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AB power amplifier design upgrades
AB power amplifier design upgrades
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Old 17th May 2019, 12:40 PM   #1
Fossilshark is offline Fossilshark
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Default AB power amplifier design upgrades

Hi! some members of this forum helped me design this amplifier a while ago and I had some boards made up for it. I was getting extreme distortion no matter how I turned the trimpots and while I do believe this malfunction is due to construction, I would like to omit the trimpots. Can I replace them with some auto-set circuitry? I know the feedback trimpot and maybe the input transistor bias can probably be replaced with a resistor but is there anything more stable? If that makes sense.

How can I make the VBE multiplier auto adjust?

I am open to changing supply voltage / going to +/- 24v

I am also including my board layouts in case I have any suspect layout issues causing ringing, any advice there is appreciated as i'm very new to PCB design.

I'm certainly not looking for a minimalistic design, just something I can assemble and play through. I also would like to build off of this circuit as much as possible.

all advice appreciated.
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Old 17th May 2019, 12:52 PM   #2
Mark Tillotson is offline Mark Tillotson
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R2 should be adjusted so that Vx is close to V+ / 2. You might then be able to replace it with a fixed value as it won't be too critical.
R5 should be adjusted for appropriate bias current through R8, the 1 ohm resistor, with no
input signal. I'd suggest 20--50mA as a possible bias setting (ie 20--50mV across R8). Its important that this can be readjusted in the future, bias is tricky to get just right.


No, auto biasing is not feasible in simple circuitry, you'd need a microcontroller to detect when there is no signal, correct the bias and store it...

R5 can be a 2k2 trimpot I think - 10k is going to make correct adjustment almost impossible to achieve unless its a 10-turn version.

Last edited by Mark Tillotson; 17th May 2019 at 12:54 PM.
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Old 17th May 2019, 02:16 PM   #3
gannaji is online now gannaji  India
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Have you followed the schematic exactly ? There is an error. The junction of C2 & R12 should not be connected to Q2 base. Check the drawings in your previous thread.
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Last edited by gannaji; 17th May 2019 at 02:20 PM.
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Old 17th May 2019, 05:00 PM   #4
Fossilshark is offline Fossilshark
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thank you gannaji, my boards are wrong! I will have to cut that trace and try again.
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Old 17th May 2019, 05:09 PM   #5
steveu is offline steveu  United States
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Quote:
Originally Posted by gannaji View Post
Have you followed the schematic exactly ? There is an error. The junction of C2 & R12 should not be connected to Q2 base. Check the drawings in your previous thread.
Yes! R12 connects to C2 only. And looking at the PCB, I see that it is indeed connected wrong. You will have to cut traces to correct the problem. Cut the traces from Q1 and Q2 to C2 and connect a jumper between Q1 and Q2.

Besides that,
1. TIP41/42 are poor choices for Q4 and Q5. Something like mpsa06, mpsa56, or even 2n3904 and 2n3906.
2. I would replace R2 with a 47K resistor.
3. Replace R12 and R13 with one resistor about 470 Ohms.
4. I would use about 1.5K for R15 and R6.
5. R5 needs about 1K in series to prevent accidentally over-biasing the output and burning Q6 and Q7.
6. C5 is placed poorly. It should go across the C-B pins of Q2, or, Q1-E to Q2-C.
7. Wind as much wire as you can around a 10 Ohm, 1 Watt resistor, solder the ends to the resistor leads and put it in series with C4, ie the output.

Good luck.

Last edited by steveu; 17th May 2019 at 05:16 PM.
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Old 17th May 2019, 05:58 PM   #6
steveu is offline steveu  United States
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Auto adjusting bias will not give as low as distortion as a carefully adjusted bias. I was going to suggest how to do that but it does not work well with a quasi-comp output because you can not cross couple the drivers as you can, had you used a TIP42 PNP instead of Q7.

Going to +/-24V would be pushing TIP41/42 so that it would probably overheat. Something like +/-15V would be reasonable. Attached is such a "autobias" circuit. This version omits output protection but that keeps it relatively simple.
Attached Images
File Type: jpg nobias.jpg (45.0 KB, 186 views)
Attached Files
File Type: asc nobiasdt1.asc (5.8 KB, 7 views)
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Old 18th May 2019, 04:43 PM   #7
Fossilshark is offline Fossilshark
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Steveu, you've given me exactly what I was looking for, I figured I would have to implement a LTP differential input. I only have some novice questions:

can I replace 3906/3904 with BD139/BD140? Do you recommend 390x for Q5 and Q2 because they have a higher Hfe than TIP4x?
I also have SS8050 and SS8055 that I bought thinking they would perform slightly better than 3904/3906?

Lastly, should I use passive values from your schematic exactly or just the parts that you modified?
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Old 18th May 2019, 07:48 PM   #8
steveu is offline steveu  United States
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2N3904/2n3906 are cheap and good performance for signal transistors under 40 Volts and 200mA. So you can waste as many as you like without any heartache. Normally I wouldn't use them for drivers but for amps under 30Volts, driver current will be under 100mA so "jellybean" transistors are fine.
BD139/BD140 are medium power transistors that are a lot more expensive and a bit slow. If you were making a huge amp then yes, they could be a good choice but slow transistors make feedback stability problems. BD139/140 are perfect drivers in large amps. They are used for cross-over bias VBE multipliers because they have a mounting hole and exposed thermal pad.
SS8050 would make a great driver transistor under 40V rail-rail, and the compliment SS8550 (not SS8055). Performance comes mostly from good design and not from using a marginally better transistor.
TIP41, TIP42 are older, quite slow power transistors, great for the output but not signal or drivers, although, a long time ago I used them as ~drivers in an amp where the final output transistors were huge, very slow TO-3's. ie the "protection" was from using massive transistors.
I suggest you download LTC spice and play with it for several weeks before you build anything. There is a wealth of knowledge to be learned from simulation, although I confess I spend years tinkering with real parts long before SPICE existed. With spice, you can predict currents and power dissipation, stability etc without wasting parts and money and much time. You can see for yourself the effect of tweaking part values.
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