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 itsjustme

itsjustme
681 posts
 
can you begin with a more precise definition of what you mean by power cycling? - and try to speak of what each components does independently:

  1. the preamp
  2. the power amp.
  3. the sound, if you are merely talking about a net results

If it is not a net result, you should be able to identify which component is doing what. If it is both, say so. If one does it and a fraction of a moment later the other does, say so. Its way to vague and i will say comments so far are merely speculation.

G
itsjustme
681 posts
 
@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.
That is very strange. Why would two independent products have the same odd behavior?  Let's assume the LEDs indicate power (green = on, red = standby?) and are triggered by various power supplies.  Its also odd, to me, that two very different products have the same LED arrangement and trigger on the same phenomenon.....
The only thing that I can think of is a power anomaly. If power, for some reason, was rapidly browning out and recovering, it could do this.  If (AC) power were to fall and recover, the power supplies would fall and become noisy, which would in fact create hum and low-volume clipping (since the rails would be lower clipping would occur nearly immediately).
But why and how would this occur?  Beats me. Some kind of power line short? maybe. But i cannot be in either box or surely they would have foudn that at the factory.
The chance of both failing simultaneously, int eh same way is near zero. So it is either a) exogenous or b) one of the components affecting the other.
I think you have a 3rd issue to find and fix.
G


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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