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"Bypass caps" and their effect on perceived sound

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Why can I hear a difference in the sound from my speakers if I put a small value plastic film capacitor in parallel with the electrolytic capacitor in a tube amp power supply?

I understand there are reasons why this might be a good thing to do. The film cap bypasses RF to ground, as the film cap will have a much lower ESR at high frequencies than the electrolytic cap. (What frequencies are we talking about here? 10kHz? 50kHz? 100kHz?)

Why is it that the amp sounds brighter when I do this?

I've tried this on a number of amps, both speaker and line level.

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In the latest amp, I put a 0.22uF 630V metallized polypropylene cap from +400V plate supply to ground, in parallel with a 47uF 450V Nichicon electrolytic.

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That makes it sound like bypassing an electrolytic cap with a film cap should always be a good thing.

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I was reading in MJ Valve Amplifiers that the film cap should be close to the active device (tube), not physically across the electrolytic cap. That's how I have mine -- at least as close to this as I could.
 
I was reading in MJ Valve Amplifiers that the film cap should be close to the active device (tube), not physically across the electrolytic cap. That's how I have mine -- at least as close to this as I could.

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.
 
I was reading in MJ Valve Amplifiers that the film cap should be close to the active device (tube), not physically across the electrolytic cap. That's how I have mine -- at least as close to this as I could.


If they shunt common voltage source, it is better to solder them closer to the star ground. Leads of electrolytics to the star ground in such case can be longer. If they shunt power of some "previous" stage powered through RC - filter, of course it may be better to solder them to the ground of that stage, to eliminate AC voltage drop on the wire that goes from the ground of this stage to the star ground, closing AC current loop through this cap with short leads. Always think about currents, and you will be happy: knowledge means freedom. :)
 
Here is a tip i found but the link doesn't work anymore:

In most electronic equipment a rectifier transforms AC to DC. So far so good. A silicon rectifier is a semi-conductor, only letting current go through it when the output voltage is more than 0,5 Volt below the input. At that very moment it switches "ON". Inside a semiconductor there is a capacitance as well. This capacitor, together with the inductance of the wiring, causes a very sharp peak voltage to occur. This peak is too fast to be dampened by the connected (electrolytic) capacitors. The solution is simple: put a resistor between the transformer and the rectifier.
In a transistor power amp the value of that resistor may be 0,1
Ohm - 5 Watt. With tube power amps it could be around 1 Ohm and in preamps
(transistor and tubes) some 10 Ohms. The result is a more relaxed amplification. It seems as if the sound comes easier out of your loudspeakers
and the stereo image is mostly improved. A
few dimes will do the job.

I now design all my power supplies incorporating this mod. Actually I have just replaced my moscode power
supply with my latest design and again I'm amazed at the clarity but this time instead of 5w resistors
I used ntc thermistors. In the link below you'll see the old supply in the photo.

Mac


Myira Moscode Amplifier kits - The Book Worm
 
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Here is a tip i found but the link doesn't work anymore:

In most electronic equipment a rectifier transforms AC to DC. So far so good. A silicon rectifier is a semi-conductor, only letting current go through it when the output voltage is more than 0,5 Volt below the input. At that very moment it switches "ON". Inside a semiconductor there is a capacitance as well. This capacitor, together with the inductance of the wiring, causes a very sharp peak voltage to occur. This peak is too fast to be dampened by the connected (electrolytic) capacitors. The solution is simple: put a resistor between the transformer and the rectifier.
In a transistor power amp the value of that resistor may be 0,1
Ohm - 5 Watt. With tube power amps it could be around 1 Ohm and in preamps
(transistor and tubes) some 10 Ohms. The result is a more relaxed amplification. It seems as if the sound comes easier out of your loudspeakers
and the stereo image is mostly improved. A
few dimes will do the job.

I now design all my power supplies incorporating this mod. Actually I have just replaced my moscode power
supply with my latest design and again I'm amazed at the clarity but this time instead of 5w resistors
I used ntc thermistors. In the link below you'll see the old supply in the photo.

Mac


Myira Moscode Amplifier kits - The Book Worm

This photo shows the new power supply:

Moscode amplifier - The Book Worm
 
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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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!

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.
 
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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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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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 ...

Frank
 
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.

Rush
 
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