Original UA console - 100D preamp, EQ, 101D program amp

[systemtruck]

The 610 seems to me to be a 'cost reduced' version of the 100 series. The 610 has both tubes set up as two stage CC with NFB which allows the channel fader to be placed between the two stages. The limited 610 EQ looks like a very cut down version of the 100 series implemented in the NFB of the second tube and because there is only one tube driving the output transformer it has a 30K primary which throws away 17dB of gain and may go some way to explaining anecdotal reports of distortion in the 610 under some conditions.

Cheers

Ian

Old thread!

Wondering about these older 100D Putnam preamps VS the original 610 preamps, since I was about to build a 610 or two but learned that maybe these sound better.

1. The 610 uses a 29k/600 output transformer, to be best of my knowledge. So I guess this 100D would use something in the ballpark of half that so a 15k/600 ? … and something that can handle DC since it’s single ended? Any ideas of an off the shelf replacement from Cinemag / Lundahl / Jensen / Carnhill / Sowter?

2. The 100D preamp has no gain / input adjustment where as the 610 has a volume pot between the two isolated NFB stages. The Putnam console then follows the preamp with a volume pot soon after. But this of course doesn’t allow adjustment of the gain/drive/overload of the preamp itself if that is needed. So I was wondering.. could maybe a pot between mic transformer secondary and input of first tube work cleanly and not throw anything off? I’ve seen examples of this in Collins and Fairchild designs. Interestingly, this document here https://funkwerkes.com/web/wp-content/techdocs/MixedProAudio/UniversalAudio/UA100D.pdf has a note about modifying the 100D preamp for more gain. The #2 option of a shunt resistor at this very point is removed where as in the unmodified schematic there is an option there for loading the transformer when using pre in booster mode. That value is 82K. Do you suppose putting a 1M pot there would work to provide some input level control without reducing overall possible gain of the circuit?