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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
unsound
7,376 posts
 
The networks keep wide bandwidth amplifiers from oscillating.
casouza
239 posts
 
You wrote:
A quiet but high pitched squeal through the speaker when no ICs are connected. Its definitely abnormal and isn't there when the preamp is on.
**THIS IS NORMAL One should never power up a high-powr amp with no ground connection thru the ICs.

2) Using XLO or MicroPurl ICs, ie not shielded, if you move, wiggle, or rub ANY part of the IC on ANYTHING, you get static and white noise in addition to the quiet, high pitched squeal.
*THIS IS NORMAL

3) Try an AudioQuest IC, ie shielded cable, the squeal is still there but rubbing the cable against the rack doesn't produce extra noise and there is no static.
*ALSO NORMAL

4) Wrapping either RCA end with my hand, without touching the middle plug, eliminates all the noise, including the squeal, on the AQ and MP but only gets rid of the static with the XLO."
DITTO, NORMAL.Your hand "shields" the spurious signal.

NEVER test a high power amp with an open input (open-ended ICs are an open input plus a long antenna to pick-up noise).

My suggestion: buy a pair of inexpensive Radio Schack RCA connectors; short positive to negative on those RCAs by bending the metal parts.
Connect the shorted RCAs to the amplifier input and measure DC mV across the amp's speaker outputs. If the DMM reading is close to zero, the amp is fine.
To double check, measure AC V (lowest range on your DMM), it should be zero.

Power down the amp and move the shorted RCA's to the preamp input and measure DC mv across the preamp's outputs. Use an RCA output to make it simpler: the external contact is minus, the center pin is plus. Adjust the volume control up and down. If the meter shows an up and down DC reading, your preamp is defective (DC on the output). If the DMM reads zero no matter how you move the volume control, the preamp is fine DC-wise.
To double check, measure AC V (lowest range on your DMM), it should be zero or close to zero.

Now connect the source to your preamp (no music playing), turn on the source and preamp. Measure DC mv at preamp output again. If the meter shows a reading, DC or AC, your source is defective.
To be certain, measure ACV (lowest range) and DC mV at the source's output.
If your meter shows a few mV DC or AC, your source is defective.
I hope this helps
dave_b's avatar
dave_b
3,159 posts
 
I must admit, I've never seen a bigger collection of lost audio nerds than on this post. Leoturetsky, don't f*** around with exotic experiments. Eliminate one thing at a time. Do have your preamp checked for DC first, then if ok take out the zobel's and try your system. Then take out the Goertz if necessary. I'm sure the problem lies within this range of eliminations. Do you have a dealer nearby?
phaelon
1,054 posts
 
Hi Leoturetsky,
I wish I could give you something more helpful, but they're not just in airplanes anymore.

http://www.motionbox.com/video/player/d39fd4b81c5a
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.
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