2. amps-preamps
  3. 1343529449

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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almarg
9,670 posts
 
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
gsm18439
895 posts
 
Leatherneck

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

Thanks
leatherneck1812 OP
116 posts
 
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.

http://www.stereorepair.net/highend.htm

Best regards,
Russ
gsm18439
895 posts
 
Thanks!
hifigeek1
770 posts
 
Leatherneck, the woofers oscillated (motor boating) because the DC operating points where destabilized.
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