2. amps-preamps
  3. 149357-109878

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!
cflayton

Showing 2 responses by mijostyn

mijostyn's avatar
mijostyn
8,223 posts
 
If you have enough gain to get to the volume you like you are in business. Forget about what position the volume control is in.
In the mean while you have reached the useful life span of most electronic equipment. You should start saving for new stuff. 
mijostyn's avatar
mijostyn
8,223 posts
 
cflayton, yet here you are. Yes, you can keep rebuilding and repairing but when things start f ing up it is best to move on. I have a 200 watt/ch Adcom amp in my workshop system that refuses to die. It is something like 35 years old. My Krell KMA 100s blew at 20 years and they cost over 10 times as much. So much for "built to last a life time." I sold them to a fellow who wanted to rebuild them. The operating term is f ing up. When the problem is weird like this it is even better not to chase your tail.
The magic of Dynaco was that you could get really great sounding stuff for dirt cheap. The switches and pots in the preamps were garbage and hardly ever went 5 years without getting scratchy. You can however keep a Stereo 70 going forever they are so simple. I am fond of class A amplifiers but with rare exception like the Boulders they run very hot and I am under no illusion that they will last as long as a similarly built AB amp. Many of the newer preamps are touch control being totally devoid of mechanical switching they should have a much longer lifespan. My TACT processor is built this way and still going strong after 20 years which is great because it is still irreplaceable. 
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