Post by keurslagerkurt on Apr 17, 2021 14:13:19 GMT
Been matching transistors by hand yesterday, as I need some for other builds (more on that soon!). I started from a transistor matching design by the legendary Ray Wilson for Music from Outer Space. However, I don't have a benchtop powersupply, so I wanted to adapt it from using a +-10V bipolar power supply to the 5V from AE.
The adaptation seemed to work fine, but the real verification will of course only follow later when I can test the pairs in a design. Thought I'd share the adaptation here if other ppl want to try their luck! It's best to also read the measuring instructions over at the MFOS website i linked above.
Here is the schematic for npn transistor matching, for pnp you can use almost the same, i marked the changes for pnp matching in GREEN. Made some scribbles in BLUE with voltages you should be able to measure to verify if you connected all well.
Here is the pretty compact breadboard build:
You can see I added some extra caps, these are decoupling caps of 0.1uF over the MCP power rails. They tend to stabilize the power supply voltage & workings of the MCP. You can check out the MFOS link above for some more info on that too!
What is the purpose of transistor matching? What happends when they are not matched ?
Most audio circuits do not need matched transistors, and the circuits that need them will state it very specifically on the schematic/build docs.
It's mainly needed in circuits where precision is very important. In audio this is specifically so for the 'tuning' parts of oscillators/VCOs. The standard is that every 1V CV corresponds to one octave (=V/Oct standard). So for this your circuit needs to be very precise, because the human ear is very, VERY sensitive to small detuning. Most VCO's with 1V/Oct have a similar transistor-based circuit to give this specific tuning response, and these always need matched transistors to give the right results.
Some circuits have transistors in pairs, sometimes even glued together, so that, whatever temperature-related change happens to one, the same but opposite change happens in the other so they don't drift in tuning so much with changes in temperature.