[sdiy] Calculating cable capacitance effect

Tom Wiltshire tom at electricdruid.net
Sat Apr 17 15:11:21 CEST 2021


I know you (Cheater) specified “hi fi equipment” but it’s perhaps worth mentioning that musical instruments are one area where the “low output impedance” assumption doesn’t hold. The output impedance of electric guitar pickups is considerable, and the effect of cable capacitance is notable as a result. Perhaps other pickup-based instruments (electric piano maybe?) show this same effect.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 16 Apr 2021, at 14:58, Ben Stuyts <ben at stuyts.nl> wrote:
> 
> You must take into account the output impedance of the device that is driving the cable. That should be quite low normally. The input and output impedance are effectively in parallel so your cut-off freq is mostly determined by the low out impedance. If that is all that is in play, you have a simple low-pass filter with 6 dB/oct falloff:
> 
> fc = 1 / (2 * pi * R * C)
> 
> Where fc = -3 dB cutoff frequency, R is the parallel resistance of input and output, and C is the cable capacitance.
> 
> Ben
> 
> 
>> On 16 Apr 2021, at 15:27, cheater cheater via Synth-diy <synth-diy at synth-diy.org> wrote:
>> 
>> Hi all,
>> if I am connecting two pieces of hi fi equipment with high impedance
>> input on the receiver, then the cable will have a capacitance and that
>> will in total create an RC filter. How do I calculate what this filter
>> does?
>> 
>> Cheers
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> 
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