dual secondary surplus trafo - ok for 4 ch use?

[linuxworks]

found a 'bargain' at my local surplus store.

toroid has 2 CT secondaries: 30-0-30 and 22-0-22. I have no data on current ;(

the photo shows the large unknown toroid (that I want to use) next to a known avel brand unit that is 30-0-30 and 80VA (I bought it new and I know its an 80va unit).

given the size (the best data I have, sigh) - does it look likely that this toroid on the left will be enough to power a pair of stereo chip-amps? ie, 4 lm3886 boards.

second question: they are not the same voltage. is that going to be a major problem? I know 30 is quite on the high side for 8ohm and not very good at all for 4ohms. if I'm actively crossing each channel over (these would be for midrange and treble only) - would the high 30v just be too hot for this, even if a tweeter amp?

finally, the fact that 2 amps would be running at diff rail voltages - is that something to really care about, given that I'm going to adjust the gain of each to match the drivers ?

[AndrewT]

The primary has me confused.
It is 3 wire, implying a centre tapped winding. That is VERY unusual in a universal transformer.
It is written: A-0-120V I don't know what that is telling us.

Is this a 240Vac or 120Vac transformer?

Secondary capability.
What are the current ratings of the windings? Are the tail wire diameters the same?
Are the winding wire diameters the same?
If the secondary current ratings are the same, then expect ~115VA from the 30-0-30Vac winding and ~85VA from the 21.5-0-21.5Vac winding.

[Professor]

Hi! You can use ohmmeter? If black wire (A) not connected to anything maybe it shield winding. If R a-o = R o-120v it 2x120V winding.

[linuxworks]

measured readings: 5.6ohms DCR across blue and EITHER black OR white. black and white seem to be 0 ohms, afaict.

I will power it up today and if I'm still alive after that fact, I'll report what happened

[linuxworks]

Originally Posted by AndrewT

The primary has me confused.
It is 3 wire, implying a centre tapped winding. That is VERY unusual in a universal transformer.
It is written: A-0-120V I don't know what that is telling us.

Is this a 240Vac or 120Vac transformer?

it definitely and clearly says 60hz. are there 60hz 240v countries? most are 50hz aren't they?

there is no mention of 220 or 240 anywhere or dual 120's either.

just this wacky A-mode stuff

if it was potted and shielded, maybe it would be a shield wire. not the case here.

Secondary capability.
What are the current ratings of the windings? Are the tail wire diameters the same?

yes, all sec's are 22ga. wire. there is no current rating ;( (bugs me, highly, that they took trouble to include a paper slip inside the wrap with colors and voltages but no va rating at all, anywhere. extra ink costs that much does it?)

[linuxworks]

one other item that may help figure this out. maybe.

the unit looked new and unused - just old. no wires seem to have been used in a circuit and the black wire was cut very cleanly yet the other 2 prim wires were factory cut and tinned and twisted, very nicely. that makes LESS sense to me - as I'd expect the 0 and 120 wires to be tinned and that wacky a-mode stuff to be commented out

a-mode trafos. maybe its from another planet. it did come from a surplus store in milpitas (lol).

[AndrewT]

power it up through the bulb tester. Don't short out any of the tappings.
If the bulb lights,indicating a fault, then start measuring to find out what the voltages are.

[Arius]

Did you buy it from ACE? That's my favourite store.

The 30V winding is too high for LM3886. The resulting 42Vdc is at the absolute 42V limit of the chip. There's no margin.

So you're left with the 22V winding. The voltage is right but we don't know what its current rating is. Usually transformers for audio have the higher secondary as their main winding (i.e. most VA out of that) with the lower voltage winding capable only of low currents (e.g. for preamp circuit).

Antek Inc seems to carry the cheapest brand new toroids. I've used them in the past before. Decent prices, well spec, no complaints.

[linuxworks]

the large a-mode toroid came from 'excess solutions' (I think that was the name of the place in milpitas). you can tell the new customers as they are not carrying a flashlight with them (lol).

I guess the 30v windings are not usable by THIS chip. maybe another?

the 22v windings - you think those might be very low current? that would bum me out. doubly.

I did a quick power-on test and did get 30v from one and 22v from another. I tied up the black wire and just used blue and white. I'm still alive to tell about it, so no damage to the known universe was done. may never know what that 3rd lead is for.

reminds me of a steven wright joke:

In my house there's this light switch that doesn't do anything. Every so often I would flick it on and off just to check.
Yesterday, I got a call from a woman in Hackensack. She said, "Cut it out."

[Arius]

Based on your resistance measurements, using blue and white sounds right. Your live tests showed that it works, so I think you're set.

I have no clue what each secondary's current capabilities are - just mentioning what is typical for audio toroids. All is not lost in your case - you can test it out yourself. Apply a resistive load and see how much current it takes for the output ac voltage to fall by 5% from no load. You'll need some power resistors or you can use an active electronic load (rectify the voltage if you use the latter).

Since your application is for an active crossover (mid and treble), I wouldn't be too worried about power. Those drivers are usually way more efficient than the woofer. So you can be driving your woofers with 20W but find that your mid/tre units only take 2W. You just need to right voltage to ensure correct peak power is available, there is no need to sustain it.

I'm mucking around with an active speaker design. It's a 2-way - the bass unit gets driven by a bridged chipamp for ~ 60W. The tweeter amp is a regular chipamp set for ~ 20W (reduced gain to match speaker SPL). Inspired by Audioengine A5 but designed to be superior to it.