seperate transformers

Yoda

Member
2001-07-30 1:35 am
I am building a 6 channel amp for my MTM biamped speakers (one amp per driver). I have researched all the possibilities for power transformers (toroidal) and realized that it would be much more economical to buy one very large transformer (1000-1500VA) and just run all the amps off of one. My power requirements are relativly low, 20w of class A /channel that consume about 100W. I want to know if I can run multiple amps off of one transformer, or if 6 smaller discrete transformers would sound better, or maybe 3 or 2 or any other combo. Thank you for responding!
 
You could use two seperate transformers, one for each channel. Each transformer would be hooked up to three bridge rectifiers with and a set of capacitors for each supply.

But considering you are driving an MTM speaker four amplifiers should do, as both bass speakers in each channel carry the same information. Just build the bass amp with a bigger output section to handle the lower impedence of two drivers in parallel.

Jam
 

djk

R.I.P
2001-02-04 4:23 am
USA
One transformer would be fine.Most torroids have dual secondaries rather than a center tap.Use two bridge rectifiers per channel, one for the + supply and one for the - supply and you will have excellent isolation. Rectifiers are cheap and the one large transformer will have better regulation as well as being cheaper than several smaller transformers.This is the same technique used by ATI on their 5 channel amps.
 
I think that the biggest problem here is that most Class-A amps have a fairly poor PSRR. Also, many designs (push-pull and single-ended/resistor source) tend to put lots of audio signal into the power supply. By using a single common transformer, and a common set of filter caps, you might end with a channel seperation of <40db.

The suggestion DJK gave for a single transformer, but seperate rectifiers and filter caps for each channel is a compromise. It will be much better than a common filter cap bank, but not as good as individual transformers. Using a PI filter in the filter bank (see ZEN Revisited at 'http://www.passlabs.com/projects/znret_2.htm') would clean it up to the point where the channel seperation would be as good as individual transformers, and the rejection of hum and line noise would be better.
 
2 transformers with 3 sets of rectifiers and caps each sounds good. I'd still use a PI filter in the tweeter power supply though. A 2mH air core inductor wound from 14 gauge wire only costs $15.20 from Parts Express (18 gauge is only $5.90, but I wouldn't go that small). It should help a lot.

Also, the transformers should be fed off of seperate power filters. You should be able to find some good, hefty ones in a surplus shop. The input to the power filters can be connected together inside the chassis. An MOV and a 0.01uF/1KV bypass cap at that point would be useful.

Good luck.