Adcom 545 II Squeal During Shutdown

[XK150]

Hi,
I just finished recapping two Adcom GFA-545 MKII amps. I replaced all electrolytics and set the bias. Both amps work great, however one of them has an annoying squeal during shutdown. The squeal occurs on both channels. One channel precedes the other by about half a second. It occurs about 5 seconds after flipping the switch off.
I can also make it squeal on powerup using a variac to slowly bring it up. It occurs when the AC input voltage is between 10 VAC and 50 VAC. The pitch of the squeal varies with the input voltage.
It appears to originate in the DC offset circuitry around the Adcom 3A IC. During power up it oscillates as viewed at pin 6 when the supply voltage on pin 7 is around 2 volts. As I bring up the input AC the supply voltage at pin 7 jumps to 10 volts and it immediately stops squealing.
Any thoughts on how to stabilize this op amp?

 

[patrick101]

The amp supply 12V to pin7 of the Adcom 3A DC servo IC.

The 12V supply by zener diode D615 and a little bit high value resistor R691, so that you can't change the 4.7uF capacitor C629 to higher value because it will affect the IC supply voltage 12V setup time and cause your problem.

Change resistor R691 to a lower value 22K~15K (1/4~1/2W) may improve DC servo circuit.

 

[XK150]

Thanks Patrick,

Before reading your post I had previously tried increasing the capacitance of C629 to 9.4 uF . It did reduce the squeal to where it was a mild brief sputter, barely audible. But, as you pointed out, this will increase the setup time of the IC. After reading your post I went back to 4.7 uF for the cap and decreased R691, but I had to bring it down to around 8K before it had much affect on the squeal. This created the need for at least a 1/2W resistor. This of course reduces the setup time. Is there a particular need to have a long setup time as in the original design (4.7 uF cap charged through a 33K resistor)?

My second thought was to remove the zener diode and replace it with a 12V 3-terminal regulator like an LM7812. Any thoughts on that?

Thanks,
Dick

 

[patrick101]

Is there a particular need to have a long setup time as in the original design (4.7 uF cap charged through a 33K resistor)?

My second thought was to remove the zener diode and replace it with a 12V 3-terminal regulator like an LM7812. Any thoughts on that?

Thanks,
Dick

I think the original design supply current is low for the servo IC and zener diode, so that the capacitor will affect the supply voltage stabilized time.

Normal servo IC curcuit, we supply 8~10mA current with a cap 10uF~47uF will be OK.

Use 3-terminal regulator is OK, but keep the original zener diode supply is not bad for a servo IC circuit.

 

[sgrossklass]

Wait a minute - if said oscillation occurs about 5 seconds after poweroff, why has the amplifier not muted its output (e.g. dropped the relay) by this point? (Bridged or stuck relay?)

 

[patrick101]

Adcom never built amps with speaker on/off relay or protection and many other USA/ European brands also.

They design the circuit, like using just enough power supply capacitor and make it power on/off with quick charge/discharge with little "pop" sound. When the amp working years, component will drift their working condition or we changed a larger power supply capacitor will cause the amp squeal when power on/off.

The solution is to readjust the circuit or replace new components to minimize the squeal.
The best way is to add a speaker protection circuit to the amp, but you must have the technique to do that.

 

[XK150]

Thanks Patrick and SGrossKlass,

I did consider adding a relay to each channel. This would be the most elegant solution. I have an old donor Sony amp which could provide the necessary relays and driver transistors. I haven't decided yet to take this approach.

Dick

 

[nauta]

Hi


Without a schematic for the amp, any number of things can be happening. If the input stage uses resistive current-sources, then low voltage will cause the gm of the whole circuit to be out of step with the compensation values and it will oscillate. Active current sources have the advantage that they assure gm of the other devices from a very low rail voltage up to full - or the other way around from full to very low. Resistive current sources are known to assure reduced power-on/off thumps, so it is a compromise choice.

 

[Phloodpants]

There is nothing wrong with your amp. The oscillation is not caused by the servo, it just happens to show up there because the oscillation is present on the input transistors, and the servo is connected to them.

There is no speaker-protection relay, and so they remain connected while the amp's power supply capacitors drain. As the voltage approaches zero, things get all wobbly and you hear it oscillating. There comes a point, as the voltage drains, where there is not enough open-loop gain to provide negative feedback to keep things stable.

Some Adcoms make the noise, some don't, and I haven't been able to discern any pattern to it. It's got nothing to do with age. The amps that make the noise will always make the noise, and the ones that don't, won't. They all make their own peculiar sound that doesn't change with age. Some go "bweeeep!", some go "Errrrrr...", some go "putt putt putt putt putt" etc.

Not to worry at all. It's got no bearing on the condition of the amp. It's random.

 

[evanc]

I have the same amp that makes the same noise. It’s been there for the many years I’ve had it. I’ve never worried about it or looked into the cause.

 

[XK150]

There is nothing wrong with your amp. The oscillation is not caused by the servo, it just happens to show up there because the oscillation is present on the input transistors, and the servo is connected to them.

....

Some Adcoms make the noise, some don't, and I haven't been able to discern any pattern to it. It's got nothing to do with age. The amps that make the noise will always make the noise, and the ones that don't, won't. They all make their own peculiar sound that doesn't change with age. Some go "bweeeep!", some go "Errrrrr...", some go "putt putt putt putt putt" etc.

Not to worry at all. It's got no bearing on the condition of the amp. It's random.

Thanks to everyone,

I do agree that there is nothing wrong with my amp. I want to sell the amp and I was concerned that a potential buyer might not understand so it would be better to eliminate it and eliminate any concerns. I must say it is a weird noise. I would describe mine as the "Errrrr" variety changing pitch as the voltage drops. By trying different caps and resistors I could change it to the "putt putt putt" variety (my Dad when working on tube amps and radios referred to this as 'motorboating').

I decided to add a double pole speaker relay using parts from a donor Sony amp and adding the necessary R-C components to get the delay I wanted. I probably spent more time on this than was necessary, but I like it better now that its quiet.

Dick

 

[NormB]

There is nothing wrong with your amp. The oscillation is not caused by the servo, it just happens to show up there because the oscillation is present on the input transistors, and the servo is connected to them.
[snip]

Some Adcoms make the noise, some don't, and I haven't been able to discern any pattern to it. It's got nothing to do with age. The amps that make the noise will always make the noise, and the ones that don't, won't. They all make their own peculiar sound that doesn't change with age. Some go "bweeeep!", some go "Errrrrr...", some go "putt putt putt putt putt" etc.

Not to worry at all. It's got no bearing on the condition of the amp. It's random.

Well, THAT’s reassuring.

I just updated a 535-II with Hoppe’s power supply, 3-wire ground, recapped it (the electrolytics), and did the input transistor matched pair upgrade and...

weee-errrr-argh-zip

is still there. Seems to go on a little longer with the higher rated PS caps, but I’m not about to return everything back to where it was to verify it’s actually longer. Just SEEMS like it.

If it CHANGES any... then I’ll worry.

Thanks for the post.

Norm