# Adcom Bias Setting



## paul port (Aug 24, 2008)

Hi All,

I'm putting together a new install using various 'Old School' equipment I have collected over the years... One of which is an Adcom GFA-4304 (A bit of a rarity in the UK)

Being a really old piece of kit, I've already been through and replaced capacitors and re-bonded power devices to the heatsink etc... 

The final step is to check the Bias condition - here is the issue!
There are two adjustments, I'm assuming Bias and Offset.
There are No resistors in the output stage (the usual voltage measurement point in a typical amp)

Does anyone have details of the Bias setting procedure? 

I see from previous posts that Adcom USED to freely supply service manuals for obsolete products - but now it appears they have sold their soul to various 'Affiliates' judging by the numerous 'Buy it from me' responses I got when trying to contact their customer service!



Thanks, Paul 


PS, for those that are interested, it will be an active system.
Dynaudio 3" Mid MD140 + Tweeter MD100. I'm having to use Focal 5ws for midbass instead of the MW160-GT I had planned due to space

GFA4304 running 4ch for the Mid / Top
2x Mission Freeway amps driving the 5ws (Mission-UK, another great home HiFi manufacturer that very briefly made car electronics!)

I'm still considering options for the crossover, but it's likely to be a DSP of some sort.

All being installed in a 2000 Boxster 986


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## dodgerblue (Jul 14, 2005)

I can help you. Drop me a pm.


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## wheelieking71 (Dec 20, 2006)

i vouch for dodger! he has personally taught me the procedure, and it does indeed make a huge difference in the performance and sonics of the amp. enjoy your ADCOM! there are none better.


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## dodgerblue (Jul 14, 2005)

wheelieking71 said:


> i vouch for dodger! he has personally taught me the procedure, and it does indeed make a huge difference in the performance and sonics of the amp. enjoy your ADCOM! there are none better.


Thanks Cyborg . I can personally vouch that Wheelie is an amp junkie


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## SchizoFrenic (Oct 14, 2011)

Hallo Paul!

Have u succeed to setup the bias?


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## Dubstep (Apr 7, 2010)

What are the bias and offset numbers to shoot for on the old adcoms, say like a 4702?


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## ATOMICTECH62 (Jan 24, 2009)

These Adcoms are very different then other amps.For some reason they omitted the emitter resistors from them so there is no way to get a voltage reading across the emitters.They want you to turn the bias controls fully ccw then increase each one so each channel pulls 1.5 amps from the power supply.So at idle with all channels ccw the amp might pull 1 amp,then adjust one channel so you get 2.5 amps then the 2nd till you get 4 amps.This is for the 4402,the 4702 might be different.I think its all up to the person on how high they want the amp biased into the class A region.
I would just set the amp at about 1 watt output with a 25khz sine wave and adjust it till the notch distortion disappears.But thats just my personal opinion.
As far as the DC offset just try to get it as close to 0mv as possible.I believe +/-10 mv is acceptable.

I have seen some info that wants the current pull for each channel at 2-3 amps.


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## Dubstep (Apr 7, 2010)

ATOMICTECH62 said:


> These Adcoms are very different then other amps.For some reason they omitted the emitter resistors from them so there is no way to get a voltage reading across the emitters.They want you to turn the bias controls fully ccw then increase each one so each channel pulls 1.5 amps from the power supply.So at idle with all channels ccw the amp might pull 1 amp,then adjust one channel so you get 2.5 amps then the 2nd till you get 4 amps.This is for the 4402,the 4702 might be different.I think its all up to the person on how high they want the amp biased into the class A region.
> I would just set the amp at about 1 watt output with a 25khz sine wave and adjust it till the notch distortion disappears.But thats just my personal opinion.
> As far as the DC offset just try to get it as close to 0mv as possible.I believe +/-10 mv is acceptable.
> 
> I have seen some info that wants the current pull for each channel at 2-3 amps.


Thanks for the detailed info!! I just got a 4702. It's clean, boards look good.. I am going to recap it with some higher end caps.. I notice, I cant find a normal 8 dip Op-Amp anywhere? Anyways. I have a small oscope, ill check out the bias settings and offset aswell.. I hope I get it right..


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## ATOMICTECH62 (Jan 24, 2009)

Yeah,there are no op-amps in them.I feel they should have used one for the DC offset like they did on the GFA-555II.That was main reason they revised the GFA-555.


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## ATOMICTECH62 (Jan 24, 2009)

There is a low pass filter built into those amps to limit ringing and protect them from damage when they are loaded down and driven into hard square wave clipping.Its not needed for people like us who run them on highs,dont like distortion and know how to set gains.
It can be modified to increase the slew rate and raise the frequency response of the amp to almost 300khz.


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## ppia600 (Jan 1, 2016)

Dubstep said:


> Thanks for the detailed info!! I just got a 4702. It's clean, boards look good.. I am going to recap it with some higher end caps.. I notice, I cant find a normal 8 dip Op-Amp anywhere? Anyways. I have a small oscope, ill check out the bias settings and offset aswell.. I hope I get it right..


Sorry for the ancient thread bump, but could I ask a few questions about the type of caps you swapped in? I'm about to recap my 5450 and want to be sure of a few things before removing it from my car and suffering without it for a few days.



ATOMICTECH62 said:


> There is a low pass filter built into those amps to limit ringing and protect them from damage when they are loaded down and driven into hard square wave clipping.Its not needed for people like us who run them on highs,dont like distortion and know how to set gains.
> It can be modified to increase the slew rate and raise the frequency response of the amp to almost 300khz.


Could you direct me to a thread or forum with information on this? Was this for all of the adcom car amplifiers? 

Thanks


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