Half-Hour Project: Building a Simple Audio Amplifier

RE: Half-Hour Project: Building a Simple Audio Amplifier

ABCd.u272 — Sun, 30 Nov 2025 04:59:30 GMT

Can you please share the simulation of the circuit???

RE: Half-Hour Project: Building a Simple Audio Amplifier

Bugsi — Sat, 19 Jul 2025 18:02:01 GMT

There is no R7 feedback resistor. Did you mean R3?

RE: Half-Hour Project: Building a Simple Audio Amplifier

shabaz — Sat, 15 Mar 2025 18:47:27 GMT

A couple of units out in the wild:

RE: Half-Hour Project: Building a Simple Audio Amplifier

jc2048 — Sun, 09 Mar 2025 12:03:18 GMT

Nice small project.

Just for reference, this is the original from the Ferranti Applications Manual. The version in your book is a straightforward copy.

This is the spec they give. With the 25uF caps, the low end frequency performance isn't that good, but it didn't need to be for an intercom. The distortion didn't matter too much, either, provided you could readily understand what was being said.

The bootstrap is interesting and crops up in a couple of the other circuits in the manual (a 2W amplifier, and a preamp, where it lifts the input impedance). This is the preamp (they claim a measured 5M input impedance for that)

If you'd asked me to explain the bootstrapping (in your circuit), I'd have described it as turning the resistor (R5) into a better approximation to a current source (only for ac, obviously) - the cap lifts the voltage at the top of the resistor by almost the same amount as the bottom moves up, leading to the resistor maintaining the current as the voltage at the base of the output transistor goes up. That keeps things balanced against the transistor (Q1) collector which very much behaves like a current sink. Essentially, the same as what you're saying, just in slightly different terms.

RE: Half-Hour Project: Building a Simple Audio Amplifier

DAB — Sat, 08 Mar 2025 18:44:17 GMT

Great project, perfect for people who want to ease into electronics.

RE: Half-Hour Project: Building a Simple Audio Amplifier

Cliffyd43 — Fri, 07 Mar 2025 22:30:44 GMT

I love doing these small projects. I wish I still had my laser engraver, it worked great on PCB's

RE: Half-Hour Project: Building a Simple Audio Amplifier

Jan Cumps — Wed, 05 Mar 2025 18:48:41 GMT

> totem-pole transistors Q2 and Q3 operate in a push-pull arrangement

are they totem-pole when one is PNP and the other NPN? It's been a while (40+ years [emoticon:c4563cd7d5574777a71c318021cbbcc8]) since my electronics studies, but I seem to recall that complementary pair is push-pull and same type totem-pole ...

RE: Half-Hour Project: Building a Simple Audio Amplifier

genebren — Wed, 05 Mar 2025 15:46:55 GMT

A fun little circuit. Thanks for sharing!

I recently built a microphone preamp for a product for one of my clients. I ended up using a MAX9814 IC for the preamp, given that it has AGC with programmable attack/release and programmable gain. Works great, but I am not a big fan of the 14-Pin TDFN (3mm x 3mm) package (not too fun to solder). I might be fun to give your circuit a try on one of my prototype boards to see how it works.

RE: Half-Hour Project: Building a Simple Audio Amplifier

rscasny — Wed, 05 Mar 2025 14:31:00 GMT

This is great!