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In Reply to: transformer-coupled pre posted by spkrplus on August 20, 2002 at 17:33:04:
Absolutely! A traditional transformer output preamplifier is excellent while a parafeed transformer output is unsurpassed in my opinion. YMMV but I doubt it.
Follow Ups:
Dave
Is parafeed simply using the transformer output in a balanced mode? What does YMMV stand for? Thanks very much for your reply.
Jim Romeyn, Petaluma CA
Jim,YMMV, short for "Your Mileage May Vary" from the automobile ads in the '70s. They would make claims for their gas mileage and then cover themselves by saying "your mileage may vary." In the Internet world it has come to mean that I have certain opinions or perhaps results that are not universal and you may have a different experience.
In a traditional transformer stage the transformer primary is connected between the plate and the B+ power supply. All of the current that flows through the tube also flows through the transformer primary because the transformer is in series with the tube.
In a "parafeed", or parallel feed, connection the transformer is connected between the plate and ground. There is a separate path from the plate to the B+ supply and then to ground. Thus the transformer is connected in parallel with the tube and hence the name parallel feed. One other thing: a transformer that was just connected between the plate and ground would draw a lot of direct current which would saturate the transformer and perhaps also fry the transformer. So to eliminate any DC there is a capacitor in series with the transformer, either between the transformer and the plate or between the transformer and ground. This is called the parafeed capacitor. All of the direct current operating the tube flows through the power supply and none of the audio signal flows through the power supply. All of the audio signal (and none of the DC) flows through the transformer. I believe that keeping the audio signal out of the power supply is one of the big reasons that parafeed sounds so good. Another big reason is associated with transformer design but I'll leave that up the transformer experts.
I hope that this helps.
I'm sold. I may be naive but it appears you know this subject well. I plan to see if the ASL Twin head is parafeed. I'd probably enjoy listening to a home-built piece, but the construciton time is very hard to find. Other than cost, why is this design not commonly known as a reference standard? (Though WAVAC & VAC have recently released transformer-coupled pres.) Are there any consumer safety issues for preamps built this way?
Jim Romeyn, Petaluma CA
Interesting thread regarding this issue on Audiogon. Titled "Transformer coupled/Capacitor coupled". Ralph from Atmosphere and Peter from Audio Note also contributed. Both by the way make transformer coupled pre-amps.
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