LM13700, why use an external buffer instead of onboard?

[Boner]

So i've been eyeballing schematics online, mostly CV controllable oscillators and filters, and I'm seeing a pattern where the designer used external op amp buffers or even transistor based buffers instead of the ones provided on the chip.

I know its weird to ask people other than the designer themselves, but why did you/he/she use external buffers? I'm lost as to why....

noise performance? Headroom?

[MrStab]

AFAIK the 13700 uses a darlington pair, which consists of bipolar transistors, which are less-ideal for guitar use than buffers with FET inputs due to their lower max impedance. also, you don't wanna be stuck with the specs of one chip when there are hundreds of op-amps out there which aren't optional afterthoughts.

[anotherjim]

There's a chance the design was originally for CA3080 which had no buffer and the minimum was changed to use the newer device.

[Keppy]

AFAIK the 13700 uses a darlington pair, which consists of bipolar transistors, which are less-ideal for guitar use than buffers with FET inputs due to their lower max impedance.

This might be a factor for input buffers, but most applications of the 13700 buffers that I've seen place them after the OTA, to convert the OTA's output current to a voltage. Once the signal goes through some active processing, guitar impedance no longer matters, plus Darlington impedance is pretty high anyway.

There's a chance the design was originally for CA3080 which had no buffer and the minimum was changed to use the newer device.

This. By the same token, some circuits might have originally been designed for the 13600 and wanted to decouple the buffers from the amp bias input (although in that case they could just use a 13700 instead, so this seems unlikely unless the designer had limited parts access).

There's also the fact that Darlington transistors have a double-diode voltage drop from input to output, which isn't tiny in 9v circuits. Headroom might be an issue.

Transistor buffers of any type pretty much require a coupling cap before the next stage, plus an emitter resistor. If you have a leftover opamp stage, it makes sense to save a component or two by using that for the buffer instead of the Darlington.

There's also corksniffing. Some people just have a favorite component for buffers. I can't really think of a reason besides this (or the 3080 thing) for choosing a separate transistor buffer.

I use the onboard buffers whenever I can, though, so I'm really just guessing at why other people don't.