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 3 responses by cflayton

cflayton OP
16 posts
 
@djohnson54  Thanks. There are no shorts on the speaker connections.

@roberjerman Back in the day, audiophiles were admonished not to turn the volume above 12 o'clock because equipment was at full gain at that point and going past 12 o'clock just pushed amps into clipping. It's good to know that may no longer be the case.

@almarg I've never used the shorting plugs; in fact, none came with the amp when I bought it used. But, I hadn't considered that perhaps ARC used them. There are no plugs on it now. I'll have to get some since I plan to run the amp unbalanced in the future.

@itsjustme The power cycling entailed the green LED on the preamp rapidly alternating from green to red. The LED on the amp was simultaneously doing the same thing.

Every time the power LEDs switched to red, the sound was distorted through the speakers, sort of harsh and "buzzy". It was as if the system were being driven into clipping, but at a low volume. It was similar to the sound of a poorly tuned FM station.

This happened whether I was using the balanced or unbalanced connections between the preamp and amp. It did not happen when I had the preamp connected to a different amp (Bryston 2B, unbalanced).

Thank you all for your time and input!
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!
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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