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Old 8th October 2012, 09:12 PM   #11
rongon is offline rongon  United States
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That's exactly right. If anything, I'd expect a reduction in perceived highs when adding the cap if the stage was previously unstable or marginally stable due to insufficient bypassing.
See, that's the crazy thing. Every time I do this, I hear "increased highs." I guess I didn't have a problem in the first place.

When I brought the amp over to a friend's workshop to check it out, we found no oscillations or funny stuff to 20MHz (the limit of his 'scope). That was before the bypass caps went in.

There's a big 10 gauge solid copper wire acting as the ground buss, which is only connected to the chassis back by the power transformer and rectifiers. There was no discernible hum, even open loop (which I was very happy about). I looked at the B+, looking for high frequency AC hash riding on the DC, and found pretty much nothing. I did find a low amplitude, dirty, AC signal (in the low MHz) riding on the AC heaters. But it's very small. Might be rectifier switching leaking in. I put .01uF 630V film caps across the heater windings, but no choke or anything.

I might remove the film caps from across the electrolytics, if I decide I like the sound better without them. What's the harm, right?

Interesting, though, that I always perceive more highs when I put these film bypass caps in. I wonder why... Power of suggestion?

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Old 8th October 2012, 09:22 PM   #12
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Originally Posted by rongon View Post
Interesting, though, that I always perceive more highs when I put these film bypass caps in. I wonder why... Power of suggestion?
I once decided to eliminate 4 uF caps that bypassed 500 uF in 800V B+ of class AB stereo amp. According to calculations they were not needed. According to measurement of frequency response and observing level of strips of distortions they were not needed. But with them soundstage was better, cymbals sounded more real and were better localized in soundstage. Then I suspected phase intermodulation and indeed found that despite of the same level of IM distortions lengths of upper and lower sidebands were slightly different!

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Electrolytics are non-linear, their dielectric is semiconductor. Even if you can't measure or hear difference in frequency response on upper audio frequencies, difference in phase intermodulation is audible.
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Old 8th October 2012, 09:36 PM   #13
rongon is offline rongon  United States
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Is there such a thing as 'incomplete' bypassing? Could it be that my 220nF bypass cap (across 47uF) is way too small a value, and that I should have something more like 1uF to 4.7uF there?

Unfortunately, this is in a Dyna ST70 chassis, and there's no room for film caps that big. Maaaaaybe I could squeeze in a couple of Panasonic ECQ 1uF 630V metallized polyester...

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Old 8th October 2012, 09:49 PM   #14
disco is offline disco  Netherlands
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Consider the cathode bypass cap on a triode (voltage amp) where you introduce distortion if the capacitance value is less than adequate.
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Old 8th October 2012, 10:02 PM   #15
rongon is offline rongon  United States
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Consider the cathode bypass cap on a triode (voltage amp) where you introduce distortion if the capacitance value is less than adequate.
Well, yes... You only partially bypass audio frequencies to ground, so you allow degenerative feedback on a frequency-selective basis. Some frequencies get affected by negative feedback (reducing the gain at those frequencies) while other frequencies get the tube's full gain because its cathode is fully bypassed at those frequencies.

So you're saying that my small value bypass capacitor is only working on very high frequencies, leaving not-so-high frequencies interfered with by the higher ESR (or inductance?) of the electrolytic cap?

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Old 8th October 2012, 10:26 PM   #16
fas42 is offline fas42  Australia
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My way of looking at it is that for the amplifier to work correctly, theoretically, it needs a "perfect" power supply, that is, the latter has zero impedance at all frequencies, it can always deliver the current asked for without varying its voltage. Electros as they're normally used are quite some way from providing that in practice, so the bypass caps assist in getting closer to that "perfect" supply, they "boost" the performance of the main caps ...

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Old 8th October 2012, 10:28 PM   #17
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Replace one less linear semiconductor device (electrolytic cap) with another semiconductor more linear one, e.g. LED
I might be beheaded for being such, but have just finished watching "Game of Thrones", so not worried.
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Old 8th October 2012, 10:34 PM   #18
Rush is offline Rush  United States
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Quote:
Originally Posted by rongon View Post
Well, yes... You only partially bypass audio frequencies to ground, so you allow degenerative feedback on a frequency-selective basis. Some frequencies get affected by negative feedback (reducing the gain at those frequencies) while other frequencies get the tube's full gain because its cathode is fully bypassed at those frequencies.

So you're saying that my small value bypass capacitor is only working on very high frequencies, leaving not-so-high frequencies interfered with by the higher ESR (or inductance?) of the electrolytic cap?

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You guys not aware of Walt Jung and Richard Marsh work on this issue back in the late seventies? Dig up the Audio Ameteur issue.
Use a 4.7 , .47, .047 (.1) uF film caps across the power supply caps.
This was after extensive testing and listening.

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Old 8th October 2012, 10:38 PM   #19
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Rush,

The technology has changed since 70's. Is the conclusion still valid?
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Old 8th October 2012, 10:57 PM   #20
Rush is offline Rush  United States
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Originally Posted by yagoolar View Post
Rush,

The technology has changed since 70's. Is the conclusion still valid?
No it hasn't changed. Power supplies haven't changed at all.
Yes it is still Valid. Folks keep reinventing the same thing, calling it new. These guys put time into this and they are engineers. Their work is documented.
Try it.
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