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kent 27th June 2006 08:17 AM

Biamped System with LM4780s

I'm just making my foray into the world of diy audio, and would welcome any tips and comments on my power amp idea. I'm familiar with basic electronics, mainly from a fair amount of work with microcontrollers, but not so much with audio per se.

I have a large pair of 3-way speakers for which the sound is rather lacking. I suspect the system is somewhat under-powered and could be vastly improved with a decent power amp rather than a stock Yamaha receiver. Although, the drivers are certainly nothing fancy, so I don't want to get too carried away.

For the drivers, each speaker has a 12" sub, a 5" mid, and a tweeter. I can only find specs on the mid - its 8ohm, 1" voice coil, 40W RMS, 80W Peak, 300hz - 8khz. The crossovers are...well, pretty much not existent. They consist of (gasp!) a capacitor to the mid and a different capacitor to the tweeter. That's it.

Now, here's what I'm thinking. Bi-amp each speaker with a LM4780, feeding an active cross-over into each amp in the LM4780 package. I'll keep about the same XO as for the cap currently used to the mid, with one amp powering the sub and the other powering the mid and the tweeter. I'll also put a passive XO between the mid and tweeter. I'll put the active XO and chipamp together on protoboard, install one PSU per speaker, and mount everything to the bottom of the cabinet. This way I'll avoid having to worry too much about cases, and will end up with all-encompassing active speakers.

I'm planning on basing my amp design on Brian Bell's LM4780 schematic, with maybe his snubberized power supply design as well.

I'm hoping the amping will do these drivers a lot of good - if not, I'll have to move on and get some new drivers to go along with the amps :).

General comments and tips are more them welcome, but I also have a few specific questions...

- Should I differ any components in particular considering one is driving bass and the other mid/high?

- Considering this isn't an uber-hifi system, is it going to make much difference if I use metal film resistors and polycarbonite caps vs carbon-film and ceramic?

- At my local electronics store, for transformers, it looks like I can get a 1 AMP 15 VAC, 2 AMP 18 vAC, or 1 through 5 AMP 24 VAC. Any recommendations?

- I think I should be alright with the passive crossover, but can anyone point me to good material on active crossover design?

Thanks in advance.

sangram 27th June 2006 11:43 AM

Your plan sounds good, except you have to figure out the active crossover correctly.

Rod Elliot has a good calculator for crossover frequencies for 24 and 12 dB/octave.

Transformers: I would pick at least a 5 amp, 24VAC but if budget is tight, then a 3A 24VAC.

Components: Use metal films and poly caps. The difference is not subtle, and can be heard on very low resolution systems too.

You should have some kind of a trimmer on the high section so you can adjust the relative level of the two sections.

At the bottom of the cabinet you'll have a lot of vibration. You might want to think about that as well.

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