tirsdag 13. august 2019

Idea: single vca based stereo panner

I just has an idea - would it be possible to make a planner using a single VCA? If one lets the VCA control the volume linearly for one channel, then subtract the result from the other, one would get louder as the other gets quieter. It could then be followed by two expo converters. EDIT: Not sure how I thought this could work, the expo converters would indeed increase the tops, but not linearly so we get distortion. Could the "Better balance control" ideas from https://sound-au.com/project01.htm be adapted somehow?)

There are some immediate problems with this - getting a correct 50% balance requires tuning, the expo conversion, if done using transistors, will require matched transistors which will also be heavily affected by temperature, and the audio signal must be unipolar or the transistors biased in a special manner.

But would it work in practice? That would be a fun little experiment :) not sure it will save us any parts compared to two lm13700 based linear VCAs controlled by a computer though.

EDIT: I've tested the LINEAR version of this and it works well.

søndag 11. august 2019

Exponential converter heater

Just something I seem to search for over and over: An oven/heater circuit for keeping the exponential converter transistor pair at a constant temperature level to prevent detuning when temperature changes:

The minibrute. Uses ca3083, the microbrute uses lm3046 combined with ca3083 in a different combination as it also has a different expo converter. Expo pair is part of u10 (first two transistors).

VCO and Digital sample player

It's that time of the year again - I'm on vacation in Nordfjord and finally have both the time and the energy to think about the XM8 project.

Some days ago I did a CEM3340 circuit design based on the data sheet and the implementations found in the Prophet 5 and Memorymoog, as well as others, found on https://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/. I have yet to test the circuit, but it has the following features:

- All three waveforms
- Soft sync
- FM modulation
- +/-5V waves
- Pulse wave hysteresis
- HF tracking and osc scale trimmers

I will try the circuit using the AS3340 come fall. I also need to see if the circuit has any issues with PWM affecting pitch, it is mentioned both in the data sheet and elsewhere, for example here.

Untested circuit

Next up is the digital oscillator/sample player. It should be able to do both sample playback (for sampled attacks, D50 style, and sampled waveforms like the PPG and Waldorf synths. Some keywords/ideas: Pitch changing using FFT, needs 24 bit for good quality. Use stereo dac, allow separate outputs to each of the output busses.