Output inductor in power amps - pro and con

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Antonio simulation shows output voltage before an inductor(.ac sim) only. Certainly it shows an influence of the cap connected after an inductor influence, but the same influence shows on the voltage after an inductor. I was trying to show the difference between output voltage(or in my case, impedance) after and before a inductor, actually a speaker line garbage suppression by showing V(out)/I(I1)/V(vout)/I(I1) in dB. Could be that my result are not good enough, I did not include any cabel resistance/capacitance, but I think it was not importand.
Sorry but it not easy for me to express my self in English technically correct.
BR Damir

I think your simulation is not a reliable indicator of RFI behaviour.
A constant current source in series with the output inductor can provide essentially unlimited volts to maintain the current as the impedance increases.
Surely this can not be realistic?

Do not worry about the difficulty to express yourself. You ideas make sense whereas many native speakers write fluent nonsense.;)

Best wishes
David
 
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JLH used 0.22R only and I like the sound of that amp.

I suspect he may also have done it 'soften' the sound by reducing the damping factor but of course I cannot be sure. But, this is still not the best way to couple an amp to a speaker. And, if the amp is not stable without the resistor, or with an inductor, then those are separate, fundamental issues that should be resolved first before inserting the resistor.
 
I think your simulation is not a reliable indicator of RFI behaviour.
A constant current source in series with the output inductor can provide essentially unlimited volts to maintain the current as the impedance increases.
Surely this can not be realistic?

Best wishes
David

Hi David,
I am not telling that I do all correctly, I am not a spice specialist, still learning to use it.
Please look again this link, it explains how to simulate an output impedance in LTspice. This is a small signal analysis and the current source is used as a load here.
http://www.cs.toronto.edu/~relu/audio/ltspice/tutorial-spice-zout.html
BR Damir
 
I suspect he may also have done it 'soften' the sound by reducing the damping factor but of course I cannot be sure. But, this is still not the best way to couple an amp to a speaker. And, if the amp is not stable without the resistor, or with an inductor, then those are separate, fundamental issues that should be resolved first before inserting the resistor.

Yes I think the bass is a bit softer and not so punchy as with my TT amp with high dumping factor.
 
Hi David,
...I am not a spice specialist...
Please look again this link, it explains how to simulate an output impedance in LTspice.

I also am not a Spice specialist, that's why I wrote with a question mark;)
My hope is to learn as we work this out.
I read the article earlier and understand it calculates output impedance but I am not convinced this is a useful way to simulate RFI tolerance.
Edmond Stuart did some RFI simulation of his new front-end and I will check there to see how he did it.

Best wishes
David
 
How do you find it images left to right and front to back compared to your TT amp?

Tough question, I am not good at this. First of all TT is my design, and JLH was modified by me. I like both, JLH with laterals is softer and less powerful, imaging is good for both. I never have a chance to have someone with "golden" ears to listen at my house and to give some opinions.
 
I also am not a Spice specialist, that's why I wrote with a question mark;)
My hope is to learn as we work this out.
I read the article earlier and understand it calculates output impedance but I am not convinced this is a useful way to simulate RFI tolerance.
Edmond Stuart did some RFI simulation of his new front-end and I will check there to see how he did it.

Best wishes
David

I just simulated output impedance before and after the inductor and devided after impedance with the one before and got ratio in dB. How good this could be interpreted in RF suppression I am not sure.
BR Damir
 
RF suppression and stability have a lot to do with the layout and track lengths. Spice will be inaccurate in the VHF if you do not include these strays.

The question asked here was how much output inductor and associated Zobels helps to suppress a disturbances coming from the loudspeaker side, not RF in general. The layout was dicussed in length in an other thread.
 
Sorry to semi hijack this thread...

Lastly I would expect some pickup susceptibility to common mode so using a dual inductor and a chassis shunt capacitor with a common mode choke right at the output to be beneficial.

Has anyone tried a common mode choke on the amplifier output?
Makes perfect sense to me, and was thinking of winding a 2uH for the output and winding a 1uH on top of it for the ground. This would (read: might) give isolation for capacitive loads and more isolation for common mode RF. I don't know what the mutual coupling is like for an air cored choke, so might even get away with just 1uH each way.

What do we think?

Brian.
 
Mutual inductance is poor (read low) for air cored chokes at audio frequencies. You'd want the common mode inductance to be much higher than 1-2uH to give more effective isolation from RF picked up on the cable. I reckon a ferrite core would be fine as the flux is very low provided the two sides are in balance.
 
I would expect common mode suppression for avoiding susceptibility to be in the neighborhood of 30-100MHz (worsened by having input and output cables wavelengths near this range).
Can test for such effects by subjecting cables to a magnetic filed, such as a current transformer around each (well worth doing rather than adding fixes for an nonexistent problem).
The manifestation or effects may be rectification or other nonlinearities to effect the audio range, but still the common mode choke these upper frequencies to remove the susceptibility not filter its effects.
SMD ceramic caps can work into this range when coupled with planes and lot's of ties to chassis.

Thanks
-Antonio
 
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The emi/rfi amplifier pickup issues here is something I am going to do some more r&d on as I have the test equipment --- RF CT's and network analyzers from sub audio to 6Ghz. Let you know..... Freq range of interest? (after I get back to my lab/home in California).

I have checked on some things now and then..... I can tell you that an unshielded twin-lead speaker cable will radiate a LOT of emi energy far from the cable. This should be reduced as it gets into nearby low level signal cables, easily. Cross-talk, etc. One more reason to measure as a System to find things you dont consider when measuring an amp alone on the bench.

A CM choke on the output - bifilar wound (1uH/1uH) will be interesting to measure.... then CM coil between a PI filter arrangement. Check its effectiveness in preventing coupled rfi back into amp's ops.

THx-RNMarsh
 
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The emi/rfi amplifier pickup issues here is something I am going to do some more r&d on as I have the test equipment --- RF CT's and network analyzers from sub audio to 6Ghz. Let you know..... Freq range of interest? (after I get back to my lab/home in California).

I have checked on some things now and then..... I can tell you that an unshielded twin-lead speaker cable will radiate a LOT of emi energy far from the cable. This should be reduced as it gets into nearby low level signal cables, easily. Cross-talk, etc. One more reason to measure as a System to find things you dont consider when measuring an amp alone on the bench.

A CM choke on the output - bifilar wound (1uH/1uH) will be interesting to measure.... then CM coil between a PI filter arrangement. Check its effectiveness in preventing coupled rfi back into amp's ops.

THx-RNMarsh

Hi Richard,

This is good stuff, and I am looking forward to your findings. Can you say a bit more about what you have in mind regarding emi radiation from the unshielded speaker wires? What in the amplifier is the source, unless the amplifier uses a switching power supply or has a very bad linear supply? I have always focused on emi ingress into the amplifier from the speaker cable and the loudspeaker, as together they form a large antenna.

Cheers,
Bob
 
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Joined 2012
Hi Richard,

This is good stuff, and I am looking forward to your findings. Can you say a bit more about what you have in mind regarding emi radiation from the unshielded speaker wires? What in the amplifier is the source, unless the amplifier uses a switching power supply or has a very bad linear supply? I have always focused on emi ingress into the amplifier from the speaker cable and the loudspeaker, as together they form a large antenna.

Cheers,
Bob

Hi Bob.... still sitting here in my condo in my shorts waiting for some window blinds to be replaced later this morning ----

The speaker leads themselves radiate a large EMI field from the high current/low z audio signals on the cable. I can pick it up with a battery powered hand held emi field test unit 3-4 feet away (no info on tester at this time/place). Perhaps shielded cable (I have tried coax and dual coax for speaker cable) or shielded twisted pair.... but then the Ls has its affects on tweeter and must be considered/measured.

Regarding the RFI pickup on the cable being fed-back into the amp.... the common-mode coils as part of a pi OPS filter is what I would use for that... going in but out as well (if any). Ferrite's would be considered if audio thd isnt increased.... might be a good solution with the proper choice of core material.

Q: What cable construction would reduce the radiated EMI with minimal impact on the signal to the speaker?

THx-RNMarsh
 
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