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In the Forum: Audio Discussions
In the Thread: Passive transformer based preamp
Post Subject: remotely possible interstage or volume controlPosted by Bud on: 10/25/2005

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actually I agree with Romy, to a degree.... If the amplifier is a push pull device, using my output and input splitter devices of course, then an interstage transformer is a much better choice than rc coupling. Using transformers designed to the criteria I use will allow a push pull amp to provide a truer musical reproduction than a single ended amplifier with the same tube set, just barely, and only in the case of all out designs.

Using interstages in a single ended design is really a bit hopeless. The requirement for high inductance to offset the dc plate current forces the use of ultra fast core material. This causes the need for slow dielectric material in the coil and a mismatch between E field and B field response. The sonic result is sound with minute but continuous headache inducing dissonances. To cure this problem requires either massive amounts of turns and commercial core, with loss of high frequencies as the result, or fewer turns, gapped core and soggy low frequencies. I am sure there are some applications where triode drivers, step down interstage and low miller effect capacitance in the output tube will allow a specific to need design to work, and work very well at that, but as Romy says you will be throwing money at the problem for a while.

As for transformer volume controls, you can get them. Intact audio has kits for sale. The sound is a bit bland compared to an active linestage and would be better if it was not a tapped autoformer but a full on isolated device with proper coupling will be quite a bit more expensive than the Intact kit. They do sound better than any of the resistor switch devices except for those using $3.50 apeice crushed foil resistors, in a discreet pair type of circuit configuration.

A very interesting possibility is a switched step output transformer driven by a ridulously overkill triode power tube in an active preamp. If a pentode driver stage is used with a CCS to allow no interstage coupling devices and the triode was used, or a pentode as a cathode follower with another CCS attached, this might be a solution worthy of the time and money.

I do thnk you will find that all passive linestage devices have draw backs musically but a transformer volume control is probably the best of the lot and Dave Slagel's kit is inexpensive enough to be worth trying.
http://www.intactaudio.com/ 

Bud

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