2. amps-preamps
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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

Showing 5 responses by kirkus

kirkus
502 posts
 
There is one thing I can tell you for sure . . . when problems defy repeated attempts at diagnosis, there is a fundamental assumption being made that is untrue. In this case, it's definately possible that there is/was more than one completely unrelated problem . . . that is, whatever's plaguing the Classe isn't necessarily the same thing that was afflicting the Adcom.

Noise that changes when cables are moved around is very common indicator of RF interference. Unshielded interconnects are especially suceptable, which is why I feel that they're basically worthless. But it's unlikely that it's another piece of audio gear putting out enough RF into the air, with sufficient field stregnth to cause a problem with anything other than radio reception. More likely it's some kind of nearby transmitting device, or something like a preamp output-stage going into oscillation and dumping RF directly into the amplifier.

DC usually manifests itself as noise that occurs when swiches and controls are being operated, much less common as steady-state noise. It also frequently triggers protection circuitry at unanticipated times.

In your system, I couldn't figure out what preamplifier/processor you've been using other than the Cary . . . problems that are volume-control dependent (with no sources connected) point pretty strongly to this component. And it's also hard to imagine that the Krell wasn't just defective in its own right . . . there's that thing about "it wouldn't turn on" that's usually a bad sign, and usually unrelated to other stuff.

Anyway, keep us posted on your progress . . . I just think that there has to be an element of coincidence and bad luck here.
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.
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.
kirkus
502 posts
 
Your plan sounds good to me.
kirkus
502 posts
 
Actually, it sounds like you've made some real progress . . . the volume-control-related noise and the high-pitched squeal are gone. Would it only be that in diagnosis of an electrical system, solving one part of the problem always solved all of it . . .

Your measurements at the wall outlet are absolutely no cause for concern . . . I would say that the Cary exhibited a failure for reasons unrelated to anything external - it simply broke.

BUT, I still have a buzz at the speakers when everything is on. Is not a ground loop because everything has a cheater plug but the Anthem. And its present even when only the amp is hooked to the speakers and the wall.

So here's where we need some clarification. You need to hook up the Classe power amp up to the speakers, and to power (WITHOUT a cheater plug), and leave the inputs (coming from the Anthem processor) disconnected - that is, no interconnects attached to the amp in any fashion. If you have any other wiring to the amp (like a 12v power-control trigger cable, etc.), then disconnect it. I'm assuming conventional passive speakers (none with any built-in powered subs, no electrostats with power supplies, etc.). I'm assuming that the the amplifier is not physically touching a metal equipment rack, or any other electrical devices, or the metal ends of any other interconnects or cables. The only possible paths for electrical conductivity, with regards to the power amp, must be: 1) the loudspeaker cables, and 2) the power connection.

If all of the above conditions are met, and then the power amplifier is turned on, then there should be NO hum . . . if there is, then there is a problem with it. (Side note - some may say that it would be normal to still hear a bit of hum unless a shorting plug was installed on the amp input - and if this were an old tube amp with a high input impedance, that may be true. But for a modern SS amp like the Classe, you should hear NO hum.)

If you don't, then add the preamp/processor, with NOTHING ELSE physically plugged into it. Plug it into the same duplex outlet or power strip as the amplifier (again, WITHOUT cheater plug) and check hum. Continue this with every single piece in your system, individually, and in isolation, until you find the exact point at which the hum appears.

Oh and if the power amp does turn out to have a problem, the cause is still overwhelmingly likely to be a coincidince. All of the stuff you own was built by humans, and humans make mistakes.
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