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Old 13th February 2010, 11:45 AM   #1
GregH2 is offline GregH2  Australia
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Default Zobel networks

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.

Last edited by GregH2; 13th February 2010 at 11:48 AM.
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Old 13th February 2010, 12:08 PM   #2
Mooly is offline Mooly  United Kingdom
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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.
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Old 13th February 2010, 12:20 PM   #3
tvrgeek is offline tvrgeek  United States
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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.
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Old 13th February 2010, 02:11 PM   #4
GregH2 is offline GregH2  Australia
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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.
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Old 13th February 2010, 04:40 PM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by swordfishy View Post
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.
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Old 15th February 2010, 02:01 PM   #6
AndrewT is offline AndrewT  Scotland
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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.
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Old 17th February 2010, 09:09 AM   #7
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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.



But
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Old 8th April 2013, 08:24 PM   #8
johnm is offline johnm  United Kingdom
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Does it matter what type of capacitor is used for an LM3875s zobel network? TI make no mention of the type in their application notes...

A member here did write the following a while back:

Gainclone Transformer

"The capacitor for speaker output zobel (output RC) included with the chipamp.com kit is the wrong type. An inexpensive little polyester/mylar dip cap is typically used for zobel, not high-efficiency polypropylene, so if you get poor quality upper treble, then replace that little cap"

Is this really the case? I have a small 0.1uF polypropylene cap here which came with my Audiosector Gainclone kit, and wondered if I should use it in light of the above quote...

John.
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Old 8th April 2013, 08:31 PM   #9
jcx is offline jcx  United States
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Zobel are funny - if they work , the amp never oscillates - then they see next to no current, power

but you need to size them as if they will see many cycles of full output swing oscillation at any frequency up to nearly the chip amp GBW

and they need to be low inductance - extended metal foil construction caps would be best - dielectric quality way secondary

the damping R shouldn't be inductive wirewound - carbon composition works fine though in this position
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Old 8th April 2013, 08:49 PM   #10
johnm is offline johnm  United Kingdom
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Quote:
Originally Posted by jcx View Post
Zobel are funny - if they work , the amp never oscillates - then they see next to no current, power

but you need to size them as if they will see many cycles of full output swing oscillation at any frequency up to nearly the chip amp GBW

and they need to be low inductance - extended metal foil construction caps would be best - dielectric quality way secondary

the damping R shouldn't be inductive wirewound - carbon composition works fine though in this position
This is the type the kit came with - assume this is OK to use:

BFC241641004 Vishay BC Components | BC2054-ND | DigiKey

I think I have have 2R7 carbon comp. in the parts box. Will go check.

Thanks
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