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Old 3rd June 2009, 04:56 PM   #1
cbdb is offline cbdb  Canada
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Default Amp sim output load capacitance, how much?

Whats a good capacitance to put accros the load resistor (8R) when simming an amp? I am not using a Zobel network in the sim yet, but am still trying to get stability. I know this has been discussed before but my searches didnt find anything. I have been using 10nf and now when i change it to 100nf I have an oscillator. If 10nf is too small I need to go back to the drawing board! Thanks in advance.
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Old 3rd June 2009, 05:46 PM   #2
Mr Evil is offline Mr Evil  United Kingdom
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2uF is often seen, because it is supposed to be similar to an electrostatic speaker. D. Self suggests that 100nF is a more difficult load, so it's probably a good value to test with if you want absolute stability. An output inductor helps a lot with this.
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Old 3rd June 2009, 10:01 PM   #3
cbdb is offline cbdb  Canada
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Did you mean 2 nf?
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Old 3rd June 2009, 10:36 PM   #4
Mr Evil is offline Mr Evil  United Kingdom
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No, definitely 2uF. I wouldn't worry if your amp isn't stable into that, because it's not a normal load. Even 100nF is very high, but I think it's worthwhile making sure an amp is as well behaved as possible into difficult loads.
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Old 3rd June 2009, 11:25 PM   #5
jcx is offline jcx  United States
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just Google for wire/cable capacitance pF/ft - 10-50 pF/ft seems to cover all but the most exotic low inductance cable designs

the 2 uF direct load is an absurd standard - leakage inductance and series resistance of the ESL step up transformer would prevent the amplifier from seeing a pure C load of that magnitude
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Old 4th June 2009, 12:56 AM   #6
kaos is offline kaos  United States
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Piezo tweeters are typically around .2 to .3 uF, so someone hooking one of those up to the amp is kind of nasty, especially if thereís no R load to help dampen things out. Electrostats are sometimes modeled as 8 ohms paralleled with 2 uF (donít ask me why, as someone already commented, real electrostats probably look quite a bit different). My current amp prototype shows the greatest over-shoot and ring around 20 nF, less than that the amp drives well by itself, and over that the output inductor progressively helps more and more.
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Old 4th June 2009, 01:28 AM   #7
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Default Re: Amp sim output load capacitance, how much?

Quote:
Originally posted by cbdb
Whats a good capacitance to put accros the load resistor (8R) when simming an amp? I am not using a Zobel network in the sim yet, but am still trying to get stability. I know this has been discussed before but my searches didnt find anything. I have been using 10nf and now when i change it to 100nf I have an oscillator. If 10nf is too small I need to go back to the drawing board! Thanks in advance.

It's good to test the amp with a wide variety of capacitance loads, but don't expect it to be stable under all conditions without the use of a series L-R network. Depending on the amplifier details, you can go as low as 1 uH in parallel with 2 ohms or so. When probing for stability of the amplifier itself, probe on the amplifier side of the L-R network.

Most people will agree that a series inductance as low as 1 uH is sonically transparent. Others think that values up to perhaps 5 uH will not degrade the sound. In any case, be generous in constructing the coil and make it air core.

The Zobel network (series R-C shunting to ground), is usually needed to stabilize the amplifier when it sees an inductive load or no load at all.

If you want to go as high as a 2 uF load test, it is reasonable to put 1 or 2 ohms in series with it.

Cheers,
Bob
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Old 4th June 2009, 02:24 AM   #8
wg_ski is offline wg_ski  United States
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2 uF is -j4 ohms at 20k. If you really wanted to make sure your amp is stable into and can drive a fully reactive load, that would be the test. If you were going to be driving electrostats or banks of piezos it might be good to check. Probably a 10kHz test (-j8 ohms) is probably sufficient for even extreme loads, however.
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Old 4th June 2009, 04:05 PM   #9
cbdb is offline cbdb  Canada
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Thanks for the answers. One more question, would I be correct in assuming most speaker/cable combinations are an inductive load? (Not including electrostatics, or pizos). So do people test stability with capacitive loads because they are a more difficult test? (I guess if you have a long high capacitance cable that isnt connected on the speaker end you dont want the amp to go unstable. So much to worry about! Oh well, I guess thats what makes this stuff so interesting.)
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Old 4th June 2009, 04:42 PM   #10
cbdb is offline cbdb  Canada
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Just tried a series R with the load cap, makes a huge differance. Adding .1R lets me go from .01uf to 2 uf. Is this a resonable thing to do ? My phase margin goes fro 70 degrees to 30 degrees (still borderline stable? but the interesting thing, my gain margin increased from 6db to 12db!
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