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How am I frying all my amps? Oscillating? Help

Hi. So I'm on my third amp this year.

I had an Adcom GFA-7000 for many years which I gave to a buddy for his birthday so I could get a Krell. The Adcom used to "hum" or "buzz" for five minutes then stop, then start again a few hours later. It wasn't noticeable with the music playing. Also, when nothing was playing, it would pop and hiss when the volume was turned all the way up as if the volume control was directly connected to the speakers and was shorting them or something. It ran for nine years and is still going strong today. It did none of these things when I first got it.

Then I got a Krell KAV-500. There was always something wrong with this amp because it wouldn't turn on when I first got it. Then it did turn on and I used it and it would hum like the Adcom and its volume control noises were even worse. Then it exploded and got sent to Krell for repair (still not back). Also, the Krell would make a horrible buzzing sound when only RCAs or XLRs were plugged into it, no speaker wire (and I tried a dozen different cables as it took only one to reproduce the problem). And when you muted or turned off the pre-amp, the thing would send a horribly loud buzz through the speakers. It did not do this when I first got it.

So I got a Classe CA-200. This amp was dead quiet and had zero problems. Two nights ago I noticed the volume control was being noisy. Last night I noticed the amp hum. So I looked around, and of course, now the thing exhibits all the same traits of the Krell except quieter: volume control noise, buzz when the pre-amp is off or muted, buzz with only ICs connected. I expect it will get louder until it blows up as per the Krell.

My system has changed dramatically between the three amps. There are two commonalities for all of them: Goertz MI2 Veracity Cu speaker cable and Thiel CS1.5 speakers. The Zobels are on the speakers ends of the Goertz cable.

In addition to the speakers and wire, the Krell and Classe have two more in common items: Cary Cinema 11 preamp and PS Audio Ultimate Outlet.

The Adcom never really died. The Krell and Classe look like its going to take about two months each but I find it difficult to believe a Line Conditioner or Preamp can cause this kind of damage. That leaves me to think they are oscillating due to the Goertz cable but I don't know nearly enough about it for this to be anything but an uneducated guess.

I leave everything on all the time. What am I doing to my amps? :-(

Thanks!
leoturetsky
atmasphere's avatar
atmasphere
11,901 posts
 
Leoturetsky, did you try a different speaker cable? I doubt the cable is an issue (I've used them for years) but its best to eliminate as many variables as you can.
leoturetsky OP
103 posts
 
I plugged everything into the one PS Audio Quintet (not UO). Every 3-prong cord got a cheater plug except the preamp (Cary Cinema 11). I have not taken out the Goertz cable because I want to make as few changes as possible at one time.

Using shorting plugs on the RCAs of the Classe amp, 1.5mV AC, 0V DC, at the right speaker output of the amp.

For the next test, before I measured I noticed the volume control on the Cary was ridiculously staticy particularly as it increased the volume.

Using shorting plugs on RCA Input 7 of the Cary, and Micro Purls to the RCAs of the Classe amp, 0.45V AC fluctuating, 0.2V DC fluctuating, at the right speaker output of the amp.

Thats 0.2 volts, not millivolts. By fluctuating I mean it was never constant but those were the high numbers. When the preamp RCAs were disconnected from the amp, the number slowly dropped to zero and held steady.

So...
1) Is this really the problem?
2) Why does the amp care about DC at the input and how does DC break it?
3) Shouldn't the Classe be shutting itself down if I turn it on while moving the volume control back and forth?

This is why I switched from ECE to CS....
kirkus
502 posts
 
My vote is that there's a problem with your preamp, and it's putting out some kind of inaudible energy that's giving your amplifier(s) fits. It could be DC, or ultrasonic AC (oscillation). There are all sorts of things inside it that could fail and cause this problem . . . a likely guess would be that an electrolytic bypass capacitor on the output of a 3-terminal voltage regulator has dried out, making it oscillate. This could then be very effectively coupled into all of the internal analog circuitry by the supply rails.

A multimeter doesn't really give you a very big window on what's going on, because depending on its internal rectifier, shunt, and sampling characteristics, ultrasonic AC could easily give a reading when the meter is set to DC. An oscilloscope would give you a better idea . . . but then again, who really cares? Your meter readings indicate to me that there's some sort of energy at the speaker that shouldn't be there. And it appears when you add the preamp. Have it serviced, and/or try another.

There may still be a slim chance that the amp-preamp interconnect is picking up a ton of RF and causing your problem (is it unshielded?) . . . but it's easy to check by swapping it with another cable, even a cheap one. Given that you're in the stix [sic], I think that this is unlikely, but it's easy to check.

Anyway, hope this helps.
atmasphere's avatar
atmasphere
11,901 posts
 
Either the preamp has a noisy (dirty) volume control (Radio Shack tuner cleaner could sort that out quickly enough) or you have DC on the volume control too.

The DC levels are not enough to trip the Classe protection circuit (I am assuming it has one). 0.2V into 8 ohms is 0.005 watts, FWIW. Given the gain of the power amp, the DC at the output of the Cary is low, about 0.2mV. Although I would not expect trouble on that account, OTOH I would not expect any DC at all, so it *could* point to a marginal coupling cap.
kirkus
502 posts
 
0.2V into 8 ohms is 0.005 watts, FWIW

And this is a TON of juice, for noise voltage. Similar to the amount of average audio power required to play speech at normal conversation levels, with an average bookshelf speaker. The AC reading was even higher, 0.45v. This is NOT normal and SHOULD NOT be there.

Also, there's no place inside a home-theater preamp-processor to spray any aerosols . . . it's sure to be a solid-state electronic volume control.
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