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 almarg

almarg's avatar
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almarg
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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

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almarg
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@cflayton,

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:

https://forum.audiogon.com/discussions/why-did-this-fix-my-problem

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


almarg's avatar
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almarg
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I took a look at the manual for the amp, which I found at hifiengine.com. 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.

Regards,
-- Al

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