IC Amplifier Question

[fraser80]

Hello. I am getting into IC and Solid State amps and have completed many successful projects. Recently I have considered building a stereo desktop amp with built-in speakers using the simple LM386N-1. That is very easy, however, I would like to build in a mini sub-woofer as well, with a separate linear pot. Will I need a third LM386 chip to do this or can I make a sister board and bridge the woofer to each of the other two amps? Also, I was curious as to how I can filter out all the the high frequency signals to get a more pure bass sound from it. Any suggestions and/or instructions would be greatly appreciated! Thank You!!

[fraser80]

Oh yeah. And it will be powered via USB, so that is only +/- 5v

[thaumaturge]

Adding a capacitor across your feedback resistor (high side) will reduce high frequency. Otherwise series a choke on the output. Capacitive reactance (effective resistance) is figured by Xc= 1 / (2pi x Freq-in-megahertz x Capacitance-in-Farads) For an inductor it is
Xl= 2pi x Freq x inductance-in-Henries.

Basically a capacitor couples more signal the higher the frequency and an inductor couples more the lower the frequency. Putting a cap around your feedback resistor will couple more higher frequency back for comparison, thus reducing that component of output.

Sorry if it is confusing, but it is.
Doc

[firechief]

USB is good for 1/2 amp so that is not an option.

[GloBug]

Go through some trash and start harvesting transformers.
You may as well built a "real" amp while your at it.
It won't be any more complicated, and won't be much bigger aside for the transformer and heatsink.
Have fun.

[pacificblue]

Originally Posted by fraser80

Oh yeah. And it will be powered via USB, so that is only +/- 5v

USB is single supply, only +5V no -.

[!]

I don't mean to be judgemental but many aspects of the project don't make much sense.

USB 5V/500mA isn't very good at all for class A/B amps, and barely suitable for class D. I mean sure you will get sound but with little to no extra parts and minor expense for higher current chips, it is as GloBug wrote, just a matter of a different transformer.

But, supposing you want it portable for use on a laptop, I could see why to do it but think there isn't a good reason to put a sub in. Output power too low, just split the difference and use a little larger L & R channel drivers.

Linear pot? Why not a log pot? It seems like a lot of complexity and added time and size relative to the result of a LM386 amp. Main point is, within the limits of USB power you will have higher output including more current available for a sub (or L/R channel bass) by going class D.

[pacificblue]

That amount of power is really small for a subwoofer. On the other hand class D usually means surface mounted, which does not suit everybody, and you don't get that much more power out of it either. 500 mA are 500 mA, whether they pass through class B or D.

Talking about desktop speakers however, the subwoofer will not be so sub. Such a miniature woofer will probably play its lowest note at a higher frequency than the usual crossing point to satellites, so not much power is needed either.

To answer post #1, either way you will need a crossover.

When you use a separate amp for the woofer, the crossover will be before the amps. The best solution would of course be active, but that means you need several op amp channels that would also feed from the 5 V 500 mA supply and reduce the available output power accordingly.

A passive crossover before the amps could be an option. It is not as big, and doesn't need extra power. Drawbacks are that it is not easy to get more than 12 dB/octave steepness, and the channel separation will suffer a bit from creating the mono signal for the subwoofer. For small desktop speakers however that is probably an acceptable compromise.

You can use two smaller woofers instead of one or a double voice-coil woofer. Then you can use two separate amps and skip the mixing part.

For your other idea to bridge the two channels into the woofer, you need a passive crossover, i.e. big inductors and capacitors between amp and speakers. Then you need to invert the signal of one of the channels to get the inverted signal for one leg of the woofer, and then invert the polarity of the corresponding satellite, so that it is in phase with the other again.

Or again, use two smaller woofers or double voice-coil instead of bridging one.

A third approach would be to use a band-pass woofer and skip the crossover. That requires 8 Ohm speakers, because you just connect the woofer in parallel to the satellites. The satellites work unfiltered and contribute what bass they can. The woofer is low-pass filtered by the band-pass cabinet. It also require to use two woofers or a double voice-coil model. And the band-pass cabinet will be slightly bigger than a closed or ported box.

[sofaspud]

The USB should be capable of powering 4 LM386's, so the easiest solution may be to passively high-pass filter the satellites (via the output capacitor, for instance), and use the bass boost circuitry found in the LM386 datasheet for dual woofers instead of just one. There's probably milliamps to spare for an active crossover, but it hardly seems worth it here, as this is a very basic, casual-listening, near-field system.

[suntechnik]

Originally Posted by fraser80

I am getting into IC and Solid State amps and have completed many successful projects.

I can highly recommend start using dual polarity power supplies Vee, Vcc. There is a serious background behind the matter.

Consider ordering a trafo and think about LM3886 for satellites and LME49830 for a sub. That would bring you to another level of sound reproduction since you have accomplished several simpler projects already.

Computer PSUs are no good for audio actually.