Electronic Design

What's All This C-R Stuff, Anyhow?

A resistor is a resistor, and a capacitor is a capacitor, right? Maybe, and maybe not. I'm doing some research in audio circuits. I've heard all the scientific claims that if two circuits measure the same, they ought to sound the same. This claim is refuted by the observation that they don't sound the same. That's a pretty convincing rebuttal. I gotta believe it.

I'm an analog and measurement guy. If somebody says some audio circuits don't sound the same, I bet I can measure the difference. I thought of the old claim (by people with "good ears") that electrolytic capacitors in C-R coupling networks don't sound the same as high-quality film capacitors in an ordinary audio circuit.

I could set up some C-R coupling circuits and measure the differences in the outputs, if any (Fig. 1a). The capacitors might be polystyrene, mylar, or electrolytics (1µF and 100 kΩ). I could use some precision differential amplifiers to see the "error voltages"—the difference between the inputs and the outputs. Yeah, I could do that.

But I'm a lazy guy. I can see that same "error voltage" if I merely-swap the R and the C and look at the voltage across the capacitor (Fig. 1b, basic low-pass filter). It's a matter of viewpoint— what point you define as ground. I could watch those small error voltages on an ordinary scope and compare them, and I could even subtract them. So I set that up.

This filter has an f (3 dB) of 1.6 Hz. Shouldn't that be far enough away from 20 Hz at the low end of audio frequencies? I put in 120-Hz sinewaves, triangles, and square waves. It was kinda boring. I couldn't see any difference. Then I cranked the square wave down to 12 Hz.

The two waveforms were different. They matched for the first 20 ms, and then the electrolytic had more curvature (Fig. 2, lower trace) as if its early capacitance was 1 µF but later changed to 30% bigger. I've used a lot of capacitors in my day, but I never expected the capacitance to change oddly with frequency like this. Would this change if I swapped scope channels? Nope.

What if I changed the R or C values? Well, I'm an old analog computer guy, and I didn't have to put in more capacitance. I just changed the virtual frequency back to 120 Hz. The capacitors may not be perfect, but when the errors are very small, can anybody hear them? And will anybody care?

That may lead to a test where even guys with good ears can't hear the electrolytics as they claim they can. I'll build up some circuits with National's new ultra-linear LM4562 audio operational amplifiers, which have less than 0.00003% distortion at 1 kHz. (For the LM4562, go to www.national.com/rap and search for LM4562 after Sept. 8.) I'll set up some A-B comparisons and measurements.

Can we hear the difference between mylar and poly? Can anybody hear the difference? I can't. But maybe I can show that in a good circuit, nobody can hear the difference between mylar and poly. Or paper. What about 1-µF ceramic? Comments later.

If you're an audio enthusiast, you may be interested in attending my Master Class at the Audio Engineering Society Convention in San Francisco, October 5-8. For more details, see www.aes.org. While I'm there, I'll present a lot of audio experiments that you can use to evaluate circuits and components. Y'all come!

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