[sdiy] sync circuit woes

Scott Gravenhorst music.maker at gte.net
Thu Nov 20 17:02:59 CET 2008

Tom Wiltshire <tom at electricdruid.net> wrote:
>On 20 Nov 2008, at 15:36, Dan Snazelle wrote:
>> the only real issue (and its not too big of a deal) is some  
>> OVERLOADING in the mixer (audio summmer) circuit.
>> its just a typical 100k summer like you would see on a vco but with  
>> a 50k pot before each sum. it seems that with all the inputs being  
>> quite loud (except the clean guitar which seems quiet in comparison  
>> even though it has almost X10 of gain) when they all get past 5  
>> there is some distortion that results.
>> is this typical of an opamp mixer?
>It is if you're using it to mix a lot of +/-5V oscillator signals.  
>You've only got (at most) 30V of headroom, so 10V signals will soon  
>max it out. If you've got +/-10V signals, it'll be even worse. If  
>you've got 100K input resistors, try reducing the op-amp's feedback  
>resistor by a factor of 2 or 3 or 4.

I'd agree - I had a similar problem as I built up my first FatMan, I added a big
mixer, but couldn't turn all input sources up to max because of too much gain in the
summation opamp.  I took a variable route - I replaced the fixed opamp feedback
resistor which sets the gain to a pot.  Works great.  The new pot is an overall
mixer gain control.

As for the guitar input, I had one too, I put an opamp preamp in front of the
mixer's input resistor for that one input.

-- ScottG
-- Scott Gravenhorst
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