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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
bifwynne
2,206 posts
 
Ok ... I admit not being a techie, so take this with a grain of pepper. If the problem with the amp is presenting in both channels, there could be a common power supply problem that is causing some type of oscillation (e.g., pulsing output power tubes). If all the output power tubes are affected, it seems more likley a problem in the power supply.

The problem is doubly weird because you say it does not occur if your Oppo 105 is directly hooked up to the VT100. Does the Oppo attenuate the signal presented to the VT100??

I have two suggestions. One -- if possible try to borrow another linestage/pre and see if the problem persists. Two -- even if the problem seems to resolve itself by changing out the linestage/pre, I would check in with Kal in ARC Customer Service. I think this situation deserves better advice than just a grain of pepper (or even a whole pepper shaker) from a know-nothing like me.

Please let us know how you make out.

Good luck.
zd542
5,339 posts
 
If your preamp is SE, you need to have the pins in the balanced input on the amp for SE to work.
macdadtexas OP
1,794 posts
 
ZD542, what is meant by SE?
almarg's avatar
Ag insider logo xs@2x
almarg
9,670 posts
 
SE = Single Ended = Unbalanced

Although per this page your MkIII version of the VT100 provides a balanced/single-ended switch on the rear, rather than requiring shorting jumpers on the XLR input connectors when the RCA input connectors are being used.

Regards,
-- Al
atmasphere's avatar
atmasphere
11,747 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...
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