Fix for Adcom GFA-555-II amplifier (with "insurance policy")

BuzzK

BuzzK

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My (former) next door had a nice set-up with the Adcom amp described and a set of Mirage speakers.

The woofer blew in the left channel. I took the amp home and stuck it on my bench, and it worked OK. I replaced the caps and tested it for a few hours -- no DC at the speaker terminals.

Being a nice neighbor, I had his Mirage reconed for a birthday present. About a day and a half later, the speaker melted again. Sure enough, there was -70 VDC on a speaker terminal.

So this time I replaced all of the discrete transistors in the channel. That seemed to fix it. But just in case the other channel blows, or heaven knows what, I bought a Velleman speaker protector unit and put it in an enclosure. That way, if DC finds it way to the speaker output(s), the speakers will be disconnected.

I really think Adcom's design is deficient in not including some form of speaker protection. So I built my own "insurance policy."
 

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Not sure how the 555 lasted so long despite the lack of speaker protection. IIRC, the new SE finally has a speaker relay.
 
I think more "audiophile"oriented amps in the past lack it than have it. Chances are one will never need it, but it's like many seldom needed safety measures like a fire extinguisher or house gun. If needed it's there if included, and if it works.
 
I think, I THINK that alot of the speakers decades ago had higher inductance and therefor though they could maybe be called 8 ohms, their DC resistance was way low. This made them more abe to blow the fuses in an amp.

That really doesn't excuse them for building gorilla amps with no protection.

Remember the times though. When I was little us kids stood up behind the front seat of the car. Car seats ? Never heard of them. Then when Dad punched it we got slammed into the back of the back seat. Loved every minute of it. And the back windows went all the way down so we could crawl out if we wanted to. Actually those "safety" windows that don't go all the way down are not for safety, there is not enough room for the window to go all the way down. Safety my (_|_), they never cared one iota unless they got sued.

Speaking of sued, does anyone know of an amp manufacturer getting sued for their product damaging speakers ?

Actually, QSC which is a fairly high end pro audio company (but their power is rated true) seems to have given up on DC coupling. They got this new deal where the secondary of the transformer floats and it effectively has both filters as coupling caps. But since it is incapable of putting out DC it needs no protection relay. It has current limiting and what they call SOA protection, but no relay. (for the uninitiated SOA means Safe Operating Area) I'm told that Behringer uses something like that as well in some amps.

There was mention of a design, in fact I wanted almost to try it but I can't source a relay within reason to do it. Simply filter and rectify the speaker voltage and use that to BREAK a relay rather then to make the contacts. I mean filter it before rectification.

There are problems though, the actuation of the relay is not solid and it might weld together, negating all protection. What's more, if the caps in the filter are not zero zero zero ESR and ESL it is too easy to false trip something like that.

And BTW, I appreciate the headsup on the Velleman. For $30 that is the way to go. Much cheaper than a high end woofer. Thanks. I might just get some of those, but I need them for 120 volts. If they ain't out there I know how to modify. Had a friend one time ask me "Do you have ANYTHING that is not modified ?". At the time I really could not think of anything. So this would only have to be converted to lower line voltage, and that is IF they don't already build a US/CSA model.

I am surprised it doesn't use a wallwart, which solves all those problems at once.
 
Pio1980 said:
I think more "audiophile"oriented amps in the past lack it than have it. Chances are one will never need it, but it's like many seldom needed safety measures like a fire extinguisher or house gun. If needed it's there if included, and if it works.
Click to expand...

Your logic is faulty. All amps will eventually pass DC. That includes protected and non-protected amps. The issue becomes whether you lose your speaker drivers as well. Audiophile is also misused in this context. Omission of a speaker relay does not separate an amp from the rest. Audiophile would be correctly applied as a subjective attribute which denotes elevated enjoyment.
 
JURB said:
I might just get some of those, but I need them for 120 volts. If they ain't out there I know how to modify. Had a friend one time ask me "Do you have ANYTHING that is not modified ?". At the time I really could not think of anything. So this would only have to be converted to lower line voltage, and that is IF they don't already build a US/CSA model.

I am surprised it doesn't use a wallwart, which solves all those problems at once.
Click to expand...

The Velleman comes with a 120V transformer. It delivers about 12VCT.

As a random aside, each positive speaker terminal has a feedback wire to the DC servo circuit. If either is open in the circuit, rail voltages are delivered!
 
Solid state amps with autoformer coupled output stages won't deliver destructive dc offset. These include most PA commercial amps and, of course, the McIntosh autofotmer coupled amps.
 
Do a search for the 555II amp on the net or here on AK. Those amps used paper circuit boards soaked with resin and cheap capacitors. The caps would leak and soak into the boards causing shorts. First signs of problems are blown fuses and speakers.
 
And then there are those that prefer no speaker protection and going commando. Wouldn't with vintage adcom though.
 
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jim249 said:
Do a search for the 555II amp on the net or here on AK. Those amps used paper circuit boards soaked with resin and cheap capacitors. The caps would leak and soak into the boards causing shorts. First signs of problems are blown fuses and speakers.
Click to expand...
Links please.
I have a 545-II, the boards are high quality with hi grade capacitors. I would expect the 555-II is similar.
 
The OP lost a driver to the 555 and came up with a very sensible solution. Even Adcom got the message with the SE.
 
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Checked the links, the postings appear confused as to which models are afflicted with leaking elyticaps, and the comment concerning the circuit board is inconsistent with my three 535.
I'm not an Adcom fanboy by any means, but there's a surfeit of misinformation on the net concerning these common plentiful amps that's not helpful. I've checked the esr on the oem boardmounted elyticaps in two of my 535 and they are still in spec, fwiw. It can be noted that the 535 operates at lower rail voltages than its larger siblings with assumed lower stress. At least one post indicates the driver devices may be operating near max rated voltage in the original 555 and upgrading considered during service.
 
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The Adcom amps soaking up electrolyte from leaking caps is a well-known issue. I had one here a few years ago...PC board smelled faintly of fish, though the caps looked OK. Swapped the caps out, and still had issues with badly drifting DC. Finally was given a hint by the guys over at DiyAudio, and pulled the board out and gave it a few dozen rounds in the ultrasonic cleaner with a strong mix of Simple Green. Cured it.

Oh, and 'well done BuzzK!'.:cool:
 
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I'm always a few steps behind So Cal Sam. I've only lost one pair speaker from an amp with no speaker protection and it was a prized pair of Spendor BC1s. YMMV. That was 25 years ago, I still thing about it.
 
EchoWars said:
The Adcom amps soaking up electrolyte from leaking caps is a well-known issue. I had one here a few years ago...PC board smelled faintly of fish, though the caps looked OK. Swapped the caps out, and still had issues with badly drifting DC. Finally was given a hint by the guys over at DiyAudio, and pulled the board out and gave it a few dozen rounds in the ultrasonic cleaner with a strong mix of Simple Green. Cured it.

Oh, and 'well done BuzzK!'.:cool:
Click to expand...
Thnx for the straight info, Glen. Which Adcom amps, btw?
 
jim249 said:
Only the 555II, 565 and 585 are affected. 545II and 535 are not affected.
Click to expand...
Thnx for clarifying, noted. The 545-II and 535 are the examples I have here, as it turns out. The 565 and 585 are supposed to need total overhaul to be reliable, afaik.
 
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