I have an interest in analog audio synthesizers and I'm in the process of analyzing and prototyping vintage synth VCOs in order to get a grasp of their inside workings. All schematics I've looked into so far like MFOS's VCO, the Bergfotron advanced VCO, Jim Patchel's No tempco VCO(see page 5) and the ASM-1 VCO use a matched BJT as their exponential converter (a 1V/Octave convention is widely used in analog synthesizers). Its task is to convert a control voltage (CV) exponentially to a current driving the oscillator core.
Now, some of these chips like the intersil CA3046 that I'm currently using are obsolete, the MAT03 is scheduled for obsolescence and who knows what happens in the near future...
That's why I want to compare my options and learn how to substitute one with the other, altering the original circuits if and as needed.
A search on mouser revealed 2 possible series. 3xxP14-U by THAT and MATxx by analog devices, the second being more or less 4 times more expensive than the 1st considering they are dual ICs against quad. The other obvious differences is VCBO which is 9V higher for the MATxx series, CE saturation voltage which is 50mV for the 3xxP14-P and 120mV for the MATxx, DC collector current and gain bandwidth product.
This got rather confusing for me when I started looking into their datasheets because (I need your verification on this) I think they are expressing the same quantities in different ways. Nevertheless, I'm feeling a bit lost and that I don't know exactly what I'm looking for.
So to make this as clear as possible, I would like some help understanding:
- What are the most important factors when comparing substitute matched BJTs (what to look for in the datasheets)
- How to deal with differences between parts (scaling input and output Vs and Is) and if this is at all possible
- Are there any 'out of the box' devices that perform exponential voltage to current conversion at the 0-10V , maybe 0-12V range or maybe +/-5V (I would love to give you a current range but i haven't managed to calculate the expected output current values)?