Bypass cap's (a long one)

[Arthur Harrison]

Yeah, these seem pretty sane comments, surprisingly objective for an
audiophile type! (;
.

-Art


-----Original Message-----
From: Edu Silva <edusilva at bahianet.com.br>
To: DIY <synth-diy at mailhost.bpa.nl>
Date: Sunday, December 27, 1998 4:34 PM
Subject: Re: Bypass cap's (a long one)


>I found the following text about bypass caps at Dan Hildebrand's page:
>http://www.qnx.com/~danh/tubenote4.html
>
>Well, it's not my opinion, but makes some sense. However, I'm still doing
>the old way...
>
>**********
>
>Tube Amp Design Notes
>
>This series of postings, by Henry Pasternack, is a discussion on the design
>of a tube power amp.
>
>From: "henry (h.) pasternack"
>Subject:  Tube Note 4.
>
>My design effort was temporarily stalled for a couple of weeks while I
>mulled over some issues of component philosophy. For years, I have
>reflexively paralleled all electrolytics with small film caps under the
>audiophile assumption that these capacitors would augment the performance
>of the electrolytics at high frequencies.
>
>The problems with using these caps are several-fold. They take up space;
>they cost money; they require careful grounding if they are to be
>effective; they have the potential to cause high-frequency ringing; they
>may be ineffective; some people have reported that bypassing actually
>increases audible coloration.
>
>In my last amplifier, I took the approach of treating the driver circuit as
>a unit, with its own ground system and all power rails bypassed at high
>frequency by 0.1uf film caps. The input signal ground and the bypass cap
>ground were run separately back to the star ground in the power supply.
>While the presence of those pretty red and blue gumdrops is very pleasing
>to the eye and soothing to the conscience, I wonder if they really do any
>good.
>
>Let's suppose there is some high-frequency noise riding on the power supply
>rails. At 100kHz, the impedance of a 0.1uF cap is about 15 Ohms. At 20kHz,
>it's five times higher. What is the benefit?
>
>I did some simple tests with a couple of capacitors, a 100 Ohm film
>resistor, and a 10-10MHz signal generator. What I found is that the
>impedance of even a lowly Sprague computer-grade electro- lytic is in the
>range of tenths or hundredths of an Ohm out to frequencies in the hundreds
>of kilohertz. The effect of a 0.1uF film bypass was entirely negligible.
>The frequency response of the film motor-run caps I'm using in this amp was
>even better. The bypass had no beneficial effect on the film cap, but did
>result in resonances at medium RF frequencies.
>
>On the other hand, putting a 10uF SCR polypropylene cap in parallel with
>the electrolytic resulted in a uniform and very low impedance right across
>the audio band and out to AM radio frequencies. Modern electrolytic caps
>are a lot better than those built two decades ago. It's probably time to
>rethink the wisdom of automatically bypassing every electrolytic with a
>small film cap.
>
>The wavelength of a radio wave in free space at 1MHz is three hundred
>meters. The length of the wire carrying B+ from the filter caps to my
>driver circuit is about three inches. This is also food for thought.
>
>Most solid-state power amplifiers are littered with bypass caps. A good
>designer will consider using tantalums or ceramics because these types,
>although they aren't that linear, have very low impedance and wide
>bandwidth compared to films. Far from being superfluous, the bypass
>capacitors keep the supply rail impedance low at frequencies in the tens of
>Megahertz. This is necessary because the open-loop bandwidth of a
>solid-state amp is often this high. As well, solid-state devices are more
>prone to producing distortion in the presence of RF noise than tubes.
>
>On the other hand, the open-loop bandwidth of a typical tube amplifier is
>only a couple of hundred kilohertz at best. They don't suffer from junction
>rectification problems.
>
>Finally, I surveyed many schematics of classic tube amplifiers, vintage and

>modern, and found that there were very few gratuitous bypass caps in them.
>Of course, it costs money to install these parts. But I suspect there'd be
>more of them if they really made a big sonic difference.
>
>The "underground" DIY literature was inconsistent. Some tweak articles
>mandated elaborate networks bypasses-upon-bypasses. Others said the
>bypasses screwed up the sound. Not much help there.
>
>I am sure there are many cases where changing a cap here and there can make
>a significant difference in the sound in spite of objective predictions to
>the contrary. The problem is, if we never build amplifiers without these
>parts, we will never have a standard of comparison for tweaked-out designs.
>
>So, I am using quality film filter caps in my power supply, and I am going
>to forego most of the bypass caps unless I can convince myself there is a
>good engineering reason for having them in the circuit. This makes the
>circuit board layout much easier and keeps the cost down, as well. We'll
>see how it sounds.
>
>**********
>
>_____________________
>Edu Silva - ES2 Audio
>Salvador-Bahia-Brazil
>