[sdiy] sync circuit woes

[Seb Francis]

>>
>>
>> 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?
>> 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.
>> thanks so much-diy

Here's the maths:

Inverting configuration:
Gain = Rfeedback / Rinput
You can have multiple inputs summing together, each with it's own Rinput 
connecting to the - input.
Thus each input in a summer can have a different gain if you wish.

Non-inverting configuration
Gain = 1 + (Rfeedback / Rground)

Lots of info on the internet about opamps.  This page is reasonably 
straightforward:
http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm

Seb