Why does it look "messy" to you? Looks pretty much de rigueur if you want digital control of a rather sensitive analog circuit (like a mic preamp).
Using the extra mic transfomer winding in the feedback circuit (presumably to adjust gain) is a new one on me.
OTOH, as Mr. Bear suggested, TI makes a replacement which is monolithic and perhaps better performing also.
This is a Studer mic amp
I just dont know if I like it. It looks messy. Or am I missing out on a great innovative design
martin
Just use TI's PGA2500.
Graham
It only looks messy if you do not understand the circuit.
-- ciao Ban Bordighera, Italy
I was trying to work out the disadvantages/advantages of running a mic transformer into a virtual earth.with the i/p z deterimed by r134 or R142. That doesnt look like a quiet solution. I havent seen this done on a mic amp before. It might mean that you can get away with having a small(cheaper) transformer.
I'd normally go with Mr Bears sugg. of a pga2500, but that doesnt quite have the headroom of a 5534
martin
That puzzled me too. Maybe it's a whacky configuration I didn't fathom straight away.
Inverting configurations are indeed invariably noisy and I wonder what kind of load it presents tothe mic.
Stick a 10dB gain stage after the PGA2500. That'll resore the headroom. Shame though. Best they could do with 5V supplies.
Graham
Thanks, thats a bit clearer, I'll do a bit more head scratching
martin
"martin griffith" "Ban"
** You do not understand the circuit - it ain't no virtual earth to the tranny.The secondary is being used in "feedback input " mode - ie the signal is introduced between the negative input of the op-amp and the reference point of the feedback network. This allows the positive input to be grounded and eliminates common mode input signal.
Can only be used with a floating signal source, ie from a tranny or a phono cartridge with modified leads.
.......... Phil
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