viernes, 21 de noviembre de 2008

Portamento Mods

In this video I show you the last mod to my synth. The actual portamento circuit has a logarithmic response since I use a simple passive RC network ((it's important to say that the circuit has a logarithmic response but when you applied to a 1 Oct/volt VCO you cancel the log effect, cause the expo response of the VCO, and you obtain a linear response in frequency, buy musical tones are distributed exponentially in the frequency range so this linear in frequency response in not musically natural)).
The result is a fast sweep at then beginning and a slow sweep at the end of the portamento effect. You can hear this effect at the beginning of Erasure’s Chorus. Many synths have this type of portamento but I don't like it. There are other electrical negative effects when the value of R is very high (R is a 1Mb potentiometer which change the portamento time), at this point the output voltage never riches the input voltage, I don't know why. Although, I suppose this is a defect of my design.

So, I decided to replace this circuit with one with linear response (exponential effect in frequency when applied to the VCOs). The circuit is very simple too. It uses an integrator Op Amp network placed in output of other op amp before its feedback network. When you apply a specific voltage to the input the output try to replicate the input but the integrator network produce a controlled linear slew rate.
I want to replace the old circuit with the new one but in a fast and simple way, so I build a little daughter board that I placed in the dip socket of the old circuit buffers Op Amp (TL 082). With this I can back to the old circuit just taking of the daughter board and placed an OP Amp in the socket.

Another possibility of the new circuit is the asymmetrical portamento. A diode placed in parallel with the rate potentiometer produce the portamento effect when you play upping phrases and cancel the effect when you play down way phrases. The new circuit has this facility but I don't use it yet because I have to put a switch in the front panel.

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