Stumped by preamp-amp issues

Hi all,

As the title says, I'm at a loss as to what is going on, so I'm turning to the community in the hopes you can help.

I have a Bryston 12B preamp that I bought new 25+ years ago. For the last year I've been running it balanced into an Audio Research D240 mkII amp, which is powering Martin Logan CLS IIz speakers. (Yes, I know this gear is old, but I love the sound.)

A few months ago, the 12B and D240 started distorting and power cycling. I started with the preamp because something similar had happened while it was still under warranty. I sent it to Bryston. Bryston replaced the power cord as a courtesy, but could find nothing wrong with the unit. It tested fine. I got it back and connected it to the amp, and the distortion and power cycling were still there.

I took the amp to Audio Research. They looked at it, bench tested, connected to their test systems, and could find nothing wrong. They did nothing else to it. The only suggestion I got from them was that the amp "sensitive to DC offset at the inputs and that can cause the unit to go into protect".

I got it back and connected it up to the preamp and speakers, and the distortion and cycling were gone.

However, now the system gain is much lower than it was. I now have to turn the volume on the preamp much higher than before (past 12 o'clock). I have a second amp that I can only run unbalanced and that gain is unchanged.

So what gives? Does anyone have any ideas or suggestions for what I might try? The system does not distort when I turn up the gain (no clipping), so I can do that, but would prefer not to.

Thank you in advance!
Have you checked the speaker cables?  I would imagine a partial short could cause what you're seeing/hearing.  The act of disconnecting and reconnecting the cables might have reoriented the cables so that they aren't shorted anymore.  Do you have any pets that might have chewed a cable?

Just a shot in the dark as I'm not sure that would cause the lack of gain.

Most volume controls are most linear from 9 to 3 o'clock. So if the sound is fine (not distorted) you are probably ok!
I took a look at the manual for the amp, which I found at I see that when its "normal" or "inverted" RCA inputs are used RCA shorting plugs should be inserted into the other of those inputs, but RCA shorting plugs should NOT be inserted when the XLR inputs are used. I’m wondering if when ARC worked on the amp they might have put shorting plugs on one of those RCA inputs, and if so if the shorting plugs may still be there. If that is the case they are most likely the cause of the loss of gain you are seeing when using the XLR inputs, as they would be shorting one of the two signals in the balanced pair of signals to ground.

Also, if perchance shorting plugs were in place in the past, when the distortion and power cycling were occurring, the short being placed on one of the two signals in the balanced pair of signals being provided by the preamp could conceivably have been stressing the output circuit of the preamp in a manner that eventually caused it to distort and/or power cycle. And that might even have caused the preamp to output some amount of DC, causing the amp to power cycle.

-- Al