Small power amplifier - Electronic Circuits Projects Diagrams Free

Dr.EM · 3rd June 2006, 09:43 AM

Although I had bought a TDA2030 chip, I realised quickly it wasn't going to fit in breadboards, or stripboard for that matter. So, I decided to make this circuit because I had the parts around:

http://www.mitedu.freeserve.co.uk/Ci...dio/2wamp.html

Well, it turned out to work quite well. I got reasonable volume and a decent frequency response, but I noticed it got exceedingly hot. I checked my test speaker and found it was actually 4 ohms and figured that was why. I might like to use this amplifier in an amplified test speaker design, but would like it to work with the 4 ohm speakers. Would this shown modification work? I don't have another set of BD139, BD140s to test anything with unfortunately.

3rd June 2006, 10:29 AM

Add a big,big heatsink.

**Nigel Goodwin** · 3rd June 2006, 10:48 AM

As far as I can remember, the TDA2030 is suitable for 4 ohms?, as suggested you should use a good sized heatsink - you should also check with a scope that it's not oscillating, layout is pretty critical on these chips - or they oscillate at high frequency.

I hope you're not trying to use a breadboard?, and if using veroboard make the layout as tight as you can, and remove all unused tracks and parts of tracks.

Dr.EM · 3rd June 2006, 10:58 AM

Yeah, I think the 2030 is suitable for 4 ohms, but its 5 pin to220 case won't fit into stripboards spacing

I thought that 2 watt circuit might be quite suitable as it has the advantage its current draw is low enough for wall wart powering. True, I probably could use a very large heatsink on it, but it would actually be cheaper to user another set of transistors on the output, so long as that would work?

audioguru · 3rd June 2006, 03:18 PM

The extremely simple transistor amplifier has only a 1.8k resistor turning on its PNP output transistors. So its negative-going output swing will have a reduced amplitude causing very high distortion and reduced power to the speaker. Bootstrapping the resistor would help but then the amplifier would probably oscillate.

Dr.EM · 3rd June 2006, 06:51 PM

I noticed that the negative swing clipped before the posistive one when I tested it with a sine wave generator. The output level was reasonable up to this point though, but are you saying that if it had to drive 2 PNPs, it would worsen the effect?

**Nigel Goodwin** · 3rd June 2006, 07:05 PM

Quote:

Originally Posted by Dr.EM

I noticed that the negative swing clipped before the posistive one when I tested it with a sine wave generator. The output level was reasonable up to this point though, but are you saying that if it had to drive 2 PNPs, it would worsen the effect?

Just bootstrap the output stage, I see no reason for that to make it oscillate (it's a standard technique used on most amplifers?). However, that is a pretty naff circuit, I would suggest improving it a little by adding the usual components that aren't fitted - such as a miller capacitor, and some HF negative feedback, and even a zobel network.

Dr.EM · 4th June 2006, 11:55 AM

Ok, i've heard of all those things, but am not sure on how any apply to this design. So far I concluded that the bootstrap would be a capacitor in parallel with the 470ohm trimpot? A zobel network is a resistor and capacitor in series to ground at the output? The miller capacitors are placed between C and B of the output transistors? For values, the bootstrap is in the order of 47uF? The zobel uses a 4 ohm resistor and a small value capacitor. Not sure on miller capacitors, i'm assuming they are quite small? The HF feedback i'm not sure how to implement.

**Nigel Goodwin** · 4th June 2006, 01:23 PM

Quote:

Originally Posted by Dr.EM

Ok, i've heard of all those things, but am not sure on how any apply to this design. So far I concluded that the bootstrap would be a capacitor in parallel with the 470ohm trimpot?

No, you split the bottom 1.8K into two series resistors (820 ohm should be OK), and connect a capacitor from their centre to the junction of the four 1 ohms (the output of the amplifier before the blocking capacitor). As it's a capacitor it's frequency dependent, between 47uF and 470uF is pretty common.

Quote:

A zobel network is a resistor and capacitor in series to ground at the output?

Yes, 3.9 ohm and 0.1uF in series should be fine.

Quote:

The miller capacitors are placed between C and B of the output transistors?

No, between C and B of the driver, the 2N3906, 15pF-47pF should be fine.

Quote:

The HF feedback i'm not sure how to implement.

Small capacitor in parallel with the 22K feedback resistor, the larger the value the greater the HF roll off - try a couple of hundred pF for a start.

I also never like the two diode bias arrangement, I'd use a Vbe multiplier instead.

Dr.EM · 4th June 2006, 01:30 PM

Thanks a lot, i'll try all of those out. I don't like the look of the diode arrangement either. In this case it seemed to work quite well though, I couldn't see crossover distortion even with the smallest amplitude sine waves. Mabye I just chose some very well matched diodes at random

**Nigel Goodwin** · 4th June 2006, 01:57 PM

Quote:

Originally Posted by Dr.EM

Thanks a lot, i'll try all of those out. I don't like the look of the diode arrangement either. In this case it seemed to work quite well though

Oh yes, it works - I just don't like it

Much prefer a Vbe multiplier!.

audioguru · 4th June 2006, 03:00 PM

Hi Dr. EM,
The diodes in your circuit don't have to be matched since they are in series and are in series with the pot that changes the idle current in the transistors and therefore how much crossover distortion.
The temperature change of the diodes match the temperature change of the output transistors so they should be clamped to the heatsink together.

Someone Electro · 5th June 2006, 01:08 PM

5 pin TO 220 can fit nicely in a breadboard. Just bend the pins out a bit. Use a breadboard as reference. I have some mosfet drivers in such a packege.This may be a bit harder if the pins are aranged in two rows.

Dr.EM · 5th June 2006, 03:00 PM

I'll have to try the chip amp out again, problem is it'll need a big heatsink, and that gets in the way with breadboard layouts.

I still haven't tried those modifications, i'll get and do that. If I ever make this on stripboard, i'll be sure to fix the diodes to the heatsink. I guessed the pot affected crossover distortion, but I couldn't see much anywhere except fully to one end when it suddenly became very distorted.

**Nigel Goodwin** · 5th June 2006, 03:08 PM

Quote:

Originally Posted by Dr.EM

I guessed the pot affected crossover distortion, but I couldn't see much anywhere except fully to one end when it suddenly became very distorted.

That's the whole point of it, you adjust the bias current until the distortion just vanishes - increasing it past that point won't have much effect on the sound, but will increase the chances of thermal runaway.

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3rd June 2006, 10:29 AM	(permalink (permalink))
Paul Obrien	Add a big,big heatsink.

3rd June 2006, 10:48 AM	(permalink (permalink))
Nigel Goodwin	As far as I can remember, the TDA2030 is suitable for 4 ohms?, as suggested you should use a good sized heatsink - you should also check with a scope that it's not oscillating, layout is pretty critical on these chips - or they oscillate at high frequency. I hope you're not trying to use a breadboard?, and if using veroboard make the layout as tight as you can, and remove all unused tracks and parts of tracks. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

3rd June 2006, 10:58 AM	(permalink (permalink))
Dr.EM	Yeah, I think the 2030 is suitable for 4 ohms, but its 5 pin to220 case won't fit into stripboards spacing I thought that 2 watt circuit might be quite suitable as it has the advantage its current draw is low enough for wall wart powering. True, I probably could use a very large heatsink on it, but it would actually be cheaper to user another set of transistors on the output, so long as that would work? __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

3rd June 2006, 03:18 PM	(permalink (permalink))
audioguru	The extremely simple transistor amplifier has only a 1.8k resistor turning on its PNP output transistors. So its negative-going output swing will have a reduced amplitude causing very high distortion and reduced power to the speaker. Bootstrapping the resistor would help but then the amplifier would probably oscillate. __________________ Uncle $crooge

3rd June 2006, 06:51 PM	(permalink (permalink))
Dr.EM	I noticed that the negative swing clipped before the posistive one when I tested it with a sine wave generator. The output level was reasonable up to this point though, but are you saying that if it had to drive 2 PNPs, it would worsen the effect? __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

4th June 2006, 11:55 AM	(permalink (permalink))
Dr.EM	Ok, i've heard of all those things, but am not sure on how any apply to this design. So far I concluded that the bootstrap would be a capacitor in parallel with the 470ohm trimpot? A zobel network is a resistor and capacitor in series to ground at the output? The miller capacitors are placed between C and B of the output transistors? For values, the bootstrap is in the order of 47uF? The zobel uses a 4 ohm resistor and a small value capacitor. Not sure on miller capacitors, i'm assuming they are quite small? The HF feedback i'm not sure how to implement. __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

4th June 2006, 01:30 PM	(permalink (permalink))
Dr.EM	Thanks a lot, i'll try all of those out. I don't like the look of the diode arrangement either. In this case it seemed to work quite well though, I couldn't see crossover distortion even with the smallest amplitude sine waves. Mabye I just chose some very well matched diodes at random __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...

4th June 2006, 03:00 PM	(permalink (permalink))
audioguru	Hi Dr. EM, The diodes in your circuit don't have to be matched since they are in series and are in series with the pot that changes the idle current in the transistors and therefore how much crossover distortion. The temperature change of the diodes match the temperature change of the output transistors so they should be clamped to the heatsink together. __________________ Uncle $crooge

5th June 2006, 01:08 PM	(permalink (permalink))
Someone Electro	5 pin TO 220 can fit nicely in a breadboard. Just bend the pins out a bit. Use a breadboard as reference. I have some mosfet drivers in such a packege.This may be a bit harder if the pins are aranged in two rows. __________________ Il give you shocking experience.

5th June 2006, 03:00 PM	(permalink (permalink))
Dr.EM	I'll have to try the chip amp out again, problem is it'll need a big heatsink, and that gets in the way with breadboard layouts. I still haven't tried those modifications, i'll get and do that. If I ever make this on stripboard, i'll be sure to fix the diodes to the heatsink. I guessed the pot affected crossover distortion, but I couldn't see much anywhere except fully to one end when it suddenly became very distorted. __________________ Angry!? I'm absolutely electrolytic! Will have to make do with myspace now I guess...