Fully balanced tube mic pre schematic

[hulahalau]

Over the course of the years, I have collected many schematics for tube mic pres, e.g., Cilletti "Electric Crayon," "Paul Stamler (in Audio Amateur), Tape Op, Ampex 300/350, Pultec MB-1. all use unbalanced topology after the input transformer and before the output transformer.

However, I have never seen a fully-developed schematic (i.e., with real values for resistors and caps) for a fully-balanced mic pre, that is, one that uses fully balanced topology after the input transformer and before the output transformer. Probably such a preamp would have to use a differential front end, and would be expensive because of the likely need to use two input transformers (or a custom wound job), dual stepped attenuator (stereo pot would not provide sufficient matching), and double the number of tubes (since each side of the balanced line requires a tube section). However, having fully balanced circuitry seems highly desireable - at least if one is looking for a clean pre resistant to interference, and where the transformers are intended to be the source of color. Fred Forsell published a differential op amp line driver in Glass Audio many years ago, and I asked him whether it could be used as a mike pre with a transformer at the front end, but he responded in the negative - said the circuit values needed to be changed to optimize its use as amike pre.

Does anyone have any info that they can share, either by referring to a URL, or posting a schematic?

[A-Barr]

Check out some vari-mu compressor schematics. They are fully balanced throughout and in fact must be in order to cancel out the "thump" of rectified voltage hitting their front end. I think some Altec comp's like this actually have a mic pre section. Otherwise, I think you could look at one and de-compressorfy it by changing the input tube (usually a 6386) to a dual triode or a pair of pentodes and nix the 6AL5 or whatever rectifier comes after the output section to feed back into the input.

That said, I don't think it's that big a deal, vari-mu comp's are the only pieces of gear I've seen that are fully balanced throughout and they don't really have a reputation for being any quieter than any other tube gear, afaik.

[brianroth]

Here are some specs and a schemo for the Langevin 117-a which had a balanced topology:

http://www.brianroth.com/library/langevin117-a.pdf

LOL....a pair of 6V6 output bottles could make this Bad Boy suitable for a guitar amplifier!!! Later model Langevin modules were somewhat downsized....


Bri

[emrr]

Vintage types

Altec 458A
Altec 459A
Collins 6M
Collins 6N
Collins 6R
Langevin 117A
Langevin 5116-B
UA 1008
UA 1016


PP output transformer saves some $ over SE with gapping for DC. Catch is input and output both have to be very well balanced designs meant for PP operation. Though actually, all the stock UTC I've tested all looks better in PP operation than in SE.