Cary SLP98P not working with Audio Research VT100


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?

If the preamp works with the Parasound, you just ruled out the preamp as the problem. Sounds like something is up with the ARC.
Ralph...the OP does state that when he hooks up his OPPO 105 direct to the ARC amp, it works fine.

I think it has something to do with the output impedance of the Cary and the input impedance of the ARC amp and the cables used to attached them? It seems to me that it is causing an oscillation (the pulsing of the tubes in the amp).

Have you tried a different set of interconnects?

Maybe Al will chime in...
OK, when I remove both from the APC S15 and plug them both into a PS Audio Duet no problems for hours.

I'll try different cables, I still have my trusty Kimber Kable Hero's.

The impedence for the amp is the same as Cary's 120S, so I'm guessing that's not the problem.

My preamp is phase inverting, and I just read the manual again for the AR amp, and it states not to use a phase inverting preamp, but does not say why, and I can't find any info on the internet about that. Seems this would have come up previously with these two companies.

I had a similar problem with SLP-98P F1. It worked fine when hooked up to a CJ tube amp but it kept shutting down my Mark Levinson amp.
Opus, did you find the cause?
Mofi... you may have just provided your own solution. I just checked the specs of a number of ARC preamps on the ARCDB web site. None of the preamp's outputs inverted phase. I have absolutely no idea what that means, so I'm hoping Ralph or Al will chime in. But that may be the cause of the anomolie. I wouldn't use the Cary pre with the ARC amp until you sort this out.
Hi guys,

The power amp has no way of "knowing" whether the output of the preamp is in or out of phase with the input of the preamp. The statement in the manual about not using a phase inverting preamp makes no sense to me, unless they are envisioning the possibility of the amp's AC current draw somehow affecting the preamp via its effects on the AC line voltage. Similar to the effect Ralph once described in this post, and his post just below that one. But it would seem far fetched that phase inversion in the preamp would have anything to do with that possibility.

Also, I recall it being mentioned in another thread a few months ago that the manuals for some ARC tube amps say in bold face type that "ARC tube amps must be used with ARC preamps or damage will occur." Which would seem to further reinforce the notion that cautionary statements in their manuals should be taken with several grains of salt.

Mac, your last post makes it pretty clear that the APC conditioner is somehow a factor. Looking at the photo of its rear panel shown on page 17 of the pdf of the manual, the one very speculative suggestion I have at this point, and it's just an intuitive guess, is to try plugging the preamp into the outlet designated for connection of a subwoofer (with the amp continuing to be plugged into the outlet designated for "amplifier"). Perhaps the separate filtering the APC has presumably been applying to the preamp and the amp is somehow involved; making that change would apparently put the two components on the same filter.

If you are using the APC's turn on/turn off sequencing provisions, keep in mind that that would be affected if you try this experiment, and you would want to manually turn the amp on last and off first.

Not sure what else to suggest at this point. Weird problem.

-- Al
Have you tried connecting the speaker wires up (from the amp) red to red and black to black? Instead of red to black and black to red?
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.
If your preamp is SE, you need to have the pins in the balanced input on the amp for SE to work.
ZD542, what is meant by SE?
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.

-- Al
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...
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.
Sorry about that. I forgot the newer units have a switch. The one I had needed the pins.
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.
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??
School of hard knocks...