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R/E/P => Klaus Heyne's Mic Lab => Topic started by: Gabriele Ponticiello on February 24, 2021, 01:51:31 pm

Title: Later release U47 fet in matte black
Post by: Gabriele Ponticiello on February 24, 2021, 01:51:31 pm
Hello Mr Heyne!
I am looking at an original, apparently early '80s, U47 fet in Matte Black (?) finish. The seller has sent pictures but unfortunately one showing the bottom is missing (so far). Seller also reported that he doesn't remember any "-6 dB" switch. While gathering info I came across a black U47 fet for sale in California and actually the switch is not present. On this one there's a sticker on the top of the bottom half (sorry about that) that says "Circuit Diagram 08". My question is: has production evolved into different revisions of the circuit? Should I expect the black I am looking at to sound like the grey, older one I already have?
Thank you very much, Gabe
Title: Re: Later release U47 fet in matte black
Post by: uwe ret on February 24, 2021, 05:08:23 pm
During the later period of the original U47fet production the switch used to configure the output transformer for 200Ω or 50Ω (= -6dB) was no longer available, and circuit versions 07 and 08 no longer featured this switch. By that time the request for 50Ω impedance microphones had disappeared, and the microphones were hard wired for the higher output (200Ω impedance). As long as both units outputs are configured the same, their technical data will be identical.
Title: Re: Later release U47 fet in matte black
Post by: Gabriele Ponticiello on February 25, 2021, 04:32:52 am
Thank you Uwe!
I guess that answers my question.
So... the bottom switch was actually an impedance selector... I am guessing switching between serial and parallel configuration of two secondaries, since the -6 dB halving of the level.
Thank you very much!!!
Best to you all, Gabe
Title: Re: Later release U47 fet in matte black
Post by: klaus on February 25, 2021, 03:47:47 pm

So... the bottom switch was actually an impedance selector... I am guessing switching between serial and parallel configuration of two secondaries, since the -6 dB halving of the level.

Correct.