2x LM3886 In Parallel - How To Add DC Offset Adjustment

Yeah, I can get another transformer like this for cheap, so if it doesnt sound as loud as Id like it to, Ill pick up a bigger (and much more expensive) transformer.

I originally wanted two 3886's, but because of the four ohm load it wasnt practical, so I scaled down to a single chip and a smaller transformer, but I think I scaled a bit too far back

mneary said:

Yes, exactly.

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Awesome Ill try it this way when I receive the amp and Ill keep you posted.

The difference between 55W and 68W is about 1 decibel. Many people barely notice.

And then if I want it louder, the only way is to use four LM3886's, because its a four-ohm load, right?

It's my opinion if you want it noticeably louder you'll have to do that. When you build the four-LM3886 amplifier, you have other things to double check:

The speaker REALLY has to be good for 250W RMS.
Your heat sink REALLY has to be prepared to dissipate 200W.
Your transformer(s) REALLY have to provide +/- 28VDC at 400VA.

So far, we've been discussing a 68W (55W) amplifier, where it's much harder to get into trouble.

When you double the power by paralleling amplifiers and using a 4 ohm speaker instead of 8 ohms (the same as using two amplifiers and two 8 ohm speakers) then the sound is only slightly louder. You might not notice that it is only slightly louder.

10 times the power sounds twice as loud. Then you also might feel the sounds.

The subwoofer amp is unlikely to ever be reproducing a continuous sinewave. They are mainly for some pops booms rumbles etc infrequently, or maybe a doosh doosh doosh if you play dance music...

I would just use the little transformer, it will probably have plenty of oomph as it is and you are unlikely to ever be pushing that 55W continuous output level anyway...

Will it cause any lasting damage if the one transformer cant produce enough current?

ParkingLotLust said:

Will it cause any lasting damage if the one transformer cant produce enough current?

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It'll get warm if you run full power for a long time. I tend to agree with MrRB. This is really unlikely unless you listen to your favorite music with way too much red wine.

So basically I just need to keep an eye on the transformer, and if its getting too hot, add the second one. Got it!

Thanks for all the help guys.

Received the chipamp stuff the other day (thanks speakerguy79!) and I got to work tonight on the power supply board.

Ive modified it so that each group of four diodes forms a bridge, the + outputs of the bridges are tied together and tied to the +VE rail, the - outputs of the bridges are tied together and tied to the -VE rail. The bridges are tied together using red wire.

The two grounds are tied together, and connected to a black wire which goes to each secondary's center tap. I took a piece of heatshrink and cut a slit down it about half-way, and slid it over each black wire where it is soldered to the PCB, to insulate it from the diode's leg that will be directly above it.

Each pair of yellow wires goes to the outside two secondary wires of each transformer. Kind of confusing in words, so here are a few pictures. Please let me know if Ive missed anything or wired it improperly; when I do something, I tend to spend a lot of time looking back at it and overthinking whether its right or not. Thanks!

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Original (Small) PCB layout:
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My modified schematic:
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I see two wires that short everything.
One connects the + to ground and the other connects the - to ground.

The / on the schem I drew represents breaking the trace. The legs that would connect the two diodes to ground have been bent up, as you can see in the first picture.

Decided to toss some very low rated fuses inline with a transformer and try it, and it worked perfectly!

Wired the boards together and tossed on a heatsink to test it all out. The heatsink is much too small and will be replaced with active cooling later. But it works! Hooray!

Pictures:
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Now, if I wanted to move on to four chips, would the power supply be fine to re-use? Also, is there anything special I will need to keep in mind when hooking four together? If anyone has helpful links that would be great, Im going to head over to National's site and read through the app notes.

If using four separate amps and PSU's there's no need (and no point) to hook the PSU's together, the only common connection required is the zero-volts/chassis connecton.

I think the configuration was called bridged-parallel that would run the one speaker off four amps? I will have to look more into it, I just havent had a chance yet.

ParkingLotLust said:

I think the configuration was called bridged-parallel that would run the one speaker off four amps? I will have to look more into it, I just havent had a chance yet.

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Yes.
When two amplifiers are bridged then the output current is doubled.
Then two amplifiers are paralleled on each half of the bridge to handle the doubled current.
The result is nearly 4 times the output power.