[sdiy] Newbie questions about summing buffers

[Tom Wiltshire]

On 24 Feb 2012, at 14:35, ganesha wrote:

> Hi,
> 
> I have 2 questions about summing buffers
> If you take this schematic:
> 
> http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Op-Amp_Summing_Amplifier.svg/200px-Op-Amp_Summing_Amplifier.svg.png
> 
> 
> -Most of the schematics I've seen so far use large resistor values of f.e. 100K for resistors R1- Rn and Rf. 
> Is there a specific reason to use 100K 0,1% instead of  small resistor values like for example 1K 0,1%? If they both are high precision resistors of 0,1% I would think the result would be exactly the same?

0.1% is very high precision, I'd say. 1% is easily good enough, and not that long ago 5% would have been normal. There's no need to go crazy for a audio mixer. Maybe for an instrumentation application, but not a mixer.

Scott J mentioned the input impedance issue, but otherwise the equations for the op-amp all work out the same. I remember doing an experiment with a 741 when I was first learning about op-amps. I built an inverting amp using 100R, 1K, 10K, 100K, 1M and 10M resistors. What I learned was that apart from the two extremes, I couldn't hear any difference. The extremes were noisy. I also learned that the 10K, 47K, and 100K values that I commonly saw in circuits were right in the middle of the range, and so I've used similar values ever since.

> -In some schematics I've seen a small capacitor in parallel with resistor Rf. Can someone explain when it is better to add a capacitor?

In general, it's better to add the capacitor. After all, in general you're not trying to amplify radio waves, so you might as well tame the top end. I usually leave space in a circuit to add the cap, even if sometimes I don't fit it. Stick in something that rolls everything off above whatever you regard as the top end of audio (15KHz, 20KHz, 25KHz?) and you're done. I tend to use a fairly low value for this since I'm not a fan of crisp bright treble anyway, and a lot of 'warm', 'analogue' gear has a frequency response that basically nosedives above 10KHz!

HTH,
Tom