[sdiy] Hand-matching capacitors for filter stages

[Antti Huovilainen]

On Tue, 9 Feb 2010, thx1138 wrote:

> I define good engineering as the ability to not have to use matched
> components is projects whenever possible.

Which in this case "obviously" means not using analog at all but instead a 
DSP with its much lower distortion and accuracy.
If the specs call for, say, "Moog filter", then we're pretty much stuck 
with the circuit topology and can only improve on it by matching and/or 
trimming.

> In some cases I find that it is easier to use 1% resistors to tighten up 
> a circuit.

If the caps have 5-10% tolerance, matching resistors to tighter than 1% 
(the basic accuracy I buy mine in for peanuts) won't make any difference. 
The accuracy is dominated by the capacitors.

> Transistors are a different case as I like to use transistor arrays like 
> the CA3046/CA3086 but these are getting harder to find in stock around 
> town.

More importantly, IMO, matching transistors directly affects CV 
feedthrough, while capacitors have no effect on that.

Now for my 0.02e:
Matching capacitors to reasonable accuracy (1-2%?) should be fairly 
trivial if you have a DMM with capacitance measurement. The absolute 
accuracy won't matter at all as long as you match only same value 
capacitors.

For material, I don't see how there would be any audible difference 
between identical value poly* capacitors. As long as the tempco is within 
your specs, any material should work fine (mica and np0 ceramic too).

In a 4 pole filter the effect of nonidentical capacitors would be slight 
shift in resonant frequency and resonance amount. This is unlikely to be 
audible except near self oscillation. The exact effect is easy enough to 
calculate when you remember that 1) resonant frequency is the frequency 
where filter phase shift is 180 degrees and 2) self oscillation happens 
when filter + feedback gain exceeds 1.0 at resonant freq.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova