Designing a classic transistor-VCA from scratch

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Working simulation on falstad: https://tinyurl.com/ygbk9l6f

In this double episode, I’ll walk you through the process of designing a classic transistor-based VCA (voltage controlled amplifier). The goal is to first de-mystify the bipolar junction transistor (BJT), its usage in a common emitter amplifier plus its two central flaws (variations between seemingly identical samples & temperature dependence), before then exploring the differential amplifier (also known as the long-tailed pair) and how we can get it under voltage control.

Inverting amp in-depth explanation:    • DIY SYNTH Quick Tip: Sawtooth-to-Tria...  
AC coupling in-depth explanation:    • Adding voltage controlled PWM to my VCO  

If you want to follow along, here’s a list of components:

2x BC548 NPN transistor (matched)
1x TL072 dual op amp (TL074 works too)
1x 100k potentiometer
2x 470k resistor
1x 200k resistor
3x 100k resistor
1x 33k resistor
3x 10k resistor
1x 1k resistor
1x 100 ohms resistor

Chapters:
00:00 Intro & Sound Demo
03:53 Voltage Dividers
09:22 Resistors vs. Transistors
14:12 Common Emitter Amplifier
19:50 Emitter Resistors & Negative Feedback
27:34 Gain Changing & Sketchy VCA
34:20 Diffamp/Long-Tailed Pair
39:11 Voltage Subtraction
43:41 Final Circuit
45:30 Sound Demo & Outro