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.
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
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
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 there’s no R load to help dampen things out. Electrostats are sometimes modeled as 8 ohms paralleled with 2 uF (don’t 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.
cbdb said:
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
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.
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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