Want to build a SS PA

[lowell]

Enzo,
How many watts is this SWR? What model amp is it from or is that confidential? I want to build a ss PA, just a PA, no preamp. Would you figure copying this would be a good place to start?

http://music-electronics-forum.com/a...r-pa-later.pdf

[Enzo]

Nothing confidential about it, it is the powr amp circuit SWR used in numerous models. Read the text notes around the page. Apparently as drawn with 75v rails, this was for the SM900 and Stereo 800. Note it lists some other models with 60v rails. It lists alternative part values when making the change. It mentions certain models will only have one pair of outputs instead of the two pairs drawn. Since power supply current is not specified, I have no idea what power it puts out. with 75v rails, it could possibly put 700 watts into 4 ohms, but I doubt it. I think that voltage is there for peaks, not continuos power. There are too few output devices in my view.

It is a good power amp, it works. If I were going to build one, I might go with something robust like a Peavey 400BH or something. SImple enough circuit and parts available. If you just want to build one, this is as good as any I suppose. Note there are no low voltage circuits like op amps, so this ought to run on most any voltage, +/-35 even. I bring that up because you could probably take the power supply from some dead consumer stereo receiver - free power transformer and stuff that way.

[lowell]

That looks complex. Anything more bare bones and around 60-100 watts?

[R.G.]

I very much like the idea of going modular. The LM3886 has proven to be a dependable workhorse with good sound quality. It's only 50-65W depending on the power supply and loading. However, it is a single power IC, needs only half a dozen external parts to work, and has simple power supply needs. It's self protecting against thermal overloads and shorts. They cost about $4 each, and the power supply can be made with a $10 transformer if you're careful. Total for a single 50-65W channel can be kept under $35 with some care. $0.60 per watt of finished custom power amp is a real bargain. Enzo's comment about power supplies is a good one. The power supply of a power amp is often the last thing an amateur thinks of, but it's where most of the money goes. The LM3886 likes a +/-32V power supply.

I think of the LM3886 as a one-amplifier-per-speaker setup. You really can make one of these be about 1/4 the size of a shoebox, including power supply and heat sink, smaller if you're careful or creative about the mechanical setup. So if your PA had to be 200w, use four speaker boxes, each containing the power amp. If your PA needed to be 400W, use eight speaker boxes, each containing its own power amp.

The LM3886 can be set up in parallel-bridged to get 200Wrms out of four of them for a single channel. That's electronically trickier to get running, but not terribly difficult in the overall scheme of things. You could go modular at 200W/channel and play arenas if you could afford the speakers.

[J M Fahey]

Agree. I also like modular, specially for the flexibility (you stack up as many as you need, from 1 to 100) and reliability (1 in 8 goes dead in the middle of a show, nobody even notices it).
You can build simple flat black painted plywood boxes, each housing a good efficient inexpensive guitar type 12" (Eminence Legend or Jensen MOD) plus 3 piezos, and mount in its back an LM3886 kit amp , with two parallel input jacks for audio daisy chaining and 2 IEC connectors (male and female) to daisy chain line power, or each with its own power cable which you plug into a multi outlet power strip nearby.
You stack as many as you want and can easily build up scary loud PA systems.
Did I mention this is easy to carry in any car?
Worst case you'll neeed to make a couple extra trips or ask for some friend's help, but you are not *forced* to use vans or trucks, plus heavy guys to carry the stuff.
If you want more bass punch you can add a few 15" woofers, powered the same way, or by bridged LM3886.
You can also build powered slanted monitors.
You can start with only 2 powered boxes, and add more whenever you have some extra $ free.

[R.G.]

http://www.generalguitargadgets.com/...lm3886_amp.pdf
LM3886 - High-Performance 68W Audio Power Amplifier with Mute
http://www.national.com/ds/LM/LM3886.pdf
http://www.national.com/an/AN/AN-1192.pdf

[R.G.]

The lowest-misery way might be this:

The Apex Junior Subwoofer Amp 250-350W for $150, fully assembled.

[J M Fahey]

Looks good and surely beats breathing solder fumes, drilling, etc.
Remember that you'll still have to mod it slightly if you are interested in any frequency above, say, 150 or 200 Hz. []
Anyway, the watts_per_U$D ratio is very good.

[R.G.]

Looks good and surely beats breathing solder fumes, drilling, etc.
Remember that you'll still have to mod it slightly if you are interested in any frequency above, say, 150 or 200 Hz.

Yeah - that probably amounts to removing the active lowpass filter from the input to the power amp, just based on the schemos they show. Looks like there is a double Butterworth crossover and maybe an adder to get the bass from both channels of a stereo. The power amp is a dead-stock version of the Linn architecture discrete power amp. I don't see anything in the power amp itself that would make it less than full range. The lowpass is probably all in the opamps ahead of it.

My view of power amps is distorted anyway. I view power amps as primarily a power supply and heat sink, with some small "interface" of power amp circuitry to make the power come out the speaker jacks properly. If that power amp didn't work at all, but had a good heatsink, mounting panel, power supply and output transistors, it would be a good deal. To me anyway. I'd probably modify anything I bought, no matter what it did.

[J M Fahey]

I view power amps as primarily a power supply and heat sink, with some small "interface" of power amp

Couldn't agree more, brother.
I find the usual Power Amp discussions *SO* irrelevant, when compared to the real world, real problems.
The difference between vanishing distortion levels (measured in millivolts) pales when compared to the 4 or 5 volts of 100/120Hz ripple *modulating the audio signal* itself plus the "envelope modulation" caused by falling DC rail voltages when driving heavy loads.
I use minimalist Linn architecture, *but* I've spent endless hours developing low internal impedance transformers, which is an unexplored area.
As of this subwoofer amp, nice find.
Yes, it mixes left and right first, and follow it with , if i don't remember wrong, 5 pole lowpass filter.
It can be easily bypassed of course.
The power amp has a turn-input-off mute/protection, but I guess this must work reasonably well.
Good luck.

[olddawg]

Looks good and surely beats breathing solder fumes, drilling, etc.
Remember that you'll still have to mod it slightly if you are interested in any frequency above, say, 150 or 200 Hz. []
Anyway, the watts_per_U$D ratio is very good.

It looks like to me that you could simply remove the pre board and run a line level signal directly into the SG input on the main board.

[Enzo]

I can't argue with LM3886s or for that matter all manner of pre-existing comercial amps. Rare that one can really build something and save money.

My impression, right or wrong, considering some other threads, was that maybe Lowell wanted to build a discrete power amp as a learning exercise.

[lowell]

Enzo,
You weren't far off at all to think i wanted to do this as a learning excercise, as with all the theory help you've been giving me lately. However that being said I want this specific project to be simple, cheap, reliable... and 60-100 watts for a specific amp I'm prototyping.

[Enzo]

In that case then, include me with the others suggesting an LM3886 or some such.

[J M Fahey]

Agree. In that case, even if you use a commercial PCB (avoiding the messiest and least "electronic" part of the job), you'll still have to wire it, get/drill a chassis, mount power supply, all connectors, and 100 other details.
Good luck on your project.