Zobel networks

[GregH2]

Hi All,

I am interested to know how one determines if a zobel network is required. I currently run my lm3875 gainclone without one and am very happy with the sound. There is no audible sign of any problems.

Firstly, in your experience is a zobel often required with a gainclone amplifier? And secondly, does not running a zobel risk damaging my amplifier and/or speakers? I prefer the idea of not adding any circuitry that is not required, but would obviously add the zobel if damaging the amplifier/speakers was a possibility.

Would I be able to hear any problems caused by not using one, or are the problems purely in the ultrasonic spectrum?

Your advice is appreciated.

Regards,

Greg.

[Mooly]

Not an easy one to answer. Some amps will run quite happily without, and needing one is definitely not a sign of a problem amp.
If you asked me if a chip amp needed one I would say yes yes yes.
It helps with stability and provides a known impedance at hf... so much depends on the phase margins and loop gain of the amp in question. And the load it works into.
As to values, what you see in any amp, 0.1uf in series with 10 ohms is fine.

You would need to test your amp and look at squarewave performance etc and stability into capacitive loads,,, which may be destructive. Your amp may be fine or running on the verge of instability... impossible to say. Very important to connect the Zobel return to the correct ground so it doesn't cause a problem rather than help stop one.

[tvrgeek]

Agree with Mooly. Solid state amps, being mostly a constant voltage source, don't usually get too upset. ( If they do, build a better amp) Tube amps can. The Zobel is more often used as part of the crossover circuit closest to the driver to make the filter network more predictable. Amp load is a side effect.

Linkwitz has a spreadsheet that helps, but I have found only testing to be accurate. I have found needing caps more than twice the calculated value, and resistors plus or minus 50%. So, if you don't have something like a WT-2 to measure the impedance, you are working blind.

[GregH2]

Alright thanks. Looks like I'm going to have to look in to this further. Mooly, when you say correct ground I assume you mean the power ground as opposed to the signal ground?

Thanks,

Greg.

[Mooly]

Originally Posted by swordfishy

Alright thanks. Looks like I'm going to have to look in to this further. Mooly, when you say correct ground I assume you mean the power ground as opposed to the signal ground?

Thanks,

Greg.

Yes... somewhere where it has no influence... which if the PCB is correct will be the speaker return point.

[AndrewT]

I'd suggest one always uses a Zobel on the amp output until one has sufficient knowledge to know when it may be omitted and what to do to check that it's omission was the correct decision.

[Analogbuff]

The zobel that fits one may be grossly wrong for another. It pays to look carefully at this, or you may have unexplainable failures with IC amps.
0.1U and 10R as a standard cure is no cure.
& here is why:
IC Audio Power Amplifiers and Zobel Networks: One Size Does Not Fit All | Digital Home DesignLine

Another matter is that the standard loudspeaker cable and loudspeaker termination can have some pretty wild resonances in the low MHz range, killing any kind of amps like flies.. A HF load like the zobel may help.



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