Adding a HPF to a Neve 1272 using a strange old Neve potentiometer

[Golgoth]

Hi guys,

I am converting a 1499 oscillator module into a 1272 preamp. I am pretty confident about the preamp part, currently waiting on a component order to start soldering. I'd like to make the most out of the module and re-use as many components and as much space as possible.

The 1499 oscillator module has a stepped potentiometer originally used for the oscillator's frequency selection. I want to use it as a HPF pot in the module. See the pot below, with the current oscillator wiring still attached:



Two questions come to mind:
1. Can I use this pot, and wire my passive HPF steps components directly onto it? If yes does anyone has info about this particular pot? It has no markings so I could not find a spec sheet but I seem to recognize rows of inputs/outputs/grounds?
2. Where should I put this passive HPF pot in the circuit? I'm guessing after any amplification occurs but not sure exactly about where still in the context of the 1272 circuit. Between the two sides of the BA283AV maybe?

Thank you very much for your help!

 

[Voyager10]

That looks like a rotary switch rather than a pot (variable resistor). You can work out the wiring with just a continuity tester. Each of the four 'layers' on the switch body will (probably) have one common contact, which is connected to each of the other contacts on that layer as you turn the shaft.

For a basic 6dB/octave HPF, the simplest idea would be to switch different capacitors into the coupling between two stages, e.g between the P and L on the BA283AV. You can keep an 'HPF out' setting where P and L are directly coupled (through the switch but no capacitor). Suitable capacitor values will be perhaps in the 47nF to 1uF range, lower values give a higher rolloff frequency.

 

[Golgoth]

That looks like a rotary switch rather than a pot (variable resistor). You can work out the wiring with just a continuity tester. Each of the four 'layers' on the switch body will (probably) have one common contact, which is connected to each of the other contacts on that layer as you turn the shaft.

For a basic 6dB/octave HPF, the simplest idea would be to switch different capacitors into the coupling between two stages, e.g between the P and L on the BA283AV. You can keep an 'HPF out' setting where P and L are directly coupled (through the switch but no capacitor). Suitable capacitor values will be perhaps in the 47nF to 1uF range, lower values give a higher rolloff frequency.

Thanks a lot for this! This is close to what I had in mind, glad you're confirming I can indeed use that switch for an HPF!

 

[Golgoth]

Finally found the time to finish the module's conversion, HPF modification and custom wooden box!
Getting a lot of gain with that T1447, sounds fantastic! Wired for use with a BAE 24V PSU.






Next modification would be more HPF points, and maybe LPF points too on the other side of the switch to take full advantage of it.