multiple secondary question

[gary h]

Hi all,

I'm new at this, please bear with me.

I'm planning on putting 8 LM3886 chip amps in a chassis using the chipamp boards and design. (I'm building a pair of actively eq'ed speakers, each driver getting an amp. None of which shouldn't drop below 4 ohms.) I was hoping to power each of the 8 rectifier boards from one transformer. Is this OK? Do I just seek out a custom made 600 VA torroid with 8 pairs of 20V secondaries? If this is recommended, what are the pros and cons?

Most of the projects I have seen only require 1 or 2 pairs of secondaries for stereo or dual mono designs but I have read about transformers with multiple windings for different voltages.

Thanks,

gary

 

[tangmonster]

the cost effective way will be to use two bridge rectifiers(one for each voltage rail) get two 35A metal casing ones, they should be very cheap.

And then put all your filtering caps on the voltage rail in parralel , I used 4 x 10000uF caps Per voltage rail for my 5 channel amp.


But , If you want a different chipamp for each speaker i guess cost effectiveness is not or worry.

use a smaller diode bridge (more than 5A) on each voltage rail for each amplifier. ( 16 x bridge rectifiers) You will have to put filtering caps on each voltage rail. You will then need the custom toroidal with A LOT of windings.


If it was me i would use number one. the big toroidal , with 2 big bridge rectifiers , with 6 x 10000uF per voltage rail. and then smaller 100 - 220uF caps close to the pins on each chip.

But the filtering cap sizes are HIGHLY debated on these forums and a lot of people will post that you must only give each chip 1000uF per voltage rail for better mids and highs.

 

[tangmonster]

You could also split things in to or more.

Still not sure how your speaker connections are going to be but.

What about 2 or 3 toroidals.


Ex: 2 x 300VA toroidals.


Use one toroidal with nice big filter caps for the bass speakers

And the other toroidal with 1000uF caps for the mids and highs.



The transformer is going to be your most expensive item , get some prices and see what the price differences are on one 600VA vs 2 x 300VA or 3 x 200VA.

 

[gary h]

Tangmonster,

thanks for the ideas. I like the last one especially. The 3-way TMWW speaker design I am building (Seigfried Linkwitz's Orion) calls for independently eq'ed drivers. I believe the woofer pairs are wired in parallel but need more power, so each gets it's own amp. Because his crossover board divides the signal up into 6 discreet sources - high, mid, bass for each channel - it doesn't really matter which amps go where. I thought it would be practical to put them in one box but two might be more managable without being much more expensive. Just one extra chassis and two less expensive transformers, each needing fewer windings.

You do bring up another interesting subject; should I plan on using different caps for the bass amps? I believe the peerless drivers will not show much less than 4 ohms resistance but I won't know for sure until I get the plans.

Thanks again,

gary

 

[GeorgeBoles]

What I did

Gary,

I have the Orions too.

I put 4 LM3886 chip amps into each of two 2U rack cases (thereby making my 8 "channels".)

Each 2U rack case has one 300VA transformer with dual secondaries. Each secondary has its own bridge. Each bridge feeds one rail of the capacitor bank - one for the positive rail and one for the negative rail.

The only "strange" thing I did was have two capacitor banks - one which feeds the woofers, and the other which feeds the tweeter and midrange.

I have an 8m * 5.5m * 2.7 m room and I am quite convinced that you only need no more than 50 or 60 W per channel (Siegfrieds comments confirm that.) Using the information on the "spectrum" of "normal" music from Rod Elliott's site, I convinced myself that 300VA transformer is at least 3 times the maximum power likely to be required before the woofers bottom out ... which I think is the limiting feature to the volume, if I read Siegfried's site correctly.

Really, don't waste money on a big custom toroidal. After a while you will probably get itchy fingers and want to try some other amplifiers, e.g. some class A amps for the mid and tweeter, and you will be building again ... well that is what has happened to me, e.g. some Pass clones.

I have put 6 foot speaker cables between the LM3886 monoblocs and the speakers (7 foot would have been better), and made long interconnects, all of which has the potential to look cool, but I have lots more work to do - check my website and click on the picuture. My wife hates it at the moment

Hope this helps,
George

P.S. The active crossover (ASP) is in a 1U case which sits on the 2U case for the LM3886's. Best seen adjacent to the Right hand speaker. The same arrangement is just out of the picture for the Left hand speaker.