8×8 Matrix Mixer

Hi! After the little demo of my new 8×8 Matrix Mixer I’ve built for Sim Hutchins I’ve been asked to explain what it’s made of, sooooo here we go! The matrix is organized in rows, each row made of 8 inputs going through 8 100k audio pots, and then into a 2-stage, lm1458-based amplifier. Very simple. The resistor R is controlling negative feedback of the first stage, I usually change this one to change the gain, in this mixer it is 220k, giving us a gain of 2.2 (approx. 7dB). Each IC has two 100nF ceramic monolithic bypass caps on both power rails. The circuit operates at +/-12V, supplied by a 317/337-based regulated power supply unit + a low-pass filter for each rail. I used a ring core transformer, mainly to have the least EMI possible. The AC power plug has an RFI filter, to further help keeping the unit isolated from external noise. In the near future (I’m waiting for delivery!) I want to try two things; transistors, good old transistors, and OPA2134 op-amp ICs, let’s see who wins! Here are some pictures of the making-of, the circuit itself is pretty simple, what was difficult was to keep everything clean and tidy, checking every small change for...

DIY vactrol – voltage-controlling a wien-bridge oscillator

Thanks to my subscribers who showed a lot of interest about my DIY vactrol project! For you I’m posting here the schematic for the circuit. It’s a pretty basic diode-stabilized wien-bridge design, utilizing half of a LM358 as the main op-amp, but it can be implemented with different ICs (that would be interesting to experiment with different ones and hear the differences). The two potentiometers that are supposed to move in sync using a dual potentiometer have been substituted with my homemade dual-vactrol (you’ll find the details in the video), enabling voltage-control on the frequency. Said frequency is also influenced by the capacitors marked C in the schematic, I used two 10n MLCC, changing their value will change the frequency range of the oscillator. I also added a parallel 10k resistor to one of the two ldr, to facilitate the initiation of the oscillation. The pull-down resistor on the negative input of the op-amp has been switched with a 10k potentiometer, enabling some control over the distortion of the sine-wave. There are some combinations of control-voltage and amount of distortion that will prevent the oscillation to start, once you decided your frequency range you’ll need to calibrate these parameters, using additional trimmers. I think I’ll dwelve more into these fine details soon. Thanks again for watching and reading, and feel free to get to me for any question, doubt or if you want to show me your...