2. amps-preamps
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Why did this fix my problem?

I have a deHavilland mercury preamp and Thor TPA 60 EL34 based monoblocks. My amps input impedance was 100k ohms and the Mercury is a cathode follower which I understand needs a minimum 10k ohm load. I was having a problem with the amps going into wildly fluctuating bias and would have to shut them down. Upon restart the bias would hold for a short period and then the whole process would start over. After 6 months of sending the amps to Thor who found them in perfect working condition I sent to a repair company that specializes in vintage and tube equipment. After a week on the bench the amps never acted up. I put the tech in contact with deHavilland and they decided to lower the input impedance of the amps to 47k ohms. It has been six months and I not only have never had one problem, the sound of these 2 components is just glorious. Anyone know why this worked?
leatherneck1812
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atmasphere's avatar
atmasphere
11,656 posts
 
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. Also, the cathode follower output of the preamp does not *have* to have a 10K load; that is probably the minimum impedance that it can drive properly. 100K should have been no problem at all, save for the fact that it revealed a malfunction of design problem in the preamp.

However, even if the power supply is regulated, it may not amount to much if the coupling caps in the preamp are incorrectly chosen. Rather than change the input impedance of the amp (which should be almost irrelevant) it would be better if you got to the bottom of this as it will ultimately sound better. Send the preamp back to Kara- I am sure she will get it sorted out.
almarg's avatar
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almarg
9,670 posts
 
Thanks, Ralph (Atmasphere). That does all sound like a plausible (and clever) explanation. Never woulda thunk it myself, though :-)

Best regards,
-- Al
atmasphere's avatar
atmasphere
11,656 posts
 
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. In that case the volume affected it as the problem was having to do with the phono stage.
leatherneck1812 OP
116 posts
 
Ralph,

Thank you for finally getting to the bottom of this. I did wonder for a while if the preamp was the problem because when I sent The amp in to Thor they could not get it to malfunction. The problem was it would only happen in one amp, the other one was fine, but eventually it did occur in the second amp at higher volumes. Kara originally offered to look at the preamp but by that time it was already on the techs bench and this is the solution that was agreed to. I will contact Kara and pass on your thoughts to her and get the preamp in to her for service. I was never comfortable with this solution. It just didn't make sense to me why the amps only had this problem in my system, but having virtually no technical skill couldn't get my head wrapped around why.

Thanks again,
Russ
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