2. amps-preamps
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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
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


almarg's avatar
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almarg
9,670 posts
 
Its also odd, to me, that two very different products have the same LED arrangement and trigger on the same phenomenon.....

Odd indeed. But if a high amplitude subsonic oscillation is coursing through both components, per the phenomenon described in my previous post, it would seem consistent with ...

b) one of the components affecting the other.

The hypothesis described in my previous post is also consistent with the fact that the problem didn’t occur when a different amp (and a Bryston amp, no less) was used with the Bryston preamp, that amp having a far lower (50K) input impedance than the ARC amp with which the problem occurred and therefore being much less likely to trigger the phenomenon I referred to.

Also, another factor pointing toward power supply instability in the 12B as being the root cause of the problem would seem to be the fact that it is 25+ years old.

Regards,
-- Al

celander's avatar
celander
1,502 posts
 
OP: call me jaded, but I recommend having an independent electronics shop check out the preamp and power amp. Sometimes Manufacturers don’t want to fix a problem that’s clearly there. 
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
892 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.
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