redesigning helios olympic mic pres

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NewYorkDave

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Jun 4, 2004
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New York (Hudson Valley)
After only a quick glance, one thing that doesn't look right to me is the placement of the gain switch. It seems to me that it should be on the other side of R4, the side that connects to the emitter of Q1. I don't have the original schematics for reference; but the gain switching arrangement, as drawn, looks completely wrong to me.
 
Hmm, I've been looking at the other schematic for the Helios line amp floating around... the only maj dif(there are a few minor value difs) I could see between that and the Olympic version was an additional 47k resistor after your R4 and before your R8... not quite sure what effect this has on the circuit... check it out here...

http://www.vintageking.com/images/schem_V69-1973-2-Var-1.jpg

BTW the gain point seems to be at the same point on the remakes as well, see

http://www.vintageking.com/images/schem_V69-2003-A3-Var-1.jpg

Justin.
 
A ha! Once you add the 47K resistor, as shown on the "official" Helios schematic, the feedback and gain switching arrangement suddenly makes sense. The schematic Butta used for his reference looks like it was someone's attempt at a reverse-engineering job, and the 47K resistor was overlooked.

Looks like that's probably your missing link, Butta.
 
The Helios I/P transformer is 200: 20000 which gives a +20dB transformer gain. it has a tap in the secondary for 200:200 that is the -20dB pad at the input.
 

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