While I’m trying to figure out the specific vga timing I decided to put down the current progress.
The plan was to add VCAs to the synth. However, without surprises a LM13700 can’t handle 1Mhz (not really designed for it). The only «ready » solution at those frequencies is the LT1228 from Linear. However it costs 8,88 euros. Needless to say, it’s not for me.
Another solution is to build a vca from discrete components. Simulation are being made with what I called the Yusynth VCA although it is a standard VCA design seen a lot in Synth design. A major drawback is the offset of 500mV at the output. I’m wondering if there will any VCA in that project after all, this will give 5 VCOs instead of 4.
This led to another constrain, the bandwith of the op-amps throughout the synth. The bandwith and the slew rate should be sufficient for the minimun 1MHZ frequency. It’s a nightmare to chose an opamp from parametric search because there’s so many. I reduced the selection by taking into consideration only parts that can be bought on aliexpress or ebay. Remained the LM6172 or the MAX4392 that works only with +5/-5V which may lead to redesigning the VCO. The synth will run with +12/-12V, I’ll stick with the LM6172 for now, besides is the same price as the MAXIM part.
The pin 3 of the
XR2206 controls the frequency. It is fixed at 3V and the design
should not pull more than 3mA out of it. The problem is to not to
exceed those specifications while modulation is applied to the input.
The first solution is to clamp the signal when it needs to be in
order to not fry the XR2206. However it requires more components
the range of the manual frequency control whereas the highest and
lowest frequencies are only obtainable with modulation applied. I took this solution.
problem applies to the « fine » frequency control
when, again, modulation is applied. Therefore I removed the « fine »
The PCB has been designed for cost and size. Jacks and switches will be linked with wires. A prototype area will be installed in the bottom empty space of the board