Helios Input Module

3 transistor series feedback circuit. Phase is coherent due to 2 phase inverting stages.

Supply voltage is negative. Note the "emitters up" style and the orientation of caps C6 and C10. Not sure why the orientation of C1 is that way.

#16 GAIN likely goes to a panel control. Since C5 is connected to the output, I would assume that grounding C5 mutes the output and increasing resistance to ground brings the gain up.

VT1 is a common emitter with series feedback amplifier. Instead of R3 going straight to the negative supply, it also gets an injection of VT3's output as series negative feedback. R5 is so large that VT2 buffers the signal before delivering it to the low impedance VT3. R8 provides dc series negative feedback, while C8 bypasses this negative feedback at signal frequencies.

R11 and C7 form a simple low pass filter to place a concrete limit on the upper end of this amplifier's bandwidth. The input's C4 is also meant to squash any RF noise before it gets into the amplifier. R1 and R2 are very high impedance so as not to load the input signal. Since the input and output are in phase, the series feedback at R3 increases the input impedance. I don't know the calculation for the VT1's base input impedance here, but I assume it is fairly high.

Hyde Street Studios history can be found here: http://en.wikipedia.org/wiki/Wally_Heider_Studios

This stationary, with the "Hyde Street Studio" name is most likely post 1978.

Helios history can be found here: http://www.danalexanderaudio.com/OutboardPic/Helios.htm

A Helios console was installed in Hyde Street Studio from 1981 to 1983. This schematic most likely is from then.

Roger Mayer worked for Olympic Studios in 1968 designing consoles. Richard Swettingham, chief electronics tech of Olympic, began the Helios company the next year, while Roger moved on to start his own company in New York.

I mention all this, because I've come across a somewhat silly accusation made several years ago that this Hyde Street document somehow casts doubt on Roger Mayer's designs, as though he doesn't do his own designs or something. Looking at the evidence myself, I completely disagree with the accuser's opinion.