Amp distortion problem

[john121]

Hi and thanks for reading.
I have the amp seen in the attached circuit diagram. The midrange and treble are reproduced fine but the amp distorts the bass.
Do you know how this problem may be addressed, what changes need to be made or components added / removed.
Q3 seems to be driven wrong. The lower output transistor has no bias. The top transistor is not receiving enough voltage (needs a bootstrap).
Can you please advise. This is a cheap and cheerful build, so am not intrested in a high performance amp, just want to get this one to work.
Thanks.

[Dave H]

The diodes are reversed. Q2 and Q3 should have emitter resistors (1 ohm?) to limit the idle current. Q1 collector should be connected to Q3 base and Q1 should have some way of setting the output to half rail voltage, perhaps add a resistor to Q1 emitter and a resistor from its base to ground?

[john121]

Thanks for the reply, much appreciated.

I also thought the diodes were reversed but the amp works. Would have thought it would not work if the diodes were in wrong way round, is this correct?

An idea i had which i want to ask you about: R2 forms nfb for dc and ac voltage. The value is selected when setting 1/2 VDC at the junction point of the emitters Q2 and Q3. If i remove D2 from its present location and move next to D1 (replace D2 with a jumper). Connect D1 and D2 in series, in the circuit diagram i included the diode directional arrows point upwards, if i change so the arrows point down and reduce R3 to 100-330 ohms.

What do you think.

[J M Fahey]

That is a TERRIBLE design, forget it, waste of time.
So many things wrong itīs actually easier to build a correct one.

Just curious, where did you get it from?
"Designer" has NO CLUE

Donīt waste time.

[john121]

I've built it now spent time and money, just want it to work. It's a simple design, can't take much more to make it stable.

I appreciate you do not like it, i am not after a high performance device just something to hook up to my laptop for a couple of extra watts.

Any advice is appreciated

[Dave H]

What do you think.

It would be simpler to use an LM386 where the bias and offset are already taken care of

This circuit is better than the one in the OP

[john121]

Thanks for the circuit. How many watts do you think it can put out.

Also is it possible a build a really simple amplifier with one output transistor, without a driver, have seen a few on the net wondering if you have tried or know any decent quality stable ones. Need about 15 watts rms. I have a preamp that can put out 3 volts so could use that and perhaps get away without the use of a driver.

Many thanks.

[Helmholtz]

There is no good solution with a single output transistor.

A push-pull stage as shown by Dave H can put out up to around 2W into an 8R speaker when powered from 12V, a 4R speaker will about double available output power.

[vintagekiki]

Need about 15 watts rms.

Why do it in a discrete technique, when there is an IC of that power
https://www.st.com/resource/en/datasheet/cd00000128.pdf

https://datasheetspdf.com/pdf/1349444/HOTCHIP/TDA2030/1

https://www.electronicsforu.com/electronics-projects/hardware-diy/14-watt-hifi-audio-amplifier-using-tda2030a

[Helmholtz]

These analog power ICs won't put out more than the discrete solution when powered from a single 12V supply.

If you need more than 2W@8R, the supply voltage must be higher. Doubling voltage will give 4 times power.

Alternatively power can be doubled by using 2 output stages in bridge configuration.

Or go for a switched mode solution.

[Helmholtz]

For any linear SS PP amp, max. achievable output power can be calculated from the supply voltage(s):

The supply voltage (difference between supply rails) defines the max. available output voltages swing. This is the max. peak-to-peak output voltage Vpp.
For power calculation we need the rms voltage Vrms = Vpp/2.83. It follows Pmax = Vēpp/8R, where R is the speaker impedance.

Example:
Supply voltage:12V = Vpp. => Pmax = 144/8*8 = 2.25W for R = 8 Ohm.

Actual output power is always a little lower because of voltage drops across emitter resistors and transistors as well as power supply sag.

[vintagekiki]

Vintage Motorola application notes with complete calculate SS amps and power supply 15 - 100W

https://conradhoffman.com/papers_lib/AN483B.pdf

https://conradhoffman.com/papers_lib/AN485.pdf

[J M Fahey]

I've built it now spent time and money, just want it to work. It's a simple design, can't take much more to make it stable.

I appreciate you do not like it, i am not after a high performance device just something to hook up to my laptop for a couple of extra watts.

Any advice is appreciated

But it will NOT work.

Itīs not a point of being "stable" , and even less of my "liking" or not, it-does-not-work.
Circuit is WRONG. Period.

Bottom transistor is not biased, does not pass current, same as if it were not there.

Top transistor is barely biased, works like a TERRIBLE single transistor Class A amp, it may put out a few milli-watts, what you hear now.

You wasted time and money.
The output pair is an excellent and expensive output pair capable of putting out some 80W RMS ........ in the proper circuit, of course.

Completely wasted in a single transistor 20mW amplifier.

Not suggesting high performance, but actual plain working ... this one does not.

ANY 12V "car radio" chip amp will work better than this with your supply.

EDIT: a few suggestions to recycle some of what you have:

1) "15W" is fantasy, but might provide 1 or 2 clean Watts with a 12V supply.
C1 polarity must be reversed.
Notice speaker goes from output capacitor to POSITIVE rail, NOT to ground

[Pixel]

That is a TERRIBLE design, forget it, waste of time.
So many things wrong itīs actually easier to build a correct one.

Just curious, where did you get it from?
"Designer" has NO CLUE

Donīt waste time.

It looks like this fine circuit came from a youtuber called Share Tech Creative. I only watched a minute or so of the video but it was enough to see that this youtuber doesn't know how to use heatsink compound.

[DrGonz78]

Sure was creative though...