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!
Any one who needs to measure a circuit to believe that diferent dielectrics (and hence caps) can sound different has no business getting involved in audio as they have to be sadly quite deaf.
These folk (e.g. BP) treat real components as if they were ideal text book items without regard to component manufacture compromises (due to structure) and, more fundamentally, the current limited knowledge of how dielectrics actually behave when subjected to complex E fields such as audio. Dielectric memory effects, structure related resonances, linearity ... hardly an ideal element.
Anonymous -April 25, 2008
"Comments later." ? Great, when? where?
Anonymous -March 28, 2007
Just for the record, the audiophile types who prefer the oil filled capacitors realize they are not perfect, they just prefer the "musical colouration" this type of capacitor adds to the signal. For those of you with long memories, the vacuum tube versus solid state debate came down to your preference or dislike for the second order harmonic distortion added by vacuum tube circuits vs. the odd order distortion products added by solid state circuitry.
Christopher Bodegraven -March 06, 2007 (Article Rating: )
Sure, Bob, everything's measurable -- but you don't get good audio designs that way. You can sure measure piezo-induced noise on ceramic caps... but it's not easy. Just imagine a kick drum shaking an on-stage audio A/D box... now how do I reproduce _that_ in the lab? And why should I bother?
It's much easier to follow "black magic" rules of thumb: no ceramic in my signal path or analog supply lines.
On Paradise -October 28, 2006
Just be careful of ceramics in an audio application- especially surface mount. They are barium titanate and, as such, are piezoelectric. I once spent two weeks trying to figure out why my audio circuit thumped every time I tapped the circuit board.
Jim Christensen -September 24, 2006
Hey Bob, in the C-R, article, listen to some synthesized music, in particular rap or more specifically, Lil flip, Game over, there is a whole video game audio to opens the song that sounds like Pac Man. That might help to decipher whether the capacitor type can "round" the typical square wave behavior that is associated with synthed music..Just your typical fan.....of Bob Pease that is...and rap... and if your wondering how to test for "square waveness" watch a voice print, maybe one on win amp, although they revamped their FFT algorithm, which leaks alot of in the frequency spectrum information into other FFT bins, and sucks. Which reminds me, I recorded a disc that had a sine sweep "for backup" to a wma format and replayed it on windows media player and it aliased in a range from 20kHz to about 15 kHz I think, more investigation needed, but I thought I would add this because music quality is critical, especially if you are an analog maniac or music phile....like myself. Just thought you should know. MP3 is the way to go!!!!
PS just liten to square wave signals if you dont want to listen to rap....
Jason
Jason Christensen -September 18, 2006
This 'Hi-Fi' website gives some enthusiastic views on Platinum foil capacitors: http://www.audiocircle.com/circles/index.php?topic=31224.0 A friend sent this to me. It is a waste of time telling him that coupling capacitors (or speaker leads) are not supposed to color the sound.
Anonymous -September 15, 2006
P.S. BP, you are THE MAN!
Lee Hardesty -September 08, 2006
Properly set up A/B/(C)/... tests often produce red faces, especially on those selling the $300/m cables and other such audio 'snake oil' remedies. If you can hear a difference there is some measurement that will reveal its cause. The real problem is setting up and conducting 'proper' listening tests. It can be done but it's not always easy, at least in my experience.
That's my $0.02.
Lee Hardesty -September 08, 2006
I have a tin ear and can't tell the difference, but have known for years that some electrolytic capacitors don't filter as well at 400Hz as they do at 60 Hz.
Art Solie -September 01, 2006
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