[sdiy] sync circuit woes

[harrybissell at wowway.com]

Tom is right about multiple stages.  Each opamp has a "gain - bandwidth
product" which in simple terms means that if you take a lot of gain in a
single stage, the bandwidth is reduced. Frequency response will suffer a LOT
with high gains in a single stage, two stages with moderate gains are MUCH better.

You can (if you want limited bandwitdh) use a single stage and get your
limited bandwidth for free.  I did this in my hex guitsr system... I don't
want too much high frequency response.  The really high gain (x500) limited
the response to about 3KHz

H^) harry




On Fri, 21 Nov 2008 11:22:36 +0000, Tom Wiltshire wrote
> 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.
> 
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Harry Bissell & Nora Abdullah 4eva