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Old 28th February 2013, 10:53 AM   #51
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The suppression of signals coming in from the speaker leads will depend strongly on these strays in the VHF and above The commonly used 100nF capacitor in the networks is self resonant at just 4MHz (EPCOS foil type)
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Old 28th February 2013, 12:55 PM   #52
dadod is offline dadod  Croatia
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Quote:
Originally Posted by davidsrsb View Post
The suppression of signals coming in from the speaker leads will depend strongly on these strays in the VHF and above The commonly used 100nF capacitor in the networks is self resonant at just 4MHz (EPCOS foil type)
In my TT amp I use SMD ceramic caps soldered direcly to the electrolytic caps on the PCB bottom side.
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Old 22nd July 2013, 06:32 PM   #53
brig001 is offline brig001  United Kingdom
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Sorry to semi hijack this thread...

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Originally Posted by magnoman View Post
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.
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Old 29th July 2013, 11:25 PM   #54
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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.
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Old 30th July 2013, 01:52 AM   #55
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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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Old 4th November 2014, 04:01 AM   #56
RNMarsh is offline RNMarsh  United States
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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

Last edited by RNMarsh; 4th November 2014 at 04:12 AM.
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Old 4th November 2014, 05:27 PM   #57
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Quote:
Originally Posted by RNMarsh View Post
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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Old 6th November 2014, 11:03 PM   #58
SSassen is offline SSassen  Netherlands
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Considering the subject this is an interesting read as well:

http://www.russandrews.com/downloads...atorDocJA4.pdf
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Old 7th November 2014, 01:40 AM   #59
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Bob Cordell View Post
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

Last edited by RNMarsh; 7th November 2014 at 02:08 AM.
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Old 7th November 2014, 10:23 AM   #60
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Quote:
Originally Posted by Bonsai View Post
I think an output resistor is totally sub-optimal.
A very little bit could be useful to prevent negative output impedance causing distortion at the speaker.
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