[sdiy] Prophet 10 op-amp swaps?

[Neil Johnson]

Tom Wiltshire wrote:
>
> On 5 Aug 2013, at 11:03, rburnett at richieburnett.co.uk wrote:
>
>> Does anyone know how they get around this problem in large analogue mixing desks where presumably they route the summing nodes of the mixing amps along the full length of the desk!?
>
> No, but I'd be interested to know too!

Depends.

I'm looking at schematics for a number of mid-range consoles (A&H,
Soundcraft, DDA).  Most of them use slow audio op-amps (NE5532) and
input capacitance compensation capacitors across the feedback
resistor.  The Soundcraft 6000 desk uses transistor mix-amps in front
of TL072, and they also add ferrites to the mix-amp inputs (not sure
if that's for EMC or to neutralise bus capacitance).  SSL also use
large (220uH) inductors on the input to the mix-amps, again transistor
pair (paralleled LM394) followed by NE5534.

Also note that while the summing nodes may be physically long (maybe a
metre or more) they tend to also be quite some way away from anything
else, certainly compared to a PCB where the next trace could be <1mm
away, or a ground plane 1.6mm away.  So stray parasitic capacitance is
more easily controlled, and therefore stands a chance of being
correctly compensated.

Any good book on op-amps should provide details on input capacitance
compensation.

Neil
--
http://www.njohnson.co.uk