Adcom’s GFP 565 Preamp Mods, Part 3 - Walt Jungwaltjung.org/PDFs/GFP565_Pt3_Galo_AX_0104.pdf ·...

In Part 2 of this series, we complet-ed the installation of a new powersupply based on Walt Jung’s Im-proved Regulator. In this install-

ment, we’ll replace the line stage with anew, high–performance design. But, be-fore beginning the line stage, there area few other options to consider.

During Victor Campos’ tenure atAdcom, they called him The BufferMan, and with good reason. Every pre-amp Adcom produced under his direc-tion had actively–buffered tape outputsto isolate the signal path from nonlin-ear loads. The GFP–565 preamp used apair of dual op amps configured asunity–gain voltage followers to bufferthe two pairs of tape outputs.

At that time, many of the IC op ampsAdcom used were manufactured by Lin-ear Technology, and bore the familiarLT logo. But, Adcom had its own partnumbers printed on these devices toconceal their identity. Now that the pre-amp has been discontinued, I won’t begiving away any trade secrets by identi-fying them. The tape output bufferslabeled Adcom 7Awere LT1057FET–input dual op amps.

During my evaluation of the 565 pre-amp for Stereophile back in 1989, one ofthe LT1057 op amps in my review sam-ple failed. The output of the device wentto the negative supply rail (−18V DC),and the non–inverting input was at a po-tential of several volts negative. This pro-duced a potentially dangerous thumpwhen you moved the recording selectorswitch to the same position as the listen-ing selector. Subsequently, anotherLT1057 failed in my other 565 preamp.

Linear Technology did failure analy-ses on both chips, and found that the

positive supply connections had failedinternally. This did not give me a greatdeal of confidence in the LT1057, so Ireplaced all of them with the AnalogDevices AD712JN, one of thebest dual op amps availableat that time (www.analog.com).

The AD712 is still a solidperformer for this application,but if you have really high-quality recording equipment,you may wish to use some-thing even better. My firstchoice is the TI/Burr–BrownOPA2604AP (www.ti.com).Other high–end replacementsinclude the TI/Burr–BrownOPA2134- PA.

Adcom used 7.5k pull–down resistors on theLT1057s, which forced theoutput stages into Class–Aoperation. Retain these resis-tors if you use the AD712. Opamp output stages have im-proved considerably sincethe AD712 was designed, soremove these resistors if youuse any of the other devices.In particular, the OPA2604 isan all–FET device, including

the output stage, so you should definite-ly remove the pull–down resistors forthis device. The modified tape outputbuffer circuit is shown in Fig. 5.

Now it’s time to add a new line

stage to the mods described in

Parts 1 and 2. By Gary Galo

Adcom’s GFP 565 Preamp Mods, Part 3

40 audioXpress 1/04 www.audioXpress.com

(Remove for OPA2604AP - See Text)

(Remove for OPA2604AP - See Text)

(*IC402a)

*(IC402b)

* = New Value and/or New TypeLeft Channel Shown.Right Channel Designatorsare in Parentheses ( ).

3

1

4

2

*IC401b

OPA2604AP

*IC401a

OPA2604AP

V-

V+

V-

V-

(R414)R413

7.5k

(R406)R405

7.5k

(R416)R415

475

(R412)R411

1k

(R410)R409

1M(R420)R419

100k

(R418)R417

100k

(R402)R401

1M

(R404)R403

1k

(R408)R407

475

FIGURE 5: The modified tape–output buffers. OPA2604dual op amps replace the original LT1057s, andpull–down resistors R405, R406, R413, and R414 areremoved. The pull–down resistors should be retained ifAD712 op amps are used.

PHOTO 22: The modified tape output buffers, using OPA2604 op amps. The pull–downresistors are removed for the OPA2604.

A-2291-5

• Remove IC401 and IC402. Replacethese with OPA2406s, OPA2134s, orAD712s (Photo 22).

• Remove the 7.5k pull-down resistorsR405, R406, R413, and R414if youuse any of the recommended devicesother than the AD712.

RESISTOR CHOICESBefore proceeding, you will need to de-cide what type of resistors you intend touse for the remainder of this project.Adcom used Roederstein/Resista MK2resistors in the GFP–565, but they havesteel end caps, and are now sonicallyoutclassed by a number of other resis-

tors. I recommend using non–ferrousresistors for all of the circuitry de-scribed later, and in Part 4 (which willcover the phono preamp).

The most cost–effective choice is theVishay–Dale CMF Type RN60 sold byMouser and Welborne Labs, which Idiscussed in Part 2. Ron Welborne saysthat he spot–checks the Vishay–Dale resistors to make sure that they arenon–magnetic, and all that I have re-ceived from Mouser have also beennon–magnetic. The Vishay–Dale RN60

resistors will drop right into the exist-ing resistor footprints on the Adcom PCboard.

Michael Percy Audio and WelborneLabs still have some old, all–copperHolco H4 resistors in stock. E-mailthem to see whether they have old-stock Holcos in the values you need be-fore purchasing. Holco H4 resistors willfit the existing Adcom footprints ifstood on end, as shown in several of thephotos later in this article. If you canget them, they are excellent.

audioXpress January 2004 41

AD744KN ALTERNATEOUTPUT CONNECTION

5

7

4

3

2

7

4

6

3

26

4

73

2

(**C252)

(With Heat Sink)

PC Board GND

(*R210)

(*R206)

BALANCE VOLUME

(VR204b)(VR201a)

JUMPERBYPASS

FROM PHONO PREAMP OUTPUT

MOUNTED ON HEADER

MOUNTED ON PERFBOARD

* = New Value and/or New Type** = New PartLeft Channel Shown.Right Channel Designatorsare in Parentheses ( ).

+

*IC201 (*IC202)AD744KN

+ To R237

-14V

+14VFrom VOL

S204MONO

TO

R C

H

TO R CH

"B" -14V

"A" +10V

RY902

RY901

(OPTIONAL)AUX OUT

MAIN OUT

"A" +10V

"B" -14V

TO R CH

-14V-14V

+14V+14V

N/C NO LONGER USED

+

*IC203 (IC204)AD811AN

**C251

1nF

50k

VR204a

50k

VR201bLISTENING

S206

SIG PROC INS205

PHONO

VIDEO/AUX

TAPE 2

TAPE 1

TUNER

CD

**R251 (**R252)

221k

*R239 (*R240)

100

**R239a (**R240a)

100

*R213 (*R214)

1.0k

*R211 (*R212)249

**R253 (**R254)2.49k

**R251 (**R252)

221k

**R257 (**R258)1.0k

**R255 (**R256)

1.0k

*R237 (*R238)

100*R209

10M

*R205

475

FIGURE 6: The new line stage.An AD811AN current–feed-back op amp buffers anAD744KN, with local feedbackaround both devices. The AuxOutput is optional.

PHOTO 23: Output resistors for the optionalAux Outputs. These resistors connect theline stage outputs to PC traces previouslyused for the signal processor outlines.

PHOTO 24: The modified output muting relay. Relay RY901, previously used to mute theNorm and Lab outputs, is now connected to the Aux Outputs with two insulated jumpers.

A-2291-6

I built my “Number 1” preamp withCaddock MK132 precision film resis-tors in all of the audio circuits. Theprice tag will be a deterrent for somebuilders, but on a high–resolution sys-tem they are well worth the expense.Michael Percy, Welborne, and PartsConnexion stock these. If cost is no ob-ject, the Vishay S–102–series bulk foilsare considered by some to be the finestaudio resistors available. But at over$11 a pop, this will be an expensivepreamp project!

I won’t refer to specific resistor typesfrom now on (with one exception)the

final choice is yours, based on your lis-tening requirements and budget.Michael Percy Audio stocks the Vishayresistors. To fit the Adcom resistor foot-prints, the leads on the Caddock andVishay resistors should be bent out at aslight angle, and then bent straightdown.

AUX OUTPUTSIn Part 1, I suggested adding a pair ofAux Output jacks for biamping. If youdon’t need the extra pair of outputs, youcan skip this section. In the followingsteps, I assume that you have installed

the jacks and connected them to the PCboard as outlined in Part 1. Now add asecond pair of output resistors androute them to the PC traces previouslyused for the signal processor outlines(Fig. 6 and Photo 23).

• Remove jumper J28.• Make a ¼″ cut in the PC trace be-

tween S205 and jumper J59 in thesame front–to–back location as theJ25 jumper. Be sure the cut area iscompletely clean and free of coppertrace material.

• Scrape ¼″ of lacquer from the end ofthe cut trace leading to J59, and tinthe bare copper.

• Solder a 100Ω resistorR240abetween this tinned PC trace and therear–most hole previously occupiedby J124.

• Connect a second 100Ω resistorR239abetween the hole left vacantby the removal of J28, and the near-est hole left vacant by the removal ofJ27. These are not the J27 and J28holes that go to the switch bankthey’re the other ones. Use sleevingon the resistor leads.

This completes the connection of theAux Outputs. But, they have no mutingrelay. This is an easy problem to fixsimply connect the muting relay previ-ously used for the Lab and Norm out-puts (Photo 24).

• Make ¼″ cuts in the two PC tracesthat go from the left and right Laboutput back to J62 and J63. Cut thembetween RY901 and J62/J63, as closeas possible to relay RY901. Makesure to remove all copper material.

• Remove C207 and C208.• Add jumpers made from insulated

hookup wire between the processorout left and right PC pads (now theAux Outputs) and the C207 and C208hole closest to RY901.

The Aux Outputs will now be muted onturn–on and turn–off just like the mainoutputs.

VIDEO/AUX INPUTSMy system hasn’t been biamped in sever-al years, so I no longer need the Aux Out-puts. I found another use for the pair ofhigh–quality RCA connectors mounted


in the old processor loop holes. I am nowusing a DVD player that plays regularCDs, SACDs, and DVD–Audio discs. Theplayer must be used as a stand–alonewhen playing SACDs, but I use my out-board digital processor for CDs and allDVDs, including DVD–Audio discs. So, Ineed an additional pair of high–qualityline input jacks.

I decided to replace the Adcom

PC–mount Video/Aux jacks with thenew jacks, connecting the new jacks tothe Video/Aux traces on the Adcom PCboard (Photo 25).• Remove jumpers J76 and J77. This

interrupts the signal paths betweenthe Video/Aux jacks and the selectorswitches S206 and S207.

• Connect the new jacks to the selec-tor switches using short lengths ofD.H. Labs BL–1 interconnect. Theshields and black center conductorsare soldered to the ground washerson the new RCA jacks (the groundwashers should already be connect-ed to the PC board). On the otherend, the shields and black centerconductors should floatcut theseleads short and use heat-shrink tub-ing to insulate them. Connect thered center conductors to the J76(right) and J77 (left) holes closest toS207. You’ll probably need to en-large the holes slightly.

Note that the old PC–mount Video/Aux jacks are not removed from the PCboardthey have simply been discon-nected. The new jacks will allow you to

connect your DVD player’s output tothe high–quality Video/Aux inputs, andyour digital processor’s output to theCD inputs, or vice versa.

THE LINE STAGEAdcom’s original line stage consisted ofa Linear Technology LT1056 op amp(Adcom 6A) buffered by an LT1010(Adcom 1A) placed inside the feedbackloop. The LT1056 is an FET–input de-vice with low enough DC offset to allowdirect coupling of the output. This linestage was an excellent performer at thetime it was designed, but there are anumber of op amps and buffers thatoffer better performance today.

A complete schematic of the new linestage, including input switching, bal-ance and volume controls, and outputmuting is shown in Fig. 6. This linestage is based on the Walt Jung designI published in the Philips DAC960 modseries in TAA back in 19921. Ahigh–performance op ampIC201provides the gain. IC201 is buffered bya high–speed, current–feedback op-ampIC203placed inside the feed-back loop, which provides 100mA of


PHOTO 25: The Video/Aux inputs connectedto new input jacks. This is an alternate usefor the new RCA jacks if the Aux Outputsare not needed.

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output current capability. In the origi-nal Adcom line stage, the voltage gainwas set at 11. The new circuit has a volt-age gain of 5, which is plenty due to themuch–improved dynamics of the newline stage powered by the new supplyregulators.

The design shown in Fig. 6 has oneimportant refinement: the addition oflocal feedback around the gain opamp, IC201. This circuit was first de-scribed by Walt Jung in his column“Walt’s Tools and Tips,” for a time aregular feature in Electronic Design2.More recently, Walt described thistopology in the 2002 edition of Ana-log Devices’ Op Amp Applications3.

The local feedback around IC201serves two purposes. First, the effectiveopen–loop bandwidth of IC201 is in-creased. Second, resistor R253 is ap-proximately equal to the impedance ofthe volume control, set to a normal lis-tening level, at the non–inverting inputof the op amp. This helps match thesource impedances at the op amp’s in-verting and non–inverting inputs. Localfeedback is also used around theAD811.

The active devices are the same asthose used in the DAC960 projectanAD744KN op amp buffered by anAD811AN high–speed current–feed-back amplifier. The AD711KN has aunique feature: the output can betaken from compensation pin 5, by-passing the Class–AB output stage.This is how the chip was used in theDAC960 project. Walt informed methat pin 5 is capable of driving thehigh–impedance local feedback net-work in the new topology. Figure 6shows this alternate output connec-tion, in addition to the conventionalconnection to Pin 6.

In the Analog Devices Op Amp Appli-cations article, Walt suggested themore recently designed AD825AR,which, on paper, seems to offer evenbetter performance than the AD744. Idon’t yet have enough experience withthe AD825AR in this design to recom-mend it, but I may report on it in a fu-ture issue.

OLD CIRCUIT REMOVALBegin by removing some of the old linestage components.

• Remove IC201 and IC203 in the leftchannel.

• Remove IC202 and IC204 in the rightchannel.

• Remove D201 (L) and D202 (R).• Remove R235 (L) and R236 (R).• Remove C201 (L) and C202 (R).• Remove C223 (L) and C224 (R).

Now change the input and output se-ries resistors (Photo 26).

• Replace R205 (L) and R206 (R) with475Ω resistors.

• Replace R239 (L) and R240 (R) with100Ω resistors. Next, you must replace a few more

resistors.• Replace R237 (L) and R238 (R) with

100Ω resistors.• Replace R213 (L) and R214 (R) with

1.0k resistors. Do not put the new re-sistors in the old locations. Instead,put the new resistors in the holes ad-jacent to the original R213 and R214footprints. This will allow adequateclearance when you install the opamp assemblies. (These adjacentholes are electrically identical to the


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ones marked “R213” and “R214.”)• Replace R211 (L) and R212 (R) with

249Ω resistors.

R209 and R210 are 10M bias returnresistors for the op amps. These resis-tors keep the DC offset at safe levels incase the volume control wiper opens orbecomes intermittent. The currentsthrough these resistors are so small

that the steel end caps in the originalRoederstein MK2 resistors are probablyquite benign. However, if you want tobe a real purist, you can replace themwith Vishay–Dale VMF type RN55 resis-tors (I did). Don’t waste money on theexpensive Caddock or Vishay resistorsin these locationsthey’re unlikely toyield any further improvement.

• Replace R209 (L) and R210 (R) with10M Vishay–Dale CMF type RN55.

Bandwidth limiting for the new cir-cuit is set to 330kHz by the R/C networkR205/C251 (R206/C252; in the assemblysteps that follow, all components desig-nators in parentheses refer to the rightchannel). C251 and C252 are solderedacross the left and right sections of the balance control, on the bottom ofthe PC board (Photo 27). The best ca-pacitor for this application is the Reli-able MultiCap PPMFX polypropylenetype; a lower–cost polypropylene alter-native is the Wima FKP–2.

• Solder C251 (C252)1nFacrossthe left and right sections of the bal-ance control, on the bottom (trace)side of the PC board.

The new op amps and buffers won’tsimply drop into the existing PC foot-prints. Gain op amp modules IC201 andIC202 must be built up on an 8–pin DIPheader with the local feedback resistorsR251 and R253 (R252 and R254). I pre-fer the Tyco Plug Adapter Assemblysold by Allied, which has gold–platedpins. The Aries header sold byDigi–Key has tin–plated pins, and will


PHOTO 27: 1000pF capacitors are installedacross the left and right sections of thebalance control for bandwidth limiting.Wima FKP–2 polypropylene caps are shownhere. Reliable MultiCap PPMFX types arean even higher–quality alternative.

PHOTO 26: New 100Ω main output resis-tors are shown at the top of the photo. Re-placement 475Ω input resistors are in thelower right, between two of the switches.

also work. Both have forked contactson the top, which facilitates solderingthe op amp and resistor to the header.Assembling the headers is relativelyeasy (Photo 28).

• Cut off the thin part of the lead frompin 2 of the AD744 op amp. Leavethe wide portion of the lead intact.

• Solder the op amp to the header, ex-cept for pin 2.

• Solder R251 (R252)221kbetweenpin 6 and pin 2 of the op amp.

• Solder R253 (R254)2.49kbetweenpin 2 of the component carrier andthe R251 (R252) lead that is alreadysoldered to pin 2 of the op amp.

• Solder the two assemblies in theIC203 and IC204 footprints. Pay care-ful attention to orientation, which isthe same as the original op amps.

If you decide to try the AD744 with theoutput taken from pin 5, you can easily

move R251 (R252) from pin 6 to pin 5 ofthe op amp. In this case, I suggestputting 100Ω resistors R237 and R238 onthe bottom of the PC board. This willmake the output connection to pin 5 eas-ier. Note that the 221k feedback resistorsmust be connected to the same pin asthe output, as shown in Fig. 6. After theline stage is completed, you may wish toconduct some listening experiments tosee which connection you prefer.

You can build the AD811 modules ontwo small pieces of perfboard, ⁵⁄₈″ × ¹⁵⁄₁₆″,or 6 solder pads wide by 9 solder padslong (Photo 29). Here’s how to build themodules:

• Solder unused pins 1, 5, and 8 of theAD811 op amps to pads on the perf-board. This will hold the op amp inplace while you connect leads to theother pins. Orient the op amps alongthe length of the perfboard, leavingtwo rows of holes between pins 4


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1

2

3

45

6

7

8

AD811ASSEMBLY

LT1010PC FOOTPRINT

PC Board GND

1

2

3 5IN

V+ BIASOUTV-

R2571k

R255 1k

FIGURE 7: Diagram showingconnections from theAD811 modules—assem-bled on small perfboards—to the LT1010 footprint onthe Adcom PC board. Thebias connection for theLT1010 is not used.

PHOTO 29: Front and back views of theAD811 modules, showing the local feed-back resistors. A clip–on DIP heatsink isused on the AD811.

PHOTO 28: Two views of the AD744 op ampmodules using Holco H4 resistors. The opamps are soldered to a header, along withthe local feedback resistors. R253 (R254)is soldered between pin 2 of the op ampand pin 2 of the header.

A-2291-7

and 5, and the edge of the perfboard.• Solder R255 (R256)1kbetween

pins 2 and 6 on the back side of theperfboard.

• Mount R257 (R258)1kon the opamp side of the perfboard. Solder one

lead to pin 2 of the op amp, or the cor-responding lead of the R255 (R256).Solder the other lead to a spare sol-der pad, and solder a 4″ length of in-sulated hookup wire to this lead.

• Solder four 2″ leads of 22AWG solid

hookup wire to pins 3, 6, 4, and 7 ofthe op amp.

• Slide clip–on DIP heatsinks ontoeach AD811. 8–pin DIP heatsinksare fine, but they are hard to attainfrom most distributors. I used theAavid heatsinks made for 14–pinDIP applications. Center theheatsink on the AD811 and use adab or two of five–minute epoxy tokeep the heatsink in place. Allowthe epoxy to cure before proceeding.

Now, install the modules on theAdcom PC board. Figure 7 shows howthe modules connect to the LT1010 foot-print.

• Bend the four bare leads of the mod-ules as shown in Photo 29. Put shortlengths of sleeving over the leadsand install the leads in the LT1010PC footprints, making sure that theconnections match the wiring dia-gram in Fig. 7 (Photo 30). Solder thefour leads to the bottom of theAdcom PC board. Note that the biasconnection in the LT1010 footprintis not used.


PHOTO 31: The completed line stage builtwith Caddock MK132 resistors. R237 andR238 have been soldered to the bottomside of the PC board, allowing easy con-nection to pins 5 or 6 of the AD744.

PHOTO 30: The completed line stage usingHolco H4 resistors. The insulated wiresfrom the AD811 local feedback resistorsare soldered to the ground pads previouslyoccupied by C201 and C202.

• Connect the insulated leads fromR257 and R258 on the AD811 mod-ules to the ground holes on theAdcom PC board previously occu-pied by C201 and C202. These arethe C201 and C202 holes closest tothe preamp’s front panel. Trim the

leads so that they are just longenough to reach comfortably. Solderthe leads in place (Photo 30).

• Alternately, you can route theseleads through screw holes in the PCboard that were used to hold theLT1010 heatsinks in place and sol-


PARTS LISTTAPE OUTPUT BUFFERS:(2) TI/Burr–Brown OPA2604AP op amps, Digi–Key OPA2604AP–ND,—or—

TI/Burr–Brown OPA2134PA op amps, Digi–Key OPA2134PA–ND,—or—Analog Devices AD712JN op amps, Newark 05F7277, or Analog.com

LINE STAGE:(2) 10M, ¼W resistors, Vishay–Dale CMF Type RN60, Mouser 71–RN55D–

10M (R209, R210)All other resistors are builder’s choice:Vishay–Dale CMF Type RN60 (Welborne Labs, Mouser 71–RN60D–Value) Holco H4 (older non–ferrous type, if available, Welborne Labs, Michael Percy Audio, Parts Connexion)Caddock MK132 (Welborne Labs, Michael Percy Audio, Parts Connexion)Vishay S–102 (Michael Percy Audio)

(2) 475Ω (R205, R206)(2) 221k (R251, R252)(2) 2.49k (R253, R254)(6) 1.0k (R213, R214, R255, R256, R257, R258)(2) 249Ω (R211, R212)(4) 100Ω (R237, R238, R239, R240) (six are needed if you add the Aux Outputs, R239a, R240a)(2) 1nF (1000pF, 0.001µF) Polypropylene Capacitors,

Reliable MultiCap PPMFX (Welborne Labs, Parts Connexion)—or— Wima FKP–2 (Welborne Labs) (C251, C252)

(2) Analog Devices AD744KN op amps, Newark 05F7342, Analog.com(2) Analog Devices AD811AN op amps, Newark 05F7667, Analog.com(2) Aavid/Thermalloy DIP Heatsink, Digi–Key HS179–ND(2) Tyco 8–pin Plug Adapter, Allied 905–3114—or—

Aries 8–pin DIP Header, Digi–Key A101–ND

MISC.Insulated hookup–wire22AWG bare hookup wireSleevingPerfboard with solder ringed holes (Radio Shack 276–147)5–minute epoxy (local hardware store)D.H. Labs BL–1 Interconnect for optional Aux Inputs

TABLE 1MODIFIED GFP-565 PREAMPLIFIER

MEASUREMENTS ON SAMPLE BUILT FOR C. VICTOR CAMPOS

S.N. AC14755

LINE STAGE THD – LEFT WIDEBAND w/80kHz LP FILTER(2V in > 2V out)20Hz 0.0037% 0.0017%1kHz 0.0037% 0.0018%10kHz 0.0042% 0.0028%20kHz 0.0055% 0.0046%

LINE STAGE THD – RIGHT WIDEBAND w/80kHz LP FILTER(2V in > 2V out)20Hz 0.0037% 0.0017%1kHz 0.0037% 0.0018%10kHz 0.0043% 0.0031%20kHz 0.0055% 0.0048%

LINE STAGE IMD SMPTE (4:1) 1:160Hz ++ 7kHz(2V in > 2V out)LEFT 0.0014% 0.0013%RIGHT 0.0015% 0.0012%

LINE STAGE FREQUENCY RESPONSE−0.2dB @ 10Hz, −2.1dB @ 100kHz (Left and Right Channels Identical)

All measurements made with Sound Technology 1700Bby Gary Galo, 11/2/2002

der them to the main ground traceson the bottom of the PC board. You’llneed to scrape enough lacquer offthe PC traces to make the groundconnection.

Photos 30 and 31 show the complet-ed line stage with both Holco and Cad-dock resistors. At this point, check allwiring and component placementsvery carefully. Be particularly carefulin checking the wiring of the AD811modules and the connections fromthese modules to the Adcom PC board.Carefully clean the Adcom PC boardaround the line stage componentswith the CaiKleen TRP DG7S–6 clean-er recommended in Part 2.

TESTINGPower up the preamp and check theDC–offset at the Main output jacks,after the muting relays have timed out.Offset will normally be 1.25mV or less.Analog Devices specifies the typicalinput offset voltage for the AD744KN as0.25mV, and the line stage has a voltagegain of 5. Maximum offset is specifiedas 0.5mV, so the worst–case situation in

this line stage should be 2.5mV at theoutput.

Check the voltage gain with a 1kHzsine wave. Set your generator output to0.5V RMS and check the output of bothchannels with the volume control fullyclockwise. It should be 2.5V RMS. Ifyou have a distortion analyzer, set thegenerator output to 2V RMS, and ad-vance the preamp volume control foran output level of 2V RMS. This is the“unity–gain” setup that Victor Camposused to measure preamp THD when heworked at Adcom.

This setup minimizes the effects ofnoise on the THD measurement. Table1 gives the measurements of theGFP–565 preamp that I modified forVictor Campos. They were made on mySound Technology 1700B analyzer, andare typical of what you should measurewith this preamp. These measurementsare quite close to the Adcom original,and I don’t believe that they are muchhelp in quantifying the sonic perfor-mance of the modified preamp. But,they will tell you whether you havemade any serious errors in performingthese mods.

I suggest “cooking” the preamp onthe benchwith the cover and bottomplate installedfor 24 hours andre–checking the measurements. If all iswell, hook the preamp up to your sys-tem, and enjoy! We plan to have at leastone third–party evaluation of the modi-fied preamp in a future issue of aX, andI welcome your comments and reac-tions. Part 4 will feature the new,servo–controlled phono preamp.


SOURCESAllied Electronics

7410 Pebble DriveFort Worth, TX 76118817-595-3500 817-595-6444 (FAX)http://www.alliedelec.com/

(Remaining sources are listed in Part 1)

REFERENCES1. Galo, Gary. “Pooge 5: Rite of Passage for theDAC960”–Part II. Audio Amateur, 3/1992, pp. 34–39,58.

2. Jung, Walt. “Op-Amp Audio: Minimizing Input Er-rors,” Electronic Design, Dec. 14, 1998, pp. 80–82.

3. Jung, Walt. “Composite Current Boosted Line Driv-ers,” Op-Amp Applications, Analog Devices, 2002, pp.6.62–6.64.

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