipment and
                                               study such matters as component ef-
                                                                                                 ture from the views generally current
                                                                                                 in the USA. The magazine does not
                    Continued from page 50     fects on sound and the question of                take advertising and unhesitatingly
                                               the relevance of measurements ver-                takes positions about relative merits
  were used throughout, not so much for        sus subjective evaluations. M. Hiraga             of equipment which includes US, UK
  accuracy as for lower noise and increas-     brings a strong Japanese interest to              Japanese and F rench gear.
  ed stability.                                the magazine and often discusses un-                     Although L’hdiophile is published
    I used LM353 bi-FET op amps in             usual Japanaese products.                              in French, the text is relatively easy to
 place of the 5558’s.                             The magazine also devotes a large                  translate for anyone with a year or
    The project finally completed, I hook-     section to Arts Sonores (sonic arts).                 two of high school language study.
 ed it up to my scope to view it. Since I      Articles deal with the acousti& of                    The written word is augmented by
 don’t have a distortion analyzer, I can       Bayreuth, the characteristics of vari-                copious illustrations and diagrams,
 only report the results of listening tests    ous musical instruments, old instru-                  whose designations are in almost all
 made when I attached it to my system.         ments and their reproduction sonic-                   cases identical with those used in
 PE reports that the unweighted noise          ally. The Arts Sonores section is                     English publications. With a simple
 level of the equalizer in the output was      edited by Jean-Marie Piel.                            French/English dictionary and a year
 66dB below 1V and when the measure-           Typical articles:                                     or two of either Latin, French or
 ment bandwidth was restricted                 Techniques        Sonores;           output           Spanish, the dedicated audiophile
 250-20,OOOHz. To exclude hum and              Transformerless amplifiers; The Sound of              can translate L’Audiophile well
 random noise, the measurement was             Turntables, a t e n t a t i v e e v a l u a t i o n ; enough for it to be a useful inpui for
 74dB below 1V.                                Thoughts on turntables; The Koetsu car                fresh ideas about sound.
    With the modifications incorporated         -------------------------_--_________
 into the project, this 74dB can be easily     Send   subscriptions to:                                                                taa Ysl
 extended through the entire audio spec-       Audio Amateur Circulation/L’Audiophile
 trum. Listening tests confirm this when       Post Office Box 576, Peterborough NH 03458                             USA
 the equalizer is switched in and out of
 the system and there is absolutely no         0 Enter my subscription to L’Audiophile
audible difference in noise level                for the next six issues                                                            S 48.00
throughout the spectrum. (This is with          0 Back Issues: (circle issues desired) -
all controls flat as I assume the PE unit         Issues 3.4,5.6 for 1978 @ 36 each ppd.                               Total
was when tested, although the article             Issues 7,8,9, 10. 11, 12 @ 97 each ppd.                              Total
does not state that this was done.) Even          Issues 13. 14, 15, 16 @ S9each ppd.                                  Total
with the lower bands turned up there is        (For a translated set of each issue’s contents pages,                   send a stamped.
no appreciable hum.                            addressed #l 0 envelope to the above address).
    Although I have never heard an un-         Please allow at least eight weeks for delivery of first copy on regular subscription. Orders
modified unit, I am convinced these            cannot be accepted from areas other than North America. Ramittancm in US S only.

modifications were more than worth the         Remittance        details:
extra money necessary to purchase the          I enclose S                 for the above in:           q     check,    q           money order
upgraded parts needed to do the job. In                                           q     MasterChargeNisaiIl                     (S 10.00 minimum)
listening tests the equalizer is clear,        card number:
responsive, and versatile enough to
tailor many rooms to your audio needs.
It is definitely one of my favorite pieces            Expire               I                   Interbank                    I
of equipment.
A LAN M. VALENTE                                      Signature
Loudonville, NY 122 11

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Description: Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a widely used in analog circuits and digital circuits, field-effect transistor (field-effect transistor). MOSFET in accordance with the "channel" of different polarity can be divided into n-type and p-type of MOSFET, usually referred to as NMOSFET and PMOSFET, the other referred to also include NMOS FET, PMOS FET, nMOSFET, pMOSFET and so on.