Amplifier improvement advice please

Thread Starter

john1211

Joined Sep 17, 2018
10
Hi And thanks for reading.

I have just put together my very first amp and would love some feedback on the circuit. The sound quality is disappointing, works ok, is there anything you can think of to improve performance. Thanks!
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Ian0

Joined Aug 7, 2020
3,252
There's probably only half a dozen stages of improvement between this basic circuit and a real hi-fi amplifier with distortion <0.001% , you could jump immediately to the last stage, but you would be better improving it a stage at a time and understanding how each change results in an improvement.
would love some feedback on the circuit.
Yes - the circuit would love some feedback!
I'd start by moving the top of R2 to the junction of Q2/Q3 emitters. That would give overall feedback instead of just having feedback around the first stage.
I would also split R3 into two 330Ω resistors, and connect a capacitor between the junction of the two 330Ω resistors and Q2/Q3 emitters. The capacitor keeps the voltage across the lower 330Ω resistor constant, so keeps the current through it constant. That increases the gain available from Q1.
I'd suggest you read John Linsley-Hoods books "The Art of LInear Electronics" and "Audio Electronics" to get the basics, and when you've understood what he has to say, Doug Self's "Audio Power Amplifier design".
 
Last edited:

Dave Lowther

Joined Sep 8, 2016
101
I have just put together my very first amp
Did you design it yourself?
Do you have any way to measure THD? I use a software package called "Multi Instrument" which works with a PC's soundcard. It can act as both a signal generator and oscilloscope. It's not free but it's free for the first three weeks.
I found it instructive to look at the THD before / after modifications in basic amplifiers like the schematic you posted.
I'd suggest you read John Linsley-Hoods books "The Art of LInear Electronics" and "Audio Electronics" to get the basics, and when you've understood what he has to say, Doug Self's "Audio Power Amplifier design".
I read them in the opposite order. First I read Doug Self's book, then a while later I read the JLH books. I got most out of reading DS's book. I also read Bob Cordell's "Designing Audio Power Amplifiers" which has an interesting section on the steps taken to improve a basic amplifier.
 

Ian0

Joined Aug 7, 2020
3,252
Fair enough - they're all good books. I'll post these 5 pages from Linsley-Hood which show the basic "discrete op-amp" around which power amplifiers are designed, and how it is improved by adding various things.
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Once you have got better than about 5% THD you won't be able to see it on a 'scope, so you will need a way to measure it.
 

Ian0

Joined Aug 7, 2020
3,252
The 2SC5200/2SA1943 is a very popular output stage, they are of the "hollow emitter" design with quite a lot of gain, but the fakes outnumber the real ones. Sometimes the fakes are the wrong polarity. If the amplifier performance is poor, that may be the cause.
@Audioguru again I think you meant "bootstrapping R2" (not R3)
 

Thread Starter

john1211

Joined Sep 17, 2018
10
Thanks for the replies, much appreciated.

Can you please write on the circuit diagram i enclosed, the component values you feel are correct and their location. I want to download the diagram and have it before me as i work on the amp. I am an amateur and want to use this amplifier project to learn and experiment from.
This is the first amplifier i have ever built, I am a total noob who gets confused.

Sorry, i now know it is a bad design thanks to your very helpful feedback, in my inexperience I trusted the source, very foolishly so.
https://hackaday.io/project/173235-2sa1943-silicon-pnp-power-transistor-utsource

I changed the orentation of the diodes and it helped improve the sound quality.

The aspect of the sound quality i find disappointing is the lack of detail, distortion as well.

Any help you can give is greatly appreciated.
 

Martin_R

Joined Aug 28, 2019
101
Don't forget to split R2 in half and connect a bootstrap capacitor of approx 470uF between the output transistors emitters and the mid point of R2 as in post #2.
Also the diodes need to be in thermal contact with the output transistors, as they form an important part of thermal tracking of the output transistors.
 
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