15W or below power amp

[zener]

Does anyone know of any power amp circuit that could deliver 15w or less. I would use it to amplify my LM386  Little Gem. I already tried the TDA2003 (same with LM383) but it didn't work well, there was a pulsating noise.

The watt i stated is because I only have a 8" 8ohm 18w speaker.

Thanks for any help.

[Mark Hammer]

I'm not sure why you want to amplify one power amp with another.  Why not just feed the preamp stage directly to another power amp?

[zener]

I first had the idea in this thread. Peter helped me to come up with a lm386 preamp and tda2003 poweramp. http://www.diystompboxes.com/sboxforum/viewtopic.php?t=21120&highlight=  Unfortunantely, it didn't worked well and it was very noisy, unstable with pulsating noise[/url]

[Lonestarjohnny]

Zener, Some time's on ebay you can pick up a small Traynor S/S amp for 25 bucks or so, these have good tone and are easily repaired, I have one with the 6 in speaker and it'll drive a 4X12 cab with no problem,
JD
Formerly Known as John Davidson  :lol:  :lol:

[Peter Snowberg]

Hmmm... I wonder what wasn't working with your earlier design. Do you have the schematic on-line?

Since you described the noise as pulsating, I'm going to guess your amp is "motorboating". This is feedback where you don't want it. Tube amp designers have to be really careful about layout and topology of power supplies but pedal makers don't usually have the same circuit demands to deal with. Check you power supply filtering and where your grounds go. You want good filtering on each stage (look at tube amp schematics to see the multi-stage designs). It shouldn't be too hard to figure out which stage is causing the noise.

Mark,

Why use two amps? One reason could be for the 386 style distortion at low volume. I don't know.

Take care,
-Peter

[zener]

Hmmm... I wonder what wasn't working with your earlier design. Do you have the schematic on-line?

Since you described the noise as pulsating, I'm going to guess your amp is "motorboating". This is feedback where you don't want it. Tube amp designers have to be really careful about layout and topology of power supplies but pedal makers don't usually have the same circuit demands to deal with. Check you power supply filtering and where your grounds go. You want good filtering on each stage (look at tube amp schematics to see the multi-stage designs). It shouldn't be too hard to figure out which stage is causing the noise.

Motorboating... yeah that sounds more like it.

I just used the schem at the datasheet http://www.ben.cz/download/121098/tda2003.pdf

For the LM386, the Ruby amp and for the buffer, the LPB http://www.generalguitargadgets.com/v2/diagrams/lpb2_sc.gif

Using my breadboard, I hooked up everything as what Peter put in the diagram. I had the jfet buffer before and after the lm386. It was very noisy. So I tried removing and changing everything 'till I only have the lm386 and the tda2003 circuits. I can't really remember every detail since I did this a month ago. I can recall however, i get the "motorboat" whenever the lm386 is feed into the tda2003, whether there were buffer/s anywhere in the circuit.

When I hooked up just the jfet buffer in to the tda2003, it was too noisy, not usable at all. The lpb is just what the tda2003 is working well with.

I was using an unregulated power supply that reads exactly 12V although it was set to 9V(that's what unregulated supply gives, slightly higher that what is stated, right?).  I can remember that I put 2 1N4148 in series to the lm386 power pin to reduce the voltage going to the lm386. That voltage is what I used to power the buffers. I also tried powering the buffer/s with battery but nothing changed.

Another thing, the tda2003 circuit is working well if I plug the guitar straight into it. That seems to be telling me that the problem is not on the tda2003 but down below the circuit.

Anyway, Peter, you talked of filtering in every stage. Does that mean that I have to put an electro cap in every V+ connections of the buffers. There was a 100uf cap in the lm386 and also in the tda2003. The buffers don't have any.

I still have to get another IC and I'll ty everything over again. I'll get back here to tell what will happen.

Thanks a lot :wink:

[Peter Snowberg]

One thing you could try doing is to reverse the connectsions on pins 2 and 3 of the 386. If you do that, you may also have to reverse the buffer stage of the ruby. Some topologies are just more sensative to rejecting signals on the power supply lines. The topology used in the Ruby is inherently less prone to feedback but swapping pins 2 and 3 will change that.

with audio circuits that pull lots of power, you want to give each stage of the circuit its own RC filtered power supply. You usually don't want to use more than two inverting stages per power supply section to avoid feedback like you're getting.

Make sure you have big wires going to the 2003 and a big capacitor at the power input, then branching off from that you might try a small resistor then feeding a second big cap feeding the inverting buffer stage. Another small resistor with a big cap would then feed the 386 stage and finally you could use even another resistor and big cap to feed the input buffer stage with clean power.

If you look at lots of power amplifier designs, you see the power supplies start to get pretty big.

Take care,
-Peter

[sir_modulus]

WHOOOO....ur lucky, as I just bought two nat. Semiconductor reference books. I have examples of use, explanaions and specs for every National Semiconductor IC. My opinion if to go for twin LM 383 or 378.

LM's

LM386 N-1 = REally crap
LM386 N-3 = 1/2 to 1 Watt
LM390 = 1 Watt Bat. Operated Amp
LM388 = 1.5 Watts
LM380 = 2.5 Watts
LM377 = 2 x 2 Watts
LM384 = 5 Watts
LM383 = 7 Watts
LM378 = 2 x 4 Watts
LM12L = 150 Watts

Tda's (look elsewhere for these, there a bunch more)

TDA2002
TDA2003

Lots of people use TDA's so look around. I'll post them if I find them.