2. amps-preamps
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Cary SLP98P not working with Audio Research VT100

HELP!

OK, a few months ago after doing a bunch of research to build a new system in a smaller room, I bought some Harbeth SHL-5's and an Audio Research VT100 MKIII amp to match with my existing Cary SLP98P F1 preamp.

When I first got the combo together they sounded amazing, but after a few hours of playing the sound started to degrade then it got really wierd:

The Audio Resarch amp started to pulse. The tubes would two at a time sequentially pulse lighter (losing thier blue hue) and the sound was terrible pulsing on and off as well. If I leave the system off for a long time then start again, the process starts again.

If the amp is attached only to the Oppo 105 I have on the rack, it has no issues. If the Cary is connected to the old Parasound amp I have on the rack, it works fine.

The Cary just came back from an update at Cary, it was doing this before the update, and this seems to have had no effect.

What could this be?

Some suggested DC from the preamp, but I just got it back, and would that build up?

HELP!!
macdadtexas
atmasphere's avatar
atmasphere
11,960 posts
 
There's a power conditioner??

OK- here is what I suspect is going on. If you move the amp to run directly off of the wall I suspect that may cure it. My theory is that the Cary has a fairly low cutoff frequency and is not particularly stable at that frequency, but normally its not too bad.

However, if there is a current limitation in the AC line it can act like a timing constant. So if some low frequency noise comes out of the preamp, the amp might react- this causes it to draw more power from the AC line, which, because a power conditioner is involved, will cause a drop in the AC line voltage for a little bit. This causes the preamp to put out a low frequency thump, probably not all that big at first but in time this develops into a low frequency oscillation of sorts.

I have seen this before, which is why I think if you plug the amp straight into the wall there is a good chance it will sort it out.

Keep us posted...
teeshot
123 posts
 
I had that exact combination for many years without a problem. Mine were plugged into the wall. Given what the OP has said, it must be the power conditioner or the cables. My VT100iii was also very finicky with after market power cables. If all else fails check that.
zd542
5,339 posts
 
Sorry about that. I forgot the newer units have a switch. The one I had needed the pins.
macdadtexas OP
1,794 posts
 
Al et al, since moving both power cable to a very meat and potatoes PS Audio Duet I dug out of the closet, no problems at all. I am about to have some other work done to my house, I'm going to have a dedicated line put in, and I'll use that for the power amp only.

Thanks for all of your help, it actually has worked perfectly for the past 4 days, and sounds wonderful, the combination is exactly what I was hoping for.

Of course it's not a set of big Maggies in a big room, but it still sounds very good. I do miss my Maggies, I may need to put an addition on the house.
bifwynne
2,206 posts
 
Fantastic detective work Ralph -- many kudos!! I kinda figured that the problem related to a power supply oscillation of some type, but you took it 4 levels beyond. Who woulda thunk??
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