So I unknowingly bought a dead-ish Adcom. Unfortunately, it should have been sold as parts and I paid pretty much the full used market value. The seller has ghosted me. Well lessons learned, time to make some lemonade out of this lemon. I had him test the outputs for DC offset, but unfortunately it was an out of the back of a car deal, so I couldn't test it further.
The amp will play music, but horribly distorted on what seems me is peak values, but how much gain is applied, doesn't seem to matter. Both channels are affected.
DC offset at the speaker terminals is -14mv/-50mV with inputs open. I've checked the bias values on each output device and they seem settle out nearby the nominal 16mV (range: ~15-17mv - one reads 20mV). Some are a little low, some are a little high, but nothing really sticks out. The power supply caps are old, and suspect some issue there, a drain test says some are definitely more than 20% out of spec. Rail voltages are about 83V
The input board on the pro model has balanced inputs (which use an opamp to convert to unbalanced) and TRS jacks for the unbalanced inputs. Given I don't have TRS adapters, I have not tried the unbalanced inputs.
Where to start?
https://hoppesbrain.com/adcom-faq/#What-are-the-most-common-issues-with-the-GFA-5x5
https://hoppesbrain.com/adcom-faq/#Bias-Adjustment
https://hoppesbrain.com/adcom-faq/#How-much-DC-offset-is-normal
https://hoppesbrain.com/adcom-faq/#...r-supply-capacitors?-If-so,-which-ones-to-get
To replace the input board, it will be a pain, since there isn't a drop in replacement since it is the pro version which is mounted by its balanced inputs. I've considered replacing the PS with a diyaudiostore universal PS V3 when they become available. This would open space to mount an input board onto the floor of the unit.
The universal PS is CRC, with 4 caps per rail with 2 each side of the resistance. I think I could use 7500uF 35mm 100V rated caps snap-in type caps and just squeak in what will fit onto one of these boards. It would allow me to significantly reduce the footprint of the PS. Drop in replacements were about $77 CAD each, so significant cost savings and a better PS.
Comments?
The amp will play music, but horribly distorted on what seems me is peak values, but how much gain is applied, doesn't seem to matter. Both channels are affected.
DC offset at the speaker terminals is -14mv/-50mV with inputs open. I've checked the bias values on each output device and they seem settle out nearby the nominal 16mV (range: ~15-17mv - one reads 20mV). Some are a little low, some are a little high, but nothing really sticks out. The power supply caps are old, and suspect some issue there, a drain test says some are definitely more than 20% out of spec. Rail voltages are about 83V
The input board on the pro model has balanced inputs (which use an opamp to convert to unbalanced) and TRS jacks for the unbalanced inputs. Given I don't have TRS adapters, I have not tried the unbalanced inputs.
Where to start?
- Input board -- https://hoppesbrain.com/product/gfa-555-mk1-input-board/
- Driver transistors
- PS caps
https://hoppesbrain.com/adcom-faq/#What-are-the-most-common-issues-with-the-GFA-5x5
https://hoppesbrain.com/adcom-faq/#Bias-Adjustment
https://hoppesbrain.com/adcom-faq/#How-much-DC-offset-is-normal
https://hoppesbrain.com/adcom-faq/#...r-supply-capacitors?-If-so,-which-ones-to-get
To replace the input board, it will be a pain, since there isn't a drop in replacement since it is the pro version which is mounted by its balanced inputs. I've considered replacing the PS with a diyaudiostore universal PS V3 when they become available. This would open space to mount an input board onto the floor of the unit.
The universal PS is CRC, with 4 caps per rail with 2 each side of the resistance. I think I could use 7500uF 35mm 100V rated caps snap-in type caps and just squeak in what will fit onto one of these boards. It would allow me to significantly reduce the footprint of the PS. Drop in replacements were about $77 CAD each, so significant cost savings and a better PS.
Comments?
Well my hopes of finding caps that will work with a diyaudio PS board have been dashed, the closest I could find were 6800uF 100V snap-ins, but these will work as a replacement. LNR2A153MSE
I'll guess I should try reflowing some items on the input board.
I'll guess I should try reflowing some items on the input board.
Hmm. Now the issue is intermittent or I have resolved it with a couple solder reflow at a couple points and/or contact cleaner. I have no way to tell now. I did too much between tests.
Not a fan of the pro design with variable gain controls on the faceplate. You can overdrive the input to ridiculous levels and never be certain of the balance. I may rebuild this thing back to a form suitable for audiophiles. I like the circuit breakers vs fuses, but not at the cost of the additional inductance the lines running to them apply to the output boards.
Not a fan of the pro design with variable gain controls on the faceplate. You can overdrive the input to ridiculous levels and never be certain of the balance. I may rebuild this thing back to a form suitable for audiophiles. I like the circuit breakers vs fuses, but not at the cost of the additional inductance the lines running to them apply to the output boards.
Well I got some RCA TS adapters (I should just replace the jacks) and well... the balanced input on the GFA-555 pro is absolute crap. Just saying. I have a better idea of the sound of this amp. Certainly ain't a FET amp. Certainly not Firstwatt. Onto replacing the Power Supply caps. Well, at least I think I can try it out with my Maggies to see if they sing a bit better.
As is, it's a pretty rough sounding amp.
As is, it's a pretty rough sounding amp.
