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
cflayton OP
16 posts
 
@almarg I think you may be on to something. Perhaps there was a power problem due to the cord and replacing it fixed the issue. I do not know whether the 12B has output coupling capacitors, but I expect it does. In addition, my current house is somewhat challenged with respect to outlets, so I had the amp and preamp plugged into the same Tripp Lite block. Thank you so much for your help.

@itsjustme You also landed on a power problem. Any thoughts on the Al's scenario?

@mijostyn It is true that the equipment is old. However, Bryston equipment carries a 20 year warranty. Of course, it's older than that now. When I sent in the preamp, I asked the Bryston tech whether he thought I should replace it with something newer. His response was: "No. But if you want to, include a note with the unit saying that you agree to sell it to the technician for $5 and I'll gladly take it off your hands." I believe that as long as I continue to have my equipment maintained, it should continue to last for quite some time. Granted, it's not Dynaco...

@celander If the gear were under warranty, I would be inclined to agree with you. However, since it is out of warranty, the manufacturers have nothing to gain by hiding something. I was paying for all repairs and techs from both companies are at a loss for what was causing the problem.

Now, if I could only figure out what caused the loss in gain...

Again, thank you all for your insights and help!
gochurchgo
910 posts
 
  1. @cflayton if I read that right both amp and preamp are connected to the same strip? Try moving them to a different outlet. Perhaps plugging the Bryston directly into the wall and see what changes.
As was mentioned earlier, power fluctuations cause all sorts of madness.
mijostyn's avatar
mijostyn
8,480 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. 
bigjoe's avatar
bigjoe
693 posts
 
Do you have another pair of speakers you can connect to the offending Amp,Preamp and source combo to rule out the possibility of the power supplies in your speakers are causing some sort of short ? 

If Audio Research found no issues and Bryston found no issues the only component in the chain thats left is your elecrostats ,id be taking a hard look into the speakers ,if you swap out your logans for a traditional speaker that dont dip below 2 ohms and all is well then youll know the previously fully functional logans have developed an issue possibly within a transformer .
itsjustme
681 posts
 
power supplies in speakers? Huh?
I would actually just go back to basics and test each unit  - power supplies, look for oscillations, frequency sweeps (or the simple "square wave and squint" surrogate), etc. It still makes little sense to me, so I'd establish facts before further speculation. Its amazing how many things work when i bring them to the lab but not in some mysterious system in the field.
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