transformer sizing and parts

I'm looking to appropriately size and wind a transformer to swap speakers in my car. Its a project to keep the factory amp and see what kind of performance I can extract from it as a challenge instead of just dropping in a nice aftermarket piece.

I need a core that can handle two primaries feeding one secondary with each a ratio of 2:1 and handle transmission of about 40W peak during transients.

The two primaries are to make use of the OEM amp that is designed to drive a dual-voice coil OEM sub and then hook the single secondary to an 8 ohm sub.

So how do I size the core (and find a source for it that's pretty cheap) and then figure how many turns of what minimum gauge wire I'll need to do this without over saturating the core? Bandwidth is maybe 30Hz-100Hz so that should keep the inductance within reason but I have no idea what numbers and sources to use.

Thanks in advance.

This doesn't sound a very good idea to me?.

As it's for a sub-woofer you're going to be looking at a very large transformer (and could well still limit your bass), and I'm not sure how well it would work because the two primary windings may well affect each other?.

I suspect it might possibly be easier to modify the amplifer, a lot depends on how it's designed - if it uses two bridged amplifiers it would be difficult. But if it uses two single ended amplifiers you could bridge them to give one 8 ohm output.

Not really, the OEM amp is some SMT circuit board all IC output devices. Not hardly a discrete device to be found. And already bridged on each output to try to compensate for not have a SMPS but instead using the 12V auto environment as the output rails.

Since music typically has a 10db+ crest factor I was hoping to try to sneak under the typical design criteria, like a 4VA capable transformer that wouldn't saturate too bad at the 40VA level. But that is my wild guess, its been too long for me to even have foggy memories of how EM theory is suppossed to work or things like that.

What is the typical transformer design process? Start with freq and VA? Is each part factored as an inductor and then somehow those are mutually coupled?

Thanks for tolerating the newbie xformer questions, there isn't as much online as I expected, most sites have a turns ratio explanation and that is where they trail off....

The trafo not prevent the AC current flow between two output stages: the stronger win, the another give up.

That's why I'm trying to describe each bridged output gets its own winding.

So the outputs stay electrically isolated, but are magnetically coupled by sharing a common core with the secondary connected to the load.

Audio transformer design is a real art, for such low frequencies you would need a large transformer - it's pointless attempting to use one ten times too small.

I'm also puzzled about the claimed output? - you suggested 40W peak?, if it has two bridged outputs they should each provide an honest 16W RMS into 4 ohms, this would give 64W peak.

40W is a guess, the actual number could be higher and I could scope it to be exact but just want close. The power and ground wires to the amp are 16ga so the output power to the 4 bridged outputs can't be too substantial.

Given the low bandwidth needs, at first I figured I might cheat with a simple step-down transformer optimized for regular mains voltage and 60Hz would still be applicable. But finding a 2:1 ratio is super rare as a cheap option with dual secondaries I could use in reverse. And I was not sure if the power losses from the excessive current would destroy enough of the efficiency to make it a pointless test, hence not buying one to find out.

Next I thought of a low-buck test of just winding some enameled inductor wire around a bolt to get a turns ratio and start empirically figuring this out. But why resort to a decades old method when I thought a better start would be to maybe start with a simple formula and then test on a torrid dognut or laminated U-shpaed core for $5-$10 and a approximate turns of 22ga wire. I just don't know if I should start with 200 turns or more and what affect increasing/decreasing the turns has exactly. I'd guess more turns rolls off the high end because of the increased inductance so the appropriate gauge (or multiple strands) is important too to handle the current without super high DCR. But I'm wildly guessing...

I don't understand why it needs to be so large? A commercially available autoformer rated for 500W RMS is not really as big or heavy like the tranformer in the power supply of some big home receivers, and this is one order of magnitude larger. Biggest problem is the autoformer is not electrically isolated so can't use it as a sum of the outputs...and it costs over $100 and I'm not that curious. 8)

Like many others I can appreciate the art aspect of a nice design. But I'm just looking for something good enough for now and something that other people with the same car may want - its asked about a lot and the lightweight factory amp has the low weight, low cost (included) and low theft factor.

So some simple help in terms of just the process of a basic transformer design would be extremely helpful.

Cliff

Cliff_J said:

I don't understand why it needs to be so large? A commercially available autoformer rated for 500W RMS is not really as big or heavy like the tranformer in the power supply of some big home receivers, and this is one order of magnitude larger.

Click to expand...

An auto-transformer is smaller because it only has one winding, a transformer has to have at least two - so you've got twice the amount of wire to fit in the core.

A mains transformer isn't going to be much use as an audio transformer, for one thing the impedances are going to be wrong, and it's not designed to work over any sort of bandwidth - in particular it's going to cripple the low frequencies, particularly for a sub-woofer, which is intended to work down low.