Fixing an Adcom GFA-5500: -20db bass attenenuation on both channels - diyAudio
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Old 20th May 2011, 04:44 PM   #1
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Default Fixing an Adcom GFA-5500: -20db bass attenenuation on both channels

Hi.

I recently purchased a used Adcom GFA-5500 and would appreciate some assistance diagnosing an issue with it. The low frequency response is extremely attenuated. This isn't a matter of splitting hairs: It is easily measured by SPL meter anywhere under
500hz, reaching more than 20db down by 30-40hz. The difference in sound is obvious, as is the difference in movement of the woofer by eye.

I've compared the response of this amp to that of my Denon receiver, using a cheap calibration microphone and laptop soundcard. This doesn't yield a flat response, but is perfectly reasonable for comparison, especially since the difference is so great. The attached plot shows two connectivities:

purple - SPDIF into the receiver, out to a Sound Dynamics 300ti.
teal - SPDIF into the receiver, pre-amp to the Adcom, same speaker

I have not moved the microphone or speaker between the two tests. Though not shown, I have also tested using my Hagerman Chime DAC connected directly to the Adcom. I do not believe there is any difference between the sources. There is effectively no low frequency in either case. Interestingly, all frequencies sound fine and have no notable distortion. Music can sound okay if my ear is placed next to a woofer.

I'm unsure what could cause this sort of problem, so I have made a few spot checks of easy and obvious measurements. I do notice that the DC rails have voltages rather greater than those printed on the silkscreen. On the right supply (left is about the same), for the "60VDC" rail, I see 77.6V and -77.6V. For "73VDC", it's 89.4V and -90.2V. They are very flat on my oscilloscope.

Voltage offsets at the speaker terminals are 4mV (right) and -8.9mV (left). Bias on left is around 40mV and close to 58mV on right. I haven't adjusted any of these.

I don't have a schematic for this amp, but have looked at the audio channel schematic for a GFA-5800. It doesn't cover the power supply though.

That the problem occurs with both channels suggests a common failure mode, or the power supply. I suppose I also can't be sure the amp has not been modified, but it does not appear so. I also can't imagine why this would be a beneficial modification. :)

I'd appreciate any assistance. Thanks! ~Jacob
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Old 20th May 2011, 05:19 PM   #2
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Looks like a very "stiff" supply. Wrong voltages are most likely bad PS caps , this would case loss of bass as well. The adcom is a high bias MOSFET creature (hot - class a for first 10W) , this would take some years off them caps. (class A stealth amp caps were "cooked to death"). Do ESR tests on caps. Being 20 years old and still working , it would be worth $120 to recap.

OS
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Old 20th May 2011, 05:21 PM   #3
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I would take a look at the input and/or feedback capacitors. I'll bet that a
previous owner altered them, perhaps to form a crossover network in
biamping.

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Old 20th May 2011, 06:45 PM   #4
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I agree with Nelson here. The difference between the Adcom and the Denon looks exactly like a high pass filter of some sort.

High voltages on the power rails could simply be by design, the voltage rails will droop significantly when under load and a slightly higher mains voltage will also raise the rail voltages.
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Old 20th May 2011, 07:52 PM   #5
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Thanks for your responses.

I have been able to obtain the service manual, so I'll do some spot checking of components this evening. Beyond a list of component values, and instructions for bias and offset adjustments, this manual doesn't provide any information for debugging components such as the power supply.

I haven't been able to find a circuit schematic. Adcom informs me that they don't provide schematics for current products. It wasn't part of the service manual I did find.

The previous owner tells me that he used it, and a couple of other GFA-5500s, without any trouble up until shipping it to me. To his knowledge, it was not modified.

~Jacob
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Old 21st May 2011, 01:32 AM   #6
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gfa-5802 schema looks just like the 5500 pix and description of VAS/OP (board/PS). link : ADCOM GFA-5802 SCH Service Manual free download, schematics, eeprom, repair info for electronics

PS .. it DOES show +/-90V for the IPS/VAS rails and +/- 80V for the "dirty" OP stage rails (2 separate windings as well).
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Last edited by ostripper; 21st May 2011 at 01:35 AM.
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Old 21st May 2011, 02:57 AM   #7
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I was able to get a schematic for the 5500, and have gone through the input and feedback portions. There don't appear to have been any circuit modifications with respect to values. I've tested the resistors, and for obvious faults of the capacitors (read: not ESR or measured capacitance) and verified that their values match the service manual. I wasn't looking as much for failures of parts as to check that they were as prescribed. I think it is unlikely that both channels would have exactly the same failure mode.

It seems most reasonable to trace the signal through the few amplification stages and check it at each point, but I'll have to play around quite a lot to get a conclusive test set up. I'd like to go after any other low hanging fruit first. Is there any?

I also obtained the power supply schematic. It does clearly state rails of 60 and 73 VDC, but I haven't seen any indication of how much these might be expected to vary. I asked Adcom's support fellow this, but he didn't know.

OS, could you explain to me a bit why bad capacitors could result in the higher voltages, and not simply less filtering? I'm not sure I follow. Additionally, if high, would this result in such low frequency attenuation? As for $150, it seems like 4x 18000uF capacitors alone will run quite a lot more. I don't have a sufficient bench supply to substitute for a simple test with the rails as spec'd.

I do have the option of returning the amp to the seller, but I wanted to put in a good effort before giving up. Other than bass, it appears to work great.

Jacob
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Old 21st May 2011, 03:55 AM   #8
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It's probably a bit off the mark, but could it be a loading problem - if the receiver has a very high impedance amplifier input and a small coupling cap to the output , and/or the input impedance is too low in the power amp, then would lose base. I'd look and see what the value of the coupling cap (assuming the use one) is in the receiver, and perhaps try a larger one if it is on the small size.
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Old 21st May 2011, 04:02 AM   #9
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Read the last paragraph of this PDF :http://www.lesterelectrical.com/serv...oltagetest.PDF
This might answer why higher voltages are read.

CAPS = 36DA183F100CC2A United Chemi-Con Aluminum Electrolytic Capacitors - Screw Terminal

$28 each. (not 150+) Still , test yours with a ESR meter.

OS
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Old 20th September 2012, 10:09 PM   #10
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hi there...I have just picked up a GFA-5500 as well..and notice the same weak bass as well...what ever became of the issue with the amp...what was it that solved the problem???...looking to try to figure out what is wrong with mine to make it have this problem....thanks for any help
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