Build a Citation 12

6

Build a MOSFET Citation 12

6~ NELSON PA S S

art by K IRK R A D E R

Audio Amateur has published a number ofprojects modifying popular kits, usuallyDynaco’s. Obviously it is much less ex-pensive to use the chassis and powersupply, pots, connectors, switches, PCboards, and heat sinks that can be hadfor the price of a kit than to buy the com-ponents individually or have themmade. Creating a one-off copy commer-cially typically costs as much as making10 copies of the same item, because thedominant costs are design and set-uptime which do not increase with quanti-ty. Small wonder then that most techni-cians and designers who write for thisand similar magazines base their pro-jects on yesterday’s readily availablekits, which, often as not, languish unap-preciated in closets and basements. Thisis one such project.

We will, however, do two things hereyou may not have seen before. We’llmodify the Harmon Kardon Citation12, a popular device which has somehowescaped the kit modifier’s attention; andwe’ll do so with power mosfet circuitryinstead of the traditional bipolar powertransistors. This project is for experienc-ed builders only, and Harmon Kardonwant me to remind you that it will voidany warranty coverage on the Citation12: .3OA

PLEASE NOTEAfter this article was prepared In-ternational Rectifier withdrew theIRF-100 from production and dis-tribution and offered an alter-native, the IRF-130 as a replace-ment.

Author Pass has tested these newdevices in the prototype and re-ports that performance is as goodor better than that using theIRF-100’s. The change came toolate to alter the diagrams so the textand drawings have been left intheir original form. In all caseswhere IRF-100 is indicated, theIRF-130 should be used.-EDITOR.

THE AUDIO AMATEUR 2/1%1

THE HARMON KARDONCITATION 12

The Citation 12, introduced about 10years ago, is based on the circuitryfound in the RCA transistor manuals(for example, see RCA data sheet file#647). This circuitry was the basis fornearly all the quasi-complementary

L-

IG. 1 fMOSFE T TUBE BIPOLAR

/ I I

Fil. 1. Morft, tube, and bipolar drawing.

IG. * lO.OOA

3.OOA

l.OOA

.0/A

.0_3A

BIPOLAR

/RF-I00MOSFET

GRID 2 =3oov

ov /V 2 v 3 v 4 v 5v 6V 7 v 8VBIPOLAR - MOSFET ‘ON ’ VOLTAGE VS CURRENT-

-35v -30v -25v -20v -/sv -/ov -5v ov

7027A GRID ! ‘CONTROL ’ VOLTAGE VS CURRENT

Fit. 2 Characteristics of Mosfts, tubes, and bipolars.

_

designs in existence, as typified by thelarge power amplifiers of the early ‘70’s.

In its time the Citation 12 was a trulyexcellent amplifier, employing anumber of concepts which have onlyrecently become popular: dual splitpower supplies, DC output coupling,and no active current limiting. Thesecharacteristics allowed it to sonicallyoutperform the Dynaco 120 which wasits popular competitor. It could driveanything with high reliability, and in thetime I spent as a repair technician Ifixed a few Dynaco, Phase Linear,McIntosh, Pioneer, a n d S a n s u iamplifiers, but I’ve never seen a brokenCitation 12.

=IG. 32 0 0

so --2N3055

VCE’4

100 --

7 0 - -

HFE 50 - -

3 0 - -

2 0 - -

I have another, nostalgic, reason forchoosing the Citation 12: it was the firstamplifier I built myself. About eightyears ago, when I was an impecuniousstudent working part-time for ESS Inc.(no, I did not design their electronics),the home-brew power amplifier built bya friend broke down, leaving me withoutmusic. Like most of you in a similarposition, I decided to do without someother necessity, and shortly my kitchentable was covered with solder blobs andthe parts to the Citation 12.

10 -1.2 .3 .5 .7 I 2 3 5 7 IO 2 0

DC COLLECTOR CURRENT ( A M P S )

Fig. 3. Bipolar current gain cwuc.

:IG.4 20A

vG5= 7 V

I decided to fire up the completed uniton a variac instead of merely plugging itinto the wall: and was dismayed todiscover that both channels exhibited ex-tremely large DC offset when I appliedAC power. After some weeks of recheck-ing my work and finding no errors, Ifinally decided to connect it up and plugit into the wall anyway?* When I did,the woofers on my speakers plunged ful-ly forward as if attempting to escape theamplifier; but after a few seconds theyreturned to normal and music appeared.Thus I discovered one of the Citation’sfew faults: an enormous subsonic turn-on thump.

/6A--

80ySEC P U L S E T E S T

i2A - -

1 6V

8A--

5v

/

4A--

4 v

/

I kept the amplifier for several yearsand ultimately sold it to a friend, only toend up buying it back. It sat in my closetfor a long time, a candidate for thatperfect pro ject , unt i l one day atThreshold we discovered a vendor hadaccidentally shipped us several powermosfets instead of diodes, and this arti-cle began falling into place. The powermosfets, reasonable analogs of thehornetaxial npn devices in the Citation12, fit quite neatly into the originallayout and provide for a state-of-the-artsuper-position upon a classic piece ofcircuitry, improving the speed anddistortion characteristics in an evenmore simple topology than in theoriginal.

3 v

0 - I I 1 I

0 IOV 2 o v 3 o v 4 o vD R A I N CURRET V S D R A I N T O S O U R C E V O L T A G E-

Fig. 4. Characterzsttc of IRF 1001.

will treat the mosfet as a black boxpossessing certain simple characteristics.In the power mosfet we have a gaindevice which combines the best proper-ties of tubes and bipolar transistors.Seen in Fig. 1, the gate is analogous tothe grid and the base, the source cor-responds to the cathode and emitter,and the drain corresponds to the plateand collector.

POWER MOSFETS

As is the case with most electronic com-ponents, an exact understanding ofmosfet operation is a formidable goal,and I won’t address it here, rather we

In all cases the current through thedevice (terminal 1 to terminal 3) is afunction primarily of the voltage be-tween terminals 1 and 3. As the voltageincreases between 2 and 3 and/or be-tween 1 and 3, so does the currentthrough terminals 1 and 3. Figure 2shows a typical relationship of theseparameters for mosfets, tubes, andbipolar transistors.

change in output current versus inputvoltage is greatest in the bipolar tran-sistor, a difference resulting from its be-ing a current gain device, where the in-put current is to be multiplied linearlyby the gain. Figure 3 gives a better pic-ture of the bipolar device’s gain and itscomparative linearity. As Fig. 2 shows,power mosfets and tubes have similarcharacteristics, but the mosfet has muchhigher transconductance and operates atbipolar voltages. Considered overall, thepower mosfet is like a tube but retainsthe high transconductance and currentof bipolar technology. Power mosfetscan also be made complementary, withthe p and n charactertistics comparableto pnp and npn bipolar transistors,where the devices’ polarity makes themmirror each other. a feature not

*rctr>mmrn<lrd .‘““1 rrrommcnded In Fig. 2 we note that the rate of available in tubes.

15OV

THE AUDIO AMATEUR 2/ 198 1 7

8

ADVANTAGES OFPOWER MOSFETSThe power mosfets used in this projectare International Rectifier IRF 130’s, a150 watt, 100 volt, 12 amp device in aTO-3 style case. They are part of arelatively new line of transistors usingIR’s proprietary hexfet process, whichcombines a particularly high voltagecapability with very fast switching timesand low saturation losses. Except for thesteep price tags, this type of device has anumber of advantages over regular bi-polar transistors in audio poweramplifier use.

First, their simple low current cir-cuitry eliminates the need for drivertransistors. Second, they are immune tothe second breakdown phenomenonwhich robs bipolar transistors of theirpower rating at higher voltages. Thisbreakdown results from the positivetemperature coefficient which en-courages local current hogging within aarea on the chip, so that at highervoltages one small part of the transistortends to do most of the work, resultingin more probable failure. In a powermosfet the temperature coefficient isnegative, and energy dissipates moreevenly across the surface of the chip,allowing full power application at thehighest rated voltage. This characteristicalso avoids the bipolar design problemof thermal bias runaway, and eliminatescostly compensations to maintain reli-able operation over a range of condi-tions.

Third, because mosfets are majoritycarrier devices (bipolars are minoritycarriers) their intrinsic speed is muchhigher. Rise and fall times are about150ns while similarly rugged bipolardevices have rise and fall times severalfactors larger.

DISADVANTAGES OFPOWER MOSFETSBasically, there aren’t any. Some ad-vantages popularly attributed to themare however, not necesarily true. For ex-ample, the negative temperature coeffi-cient does not in itself guarantee thedevice is indestructible. While this cha-racteristic eliminates the second break-down mechanism, it does not offer bet-ter reliability at lower voltage levels, sothat in many well designed bipolaramplifiers (including the Citation 12)this effect does not provide for signili-cantly better reliability.

In many applications the powermosfet will exhibit greater linearity thanbipolar devices. An important exceptionto this is when the device is used as afollower driven by a low impedancesource. In this case, the higher intrinsictransconductance allows for more ac-curate voltage following and experimen-tally, bipolars show about one-third thedistortion of mosfets.

Also contrary to popular conception,

THE AUDIO AMATEUR 2/1981

power mosfets may not necessarily beparalleled without the use of sourceresistances to equalize the current drawbetween devices. Unless the devices arematched, the designer cannot assumethat one device will not take on more orless than its share of the load. In morethan one commercially available ampli-fier paralleled mosfets are accompaniedby source resistors, to ensure that theircharacteristics are equalized.

without an additional g>in stage.

One more aspect affecting thesedevice’s audio quality is their relativelyhigh intrinsic capacitance, on the orderof 500-lOOOpF, which makes specialdemands on any circuitry which wouldrealize the high speed capability. For ex-ample, the current required to slewlOOV/$S into 1OOOpF is .lA, a c o n -siderably higher current than most frontend circuits are designed to source

Fortunately, in actual use much of thecapacitance is associated with the gate-source, which sees only a small fractionof the gate-to-drain voltage swing. Inour project amplifier, .003A from thefront end circuitry will drive the positiveoutput transistor’s capacitance at about4OVI@ for an effective capacitance ofsome 100pF. This is about five timesbetter than the raw capacitance figurequoted for the device, indicating thatmost of the effective capacitance is gateto source.

tiered power supplies to feed the

I must mention one final and actualdrawback. Because the transconduc-tance is low compared to bipolars, thereis significant loss in maximum power fora follower unless the source voltage candeliver several more volts than the sup-ply to the output transistor, resulting ineither lower efficiency, or the use of

:IG. 5

R 3

RI : Q6

Q 4

,b \/

R2

Fig. 5. Citation 12 conceptual schematic.

(,

Q 3 Q 7

,b

R5

ZIG. 6

P 3

RI X Q6 D

R 4

C l E,0

Fig. 6. Mozfet 12 conceptual schematic.

FIG. 7

03

D U A L D I F F E R E N T I A L

T R A N S I S T O R

D U A L D I O D E

4 O U T P U T

Y U S E D lN

L A T E R V E R S I O N

Fig. 7. Citation 12 schematic.

‘IG. 8

Q6

/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

U N L E S S O T H E R W I S E S P E C I F I E D

Fig. 8. Mm&-t 12 schematic.

THE AUDIO AMATEUR 211981 9

amplifier’s front end several more voltsthan the output stage.

C I R C U I T R YThe IRF 130’s selected for this projectare sufficiently similar to the originalHK-12 devices that they lit into theampli f ier ’s quasi -complementarytopology with a minimum of modifica-tion, as seen in the simplified schematicsof the HK-12 and the project circuit inFigs. 5 and 6.

In Fig. 5 we see the standard differen-tial input pair (Q, Qz) driving an npnvoltage gain transistor (Q) where Riand Rz are used to properly bias thethree transistors. The output of Q seesa bootstrapped current source (Rs, Rh,and C,) and a bias voltage source for theoutput stage. Q provides a level shiftingdevice which elicits complementary ac-tion from Q, the negative output tran-sistor, and Q+ provides the follower cur-rent gain to drive Q, the positive outputtransistor. The Fig. 6 circuit is basicallythe same; but Q and Q have becomepower mosfets and Qt has disappearedbecause the additional current gain isnot required to drive Q.

THE ACTUAL SCHEMATIC

In Figs. 7 and 8 we see the actualschematics of the two power amplifiers;for clarity the part numbers correspondto Figs. 5 and 6. In Fig. 8, Rii and CTfilter the power supply line to preventripple and transients from modulatingthe bias, causing noise and distortion.RI2 and C2 form a low pass filter to pre-vent spurious high frequency inputsignals from being amplified.

Resistor RI feeds 2mA of current

equally to Q and Qs. The 1mA of cur-rent through Q then goes through RZ toprovide the approximately .65V bias be-tween the base and emitter of Q. ThisDC current with an AC componentmodulates the current through Q,which, passed through the high im-pedance of R3 and R+ (bootstrapped byC,) produces a large voltage gain at thecollector of Q. This voltage gain isfollowed by Q, and on the negative halfof the circuit, by the Q,, Q combina-tion. In this scheme of things, Rs’s ac-tion is similar to Rz’s, and Rg limits Q’sgain.

The combination of Ri6, Ris, and Riodamp out the resonances which woulddevelop from the combinations of gaindevices and internal capacitances, andprevent parasitic oscillation. CS is inseries with RI0 to ensure that thisresistor is not driven by audio frequen-cies, which would overheat and destroyit. C*, a damping capacitor, enhancesthe circuit’s stability by providing asecondary high frequency loop, causingthe feedback loop to ignore the output’shigh frequency operation in favor of thefront end’s low-order output at frequen-cies above 800kHz. This allows theamplifier’s input stage to dominate thehigh frequency response with a less thantwo-pole characteristic, providing forhigh stability under transient condi-tions.

The bias network of Figs. 5 and 6 hasbeen replaced by Qp, Ri7, Ris, and Cg,forming a constant-voltage source trim-mable by R17 and frequency-stabilizedby Cs. DS ensures symmetric clipping,and D,, D2 provide a current path forthe flyback energy which an inductive

load may produce. Re, RT, and C3 formthe feedback loop, with a low frequencyrolloff of .7Hz.

A word is appropriate here to explainthe operation of the bootstrapping cir-cuitry of RB, R+, and Ci. The idea is tocheaply achieve the effect of a constantcurrent source load for Q, where theAC impedance of the load is very high,giving maximal gain for that stage, andw h o s e D C i m p e d a n c e i s RS + Rh(9.4kG). This provides about 3mA ofDC class A bias current for Q withoutloading the gain stage.

This effect occurs because the pointbetween RS and Rd is bootstrapped byC 1, which is connected to the amplifier’slow impedance output and causes thenode between RS and R+ to follow theoutput. Because the output voltage isnearly identical to the voltage at the col-lector of Q, the voltage across Ra isnearly constant, resulting in effectivelyconstant current through R+, the actionof a constant current source. In thisway, Q can swing full output into an 80load with only about 10 percent varia-tion about the bias current, as opposedto the f 100 percent variation thatwould occur without the bootstrapping.This contributes greatly to the system’sgain and linearity and achieves thehighly desirable effect of the constantcurrent source with great simplicity andlittle cost. The effect is good across theAC band, rolling off at about .14Hz onthe low end and at about 300kHz at highfrequencies.

CONSTRUCTIONStart by removing the PC board andstripping off all components except the

FIG. 9 r’eovR I G H T C H A N N E L T l +42v

3 AB L K

4 :,__z t_ 6 0 0 0

BLK/WHT I --IOOV _-- SO”

I

I,I,

9 COMBRN/ WH T

I

IL E F T iHANNEL ~+42V

Fi’. 9. Power supp(p schematic.L

10 THE AUDIO AMATEUR 2/1981

connectors. Use solder wick or a soldersucker, and take great care not toseparate the copper foil from the PCboard. You will also have to removemany of the components from theunderside of the chassis (see Fig. 13), in-cluding the heat sinks. While you mayelect to retain the original input and out-put connectors, I replaced mine withhigher quality gold-plated types.

You may also elect to retain theoriginal amplifier’s output breakersand/or thermostats; however, I removedthem as unnecessary. Personally, I don’ttrust the quality of connection offered bythe output transistor sockets (especiallyold ones) and I decided not to use them,preferring solder connections to theTO-3 pins. Remove and discard all thecapacitors on the underside of thechassis except the four computer gradeelectrolytic 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 deviceson the heat sinks, using mica TO-3 in-sulators, silicone grease, plastic shoulderwashers, 6-32 screws/washers/nuts, anda solder lug for electrical connection tothe transistor case (drain).

Figure 14 shows the output transistorwiring with the lead wires soldered tothe pins. The wires from the outputsfeed down through the holes in the

FIG. 10

PARTS LIST (one channel)

Rs, Ris 1onRN 3303R7 4703RB, Ru, RIS 68OQRI3 8203Rs, R,I lkRl 3.3kR3, R1, RIG 4.7kRs, RIO, Rlr 10kRI 18kR20 1OQ 1W 5% carbon

camp.R17 5k CTS vert. PC mount

PotAll resistors are % W f 2% metal filmRL-07, or * 1% RN-55D or equiv.unless otherwise specified.c1 470/5OV Radial Electrolytic

c”:390pf f 5 % Dipped Silver Mica20pF f 5% Dipped Silver Mica

c”:470/16V Radial Elect.4.7125V Tant.

CS 47l5OV Radial Elect.C7 .068/1OOV MylarQ, Q, Q MPS-L51 M o t o r o l a

Z:$MPS-LO1 MotorolaInternational RectifierlRF-130

D1, Dz, Ds lN40042 .01/1.4kV CAPS INPOWER SUPPLY4 .47/1oov

FIG. 11

Fix. Il. Component side of PCB.

FIG. 12

Fix. 12. Foil side of PCB

THE AUDIO AMATEUR 2/1981 11

chassis to the connector pins as shown inFig. 13. After mounting the outputdevices to the heat sinks, check first forpossible connection between the tran-sistor case or either of the two pins andthe heat sink. Use an ohmmeter withone probe connected to an unanodizedportion of the heat sink, (possibly in thetapped holes) and the other probetouching the various parts of the tran-sistor. The transistor must not connectwith the heat sink.

When you have mounted and wiredthe output transistors, reattach the heatsinks to the chassis and solder the wiresto the PC board connector pins. Note inFig. 13 that some of the pins are hard-wired together to achieve the best possi-ble connector contact by paralleling con-nections to the PC board. Also shown inFig. 13 are the .47pF/lOOV capacitorswhich I placed in parallel with the powersupply capacitors to achieve lowerdissipation factor at the highest frequen-c ies . These are opt ional and the

amplifier will run quite well withoutthem. I also placed .OlpF capacitorsacross the dioded bridges to suppress rfemission by the diodes; these too are op-tional.

Figures 11 and 12 display the compo-nent layout on both sides of the PCboard and are self-explanatory. Pleasenote the 18 gauge jumper wires on theback. Two replace the coil/resistor inseries with the output of the original cir-cuit (a standard feature on 99 percent oftoday’s solid state amplifiers), and toreduce the distortion inherent in theoriginal PC board. The original PC art-work, of which more than one versionapparently exists, takes the feedbackfrom a point separated from the actualoutput node by a short length of copperwhich carries an asymmetrical portionof the output current. The feedback thusoperates off a voltage several milivoltsremoved from the actual output,generating even-ordered distortions.

As always, take special care to orient

the polarities of the components andavoid damaging the components byoverheating the leads when soldering.As this project is for the more advancedhome-brewer, I will not mention thatthe use of a solder gun is forbidden.Leave the TO-92 transistor leads as longas possible. Q and Q require press-fitheat sinks.

TURNING ON THE AMPLIFIER

When you first switch on the amplifier,turn the bias pots so as to exhibit max-imum resistance. This minimizes theamplifier’s initial bias current when firstfired up, a valuable safety precautionwhich should be checked with an ohm-meter. The pot’s wrong extreme settingshould read about 0 ohms and the rightsetting will read 5000 ohms, dependingon the polarity of the probes (note the pnjunction of the bias transistor).

The following test is important, andshould be carefully performed, first for

Continued on page 14

ZIG. 13

INPUT OUTPUTFUSE FUSE

3 A P I L O T 3A OUTPUT INPUT

GND

I

t. t-

‘~_. _- _ _.. .- ---.__._.__-./

(L3LINE CORD

Fig. 13. Underside of chassis.

1 2 THE AUDIO AMATEUR 21198 1

BUILD A MOSFET CITATION 12Continued from page 12

one channel and then for the other.Remove the AC line fuse of the channelnot being tested and install a 1A fastblow fuse in the channel being tested.Drive the channel at a low level (. IV) at1kHz by an oscillator whose ground is

TABLE 1Distortion Figures of the Two Units

STOCK HK 12 MOSFET HK 12

Left1w20H ,025

1OOH ,0241kH ,0185kH ,02320kH ,045SMPTEIM ,012

low20H ,025

1OOH ,0221kH ,0155kH ,02120kH ,095SMPTEIM ,034

3ow20H ,0221OOH ,025

1kH ,0225kH .027

20kH ,190SMPTEIM ,060

60W20H ,075

1OOH ,0901kH ,0855kH ,075

20kH ,390SMPTEIM ,140

Right Left Right

,027 ,010 ,010,023 ,009 ,009,022 ,006 ,007,024 ,016 ,019,060 ,050 ,064

,015 ,020 ,024

,028 ,008 ,008.023 ,006 ,007,020 ,005 ,006,025 ,015 ,018,120 ,050 ,058

,040 ,012 ,015

,028 ,007 ,008.028 ,006 ,007,024 ,004 ,005,031 ,013 ,016,240 ,044 ,052

,065

,050,070.080,076,540

,130

,007

,007,007,004,013,041

,007

,009

.008,007.004,015,055

,007

FIG. 14

DRAlN

R E D L E A D

G A T E

G R A Y L E A D

S O U R C E

B L A C K L E A D i

-l0

I N T E R N A T I O N A L R E C T I F I E R /RF-/00

Fig. 14. Mosfet wiring diagram.

floating with respect to the oscilloscope(you can do this by using a 3-to-2 prongcheater adaptor on the oscillator’s ACline if it has an earth connection). Theosc i l l o scope i t s e l f shou ld be ea r thgrounded. Measure the output withouta load, connecting it and its ground tothe oscilloscope input.

Set the scope vertical scale at 5V/divand the horizontal at .2mS/div. Using

only a variac, slowly raise the line voltageto the channel while watching the out-put. The output voltage should exhibitDC offset at first and then settle into a*2V sine wave; if it does, continue to

slowly raise the voltage until the full linevoltage is achieved. If you manage thiswithout blowing the fuse, the next step isto slowly increase the input voltage from


FIG. 152608

2 2 LIB --

FIG. 16

- 3o”--

1 1 I 1/ II /O /co ,K IOK IOOK

_

1 4 THE AUDIO AMATEUR 2/l%? 1

FIG. 17

/ - -

.3 -- /

- 6 0 W - R T-6OW- LT

. / -- i- - - - _ _ _ _

- -- ---_--- _ _ ,-/W-RT

.03 - -

. 0 / - -

, 0 0 3 - -

,002 RESIDUAL DISTORTION____________________--______-___---

,001 I , 1 4 I 1 I I I1 I 1 iIO 30 100 300 lK 3K IOK 3 0 K

Fig. 17. HK-I.? distortion curues.

FIG. 18

I - -

.3 - -

. / -- &&k x:

/.03 -- p z

/ /

.Ol --/ y

=_z_-_~.=Y,_ _ _ _ _ _ ’ 2

-1 - - - - - - I ’

,003 - -

.OOZ R E SIDUAL DIS T O R TIO N_ _ _ _ _ _ _ _ _____~~_-_~~. _._~~~.___

.oo/ I / I I I I II I I I I IIO 3 0 IGC 3CO lK 3K IOK 70

Fig, 18. MF-12 distortion cumes.


the oscillator until the output reachesclipping (at about f 3OV), watching foroscillation or severe distortion.

Having accomplished that, repeat thetest for the other channel. When both


Fig. 21. 20k THD wauc HK-12

Fig. 22. 20k square waue HK-12

Fi’. 23. 20k square ulaue MF-12

Fi,. 19 1kHz THD uxue MF-12. Fig 20. 20k THD W~UE MF-12

1 6 THE AUDIO AMATEUR 211981

Fig. 24. Current through outpuf stage MF-12.


channels operate successfully without aload, repeat the test for each channelwith a 4A fuse and an 89 load, stillwatching for waveform problems or fuseblowing. You will see crossover distor-tion at this point, which is normal for anunbiased amplifier.

When both channels can successfullydrive an 83 load at full power, it is timeto bias the amplifier’s output stage. Putmost simply, we want to adjust the biaspotentiometer so as to remove crossoverdistortion, a condition which cor-responds to about lOOmA of idle currentthrough the output stage. We can do thisas follows.

Using the test setup shown in Fig. 2.5(see following paragraphs) and with thedrive channel at +8V into the 8h2 load,look at the notch distortion on the otherchannel and adjust the bias poten-tiometer until the spike just disap-pears--and not more. Be careful here: it iseasy to set the bias too high in search ofperfection. Disconnect the load andmonitor the operating temperature of

the channel for 15 minutes by placingyour hand on the heat sink. Ideally thechannel should operate slightly warm atidle. If it becomes hotter than this, ad-just the bias current down. If it stayscool, you may with care, slightly in-crease the bias.

After 15 more minutes recheck eachchannel’s temperature and distortion,readjusting as necessary. If you are for-tunate enough to possess a current pro-be, this entire procedure can be ac-complished by simply setting each chan-nel to draw lOOmA from the supply,again rechecking after 15 minutes.Alternatively, you may monitor the biascurrent by inserting 1Q in series with thepositive power supply lead and measur-ing 1OOmV DC across it.

USING THE AMPLIFIERTO ANALYZE ITSELFIt may surprise you to know that onecan quite easily perform cursory distor-tion analysis on this (and many) poweramplifiers using an 83 power resistor,an oscilloscope, oscillator, and the am-plifier itself. The oscilloscope need notbe of particularly high quality and theoscillator need not have low distortion.

Figure 25 shows how one amplifierchannel (A) uses the oscillator’s signal tosource current into the other channel (B)through an 83 resistor. At the output ofchannel B we will find that channel A’svoltage output has been divided bychannel B’s damping factor. In thisamplifier the broadband output impe-dance is .0640 (83/.0643 = 125 damp-ing factor), resulting in a conditionwhere channel A’s voltage output isreduced to .8% at the channel B output,plus the distortion of channel B causedby sinking the current.

At channel B’s output we thereforesee the channel’s notch and other distor-tions in response to channel A’s driveconditions, but much less obscured bythe fundamental voltage. Because in thisand most other amplifiers the distortionsare dominated by current fluctuations (aconcept addressed b y t h e S t a s i samplifiers) this serves as a useful and in-expensive bench technique and for themost part it is easy to see the distortionadded to a .8% fundamental. Thisprocedure is useful in biasing theamplifier as You can clearly seecrossover notch and other effects.


FIG. 25

S H O R T I N G

P L U G

Fig. 25. Distortton hookup.

18 THE AUDIO AMATEUR 2/1981


This test will also reveal distortionscaused by poor connections between thePC board and the connectors (some-thing I encountered in the actual unitand which I cured by cleaning andreseating the connectors). To evaluatethe percentage of distortion rememberthat the voltages seen by the ‘scopedivided by the output voltage of theother channel will equal the fraction ofdistortion. For example, if channel A’soutput is at + 28V (20VRMS) throughthe 8 Ohms into channel B, then chan-nel B’s output will show a signal com-prised of channel A’s fundamentaldivided by the damping factor (for

* .225V sinewave) plus an equivalent ofchannel B’s distortion when operated atthat level into 8ti. In this case a .28Vpeak distortion spike would indicate a1% peak distortion (not uncommon inamplifier’s with .1 ‘$J average distortion).

PERFORMANCEFigures 15-24 document the performanceof the prototype amplifier, which wasbuilt without selected components. Ofparticular note are the distortion andsquare wave comparisons against theoriginal, showing a significant improve-ment in distortion characteristic andtransient response. Figure 24 shows thecurrent waveform through half the out-put stage. Note that the current throughthe output device idles down instead ofshutting off abruptly as in bipolar Class

B and AB amplifiers. This gives reducedcrossover distortion and the quasi-ClassA operation at lower levels which other-wise we could only achieve in a bipolarcircuit with dynamic biasing.

The amplifier is sonically a significantimprovement over the original, par-ticularly in the high end where the Cita-tion 12’s veiled characteristic is replacedby a detailed, somewhat sweet sound.The imaging and midrange definitionare also much improved; but the bassresponse (one of the Citation 12’sstrongest points) remains much thesame-ideal for planar loudspeakers likeMG II’s, less so for acoustic suspensionwoofers.

In conclusion, I hope many of youwill try this one and enjoy yourselves.No amplifier sounds as good as the one

POWER MODIFICATIONSfor the ST- 150-BJ- 1,DH-200, and ST-400

Continued from page 45

signal across the power supply caps whendriven hard; this signal is the combina-tion of both channels of load, and itcouples the two channels together,resulting in a variety of distortion pro-ducts. The stock regulator reduces thissignal by 60dB (a l-volt signal is reduc-ed to 1 millivolt at the regulator output);Walt’s additions to the circuit reduce itby 80dB (1 volt is reduced to 100 micro-volts at the output). Either way you aregoing to see a substantial reduction inproblems from this source.

IMPLEMENTING CHANGES

Pat used the Old Colony kit for the Passf 32V supply as the basis for his

regulator; he wanted a 50 volt output, sohe substituted a 33V zener for the 15Vzener supplied at Z2 (see Table 1). Patchanged the value of Rs to 0.333 to givethe regulator a 9.5A max current (seemy TAA l/80 article for details). He alsoadded a 2200pF cap to the output to flat-ten the impedance curve at a low fre-quency. The Old Colony kits turned outto be quite a bargain, as Pat ordered twokits and received two complete plus andminus regulators in each kit. The kits allcontained LM340T regulators, howeverthis is the result of my own drawingerror in the l/80 article which showed anLM340 regulator on the negative sche-matic, where an LM320T-15 shouldhave been. (Present kits include LM340Tand LM320T regulators. - E D . /

We swapped the ST-150 output tran-sistors with the regulator transistors(TR,) because the TCG180 and 181’ssupplied by Old Colony are 2OOW,lOOV, 30A devices, and the amplifieroutput transistors will see larger powerproducts than the regulator passelements. This change just gives a little

4 6 THE AUDIO AMATEUR Z/1981

margin to the design and was not reallynecessary. Watch the lead length on thetransistors themselves if you swap,however. The ST-150 heatsink is sothick that the leads barely make it intothe sockets, and some brands of tran-sistor make marginal contact. If indoubt, use the old devices.

Pat’s listening comments are morethan supported by my own experience; Ithink the combination of changescovered in his article turned a ratherlackluster, low/medium power amp intoan o p e n , d y n a m i c , e x c i t i n ghigh/medium (140W/chan) amp whichis sonically the equal of anything I haveheard. The photos show the care andcraftsmanship Pat has applied to thechanges, and demonstrate that theST-150 was indeed an ideal candidateboth physically and electrically for theaddition of full regulation.

RESULTS AND SIGNIFICANCEThe test results show something evenmore interesting than low distortion orhigh power. The distortion of theregulated amplifier is nearly constant atall power levels, and across the entireaudio bandwidth. Let me stress the im-portance of this finding.

The normal pattern of transistoramplifier designs, and of the unmodifiedST-150, is to have the THD measure-ments fluctuate by 50% of their 1 kHzvalues, or more, at various points in thefrequency spectrum. Also, the varia-

tions get more pronounced at higherpower levels, since the output im-pedance of the unregulated supply isfairly high, and also strongly frequencydependent.

The fact that we can control sine wavesteady-state distortion with regulationalmost certainly means that operating-point-dependent distortions which arethe result of supply variations, such asdynamic compression and some types ofTIM, are being controlled as well. Wehave solid, measurable evidence thatregulation of the supply can make asignificant contribution to overall sonicperformance.

TEST RESULTSWe performed tests into an 8fI nonin-ductive load, both channels driven, aftera 1 hour warmup period at 25W and1kHz signal. Unregulated readings aretaken from Dyna supplied charts.

IM vs. POWER

POWER REG. AMP UNREG.(W) IM % AMP’ IM %

.1 .Ol ,0111 .Ol ,018

10 ,014 ,02250 ,016 ,04075 ,017 .050

100 ,018 n/a140 ,020 n/a

THD vs. FREQUENCY, SEVERAL POWERS

Regulated Amp Unregulated Amp

Power .75W 7.5w 75w 140w .75w 7.5w 75wFreq.20Hz ,030 ,030 ,030 ,033 ,035 ,035 ,0451OOHz ,030 ,030 ,030 ,033 ,035 ,035 ,0451OOOHz .030 ,030 ,030 ,033 ,030 ,030 ,0401OkHz ,030 .030 ,030 ,033 .031 ,031 ,03520kHz ,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,there is probably an error or two; 1recommend that you cross-check thrparts list. schematic, and pc boarclayout.

My amplifier has performed well fo1six months, as of this writing, and I en-countered no exotic problems during itsconstruction and testing. Nevertheless,inevitably some of you will need help,which you may obtain by calling orwriting me at Threshold Corporation,1832 Tribute Road, Suite E, Sacramen-t o , C A 95815-(916) 927-5061.Although I am pleased to help, those ofyou who call should be reconciled to thepossibility of only a short conversation.

Good luck, and have fun. 0

Audio AidsContinued from page 50

were used throughout, not so much foraccuracy as for lower noise and increas-ed stability.

I used LM353 bi-FET op amps inplace of the 5558’s.

The project finally completed, I hook-ed it up to my scope to view it. Since Idon’t have a distortion analyzer, I canonly report the results of listening testsmade when I attached it to my system.PE reports that the unweighted noiselevel of the equalizer in the output was66dB below 1V and when the measure-m e n t b a n d w i d t h w a s r e s t r i c t e d250-20,OOOHz. To exclude hum andrandom noise, the measurement was74dB below 1V.

With the modifications incorporatedinto the project, this 74dB can be easilyextended through the entire audio spec-trum. Listening tests confirm this whenthe equalizer is switched in and out ofthe system and there is absolutely noaudible difference in noise levelthroughout the spectrum. (This is withall controls flat as I assume the PE unitwas when tested, although the articledoes not state that this was done.) Evenwith the lower bands turned up there isno appreciable hum.

Although I have never heard an un-modified unit, I am convinced thesemodifications were more than worth theextra money necessary to purchase theupgraded parts needed to do the job. Inlistening tests the equalizer is clear,responsive, and versatile enough totailor many rooms to your audio needs.It is definitely one of my favorite piecesof equipment.A LAN M. VALENTELoudonville, NY 122 11

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