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!
Have you checked the speaker cables?  I would imagine a partial short could cause what you're seeing/hearing.  The act of disconnecting and reconnecting the cables might have reoriented the cables so that they aren't shorted anymore.  Do you have any pets that might have chewed a cable?

Just a shot in the dark as I'm not sure that would cause the lack of gain.

Most volume controls are most linear from 9 to 3 o'clock. So if the sound is fine (not distorted) you are probably ok!
I took a look at the manual for the amp, which I found at I see that when its "normal" or "inverted" RCA inputs are used RCA shorting plugs should be inserted into the other of those inputs, but RCA shorting plugs should NOT be inserted when the XLR inputs are used. I’m wondering if when ARC worked on the amp they might have put shorting plugs on one of those RCA inputs, and if so if the shorting plugs may still be there. If that is the case they are most likely the cause of the loss of gain you are seeing when using the XLR inputs, as they would be shorting one of the two signals in the balanced pair of signals to ground.

Also, if perchance shorting plugs were in place in the past, when the distortion and power cycling were occurring, the short being placed on one of the two signals in the balanced pair of signals being provided by the preamp could conceivably have been stressing the output circuit of the preamp in a manner that eventually caused it to distort and/or power cycle. And that might even have caused the preamp to output some amount of DC, causing the amp to power cycle.

-- Al

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.

@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!

This brings to mind an issue that was discussed in the following thread, in which someone was seeking an explanation as to why changing the input impedance of his amp from 100K to 47K cured a problem in which the amp’s bias would fluctuate wildly:

Some excerpts follow from the posts in that thread by Ralph Karsten of Atma-Sphere. Note the reference to an ARC amp. Also note that the input impedance of your amp is 300K balanced and 150K unbalanced, which is much higher than the input impedance of both the D-150 Ralph mentions and the amp used by the OP in that thread. Everything else being equal the very high input impedance of your amp increases the likelihood that the phenomenon Ralph describes is what was going on when the distortion and power cycling occurred. (In saying that I’m assuming the preamp has coupling capacitors at its outputs; I couldn’t find a schematic for the 12B but schematics I looked at for several other somewhat more recent Bryston preamps do indeed show output coupling capacitors, which in fact have relatively large values such as 47 uF which also increases that likelihood substantially).

Atmasphere 8-3-2012
The problem is that there is a power supply instability in the preamp. The output coupling cap, when driving a 100K load, represented a frequency pole that was lower than the frequency pole in the preamp’s power supply.

The result is low frequency instability. With many amps this may not manifest with anything, especially if the amp does not have good LF bandwidth, but I think the interaction occurred due to the fact that you do have enough bandwidth in the amp and the power of the amplifier was able to mess with the AC line voltage, which in turn exacerbated the LF instability of the preamp.

So lowering the input impedance of the amplifier solved the problem by knocking off an octave of LF bandwidth.

IMO/IME there are much better ways to solve this problem. The power supply of the preamp needs to be either repaired, redesigned or perhaps regulated, depending on why it has the instability....

... I first saw something similar occur in the 1970s with an ARC D-150 and an ARC preamp that had a bad regulator in the power supply.

The preamp would make some low frequency noise, the amp would amplify it, the AC line would sag, that caused the preamp to do it again, repeating the cycle.

If the distortion/power cycling problem should re-appear it would seem to be a good idea to call this possibility (which I think is a very strong possibility) to Bryston’s attention.

Regarding the gain reduction that occurred, I have no thoughts at this point.

Regarding shorting plugs, a search at eBay for "RCA shorting plugs" will turn up a number of inexpensive sources.

Good luck. Regards,
-- Al

If you have enough gain to get to the volume you like you are in business. Forget about what position the volume control is in.
In the mean while you have reached the useful life span of most electronic equipment. You should start saving for new stuff. 
@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.

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.

-- Al

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. 
@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!
  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.
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. 
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 .
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.
@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):
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.