Adcom GFA-555 questions...

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I have just purchased a GFA-555 and have just read 15 pages of threads on this amp but still have a couple of questions.

1)Amp serial # is 6709 can I determine the build date from this?

2)offset is 38Mv and 70Mv, is this a problem?

3)bias current measured at the fuse blocks is about 48Ma
Is this ok?

4)the psu caps are 15,000uf 80v, the measured rail voltage is 80vdc with no by-pass caps present at the cans.
There is a black crust around the vent holes of the caps.

I want to replace the ps caps.How much can I safely increase the capacitance?

I read long debates on By-passing the ps caps and I would like to try it for myself and report, however I would like some advice on by-pass cap values from other experience...

I also think that I will replace the electroltyics on the driver board.
Are there any other replacements/improvements that I should consider?

Thanks
 
As an update I measured the mv across the emitter resistors and
one channel was 7.5mv and the other measured 5.2mv

The service manual calls for 16mV.

That is quite a long way out.
I'm alittle leary of making such a large adjustment.
The amp works fine now but possibly alot better with the correct bias adjustment???
 
rtate said:
As an update I measured the mv across the emitter resistors and
one channel was 7.5mv and the other measured 5.2mv

The service manual calls for 16mV.

That is quite a long way out.
I'm alittle leary of making such a large adjustment.
The amp works fine now but possibly alot better with the correct bias adjustment???

You may have some improvements by setting the bias levels to factory ones. But do not expect major changes in audio quality.

That may come from improving some of the passive parts on it, particularly the capacitors, and perhaps by using supply regulators like the ones designed by Kit Ryan.
 
RE: Bypass Caps on PS
I used 22 or 44uf on each cap (i forget which). It does tighten up the bass a bit. BTW, I have never been able to find a clear explanation for this effect.

RE: bias
The trick on the bias, as you probably already know, is that the amp must be fully warmed up. The service manual (Adcom will e-mail you a copy), outlines the procedure for this using a power resistor.

Good Luck.
 
Thanks WithTarragon!! That's the kind of info I need!!
With 22 or 44uf can i assume that they were electrolytic caps??

I have some Wima 10uf 100v caps that I was thinking of trying..

So what is the benefit of using higher values??
 
There are different philosophies on this one. I would not bother using a smaller size. There are some designs which will use a very small cap for the bypass also. If you do this the amp may actually oscillate. It would need to be located very close to the output. I would not bother.

Incidentally, some of the later ones in the 555ii series already have bypass caps.

Yes, these are polarized electrolytic caps. If you don't know what you are doing, then have a friend discharge the big caps for you (with something other than a screwdriver). These store huge voltages and it is not a joking matter (I don't recall if there are bleeder resistors or not).

The easiest thing to do is to get some bipolar (non-polar) electrolytics used for speaker crossovers. There is no need to spend more than a couple of bucks per cap

Good Luck
 
My amp is not the MK-II version and although the service manual schematic says that there are .1uf 100v film caps across the large caps there are none present. There are 3.9k 2watt bleeder resistors present.

I think that the large caps have been changed at some point and the by-pass caps were left off.
 
Bypass Caps

RE: Bypass Caps on PS
I used 22 or 44uf on each cap (i forget which). It does tighten up the bass a bit. BTW, I have never been able to find a clear explanation for this effect.

RE: bias
The trick on the bias, as you probably already know, is that the amp must be fully warmed up. The service manual (Adcom will e-mail you a copy), outlines the procedure for this using a power resistor.

Good Luck.

Hi WithTarragon,
I have the first version of the GFA555, with one channel dead. After changing some components from the main amp board and all the output transistors together with the driver, I put a 100uF/100v after the 15A fuse and bypass with 0.1uF/250v wima cap. Do you think the the value 100uF is just fine?
Also, measured the dc offset at the speaker terminal, I got 10mV from the repaired channel while 5mV from the original working right channel.
Do I need to re-adjust their bias? To what voltage value?
By the way, the heatsink stays cool for almost 3 hours, having no input and a 22-ohm 20w dummy load.

Appreciate very much some advise and helpful tips. 🙂
Thanks in advance.

>Joel
Philippines
 
Let's clear up some of the confusion from the last few posts.

1) 5 or 10 mV of DC at the outputs is not much and I would not worry about it. They are both small values.

2) The orginal Adcom 555's did not have the bypass caps on the power supply. The the bypass caps are found on some of the series 2 versions (but not all of them).

3) For some reason (this is emprical and I have not read a good explaination of "why"), the bypass caps are the most likely to "wear out" first. So the big caps (cans) may still be fine but the smaller caps can be bad.

4) The recommendation was for a bypass cap (22-66 uF, or so) for each of the four large caps, ie, a total of 4 (not just one for each side).

5) Be careful if you use an additional small cap (in addition to item #4). If you do use one, then make sure it is (electrically) as close to the output as possible. You are running the risk of an oscillation (due to inductance in the traces of the PCB). This oscillation will be difficult to see unless you have a 'scope. It could make the amp run very hot.

6) If your heat sinks are not warm at all, then you need to check the bias (read the service manual). You may have it biased too low. But the true measure is not the temperature, it is the voltage (as specified in the service manual - the test points are labelld on the the PCB). Wait, I re-read your post, I hope you had an input of the specifed wattage when you set the bias. Again, follow the directions in the service manual

The benefit of the bypassing the PS caps is on the sound of the bass (a "cleaner sound"). The effect is audible but I would not say that it is "dramatic". However, people use words differently ....
 
Last edited:
Let's clear up some of the confusion from the last few posts.

1) 5 or 10 mV of DC at the outputs is not much and I would not worry about it. They are both small values.

2) The orginal Adcom 555's did not have the bypass caps on the power supply. The the bypass caps are found on some of the series 2 versions (but not all of them).

3) For some reason (this is emprical and I have not read a good explaination of "why"), the bypass caps are the most likely to "wear out" first. So the big caps (cans) may still be fine but the smaller caps can be bad.

4) The recommendation was for a bypass cap (22-66 uF, or so) for each of the four large caps, ie, a total of 4 (not just one for each side).

5) Be careful if you use an additional small cap (in addition to item #4). If you do use one, then make sure it is (electrically) as close to the output as possible. You are running the risk of an oscillation (due to inductance in the traces of the PCB). This oscillation will be difficult to see unless you have a 'scope. It could make the amp run very hot.

6) If your heat sinks are not warm at all, then you need to check the bias (read the service manual). You may have it biased too low. But the true measure is not the temperature, it is the voltage (as specified in the service manual - the test points are labelld on the the PCB). Wait, I re-read your post, I hope you had an input of the specifed wattage when you set the bias. Again, follow the directions in the service manual

The benefit of the bypassing the PS caps is on the sound of the bass (a "cleaner sound"). The effect is audible but I would not say that it is "dramatic". However, people use words differently ....

wow, great advice and thanks!
well, any explanation? how come the repaired left channel (-10mv) stays as cold as the room temperature whilst the original working right channel having -5mv dc offset is a bit warmer than the other after a couple of hours?
Q2: is it still safe if I bias the original normal working right channel to an offset of -10mv same as the repaired one? can I adjust them further to about -25mv? or what value??
will the 100uF and the 0.1 uF in parallel with the + & - rails on the output pcb not cause any oscillation on the said circuit.
As mentioned earlier, the RCA input jacks are open, like nothing is connected as input signal as i still don't have the signal gen, but i have the dual scope comparing both outputs. I also have the 20w 22-ohm dummy load connected on the speaker posts.
Thanks WithTarragon!
 
bypass caps

The .01uf/100v caps are only on the 555 mkII that I have seen. I had installed them on one of our first edition 555 but sound the same with out with out the caps. They are like the orange type / film caps I never seen el's on this circuit.

@ embrown057
not only a 0.1uF polypropylene caps i have added but with a 100uF in parallel in the + & - rails on the output pcb.
some noticed a more tight bass response. haven't heard any changes on the higher frequency level.
 
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