#007: Incredible Synth Filter, SMD MIDI Optocouplers, and Woes of Production

Pocast 007 : Incredible Synth Filter, SMD MIDI Optcouplers, and Woes of Production

Last week’s video:
LTspice Potentiometer Download and Install

— Filters
—– Scowering the web
——- found interesting schematic
——- built it up. It acted weird.
——- I simulated it in LT Spice
——–I It was still weird. It wasn’t a LPF at all, but some kind of goofy resonant peak thing.
——–Later on, I re-read the description and it said, “Fake LPF”. Sigh.
— No forum schematics? Fake news?

— Incredible video on synth design
——-DIY Synth Design Tutorial Series – 003: Analog Filter and Power Supply
———- This guy knows his stuff. He gets into the real design. the.room.dis.connect

—— Introduces LM13700 transconductance amplifier
———- We all know what an RC filter is?
————– The R is little more than current control.
————– The cap is a bucket and the “current” is the flow of water to fill up that bucket.
————— What if we wanted to control R on the fly?….potentiometer
—————— What if we wanted to control a stereo 4-pole filter. We’d need an 8-gang potentiometer.
———- This led the search for a VCR (voltage controlled resistor)
—————I knew a MOSFET’s Rds does this, at least to a certain degree.
—————This would require some real chops
———- Enter the LM13700 transconductance amplifier.
—————— Essentially a VCR… voltage in controls current out.

Filter conclusion: Transconductance amplifiers allow us to control filter frequency.
———— allows creative options as CV

Back to LM13700
— About $1 in quantity
— Plenty of them on Digikey and LCSC
— A mono, four pole filter needs 2.
—- The datasheets “Application Information” is sensational.

SMD Optocoupler For MIDI TLP2361
This guy made a super helpful blog post showing how he made his Teensy MIDI board using TLP2361 opto coupler