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
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
682 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.
cflayton OP
16 posts
 
@gochurchgo Yes, I plugged the ARC directly into the wall. No difference. I can try the preamp. However, right now I'm not having the issues with oscillations, so I doubt I would see/hear a difference. At this point, I'm just trying to figure out the cause of the drop in gain through the ARC.

@mijostyn Well, I don't really have a problem. It's more of a mystery at this point. But to each his own as far as repair vs. replace.

@bigjoe Yes, I have a pair of Heresy's I can connect. That's a good suggestion. I've had the Heresys connected to the preamp and 2B and everything has been fine. I have not tested them connected to the ARC amp, but I should have thought of that.

@itsjustme Yes, the speakers have rather large power supplies. Here are a couple of links to pics (not mine): 
https://www.canuckaudiomart.com/details/649399445-martin-logan-cls-iiz-electrostatic-speakers/images/1726284/
http://www.hifishock.org/gallery/speakers/martin-logan/cls-iiz-2-martin-logan/
Unfortunately, I don't have the equipment to run such tests. I used to design user interfaces. I learned a lot each time I ran tests with real people and saw how they interacted with the designs. There's nothing like empirical data.
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