[sdiy] sync circuit woes

[Tom Wiltshire]

On 20 Nov 2008, at 17:23, Dan Snazelle wrote:

>
>
> Ok
> I thought that in summer circuits, the feedback resistor HAD to be the
> same value as the summing resistors! i thought that was what made
> summers work properly. i feel very stupid right now.
> Ok, so if i was using 100k, i can maybe try a 47k?

Yep!

> also...in my GUITAR PRE opamp, i am using a non inverting opamp with a
> 10k to gnd resistor and a 100k feedback resistor for a gain of X10 but
> it still isnt really loud enough compared to the other mixer  
> inputs. SO
> i was thinking of bringing it up higher BUT i dont want any  
> distortion.
> (there is already a dirty switch for that)
> so what do you guys think i could get away with before distortion?
> maybe 15x? i was thinking of putting in a 150k on the feedback
> resistor.

Yeah, x15 sounds ok. Give it a try. With guitars it depends a lot on  
the individual guitar and also the pitch played. Some humbucker  
pickups blast out quite a lot of signal, and some single coils are  
much quieter. Bass notes tend to be louder than higher pitches.

If you've got a preamp on the guitar input channel, I'd be inclined  
to split the gain required between the preamp and the mixer. Hence  
you could have x10 or x15 in the preamp, and then reduce the summer  
input resistor for that channel too, to give you another x2 or x1.5  
or whatever. The reduced input resistor will give that summer channel  
more sensitivity (others have given you the gain equations for that).
I'm not entirely clear why, but building gain up in this way seems to  
give better results than just whacking a signal through an opamp with  
a massive gain. I suppose you're not pushing any one stage as hard.

T.