DIY Class A amplifier - Need some help. - Page 1
Forum section: electronics / projects

author: ShaunG01
author_rank: Newbie
timestamp: July 13, 2021, 03:15:34 pm
content: Hello,this is the first time I'm posting here, so if I'm breaking any rules, sorry about that!Anyways, the reason that I'm posting here is that after several attempts of troubleshooting, I have yet to find what is the problem on a project that I was working on.The project that I am working on is basically a Class A audio Amplifier that is based on the project seen in the 2008 release of "Everyday Practical Electronics" magazine, by Leo Simpson & Peter Smith, where I used the same exact circuit design to create my own PCB with SMD components.(Link to a PDF:http://www.handsontec.com/ezine_category/Audio/Class-Amplifier.pdf)Here is what it looks like.A general overview. (The big circle is the toroidal transformer.)Here is the circuit that I have got down from the pdf.The power supply circuit and the speaker protection circuit seemed to work as intended, but when I used the amplifier circuit standalone, all I got random pops and cracks.Here is the link to the EasyEDA project that I have been working on if it might be useful.https://oshwlab.com/shaungoda/20w-class-a-amp_copyAnd here is the BOM for the components I have used on the project.https://docs.google.com/spreadsheets/d/1x7JyUI8bSqp_fWkebT1AwnCGDFgJYKQwx4kK-Xuthec/edit?usp=sharingAfter all, I already have them made, but if you guys could give me some suggestions or fixes to make for the PCB design, I would gladly like to head that as well... I will try to fix it all on Rev.2.I was not so sure about component placement and component selection as well so if people have some suggestions about that, I would appreciate a comment on that too.I'm just a student who started learning EE recently so if I made some dumb mistakes, please go easy on me...haha.Thank you.

author: TimFox
author_rank: Super Contributor
timestamp: July 13, 2021, 03:31:27 pm
content: Can you be more specific about the incorrect behavior of the constructed amplifier?Is the DC voltage reading in the photograph the output of one side of your power supply, or the output of the amplifier with the bipolar power supply connected (with intact fuses)?If so, when you measured that voltage, what is connected to the input of the amplifier and what load is connected to the output, and have you looked at the output with an oscilloscope?

author: ShaunG01
author_rank: Newbie
timestamp: July 13, 2021, 04:08:06 pm
content: Hello, the readings on the multimeter represent the DC supply output. It shows that a correct voltage (20~22v) is being supplied to the amplifier itself.I have tested the amplifier with 1.5 ohms on F1 and F2, and nothing seems to be burning or excessively hot.Here are the readings of power supply output.Amplifier output with 1khz sine wave inputted.In better resolusion.Although there was a noticeable pop when I turned on the signal generator, the readings on the oscilloscope seem as if it's not doing anything.I tested both Left and Right channels but both of them seem to have the same problem.

author: TimFox
author_rank: Super Contributor
timestamp: July 13, 2021, 04:27:15 pm
content: Again, what load is connected to the output, and what (if anything) is connected to the input when you did the oscilloscope measurement? The waveform might be an oscillation, or garbage from a switch-mode power supply, since there is an obvious periodic component at roughly 66 kHz (15 us period).

author: ShaunG01
author_rank: Newbie
timestamp: July 13, 2021, 04:52:24 pm
content: The load on the amp is 8 ohm when I was measuring it, using the oscilloscope.The readings from the second and third images are obtained by inputting a 3V sine wave at 1khz while using an 8-ohm load, probing the output.However, the reading does not seem to be affected except for the occasional pop and cracks produced when turning on the signal generator.

author: vad
author_rank: Frequent Contributor
timestamp: July 13, 2021, 04:59:00 pm
content: What is the voltage across R21 and R22, when input is grounded?

author: TimFox
author_rank: Super Contributor
timestamp: July 13, 2021, 05:21:08 pm
content: What is the waveform at the collector of Q9.2, seen with a DC-coupled oscilloscope, when driven by your 1 kHz sine wave? Also as a sanity check, what is the waveform at the base of Q1?

author: ShaunG01
author_rank: Newbie
timestamp: July 13, 2021, 05:32:10 pm
content: Hello, I've read only 0.001V across both R21 and R22 on both Left and Right channels.The voltage at R21 and R22 (ref. at GND) was 20.7v for the right channel and 8.5v on the left for some reason.This may be indicative of a problem but I'm not so sure how to troubleshoot it.

author: vad
author_rank: Frequent Contributor
timestamp: July 13, 2021, 05:44:04 pm
content: Does adjusting 1K pot make any difference on the voltage across the resistors?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 13, 2021, 08:10:48 pm
content: The original article gives a variety of voltages that indicate the current through the branches. How do your voltage compare to those in the original article?

author: Gyro
author_rank: Super Contributor
timestamp: July 13, 2021, 08:37:21 pm
content: It hopefully goes without saying, but make sure you set the bias pot for lowest quiescent current and keep the resistors in place of the fuses at every step. If you suddenly fix the problem with the quiescent current set too high and no PSU current limiting, you will blow the output stage(s)!

author: ShaunG01
author_rank: Newbie
timestamp: July 13, 2021, 10:43:41 pm
content: These are some pictures of the scope.Base of Q1Collector of Q9.2At output

author: xavier60
author_rank: Super Contributor
timestamp: July 14, 2021, 11:51:08 am
content: The voltage across R13 can be used to calculate the VA bias current.

author: TurboTom
author_rank: Super Contributor
timestamp: July 14, 2021, 08:27:50 pm
content: C15 directly to ground may be a problem. Better put a small resistor in series (4.7~10 Ohms). Without the resistor, it may cause too much phase shift in the output stage which will result in positive feedback above a certain frequency -> oscillation.Moreover, the interconnection of Q9.1 / Q9.2 looks strange. It appears to be used as a darlington pair, but with Q9.1's collector wired to ground and no additional current limiting measures, there's a low impedance path from ground to the negative supply rail if the transistors are driven.

author: bson
author_rank: Unknown
timestamp: July 16, 2021, 12:14:09 pm
content: Are you sure Q7 and Q12 aren't swapped? Or are you aiming for an inverting output?

author: TurboTom
author_rank: Super Contributor
timestamp: July 16, 2021, 01:10:51 pm
content: Quote from: bson on July 16, 2021, 12:14:09 pmNo, these are actually correct. The configuration is called "Sziklai Pair" and they work more or less as a conventional darlington transistor with the polarity of the driver. They feature slightly better thermal stability and may have bandwidth advantages.