Adcom Amp "Whines" when shut off

[Whell]

Adcom Amp "Whines" when shut off

I have 2 Adcom amps: a 555 and a 545. When powering down the 545, I've begin to notice that it makes a "whining" noise several seconds after it is powered off. Starts off modertely loud, then trails off to silent after several seconds. The 555 does not make a such a sound.

I know that it is normal for some amps, including Adcoms, to have current slowly drain from the capacitors when they are powered off. I'm thinking that the noise is related to this.

Otherwise, the amp functions flawlessly and sounds very good. I picked this up a few weeks ago from a co worker for $40. Its cosmetically pretty ugly, but it sounds fine, other than this anomaly. Looks like someone opened the amp up at some point, at least to replace one of the two binding posts - the posts do not match, and there are several machine screws missing on top.

Is this "normal" for this Adcom, or is this an indication that the amp needs the attention of a technician?

Thanks!

 

[mcp]

Some amps make funny noises when powered off.

This is attributed to the circuit becoming unstable as the filter caps discharge. This instability results in a burst of oscillations, which is what the "whining" is.

The amplifier itself is not faulty, just impolite. Such behaviour are known to cause some embarrassment in the presence of visitors.

 

[Nelson Pass]

Usually it's a case of find the voltage source that biases the
front end current sources (diff pair and VAS) and put a cap
across it. Too small a cap and the amp still burps on turn
off. Too large a cap and the amp burps on turn on.

 

[Whell]

Got it. Since I don't have much ability with electronics repair, I'll simply accept that the whine is "normal" behavior for this amp and leave it alone. It sounds just fine otherwise.

By the way Nelson, I know that there are voices both pro and con as it relates to this "vintage" of amp. I must tell you that in my system, both the 555 and the 545 sound great. Thanks!

 

[Nelson Pass]

Don't thank me. Adcom paid top dollar for schematics and
working prototypes, but they also chose parts and did their
own artwork in house, and usually ended up modifying the
design (for better or worse) over a period of time, so I'll take a
small tip of the hat

 

[slowhands]

I put a higher voltage supply on the front-end of my amp for more output, and serendipitously discovered it quenched turn-on and turn-off thumps and groans.

The front end seems to take control and stay in control of the output from fairly low line voltages on up, which is not surprising since it ramps up much faster than the output supply on turn-on. On turn off, the front end stays up just a little longer than the outputs, so again is in control and things stay quiet. (I used a doubler and regulated down, so the front end runs with 10v higher rails than the MOSFET outputs, but the voltages kick up really fast on turn on and hold up nicely on turn off too.)

Nice to get rid of annoying turn-on/off transients, without resorting to relays.

 

[Nelson Pass]

When you give front ends more voltage than follower output
stages, you can drive the outputs into saturation, and their
slower recovery causes Cross Conduction Current (from V+
through the output stage to V-). Don't clip this amp repetitively
at high frequencies.

This is why on BJT output stages I often like to have the front
end clip first.

 

[fab]

For Adcom amps 555 and 545, have they output BJT or mosfet?

 

[EchoWars]

Bipolar output.

 

[fab]

(I used a doubler and regulated down, so the front end runs with 10v higher rails than the MOSFET outputs, but the voltages kick up really fast on turn on and hold up nicely on turn off too.)

I am trying to use also a doubler in a mosfet output amp but to reduce the resulting voltage so it is only about 10 volt DC higher than the normal rails, I have a lot of power to dissipate in the doubler circuit. I have to use 10W power resistors and several 1W diodes in series. Can you suggest an efficient circuit.

 

[Nelson Pass]

Doublers don't tend to be all that efficient, but then again,
10 volts extra is usually plenty for a front end.

You could consider an extra transformer with 2 secondaries,
each producing a DC which rides on top of the mains. Also this
could be done if you can wind a few turns of wire through the
toroid to form a couple of extra secondaries and do the same
thing.

 

[fab]

Doublers don't tend to be all that efficient, but then again,
10 volts extra is usually plenty for a front end.

You could consider an extra transformer with 2 secondaries,
each producing a DC which rides on top of the mains. Also this
could be done if you can wind a few turns of wire through the
toroid to form a couple of extra secondaries and do the same
thing.

For the doubler I got the idea from Pass A75 power supply amp! My amp does not have a toroid transformer. Using an extra transformer was my original idea but I did not find one that I could incorporate inside my amp. Ok, I will search again...