Another transformer question

[kneadle]

Hi all,

First off, I know from searching that there are zillions of transformer threads, and if I missed therein the info I need, let me grovel in apologies from here to eternity.

I like the basic LM3875 chipamp as my main power, and for stereo applications driving fullrange speaker systems (meaning one amp per driver), I've been quite satisfied with the Avel Lindberg Y236750 (330/25+25) to power a single bridge and both amps. Quite satisfied.

Well, now I'm building a two-driver system, and I want to biamp them.

For the preamps I'm using the OPA 2132.

So that gives me a total of 2 preamps and 4 power amps (wait, is that right?).

The last time I built a preamp, I used a separate transformer with its own switch. Is that the way I should go? Or should I aim for a bigger transformer?

Well, I've got to double-check the design structure, but the question is going to remain the same: how many/how big of transformers?

Thanks,
Dave

[jaudio]

Why two preamp? How chip amp(lm3876) in total?

[Nigel Goodwin]

Power consumption of a preamp is very low (compared to a power amp), so consumption of the preamp isn't really a concern.

A common method is using voltage regulators from the poweramp supplies, or (as you've done previously) a seperate small transformer for the preamp.

Technically, the seperate transformer solution is the better one, don't forget the preamp only requires a small transformer.

[kneadle]

Hi Nigel,

Thanks. I think I'll go with the separate transformer.

Dave

[Nuuk]

Kneadle, can't you use the PSU that you used for your previous pre-amp?

[kneadle]

Oh, absolutely. That's the plan; I just didn't know whether to power it with its own transformer or with a larger tx or what-have-you.

I want the thing to power four channels smoothly. Isn't that the right concept? One pre-amp (2 x opa2132) and one amp (4 x lm3875) per driver for a four-driver system?

So I need a transformer (or more than one) to provide for the various power supplies. Eh?

Dave

[Nuuk]

The preamp PSU powers the pre-amp. I think I am correct in saying that it doesn't mater if you send the signal from the pre-amp to one or two power amps, it doesn't increase the PSU requirement of the pre-amp.

I am currently using one of my pre-amp PSU's to power both my pre-amp and the active crossovers. And the pre-amp feeds both the GC for the main drivers and the woofers amp at the same time.

[kneadle]

You know (and Nuuk, you know me too well, I'm afraid),

It seems I have a block for distinguishing terms.

It's the active crossovers I'm trying to power-up. I've been calling them, inaccurately, "preamps," but what I mean is "active crossover circuit."

One PSU should power the preamp for all four circuits; is this the right understanding? One transformer for this one preamp, and one transformer for each pair of GCs.

Like so:

Small tfo-->PSU-->preamp-->x-overL1, x-overH1, x-overL2, x-overH2

Big tfo-->PSU------------------->amp, amp, amp, amp

Or 2 big transformers and two PSUs, one per 2 amps.

My kingdom for a clear understanding!

Dave

edit: my little picture won't quite work right in primitive text format...

[Nuuk]

Yes, if I remember, you built your pre-amp PSU like the one I describe on DD. In that case it has plenty of spare power to provide an awful lot of opamps with their 'juice'. I assume that your crossovers are based on opamps!

So to recap, the PSU for the pre-amp also supplies the active crossovers and the larger PSU supplies the amps.

[kneadle]

Whew! I'm starting to get my mind around these things.

Yes, the OPA 2132, to be exact, is the active crossover circuits' heart and soul.

And, yes, the preamp PSU on DD is the prototype for this project.

Dave