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Old 8th October 2012, 08:12 PM   #1
rongon is offline rongon  United States
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Default "Bypass caps" and their effect on perceived sound

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.

?

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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Last edited by rongon; 8th October 2012 at 08:14 PM. Reason: details...
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Old 8th October 2012, 08:30 PM   #2
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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, 08:35 PM   #3
rongon is offline rongon  United States
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That makes it sound like bypassing an electrolytic cap with a film cap should always be a good thing.

?

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.
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Old 8th October 2012, 08:52 PM   #4
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However they are the last thing you should update.
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Old 8th October 2012, 09:04 PM   #5
SY is offline SY  United States
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Quote:
Originally Posted by rongon View Post
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.
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Old 8th October 2012, 09:17 PM   #6
TheGimp is offline TheGimp  United States
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Some times it is easier to place the film cap across the last filter cap rather than at the tube, such as when you are using point to point wiring and the anode load resistor is in a terminal strip, or otherwise located away from the tube itself.
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Old 8th October 2012, 09:27 PM   #7
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Quote:
Originally Posted by rongon View Post
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.
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Old 8th October 2012, 09:30 PM   #8
balerit is offline balerit  South Africa
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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

Last edited by balerit; 8th October 2012 at 09:33 PM.
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Old 8th October 2012, 09:36 PM   #9
balerit is offline balerit  South Africa
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Quote:
Originally Posted by balerit View Post
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
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Old 8th October 2012, 09:44 PM   #10
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I know tektronix used to do that but they put the resistors Behind the Electrolytics Besides this purpose . they made Good fuses & Shunt resistors
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