Big film caps and noise

[xiphmont]

I'd brought this up in responses to other threads... I thought I'd document it here seperately with a hard measurement. The ubiquitous advice with respect to caps is 'film is always better-- if you can afford it'. That's not actually true; you'll probably not be able to measure the THD contribution from just about any cap, ceramic and tantalum excepted. Also, huge film caps pick up noise. Case in point below.

I've been constructing a small fleet of gainclones for some self-powered monitors. These are intended to be refined, thoroughly debugged 'milking what you can from a great little chip'. I'll post a schematic if AndrewT demands, but for now suffice to say it is a pretty standard little gainclone with a Panasonic FC blocking cap for Ci and an Orange Drop .22uF input coupling cap (big dipped polypropelyne film/foil).

The initial prototype fired up and with the inputs shorted produced the following self-noise graph:



[The vertical scale is dBV mesaured at the driver. The gray line is the measurement, the red line is the sampler self-noise reading]

Good-- but not great. That's just audible in a quiet room.

The foil cap turns out to be coupling hum and hash noise coming from the PSU. Although shileding the input cap helps, eliminating it entirely or using an electrolytic yields:



...much better. That's the output of 30dB of amplification from a chipamp with +/-35V rails. The only difference is avoiding using a obscenely large film cap.

Presented for nothing more than illustrative/documentation purposes.

[peranders]

My statement is: Better to use a physically small cap than a large one if HF propertiers are essential. I'll guess you proved me right here.

[sharpi31]

I find these results very interesting.

I (like many others) have found teflon caps to have the cleanest HF reproduction of any type of cap I've tried. But teflon caps are huge compared to other types... hmmm...

All the teflon caps I've tried have been russian military types with a metal case. Perhaps the case helps to shield the cap against noise...?

Either way, non-shielded teflon caps also seem to be liked for their clean HF reproduction (not that I've ever listened to one).

Please don't think I'm questioning your results - I'm genuinely intrigued :-)

[xiphmont]

Originally posted by sharpi31

I (like many others) have found teflon caps to have the cleanest HF reproduction of any type of cap I've tried. But teflon caps are huge compared to other types... hmmm...

All the teflon caps I've tried have been russian military types with a metal case. Perhaps the case helps to shield the cap against noise...?

A metal can provides shielding if the can is grounded to the chassis, yes.

The problem with claims like 'teflon has the cleanest HF' and 'film is more transparent than electrolytic' is that in small signal coupling applications, I can measure zero-- and I mean zero-- performance difference between them up to 50kHz. The tools can tell the capacitors apart when measuring to performance extremes that have nothing to do with audio (amps of current flow, nonlinearity at 200V, impedence differences at radio frequencies), but in an audio signal coupling application the measurement tools say there is no difference whatsoever (measurement depth of 20 bits).

Of course, at high power or other applications there are real differences. But for small signals, all the film and electrolytic caps are ridiculously overqualified.

[gmikol]

Slightly OT, but...

I don't recognize the look of the software. Judging by the UI, I'd say it's a Linux-based application?

What is it?

--Greg

[sharpi31]

But for small signals, all the film and electrolytic caps are ridiculously overqualified.

Again, I have no grounds to doubt your measurements but I have always considered it incredibly easy to hear the differences between different coupling capacitors. Judging from the prevalence of threads discussing these differences, it seems I am not alone.

Xiphmont: Ignoring your measurements, do you actually hear a difference between different types of cap?

[AndrewT]

Originally posted by xiphmont
But for small signals, all the film and electrolytic caps are ridiculously overqualified.

what does this mean?

[xiphmont]

Originally posted by gmikol
Slightly OT, but...

I don't recognize the look of the software. Judging by the UI, I'd say it's a Linux-based application?

What is it?

--Greg

Yes, it's a program I wrote years ago when I was doing alot of speaker array building and testing for some local theater groups. I never finished it (just got it good enough to work for what I needed right then). I've been working on it again recently along with a suggested schematic to plug into the soundcard to use along with it. I'll post a thread about it when I get a little farther along.

The source is at http://svn.xiph.org/trunk/spectrum

The version there runs now but is incomplete (that's what I used to make the graph)

Originally posted by AndrewT
what does this mean?

I mean that most people are using 600V capacitor capable of full amps of RMS ripple with HF characteristics suitable for use into tens or hundreds of megahertz for audio coupling, an application that doesn't scratch a tenth of a tenth of a tenth of a percent of the capacitor's rated/intended capabilities. The nonlinearities you see caps exhibiting out toward their rated limits are damned near undetectable in an audio application. 'Soakage' is about as real a phenomenon in these caps as 'skin effect' in speaker cables. It exists, but you'll never measure it in audio with even the most sensitive instruments.

Even ceramic caps, which are avoided for some good reasons, show barely detectable THD contributions when used to couple microamps of line-level voltage. The noise coupling and microphonic characteristics of many caps contribute way more to signal degrdation than any other imperfection in the cap. I can easily measure a ceramic cap picking up room noise around it (acting as a condensor mic).

And no, I don't hear a difference between the caps. Honestly, I've never gone looking to hear a difference. I already know my ears are not as sensitive as my measuring tools, and if the tools can't detect a difference, I'm fully confident any difference I hear will be imagined. Placebo effect is strong and unavoidable, even in people who are aware of it. It is human nature to find patterns and detect differences where none exist.

[Panelhead]

I used to use huge film caps for coupling. The US made teflon caps in metal tubes were big faves. The surplus teflons seemed better than the approved boutique films caps. Now use many varients, small electrolytics such as Silmics and BG, 50 volt polycarbonate film, and the lowly Epco mylar.
To my ears, the smaller the coupling cap the better. Decoupling is the same. I use a lot of 20 cent ECR 1% polycarbonates in recent builds. They have mid 80's date shift codes.
I used to throw all steel leaded caps in the trash too. Cannot honestly hear the non linearities of coated steel lead component versus a copper lead. Short lead length may help here.
A better design and layout is much more important than the brand of caps used to build.


George

[hermanv]

Originally posted by xiphmont

<snip> I already know my ears are not as sensitive as my measuring tools, and if the tools can't detect a difference, I'm fully confident any difference I hear will be imagined. Placebo effect is strong and unavoidable, even in people who are aware of it. It is human nature to find patterns and detect differences where none exist.

What makes you think here's something wrong with your ears? In an anechoic chamber, human ears can hear the noise of air molecules bumping into one another (about 0dB) and can tolerate 130dB for short bursts. Measuring equipment with a 130 dB dynamic range is beyond the average engineering lab.

You assume that the quality that makes one type of capacitor sound better than another is THD. There's little evidence to support this assumption, but if true, many have measured significant distortion differences in various capacitor types. See this thread in this forum Capacitor Distortion