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Old 18th June 2013, 11:13 PM   #1
M Gregg is offline M Gregg  United Kingdom
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Default Cable compensation

Just thought about this,

many years ago Richard Brice (HIFI world mag) talked about Perfect Pitch and made a passive preamp with cable compensation using ideas from oscillscope leads..it was called the J-solate compensated passive preamp.

You don't hear much about it I guess it was one of those ideas that didn't take off...

This sort of idea..

Application Note: How to Tune x10 Oscilloscope Probes

He used a 100K pot in series with a 330pF cap from the top of the volume control to the wiper. To tune out the cable effect on frequency response.

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M. Gregg
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Old 19th June 2013, 02:36 AM   #2
DUG is offline DUG  Canada
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If your signal source has an equivalent series output impedance then it will accomplish compensation.

If there is no significant equivalent series output impedance then you will have a treble boost control.

Have fun.

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Old 19th June 2013, 09:37 AM   #3
DF96 is offline DF96  England
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To do a wideband compensation for a capacitor in parallel with a resistive load, you need to add a capacitor in parallel with your resistive source (as done in a 'scope probe). If (as with a passive preamp) the source resistance varies then the capacitance has to vary too.

Adding a cap in series with a resistance won't do it, although it might do a reasonable approximation over a narrow range of frequencies and a narrow range of volume settings. It didn't take off as an idea because it doesn't actually work!

Having said that, in the world of audio the fact that something doesn't work seems to be no bar to it taking off as a popular meme.

PS under certain circumstances you might make more progress by adding an inductance in series with the source - as Heaviside suggested for telephone lines

Last edited by DF96; 19th June 2013 at 09:39 AM. Reason: add PS
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