audio amp study

[Herschel]

I am making a single supply audio amp according to this circuit. even though the amp is biased by two diodes, here i used 1n4148 and a 22R resistor, why is this 200k potentiomete for? is it to control the output of the amp? not sure...
also the website dosent say anything about it.

https://www.eleccircuit.com/50-watt-audio-amplifier-circuit-with-pcb-using-2n3055/

[MrAl]

I am making a single supply audio amp according to this circuit. even though the amp is biased by two diodes, here i used 1n4148 and a 22R resistor, why is this 200k potentiomete for? is it to control the output of the amp? not sure...
also the website dosent say anything about it.

https://www.eleccircuit.com/50-watt-audio-amplifier-circuit-with-pcb-using-2n3055/

Hi,

I did not look into this in depth but it looks like it could be for adjusting the crossover distortion.  That would control the quiescent current in the output transistors so that neither transistor could turn off completely even when the output signal polarity changes.

[Herschel]

how am i supposed to adjest it? using a multimeter. is it possible to do that with a multimeter?

[mariush]

Not gonna comment on that 200k resistor, but it would be much easier to just grab a couple TDA2050 amplifier chips, and make a stereo amplifier with them.

They're less than 60 cents on LCSC : https://www.lcsc.com/search?q=tda2050

You have example of single power supply circuit on page 6 in the datasheet : https://www.st.com/resource/en/datasheet/cd00000131.pdf

Or you could use a couple of them in bridge mode like in the TDA2030 example in this datasheet, on page 7 : https://www.lcsc.com/datasheet/lcsc_datasheet_2106070404_HGSEMI-TDA2030_C2683154.pdf

Or you could use 2 or 3 tda2050 amplifier chips, one for bass, one for mids and one for highs like in the tda2030 example on page 9 in the same datasheet above.dk52

[Zenith]

Usually, the maker/designer specifies a quiescent current, which is a trade off between low distortion and low power consumption. You should be able to set it with a multimeter using their instructions.

You might contact the website and ask what they recommend.

Otherwise, in the absence of other test equipment, set it by ear. Try to find the lowest quiescent current that sounds acceptable.

[magic]

VR1 provides (the only existing) DC feedback to Q2 base and thus sets the quiescent output voltage. Tweak VR1 so that Q6 collector sits at half the supply rail voltage with no signal playing and after the amp is warmed up for a few minutes (immediately after a cold start the output will be somewhat higher and slowly move down).

[Herschel]

Not gonna comment on that 200k resistor, but it would be much easier to just grab a couple TDA2050 amplifier chips, and make a stereo amplifier with them.

They're less than 60 cents on LCSC : https://www.lcsc.com/search?q=tda2050

You have example of single power supply circuit on page 6 in the datasheet : https://www.st.com/resource/en/datasheet/cd00000131.pdf

Or you could use a couple of them in bridge mode like in the TDA2030 example in this datasheet, on page 7 : https://www.lcsc.com/datasheet/lcsc_datasheet_2106070404_HGSEMI-TDA2030_C2683154.pdf

Or you could use 2 or 3 tda2050 amplifier chips, one for bass, one for mids and one for highs like in the tda2030 example on page 9 in the same datasheet above.dk52

honestly im fed up with tda2050s why? the single supply circuit in tda2050 datasheet creates too much distortion. actualy i own 10 tda2050 ic now 5 of them only work. also they can only give a max of 10 watts at the output without distortion and overheating
and my speaker are 4ohm 40 watt rms 12 inch woofer set

[dietert1]

Your discrete amplifier will also create distortion, as the bias current adjustment for the output stage is missing/wrong. The two diodes don't generate the necessary 2 V but about 1.4 V. Output bias current needs to be adjusted to 40 or 50 mA in order to get low distortion at low output levels.
Also the bandwidth limiting networks are missing, so the circuit will probably oscillate.
Another point is the gain set at 39 K / 100R = 390 times. At 30 V supply this means an input sensitivity of 9 Vrms / 390 = 23 mVrms. Very unusual.
Schematics found on the web aren't always complete for building something.

Regards, Dieter

[CaptDon]

That adjustment is not there to adjust the centerpoint voltage, it is there to set the bias of the output stage and is completely typical of this style circuit. The two series diodes are often placed on the heatsink near the output transistors for thermal compensation and stability. The adjustment will set the idle bias current, if to little current you hear very objectionable crossover distortion and very pronounced at lower listening levels. To much bias current and the output transistors will run excessively hot with possible thermal runaway.

[dietert1]

That adjustment is not there to adjust the centerpoint voltage, it is there to set the bias of the output stage and is completely typical of this style circuit. The two series diodes are often placed on the heatsink near the output transistors for thermal compensation and stability. The adjustment will set the idle bias current, if to little current you hear very objectionable crossover distortion and very pronounced at lower listening levels. To much bias current and the output transistors will run excessively hot with possible thermal runaway.

No, have another look. magic was right.

[Herschel]

Your discrete amplifier will also create distortion, as the bias current adjustment for the output stage is missing/wrong. The two diodes don't generate the necessary 2 V but about 1.4 V. Output bias current needs to be adjusted to 40 or 50 mA in order to get low distortion at low output levels.
Also the bandwidth limiting networks are missing, so the circuit will probably oscillate.
Another point is the gain set at 39 K / 100R = 390 times. At 30 V supply this means an input sensitivity of 9 Vrms / 390 = 23 mVrms. Very unusual.
Schematics found on the web aren't always complete for building something.

Regards, Dieter

actually i have replaced the two 1n4148s and 22R resistor with a single 1n4007 and a 2.2k pot " The two diodes don't generate the necessary 2 V but about 1.4 V" yeah, i had already noticed that so i did that modification. i had to wait for some time to get the amplifier a bit warm to do that

"Another point is the gain set at 39 K / 100R = 390 times. At 30 V supply this means an input sensitivity of 9 Vrms / 390 = 23 mVrms. Very unusual.
Schematics found on the web aren't always complete for building something." I only noticed it now. but i followed the second circuit where they are using 3.9k instead of 39k i noticed that first but i folowed the old circuit (2nd one)

[CaptDon]

Should the top end of that VR have gone to the junction of the 1K/3.9K above the capacitor to be a proper bias adjustment?

[CaptDon]

Don't you love how the input balance pot shows a variable short circuit to the input signal?

[Herschel]

That adjustment is not there to adjust the centerpoint voltage, it is there to set the bias of the output stage and is completely typical of this style circuit. The two series diodes are often placed on the heatsink near the output transistors for thermal compensation and stability. The adjustment will set the idle bias current, if to little current you hear very objectionable crossover distortion and very pronounced at lower listening levels. To much bias current and the output transistors will run excessively hot with possible thermal runaway.

i think its there to do the adjustment in centerpoint voltage, I found a similar circuit in youtube, where he is using a fixed resistor insted of a variable one. the biasing is alredy done by the 22R and 1n4148s which i replaced with a 2.2k pot and 1n4007 to adjust the bias viltage to 2.1volts to prevent the transistors from overheating...

[Herschel]

That adjustment is not there to adjust the centerpoint voltage, it is there to set the bias of the output stage and is completely typical of this style circuit. The two series diodes are often placed on the heatsink near the output transistors for thermal compensation and stability. The adjustment will set the idle bias current, if to little current you hear very objectionable crossover distortion and very pronounced at lower listening levels. To much bias current and the output transistors will run excessively hot with possible thermal runaway.

as you said, it does create a thermal runaway at some point but not too much dne by the in4148s

[CaptDon]

Sorry for my incorrect information. I glanced at the circuit and thought "Yes, that looks like a comp-sym circuit with a bias adjustment" and you folks are correct, it isn't. I looked at the website, many useful circuits and some that are really crude and half baked. The text is nearly incomprehensible.

[Herschel]

Sorry for my incorrect information. I glanced at the circuit and thought "Yes, that looks like a comp-sym circuit with a bias adjustment" and you folks are correct, it isn't. I looked at the website, many useful circuits and some that are really crude and half baked. The text is nearly incomprehensible.

i looked through the whole circuits in that page and this was the only circuit i found convincing. other circuits also maybe correct but they are too large for my project. i was sure that website, maybe that page was made by a person who don't know english fluently. its 100% google translated

[dietert1]

The DC adjustment appears to be gone.
The 10 Ohm resistor of the output snubber is missing and the 0.47 uF capacitor was meant to be 47 nF instead.
Don't forget the output capacitor when powering this up!

[MrAl]

Hello again,

The 200k pot may in fact adjust the output zero center point, but that looks like it's just a secondary effect.

As you adjust that pot and the output DC operating point changes, it also changes the current through the two diodes and 22 Ohm series part which changes the voltage drop across that little network, and also causes currents from Q3 to Q4 and from Q5 and Q6 (of the second drawing) to change.  The effect would be that the output DC center zero point would change, but more importantly the crossover distortion would get worse or better with the adjustment.

I say this for now mostly because the crossover distortion is a major concern in these amplifiers, whereas the DC center point is only of concern if you need to get the absolute maximum output voltage range and that would only be a concern at maximum volume, and probably still not that much of concern.

A question might be, is the innate output DC zero bias point so far off that it needs an adjustment, or is this just like any other typical bipolar transistor power amplifier that needs to reduce crossover distortion while keeping quiescent current low.
An in-depth analysis would tell us more about both of these functions, but my bet is on the cross over distortion issue, and that stands even if the designer thought the adjustment was going to be for the output DC zero center bias point.
Still, this amplifier does not have a complementary pair output, so that could present a DC zero center bias point issue.
To be sure, we'd have to do a complete analysis for both the DC zero center point bias AND the cross over distortion.  Then, see how the adjustment affects both.

What also shifts my opinion toward the crossover distortion issue (without doing an in-depth analysis) is that I always read about people complaining about crossover distortion, while I do not think I have ever read one post about someone complaining that their DC center bias point was way off.

[magic]

Center voltage is the primary effect and bias is secondary.

For reasonable operation the output has to be near 15V (on a 30V supply). This means 3mA through R7+R8, and you could tweak it from 2mA to 4mA by shifting the output between 10V and 20V, but why? If you want to play with bias, change the value of R13 or R8.

This output stage is not particularly great anyway. Common wisdom is, if you really must use this quasi-complementary configuration, at least add a Baxandall diode and make the emitter resistors more symmetric.

[Zenith]

It's a bit late since you are so far along with this, but there just have to be better described and worked out designs for similar amplifiers out there.

I found this without much difficulty.

https://www.sound-au.com/project3a.htm

The author does appear to know what he's talking about and gives set up instructions.

There must be loads of others.

[dietert1]

Another method to find a complete schematic is looking into the service manuals of vintage audio equipment, e.g. Denon, Sony and Technics. Or get one of those amplifiers at ebay.
I know that you will feel like the master of the universe when your DIY amplifier starts playing music. Good luck!

Regards, Dieter

[jasonRF]

It's a bit late since you are so far along with this, but there just have to be better described and worked out designs for similar amplifiers out there.

I found this without much difficulty.

https://www.sound-au.com/project3a.htm

The author does appear to know what he's talking about and gives set up instructions.

There must be loads of others.

Yes, that is a legit site.  Folks building that amp can also get help on diyaudio.com - search "ESP P3A" and you get a lot of hits.

jason

[themadhippy]

I found this without much difficulty.

https://www.sound-au.com/project3a.htm

But its a dual rail supply when the op clearly states

a single supply

[Zenith]

I found it without looking very hard. There must be loads of others including single supply, both commercial and amateur, all better worked out, documented and discussed than this one.