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Old 13th January 2011, 05:22 PM   #1
BigE is offline BigE  Canada
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Default 4 pole caps and bypass caps

Does anyone use low value bypass capacitors with Jensen 4-pole caps?

I've replace the 5600 uf caps in my phase linear amp with 10K 4-poles @ 80 volts.

I'm thinking about installing a 0.47 uf between each supply rail and ground?

Will this be safe or will it overheat the 4-pole cap?

Thanks.
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Old 13th January 2011, 05:26 PM   #2
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The .47 uf caps shouldn't cause any problems and will certainly reduce the ESR at high frequencies.
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Old 13th January 2011, 06:58 PM   #3
BigE is offline BigE  Canada
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Quote:
Originally Posted by Frank Berry View Post
The .47 uf caps shouldn't cause any problems and will certainly reduce the ESR at high frequencies.
Thanks Frank.

Would there any point in putting a 10uF cap to feed each output and driver transistor instead?

I've read that this is a "good idea" but I don't know if it really matters.
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Old 13th January 2011, 06:58 PM   #4
Gopher is offline Gopher  United Kingdom
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Really Frank? I think you mean impedance not ESR since ESR is a fixed value that does not vary with frequency. The much larger ESR of a film cap in parallel with the much smaller ESR of a large electrolytic will make no discernable difference to the ESR of the electolytic on its own. Do you have specific manufacturer's data to back up this commonly held fallacy about HF impedance?

Last edited by Gopher; 13th January 2011 at 07:06 PM.
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Old 13th January 2011, 07:08 PM   #5
BigE is offline BigE  Canada
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Gopher, there is a graph that shows what you are looking for here:

http://www.intersil.com/data/an/an1325.pdf
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Old 13th January 2011, 07:21 PM   #6
Gopher is offline Gopher  United Kingdom
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Big E

That graph only shows the way to prevent resonances when using multiple small value LOCAL bypass caps. The impedance of a large (10,000uF) reservoir electrolytic cap is several orders of magnitude less than that of a small (0.47 uF) film bypass cap at frequencies up to several hundred kHz. Adding the bypass cap in parallel does NOTHING to improve HF impedance (just do the sums) and can actually introduce audible resonances in the audio band.

Last edited by Gopher; 13th January 2011 at 07:26 PM.
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Old 13th January 2011, 07:34 PM   #7
BigE is offline BigE  Canada
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Thanks Gopher!

Interesting note, is that I saw this configuration in the service manuals for one revision of the phase linear 400, except the main PS caps were 5.6K uF and the bypass was 0.33 uF.

Odd that Bob Carver would fall for this.

The next revision did not include the bypass caps.
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Old 13th January 2011, 07:41 PM   #8
Gopher is offline Gopher  United Kingdom
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Like you say, the bypass was removed. Maybe he listened instead of just believing the urban legend! Your Jensen 4 pole caps are specifically designed for very low inductance/HF impedance. If you add bypass caps you could screw that up.

What's far more important is having substantial LOCAL elecs at or near the output transistors in addition to the main reservoir caps. Anything from a few hundred to a couple of thousand uF should do nicely, depending on the power output. A small value resistor before the local caps would help too.

Last edited by Gopher; 13th January 2011 at 07:47 PM.
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Old 13th January 2011, 10:19 PM   #9
tatus is offline tatus  Germany
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Quote:
Originally Posted by BigE View Post
Gopher, there is a graph that shows what you are looking for here:

http://www.intersil.com/data/an/an1325.pdf
If you refer to what they have done in that app note on pages 6 and 10: that is complete nonsense: they are adding magnitudes of complex functions (real and imaginary parts) over frequency. What happens in reality is that small capacitances together with stray inductances of large capacitances form a resonant circuit, creating very high impedances at certain frequencies. As a result, adding capacitance without clearly knowing all relevant parameters of all parts (for each capacitor at least besides C: stray inductance and ESR), things tend to get worse than before. The stray inductance is normally not specified but can be calculated from the resonance frequency when the impedance graph of the part is known. Then run a spice simulation to see what makes sense.

Furthermore: small bypass capacitors only make sense very very close to the load. If placed in parallel to the large supply capacitors, the long lead to the amplifier creates another inductance that renders all positive effects void.
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Old 14th January 2011, 01:39 PM   #10
BigE is offline BigE  Canada
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Thanks!

If anything, I may try adding reservoir caps right at the output transistors.

This site is fantastic! Maybe one day, I'll know enough to contribute....

Thanks again!
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