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Amp pick up interference
Amp pick up interference
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Old 11th October 2018, 07:56 AM   #1
Hicoco is offline Hicoco  France
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Default Amp pick up interference

I pick up interference with my audio input cables, I read that it was possible to install a capacitor at the input to filter it.
What is the value (1nF ???) and how can I install it ?, Is it between each input ground terminal and chassis or positive side to ground?
Thanks for your help
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Old 11th October 2018, 03:00 PM   #2
Hicoco is offline Hicoco  France
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Quote:
Originally Posted by Hicoco View Post
I pick up interference with my audio input cables, I read that it was possible to install a capacitor at the input to filter it.
What is the value (1nF ???) and how can I install it ?, Is it between each input ground terminal and chassis or positive side to ground?
Thanks for your help
Click the image to open in full size.49 views but nobody answer.....!!!!!!
May be no one have been this trouble before!!!!!! or this thread does not have any interest...thanks anyway for the click on the link
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Old 11th October 2018, 03:37 PM   #3
FauxFrench is offline FauxFrench  France
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Interference may be removed using a few simple measures (assuming an RCA connection):
First, check that the cable shield is well connected to the RCA ground contacts in both ends. Eventually turn the male plugs a bit around while they are inserted in the female connectors.
Second, check that the interference disappears when you disconnect the cable at the source end and connect the signal wire to the cable shield/screen through a 1KOhm resistor. Be careful not to touch the resistor during test because then you introduce hum. If this test is negative, the cable may be defect or the problem is in the amplifier.
If second test was a success, sensitivity to noise depends on the impedance level. You may use a 10KOhm resistor connected between the cable signal conductor and cable ground/shield (screen) or a capacitor connected between the same two. The size of the capacitor depends on the impedance level and should not reduce treble. 330pF-470pF should be a good start.

Last edited by FauxFrench; 11th October 2018 at 03:42 PM.
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Old 11th October 2018, 03:57 PM   #4
Hicoco is offline Hicoco  France
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Quote:
Originally Posted by FauxFrench View Post
Interference may be removed using a few simple measures (assuming an RCA connection):
First, check that the cable shield is well connected to the RCA ground contacts in both ends. Eventually turn the male plugs a bit around while they are inserted in the female connectors.
Second, check that the interference disappears when you disconnect the cable at the source end and connect the signal wire to the cable shield/screen through a 1KOhm resistor. Be careful not to touch the resistor during test because then you introduce hum. If this test is negative, the cable may be defect or the problem is in the amplifier.
If second test was a success, sensitivity to noise depends on the impedance level. You may use a 10KOhm resistor connected between the cable signal conductor and cable ground/shield (screen) or a capacitor connected between the same two. The size of the capacitor depends on the impedance level and should not reduce treble. 330pF-470pF should be a good start.

Thanks for your answer, i did the test already and when i disconnect the input cable the Amp is silent, no humm....... so if i understand well i can install a 330pF-470pF capacitor at each input terminal, from the positive side to ground..right (The input impedance is 45Kohm)

Last edited by Hicoco; 11th October 2018 at 04:00 PM.
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Old 11th October 2018, 04:05 PM   #5
FauxFrench is offline FauxFrench  France
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I would do it at in the male plugs near the amplifier input. From the signal wire (positive?) to ground inside each male plug. Or, on the female input connectors of the amplifier if possible.
I would start with a 10KOm resistor instead because it is likely to be more efficient. Eventually try both to see what is best.
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Old 11th October 2018, 04:25 PM   #6
Hicoco is offline Hicoco  France
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Quote:
Originally Posted by FauxFrench View Post
I would do it at in the male plugs near the amplifier input. From the signal wire (positive?) to ground inside each male plug. Or, on the female input connectors of the amplifier if possible.
I would start with a 10KOm resistor instead because it is likely to be more efficient. Eventually try both to see what is best.
ok i will try that, thanks again, note my ground input connectors are isolate from the chassis.
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Old 11th October 2018, 04:28 PM   #7
FauxFrench is offline FauxFrench  France
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In case of continued problems, try with 100nF between signal ground (and cable shield/screen) and chassis.
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Old 11th October 2018, 05:51 PM   #8
academia50 is offline academia50  Argentina
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Quote:
Originally Posted by Hicoco View Post
Thanks for your answer, i did the test already and when i disconnect the input cable the Amp is silent, no humm....... so if i understand well i can install a 330pF-470pF capacitor at each input terminal, from the positive side to ground..right (The input impedance is 45Kohm)

it is not specified in which amplifier input the buzzing appears.
Presumably at the entrance of phone to be the most sensitive
A cable has a certain capacitance, this makes it act as a detector, tuned to the frequency of the noise. Very unlikely in high level entries.

Change the cable, measure the capacitance of the cable and replace it with another that is quite different.
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Old 13th October 2018, 01:57 AM   #9
nigelwright7557 is offline nigelwright7557  United Kingdom
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Just putting a capacitor across the input is a bit random.
To make a proper RF filter you need and resistor and a capacitor.
Formula is 1/(2 pi r c)=f
Most amps have them anyway.
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Old 13th October 2018, 02:13 AM   #10
MAAC0 is offline MAAC0  Portugal
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Hicoco


First of all .. What kind of Interference are we talking about ??
PSU Hum, Radio, TV, Cellphone, Buzz ??
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