Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual
Page 21
Instead of adding the bias at the base, we can just subtract it from the voltage
below the emitter resistor
. Because remember: the only thing that matters for the
transistor’s operation is the voltage di
ff
erence between base and emitter. And by pulling
the emitter resistor’s south end below ground (so below 0 V), we can expect the voltage
at the emitter to drop proportionally, too. This will increase the di
ff
erence between base
and emitter voltage – which in turn should push up our amplifier’s gain. To try this out,
we’ll first set up a potentiometer as a variable voltage divider. This will be our control
voltage source. Then, we’ll invert and scale down that control voltage with an op amp.
By turning the potentiometer, we can now adjust the voltage at the op amp’s output to
any value between 0 V and -7 V. Going any more negative would increase the collector
current too much and thereby starve our transistor – that’s why I’ve chosen this range
here.
Lastly, we’ll need to scale our input signal down to the appropriate levels. Since the
intention here is to pair this circuit with eurorack-compatible oscillators (which push out a
10 V peak-to-peak signal), I’m using a 100k/100 ohms voltage divider for this purpose.
This way, we’re e
ff
ectively feeding our transistor a 10 mV peak-to-peak input. Be aware
that this is an active circuit –
so you’ll need to hook your breadboard up to either a dual
power supply or two 9 V batteries.
Also, make sure that the TL072-chip (which
houses two standard op amps) is set up exactly as shown here –
if you reverse the
power connections, it will heat up and die!
If you need help finding the components in
your kit, check the components & concepts appendix.
Read more about op amps in the components & concepts appendix (page 39/40).
If you want to use batteries, you can find a quick setup-guide in the components & concepts
appendix (page 32). Keep in mind that even though I specify +/-12 V as our supply voltages in
these schematics, running the circuits with +/-9 V will still largely work the same way.
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