Amp Project Fails - Page 1
Forum section: electronics / beginners

author: pivr
author_rank: Newbie
timestamp: August 28, 2025, 03:33:05 am
content: Hello, experts!First post here, so here goes....I'm pretty new at this and trying to teach myself electronics and making some progress, but when it comes to making a Class A and Class AB amplifier, I'm not having much luck and not really sure where to go from here.Attempt 1 - a very simple common emitter Class A headphone amp. I saw a video from the Great Scott YouTube channel and made the exact circuit. I built it on a breadboard and used an iPod for the audio source. It was awful. I have a tiny speaker that came with my parts kit and it sounds better/louder coming straight off the iPod. It's super distorted and after some google'ing I read that breadboards can give poor results, so I soldered it up on a project board without any improvement. I hooked my DIY signal generator to it and used a scope to see the results (image below), and it looks pretty "fuzzy".Here's the project page and schematic I use:https://www.instructables.com/Building-the-Simplest-Best-Sounding-Yet-Most-Ineff/https://content.instructables.com/FSE/1KLC/KKML1ZZ9/FSE1KLCKKML1ZZ9.pdfI also tried to mock up the circuit and the output waves look very deformed.https://everycircuit.com/circuit/5655774785306624Reading through the comment on that YouTube video I see feedback from people that successfully built the amp, so, not sure what I'm doing to mess things up.I figured I screwed something up, so I punted and tried to build a Class AB amp after watching some videos about them. I made my schematic using 3904/3906 transistors and did all of the calculations myself to bias the transistors and set the gain values. But it's also a fail.My Falstad simulation is here (just look at those sinewaves!):https://tinyurl.com/2cl8fsluSo, I'm at a loss. I'm not sure how to solve these "puzzles" and any help or suggestions would be greatly appreciated.Thanks in advance!

author: pqass
author_rank: Super Contributor
timestamp: August 28, 2025, 06:52:50 am
content: For everything audio (and more), I recommendsound-au.com.See project113for a headphone amp (project indexhere) or articles on amplificationhere.See W2AEW's videos on transistor circuits like #273 (class-A design), and also #114, #113, #67, #185.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 28, 2025, 10:09:22 am
content: Quote from: pivr on August 28, 2025, 03:33:05 amA single-transistor amplifier is not a good idea for a power amplifier. This is a voltage amplifier designed to work with loads larger than the collector resistor value, meaning in the tens of kilohms range. For it to work correctly, you should set half the supply voltage on the collector. The waveform is heavily distorted when the collector voltage is too close to the supply voltage or ground.The headphones have a resistance of several dozen to several hundred ohms and require a power amplifier. This power is small, but greater than what a voltage amplifier can provide.The noisy wave is due to interference from a low-quality switching power supply.The emitter resistor determines the gain; the lower the gain, the lower the distortion.It's easy to build a correct one-transistor amplifier. I showed the waveform on the collector without a capacitor so you could see the distance from the power supply.It won't be a hi-fi amplifier. For gain of 5, it reaches 1% THD, it's suitable for testing.

author: MariuszD
author_rank: Regular Contributor
timestamp: August 28, 2025, 10:15:12 am
content: With incorrect R3 distorted waveform look like this.

author: pivr
author_rank: Newbie
timestamp: August 29, 2025, 05:19:00 am
content: @pqass, thanks for the links! I'll have to spend some time going through those and find my next project. Nice to have some reference designs to compare values with, too.

author: pivr
author_rank: Newbie
timestamp: August 29, 2025, 05:39:20 am
content: Quote from: MariuszD on August 28, 2025, 10:09:22 amThat makes total sense. Is this the same as setting the Q-point?QuoteThat's great to know! I think it was $50 on Amazon, so yeah, you nailed it. I guess I now understand why the good ones cost more.QuoteAgain, great piece of info! I have been trying to learn about setting the values of R1-R4, but didn't know which one did what. Thanks for the tip.QuoteI figured that wouldn't be anything special, but wanted to start off with a simple amp and start making better variants as my understanding grows (hopefully). I really appreciate you taking the time to give me such a detailed response! It's super helpful.And if you know of any documents/videos/etc. that cover how to correctly calculate the R1-R4 values could you point me toward them? I must have used a bad example when I tried my own CE amp. I've seen several different ways to do it that all seem to come up with different results. Being very new at all of this, I'm not sure where to get reliable info.Thanks again!