gEDA-user: OT: low noise microphone amp

[Greg Cunningham <greg@xxxxxxxxxxxxxxxxxxxx>]

Seeing there has been some discussion about low noise design, I'll throw 
this in.

I had to repair a Behringer 24/4/2 mixing desk this week.  I found the 
following two points interesting:

i) Only one IC type was used throughout the desk.  A dual op-amp SIP 
NJM4580L.  Channel input, EQ, output, headphone etc.  Possibly used as 
bus summing amp too, but not sure without a circuit diagram - there were 
a couple of discreets around the bus master area.  Maybe not optimal, 
but it sure would cut down on BOM items;

ii) Mic input diff amps are discreet. No fancy low-noise instrumentation 
amp.  Each channel had 4x A1084 PNP transistors and 4x MPSA06 FETs, 
feeding a NJM4580L op-amp.  I guess the PNPs were the amp & the FETs 
were current sources (???).
--
Greg


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