Hi guys,
hope you are doing well. Currently I design a MS20 style filter and definetly want to use LM13700. I already got a working breadboard version, but I want to really dig into the details before deciding the PCB design.
I am looking into some schematics I found all over the internet:
The original Korg MS-20 did not use an OTA for its filter (or anywhere else really), and relied instead on the proprietary “Korg 35”, a discrete BJT/FET module in a SIP package. I think that later versions used an OTA, and this is where these “Korg MS-20” filter schematics alegedly come from, althought I haven’t been able to see a service manual of the MS-20 showing an OTA.
Either way, the darlington buffers of the LM13700 do not appear to be used in the alleged “late MS-20” filter, so you can either use one LM13700 (or LM13600 as they only differ in the buffer section), or two CA3080’s.
I had a closer look at the “Korg 35” (apparently, some people were curious enough and reversed engineered them) and it seems that it uses the JFET in place where the buffer after the OTA would be. In this respect, it made sense that (allegedly Korg) and others put a TL7* there as these are JFET input op amps. So a Darlington buffer would not work there (or at least not work as Korg intended).
When do you use the Darlington Transistor Pair inside the LM13700? What could be applications?
They can be used as a buffer. The output of the OTAs in the LM13700 is a current, which in the OTA MS-20 filter is charging or discharging the capacitor to create the filtering. If whatever comes next in the circuit pulls or pushes any current to that capacitor, the voltage will be affected messing up your signal. The darlington buffers can be used to buffer the voltage at the OTAs output so it can be used by other circuitry without affecting the voltage.
When can the Darlington Transistor Pair be an alternative to an OpAmp?
When you need a buffer. An opamp can do lots of things, the darlington buffers in the 13700 are only usable as buffers because their collectors are internally tied to the positive supply voltage.
Why doesn’t Eddy Bergman’s design use the Darlington Amp built into the LM13700?
There are a couple downsides to the darlington buffers. Their output voltage is always 1.2-ish volts lower than the input, which you need to deal with somehow, and because they’re a “class A” amplifier they waste a bit of current. An opamp buffer is better in these respects, but takes more board space and costs more than just using the darlingtons that are already built into the 13700.