Wednesday, January 15, 2014

Chip burning, but success in the end

LM13700 bias input

After thinking about it for a while, I realised that the bias current input of the LM13700 isn't referenced to ground, but to the negative rail. That explains the doubled amplitude of the VCO waveforms. Looking in the data sheet you can see that the bias input sits a current mirror over the negative rail. It seems like there should be two diode drops over V-. But since I'm there messing around and changing the current setting resistor, I may as well measure it properly. So, voltage between positive and negative rail is measured to V+-V-=18.034V. The resistor is 38.33kΩ and the current IA=0.4371mA, so: 18.034V-0.4371mA·38.33kΩ=1.28V, which we, with confidence, can say is two diode drops.
 Changing the current controlling resistor for IA, I made a calculation error, and since I stopped thinking for a little while, I used a too small resistor and killed the chip with 3mA on the current bias input. This is about 1 mA over the maximum current. I did get a new chip today (they are quite expensive) and wired everything up again, and now it actually works like it should...

Completing the VCO

By wiring the log converter to the VCO and connecting the input of the log converter to the VCO, I now can play some music... By connecting the MIDI2VC output to the log converter and the VCO output to a speaker, I can now play using Midi Ox as MIDI source. The VCO needs some tuning, so I added a tuning pot that adds or subtracts some voltage from the input stage inverter in the log converter. Here are some pictures:
Playable system. MIDI2VC to the right. Log converter and VCO on the upper left. To the lower left is a mixer mentioned in earlier posts.
And here's a video where you can hear some random notes from the system (sorry about low sound volume):

Logarithmic VCO, final schematic

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