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profdc9 4th September 2020 05:08 PM

Stability of an amplifier under capacitive loading
When driving a speaker on a long cable, what is a safe capacitance to design an amplifier to be stable driving? Would 10 nF be a sufficient maximum capacitance or 100 nF? I measured some audio cable at about 400 pF/m, but I would guess that the variations in setups might produce higher capacitance loads. Are audio amps ever designed to be unconditionally stable with respect to capacitive loading, and if so, how? Given that oscillation of an amplifier could produce a catastrophic effect on a speaker if the amplitude was high enough, I would think this might be a consideration to maintain SOA.

duncan2 4th September 2020 06:44 PM

VERY touchy subject !

This has split audio designers right down the middle just ask D.Self (or read his article in EW ) where in reply to several articles by another engineer cut him to ribbons by presenting an article "proving " that ring mains electric cable was just as good .

It didn't end there verbal fisticuffs ensued and the Editor had to calm things down .
Naim audio specified that their amps required special cables and that other types of cables could cause "problems ".

Those were the days of low capacitance cable/low inductive cable /low resistivity cable etc , from what I remember during the 80,s/90,s in EW .

I hope calmer minds are at work here as I was engrossed by each monthly episode of EW in the Letters & articles printed , things are a lot more duller now there.

Loudspeakers: Effects of amplifiers and cables – Part 4 | EE Times

Mark Johnson 4th September 2020 06:52 PM

Some people also say that some types of loudspeakers are themselves capacitive. So even if the cable length is 10 centimeters, the capacitance load driven by the amplifier is high.

One reason why some amplifier designers install an inductor in series between the amplifier output and the loudspeakers, is to improve stability when the load is capacitive. Other amplifier designers brag that they have no / need no series inductor.

cbdb 4th September 2020 07:24 PM

With all the wacky (audiophool) speaker cable out there why would any amp maker exclude the zobel? I guess to market the thing too the fools that buy that sort of cable? So marketing.

rayma 4th September 2020 07:35 PM

Depending on the output network, many amplifiers will be stable for very large and for very small capacitances,
but not for a certain range of intermediate values. This is typical for many types of feedback amplifiers in general.

duncan2 4th September 2020 08:23 PM

What helped get all this going see -WW+EW -December 1996.

Even earlier---February 1995 relating to Mark,s post.

rayma 4th September 2020 08:55 PM

Very nice notes:
Practical Techniques to Avoid Instability Due to Capacitive Loading | Analog Devices
Op Amps Driving Capacitive Loads | Analog Devices

profdc9 4th September 2020 10:03 PM

simulation of C300 amplifier
3 Attachment(s)
I am working on building up Michael Chua's C300 amplifier which does include a Zobel load on it. I have a SPICE simulation of it going. I am trying to figure out what could go wrong with it through simulation. I have been able to put 10 to 20 nF on it without problems, but once 100 nF is directly loaded on it, oscillations start. I can increase the Miller compensation capacitor to bring the pole down to be stable, but if the loop bandwidth is reduced too much, THD increases. It seems to be in the neighborhood of 0.05% to 0.1% for 20 kHz @ 200 W output to an 8 ohm load (at least in simulation).

The attached SPICE file is used with Qucs-S/ngspice and uses combinations of faster transistors to try to increase the loop bandwidth so that the dominant pole is the Miller capacitance. If anyone uses Qucs-S you can load this model up and try it for yourself.

This is why I need to have some idea of what a reasonable capacitance to design for is. 100 nF seems like a lot of capacitance for a cable, but perhaps someone has some ribbon-like audio cable that works that way.

steveu 4th September 2020 11:30 PM

optimizing feedback
Unfortunately, there will always be a feedback arrangement that provides a bit less distortion at the price of stability. You have to do everything you can to stabilize your feedback and then look to other means to reduce distortion. For example, replacing R31+R32 with a single resistor and perhaps a capacitor (no connection to the final output) in order to cross-couple the drivers will improve the cross-over and high frequency performance.
C6 should be limited with a series resistor, 100 to 1k depending on R14. C6 causes RF rectification problems because it is a direct path from the speaker cable antenna to the LTP. C6 also exaggerates unpredictable problems that result from stray capacitance and inductance.
There are many complicated compensation methods but complicated compensation is vulnerable to unexpected variations that cause instability. My favorite plan is KISS, a single dominant pole. So I would not use R5+C3, and simply adjust C9 and/or R6...R10 until the circuit is stable.
Not using a series inductor +resistor in the output is leaving the feedback wide open vulnerable to all kinds of stability issues. I would only omit the inductor if the amp is integral to the speaker, ie no speaker cables to speak of, and the speaker impedances well defined.

rayma 4th September 2020 11:33 PM

"Not using a series inductor +resistor in the output is leaving the feedback wide open vulnerable to all kinds of stability issues. I would only omit the inductor if the amp is integral to the speaker, ie no speaker cables to speak of, and the speaker impedances well defined."

Definitely, who wants full power oscillation at 200kHz?
That lower distortion is illusory if it is only available along with conditional stability.

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