Issues with Adcom GFA-545

[cayovelez]

Hello all,

I am fairly new here and a very amateur tinkler, so please be nice, I need help. Here is my issue: My GFA-545 is in perfect cosmetically and sonically for the most part. The inside is spotless without a single sign of any cap leakage, or burn out. The fuses are all original and have never been tripped. It is mint looking, and sounding. The off-sets are .5 on both channels, and the bias are perfect as well. So here is my worrisome issue: when I shut the amp off, there is a short winding sound for about 2 seconds. Any ideas?

Thanks,

Carlos

[djk]

Caps are dried out.

[cayovelez]

Originally Posted by djk

Caps are dried out.

If they are dried out, how come the amp sound great otherwise, and the DC offsets are right up to specs? I don't understand.

[dave_gerecke]

Hello cayovelez, it the sound you are hearing upon turning off the amp a new sound, or has it done this since you got it. If it's been doing it since you bought it, then it's most likely just the Electrolytic Caps in the power supply discharging after power is disconnected. If it's something fairly recent, then you do have a failing component(s), most likely a cap(s) somewhere.

I know that the Adcom GFA-6000 I own would make a staticy whining noise when I powered it off, and it did this since I bought it when it was about 3 months old. Any amp that doesn't have a relay to disconnect the speaker from the amp when the power is shut off can do this. And the larger the capacity of the cap, then the more likely it will do this, and the longer the noise may last.

Peace,

Dave Gerecke

[cayovelez]

The issue never happened with the amp on its own, there are no weird sound on its own at all, however when I pair it to my gfa-555mkii for bi-amping is when it happens. I have tried 2 different pre-amps an adcom gfp555mkii and a conrad johnson pf-1. Both amps sound fantastic, and their offset are around .5. When I shut the 555 no issues, only when I shut the 545 that I get the weird wind down like noise only in my upper Panels that it amps. I tried both amps on their own, and no issues at all what o ever. What gives?

[anatech]

Hi cayovelez,
Dave is right on most counts there. Most amplifiers that do not disconnect the speakers when turning off or on will become unstable and make some kinds of noises.

These amplifiers have issues where some capacitors can leak - a defect in the original parts. The electrolyte is not visible to the naked eye, and it's extremely corrosive. If your amp is truly untouched, get it into a really good audio technician for repair right away! This tech will probably not repair TVs / VCRs / Microwaves - just audio. They will also be older, past 40 for sure. You may find this person working out of home, rather than in a service shop these days.

If they are dried out, how come the amp sound great otherwise, and the DC offsets are right up to specs? I don't understand.

Because djk is an experienced technician and you are not. Defective capacitors still have some capacitance, and this should not affect the DC offset anyway. The readings you get would then depend on whether the amp is oscillating or not, and how your meter reacts to high frequency AC on a DC scale.

Are your DC offsets 0.5 mV or 0.5 V? What is the bias current sitting at, how and when did you measure them? What meter did you use, and are you aware of the accuracy of that range and whether it's ever been calibrated or not?

I'm not trying to be hard on you, just explaining some things that are important that I don't think you are aware of. When you are measuring in the mV range, the quality of the meter you use becomes important, as is your understanding of whether the LSD means anything or not. With a cheap 3 1/2 digit meter, the last digit is often pure fiction. Is that 0.5 really 0.9, 0.1, 1.3 mV? When measuring 1.9000 VDC, a reading of 1.98 may be in tolerance for some meters. Imagine one of these that is out of tolerance and was never checked or calibrated! Just because you have 3 1/2 digits does not mean they are significant.

-Chris

[anatech]

Hi cayovelez,
What is the resistance between the input jack grounds to the chassis, to each other? I don't have this schematic, so I can't say. Other models used a 100 R resistor between signal common and the high current ground. Other amplifiers have the ground connection continuous between signal and output stage / power supply common.

The issue never happened with the amp on its own

Okay, so now you know the issue is not with the amp itself - right?

Are you using an electronic crossover? Doesn't sound like it. Are there any common connections between the woofer and high frequency speakers (dangerous)? Are these amps both plugged into the same power bar or outlet?

Remember that the chassis of each amp are connected to AC ground in your house, and also the grounds between RCA shells are connected. That's a ground loop, and probably the cause of your problems. You may have to modify the chassis ground to circuit common connection in order to break that loop. Do not run the system with an AC ground interrupted!

-Chris

[cayovelez]

Anatech, Thanks for your comment. My meter is a good old trusty Simpson 461 freshly calibrated, a buddy of my that works for the air force as an electronics tech calibrates it for me often. My dc offsets on my 545 is .5mV on one channel and .7mV on the other, both freshly turned on and after a while on. The bias are very close to spec considering it's age, the channel with .5mV the bias is 10.2mV and the one with .7mV was 11.4mV. So I adjusted both of them to spec after some warm up and kept on re-checking them and after 5 plus hours of warming up and playing tunes the bias kept at 10mV. The next day I checked again my DC offset and the same, and the Bias was steady at 10mV. The noise only come on when I bi-amp running the 545 for ESP and the 555 for the lower. Each amp on their own sounds fantastic, the 545 only acts up when bi-amping.

[cayovelez]

Anatech, thanks for the ideas. Problem solved, it is a ground issue. Both amps were in the same circuit, once I switched one to another circuit viola...problem solved. I feel very stupid to have even started this thread for a grounding issue. Thanks all I really appreciate it, Carlos

[dave_gerecke]

Hello Carlos, Don't feel stupid about grounding issues. They are actually some of the most common, and also difficult problems to work on. Many people, including experienced EE's, get tripped up by ground loops.

Peace,

Dave

P.S. Any info from Anatech is worth it's weight in gold. From reading a little of his bio, scattered throughout diyaudio.com, he's got an incredible amount of hands on experience with just audio equipment.