MOD - Adding variable HPF and polarity switch on a Soundcraft 500 desk

TAPESOURCE

TAPESOURCE

New member
Hi,
I'm a newbie here,
It's been 3 years now that i own a Soundcraft 500 mixer and I'm pretty happy with it.
The only things i've done is recapping the entire board and the power supply, and replace the old ribbon cable by a new one.
I would like to use the desk to mix tracks that i record.
I use to work a lot with HPF and LPF, and i was wondering if there's a way to modify the fixed 100Hz HPF switch to a variable HPF potentiometer.
I made some research but haven't found anything on this.
The questions here are : Is it possible ? Is there someone here who have already done this? Is it simple ? or do i need to forget about this mod?
The other mod i would like to do is adding a polarity switch on every channels. Is there an easy way to do it?
If some of you have links to share to learn about this would be great.
Thank you!

r . 0 0 1
 
I am sure someone will come up with the relevant schematics shortly so this is only a basic educated (not a lot) guess.

The HPF stage is likely a "Sallen &Key" filter and yes, you could make it variable if the two resistors are of equal value and you inserted a dual gang pot. If the slope is 12dB/octave it is likely they are equal.

Polarity inversion: Where in the channel would you put it? If both line and mic inputs are balanced* then you can flip the inputs to the mic pre (most mixers use the same amp for line and mic) Later on in the circuit will not be balanced and so you will have to insert an inverting op amp stage. the NE5534 would be ideal.

*Since most folks only ever want to P flip mics, just buy a few XLR 'flipping ' adapters?

If you can do without the switch they can be had at half that price.

Dave.
 
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I had a friend who made up little filters and put them inside tubes he glued to 3 circuit jacks - and he just picked one amd plugged it in via the insert socket. Fixed ones and variable with aminiature pot on the top. This was before surface mount devices were popular so could be smaller nowadays.
 
rob aylestone said:
I had a friend who made up little filters and put them inside tubes he glued to 3 circuit jacks - and he just picked one amd plugged it in via the insert socket. Fixed ones and variable with aminiature pot on the top. This was before surface mount devices were popular so could be smaller nowadays.
Click to expand...
It's certainly a way Rob but even a modest 12dB/octave is going to cause some signal loss and consequently possibly more noise. An active S&K with a suitable op amp won't. The turnover frequency will also be at the mercy of the return input resistance and that is generally unknown, as is Rout.

Dave.
 
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