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?
I guess there was an impedance mismatch between your amps and your preamp. It sounds like either the amps were oscillating, i.e. motor boating, or the mismatch was forcing the preamp to oscillate pulsing the amps . That's the reason why the bias would fluctuate the way it did. It good you solved the problem because issues like that can cause output tubes to arc and woofers and tweeters to seize, depending on the oscillation frequency.
Hifigeek1, The woofers oscillating is where I saw the problem in the first place. I thought it was DC leakage at first. I did lose 1 kt77 tube. I tried everything I could think of..swapped tubes in amp and preamp AND Modwright Logitech transport, cables, even brought out an old NAD integrated that I could use the preamp section to run the amps which is when I finally got it to settle down.

What I don't get is what did lowering the impedance do to make them compatible?
I assume that the interconnections between the two components were via unbalanced (RCA) cables, since the preamp does not provide balanced outputs. I see that the amp has both unbalanced and pseudo-balanced inputs. I presume that there were no jumpers or other connections on the XLR inputs.

This is just a shot in the dark, for which I can't formulate a precise explanation, but perhaps the problem would not have occurred if the amp's inverting input on XLR pin 3 had been grounded (e.g., with a jumper connecting that pin to XLR pin 1).

With the original 100K input impedance, the unconnected inverting XLR input would have more closely approached a "floating" condition, compared to when the input impedance was subsequently changed to 47K. That would perhaps have made it more likely that the amp would behave in an uncontrolled manner. Without detailed design information on the amp it is probably not possible to say what that behavior might have been, but an oscillation of some sort seems conceivable to me.

The different (and probably lower) output impedance of the NAD that you temporarily connected would have changed (and probably reduced) the impedance between the non-inverting input signal path of the amp and ground. Again, without having specific knowledge of the design of the amp's input circuit it's hard to explain precisely how that might have prevented the oscillatory condition, but it seems conceivable to me that the changed circuit conditions might have had that result.

-- Al
Al, Thanks for chiming in! I have read many of your posts over the years and your thoughts are always insightful. My amp doesn't have any xlr inputs though. I am assuming you read the review on the Crescendos in Stereo Times, I have read that also. There aren't any extra holes so this pair never had xlr inputs. These are only identified as TPA-60, not Crescendo. The amps are a push/pull design and Direct Coupled. That is about as much info as I have.
My amp doesn't have any xlr inputs though. I am assuming you read the review on the Crescendos in Stereo Times ....
Right you are! What misled me was the reference in the review to "... the TPA-60, now known as the Crescendo," together with the indication that a pseudo-balanced input is provided.

So I have no further theories to offer, other than a general suspicion that some kind of design marginality existed in the original 100K configuration, that in turn was sensitive to the output impedance of the connected component.

Best regards,
-- Al

Which company "repaired" (or, maybe I should say, "modified") your amp?

Gsm18439: The company is "The in house service company" at the address below. I am very happy with their service and prices. They quoted me a bench service price plus hourly. After my amps arrived and they got them on the bench they called, and we talked. They asked about talking to the maker of the preamp Kara Chaffee at deHavilland. I gave them her number and they worked out the details with her. She called me and said they were very knowledgeable and a solution was arrived that they both thought would work and it did! It did take a cpl of weeks before they even looked at it so I guess they keep busy.


Best regards,
Leatherneck, the woofers oscillated (motor boating) because the DC operating points where destabilized.
And the DC operating points were destabilized because???
How did lowering the input impedance from 100k to 47k stabilize the DC operating points?
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.
Thanks, Ralph (Atmasphere). That does all sound like a plausible (and clever) explanation. Never woulda thunk it myself, though :-)

Best regards,
-- Al
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.

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, the solution you have right now is a jury-rig. You need to get the preamp sorted out- and when you do, I think you will appreciate the improvement.
I sent the information off to Kara and she is going to run some calculations and contact me when she gets home from an audio show.

I do have one more query. Is it possible that the preamp is passing on this signal from a source? My Modwright-Logitech transporter is tube rectified. I have experienced woofer modulation in one speaker with certain tubes. The modulation moves to the other speaker when the tubes are swapped right for left. I just assumed I had a failing tube but these are tubes I was using when all of this started. As I have stated the amplifier modulation has never occurred since the 47k resistor was installed but I can still get the woofer modulation when specific tubes are placed in the transporter. Is it possible that this modulation was being amplified by the preamp and sending the amps in to motorboating? If this is the case does it still point to the preamp being the problem because the circuitry is designed to have such a minimal impact on the signal that it lacks the sophistication to correct this?
Huh! OK- yes, its very possible (and with this new information, very likely) that indeed the preamp is simply passing the LF noise along from the source. However there is a feedback mechanism that is also in play and its hard to know if that is the preamp or the CD player.

BTW, the preamp is not supposed to have anything in it that would 'correct' this!

If it were me, it would re-install the 100K resistors, and then run the preamp with the source, with the volume all the way up, and see if you can provoke it. Once it starts I would disconnect the source and see if it shuts up.

It seems highly coincidental (and a bizarre explanation) that you would have this phenomena in the CD player and it not also be the source of the problem! So right now, barring further information, in my mind the preamp is tentatively off the hook and the CD player should be under scrutiny instead!

FWIW I would not expect any LF noise from the digital system unless the tubes you are using are noisy/defective.
Thanks Ralph,

I will put the 100 ohm resistors back and see what happens.

Thanks again,
Correction- 100K...