LM13600/13700 Supply Voltage?

[kingsteven]

The datasheet for the LM13600 states absolute maximum supply voltage as 36VDC or ±18V... I see a lot of audio circuits which use these I.C.s at lower voltages (the example circuits in the datasheet run at ±15V, Some of the Maestro FSH1 (low-pass filter) clones use ±8V). This is probabbly quite simple, but I can't figure out from the datasheet what effect using a lower voltage has on the output... can someone please explain?

Thanks,
Steven

[Enzo]

The potential output signal level will be limited by those several volts. Our audio is not very demanding, so any response differences I would think to be inaudible. Link your data sheet.

[kingsteven]

Thanks Enzo, I thought it may be something like that - here's the datasheet:
http://www.national.com/ds/LM/LM13700.pdf

If a lower voltage limits the potential output, could you explain the significance of the step down from ±9V to ±8V to power the chip in this filter shematic? (LM13600 is IC1)

[Steve Conner]

I imagine that RC filters had to be included in the supply rails to reduce noise from the charge pump, and the 1V drop is an (unwanted but not very important) side effect of that.

I have one of the old Maplin kit wah-wah pedals with a LM13700 running straight off a 9v battery, and it seems to work fine. I modified it along the lines of a synth filter, with a choice of lowpass or bandpass, and a resonance control.

[kingsteven]

I thought that might be what they're doing - but it seems superfluous as the the chargepump (MAX1044) is running in "boost" mode which means the oscillator is running at at least 30kHz anyway!? Could it be that they are trying to filter the noise generated by the S&H circuit?