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Otherwise 6th July 2003 06:38 PM

Good&simple chip amp for 2-ohm load?
The project I have in mind is to use my old transmission line speaker cabinets with 8" subwoofers left over from my older things. On top of the TL's I would put Vifa's 3 inch full-range units, maybe in TL's too, but smaller of course. So it would be a "1"- way speaker system. The Vifa's would be driven with Rotel ra-931. Active x-over in between.

But the subwoofers still need some planning. They have twin voice coils, 4ohm each. I think i'm going to to wire 'em parallel, so it's two ohms per speaker. The question is, can for example LM3875 drive this low impedances? Or should I just wire the coils in series?

Also the power supplies should be made from the transformers I already have. They're 2 x 33,6 VAC something around 120-150VA, 1 x 33 VAC around 120-150VA too, and a big 1200VA which has 1 x 48, 1 x 42 and 1 x 36 VAC outputs.

All advices and thoughts are welcome :nod:

UrSv 6th July 2003 07:40 PM

Since the transformer best suitable for a GC is the dual secondary one I would go for LM3886 rather than LM3875 and regulate the supply down to safe levels and wire the coils in series to 8 Ohm. The 33.6 VAC is too high to use straight for the chip. To drive the 2 Ohms configuration would be OK if you parellelled 3 chips of the LM3886 falvour but unfortunately then you don't have any suitable transformer as 150 VA is too low IMHO.

daatkins 6th July 2003 08:37 PM

Using chip amps with a 2 ohm load won't be easy. The problem with chip amps is their reletivly low output current when compared with traditional discrete circuits. For example, an LM3875 has a guarenteed Imax of 4 amps so...

P=I^2/R = 32W.

The LM3886 is better with Imax = 7A and the meaty LM12CL has 13A. Paralleling devices will obviously give you more current capability.

When driving such low impedance loads, you need a low voltage, high current transformer to keep the dissipation of the amplifier down. The DC rails need to be just a little than the voltage needed to drive maximum current. Using the LM3886 as an example V=IR = 7*2 = 14v so a transformer that would give +/-16v to 18v would be okay, assuming the LM3886 device dissipation isn't exceeded.

Finally, as you're planning to drive subs using this amp, you need to take into account the real impedance of the driver over then range you intend to use it. You'll probably be driving the sub around it's resonant frequency so the impedance would be higher than 2R.

Nice one,

pinkmouse 7th July 2003 11:19 AM

The other option is to build 2x 4ohm capable GCs, and use one to drive each VC on the driver.

You might want to look at using the OPA549, as it can sink 10A peak, and has current limiting built in to make sure nothing goes bang:)

Otherwise 7th July 2003 03:20 PM

It might be that my project is going to change a lot (again :rolleyes: ) but if i did two LM3875s or 3886s to drive one sub (4 + 4ohm), would it be more reasonable to use the transformer with 2 x 33,6 vac = approx 2 x 45 vdc regulated OR the 1200VA one with 1 x 48 vac = 68 vdc and build the single supply thing?

Does the single supply application have more bad aspects than that it has a few more parts :scratch: ? The 1200VA transformer just "waits to be used" :yummy:

daatkins 7th July 2003 03:52 PM

Neither are really ideal, but both would work...

The 2x36.6v is only 150VA so your supply rails would drop when it's delivering loads of current. The 1x48v @ 1200VA would only give a single power rail so it will require a large output (DC block) capacitor to give good bass.

You could use the 1200VA xformer with a circuit to generate split rails but that would add a lot of complexity and cost.

I don't know why I didn't suggest using two amps to drive a DVC sub - I'm building that very thing at the moment!!

Nice one,

pinkmouse 7th July 2003 07:23 PM


Originally posted by daatkins

I don't know why I didn't suggest using two amps to drive a DVC sub - I'm building that very thing at the moment!!

I was planning to build a version of the matching sub for the JBL speakers I got that use LM3886. I will use the control circuitry they have, but with chip amps to drive each VC, as the real driver is just about impossible to obtain at the moment, and I can get an Audax DVC driver fairly easily.

The JBL Schema for the input side looks fairly flexible for use with any sub, it is called the LSR12P, and the schema is available here along with loads of other goodies.

The actual power amp circuit that JBL use has dual rails, I may build one one day, but not yet!

Otherwise 8th July 2003 08:21 PM

Ok. So I don't exactly know which amp I'm going to build, but it's going to work with single supply voltage :rolleyes: . Have I understood right, that if, for example, the datasheet says the minimum supply voltage is 20 volts [-10]+[+10] then with single supply it must be at least that 20v? Then with LM3875 it would be ok to use 60v (single)supply voltage?

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