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Passive pre for proprius ?

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franklin View Drop Down
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    Posted: 03 May 2015 at 2:02am
Not sure is it suitable to do that?

I've just built a passive preamp between my Genera and my Proprius, 
it is just a selector and then a 20k volumn.
However I found the sound become flat and bass is gone, but , if I turn the volumn to max on 
the preamp and use the proprius volumn, the sound is ok again.

May be the resistance of the preamp volumn is not right?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2015 at 10:15am
We have a name for passive preamps: "pot in a box". It is rather an unkind name - sorry.

At half setting, somewhere around 3 o'clock on an audio taper or log pot (a pot designed for volume), the series resistance is going to be 10k (with a 20k pot).

The following interconnect cable has some capacitance. It might not be much, but it is there. Combined with the series resistance of the pot (10k max) it constitutes a low-pass filter.

It might be at a frequency as high as 150kHz using a low capactance cable, but it will be at a much lower frequency with a higher capacitance cable.

Most people would think that didn't have much effect. We cannot hear much above 15kHz anyway.

But we hear the timing of the music, and if we alter the phase between lower and higher pitch parts, we hear the music differently.

A 150kHz filter results in flat phase to 15kHz (-6 degrees). If the cable is more capacitive - say twice as capacitive - the flat phase is only to 7.5kHz.

The result may be a lack of impact to the bass and other frequencies, and that may result in a flat sound.

With the pot set below 12 o'clock it might be more acceptable? This is because the series resistance is much lower at around 1.8k. At full volume there is no series resistance, and the pot is virtually "out of circuit".

Hope this helps in understanding what's going on?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote franklin Quote  Post ReplyReply Direct Link To This Post Posted: 03 May 2015 at 2:01pm
Yes, I can clearly understand it now. The series resistance limited the current to charge the cable capacitance.(low pass filter) 

Bloody hell , a volumn pot should not be placed "before a cable" at all. 
This thing is so wrong...Cry

And the sound is a phenomenal result. My Proprius and Harbeth are amazingly revealing that.
It is quite funny for me to hear a low pass filter making bass flat. 
In the other world I always see people saying super tweeter make the bass better...

Thank you Graham, your reply is inspiring me. 

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