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Old 28th February 2013, 09:53 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, 11:55 AM   #52
dadod is offline dadod  Croatia
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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, 05:32 PM   #53
brig001 is offline brig001  United Kingdom
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Sorry to semi hijack this thread...

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?

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Old 29th July 2013, 10:25 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, 12:52 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.

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