Custom Transformer Construction?

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So I've been thinking about it, and I've decided to try to use transformers to change the apparent ohm load of speakers to amplifiers.

the application will be purely with subwoofers.

one application, the one I'd like to work on first, is to either find or build a transformer to make a 4ohm subwoofer appear as an 8 ohm load on a 167w HTIB powered output.

I know that I'll need a 1:0.7028625 winding ratio for this application and that it should leave me with about 165w of the original 167w for use on the sub.

what I'm not sure about is what gauge wire to use, what number of windings would be best for sound quality, and what size iron core I should use, nor where to source the iron core.

a future application would be to build a pair of transformers to make a pair of DVC subwoofers wired in parallel (2 ohm load) appear as 4 ohm loads on each channel of a 375w/x2 car audio amplifier (750w total). this second application would be to optimize clarity and reduce distortion, as well as to reduce the strain on the car audio amplifier. obviously I could wire each DVC speaker to 2 ohms, then wire both DVC speakers in series to the amp in bridged mode, this would be typical, but amps distort more when bridged, as well as not putting out the total power. I think the transformers would make for a much more optimal setup.

if transformers are available with appropriate specs for these applications at a reasonable price, I may opt for that, but I'm new to the idea of using secondary transformers to tweek an audio system.

if anybody has experience in this area, I'd love the input.

Thanks!

[budm]

Re: Custom Transformer Construction?

I do not know how to design transformer, but what you need is called speaker matching transformer, in your case it can be autotransformer type. To understand AUTOTRANSFOMER, just Google "AUTOTRANSFORMER":
http://www.allaboutcircuits.com/vol_2/chpt_9/5.html
Just Google "speaker impedance matching transformer"

[Home_Command_Center]

Re: Custom Transformer Construction?

I'me searching prebuilt speaker impedance matching transformers online, but all of the ones I'm finding have too high of an opperating frequency for a subwoofer application. the bottom cut out is around 50hz on most of them. they go up to 20Khz, so they'd be great if I was trying to up the apparent ohm load of some tweeters or midrange drivers, but nothing seems to be available for an application that needs a bottom frequency range at 20hz or hopefully less...

[redwire]

Re: Custom Transformer Construction?

One thing you need to consider is the low-frequency rolloff with audio transformers. If you want full power at 30Hz, it's going to be a big, heavy part. Roughly 20lbs/200W. If you feed the transformer some (low) 20Hz earthquake, it is no longer 8 ohms and can take out your power amp!

I find much cheaper to just replace the woofer with an impedance that works with my amp.

[Home_Command_Center]

Re: Custom Transformer Construction?

or throw out the amp and get one that works with the sub. or buy a 2ohm stable home monoblock amp and run it from the line level sub output. there's a lot of ways to skin a cat.

I assume when you say "If you feed the transformer some (low) 20Hz earthquake, it is no longer 8 ohms and can take out your power amp!" that what your talking about is soaking the iron, correct?

theres a company that sells some autoformers called ZEROS, but they're $450 a piece and would cost more than the combined cost of basically every other part in the system I'm working on...

[edit] I'm thinking a transformer with 800 primary windings and between 562-563 secondary windings would be appropriate, but again, not sure about the size of iron I'd need. are you saying I need about 17lbs of iron for the core? would it have to be specific iron for a transformer, or would some other form of iron work? I know with lower grade iron you need more of it, but not sure about the details there...

would a 25lb weight lifting iron plate do the trick? then wind it with about 12g wire? just wondering how home brew this part could be...

[budm]

Re: Custom Transformer Construction?

The lower the frequency the lower the impedance, if you do the frequency sweep of your speaker, the lower the frequency the low its impedance will be so more load to the amplifier, to have 20 Hz frequency at over 100W, the transformer will be so big. To design the transformer, it will have to consider the frequency requirement, the core lost factor, the power requirement, the impedance (frequency dependent, 8 Ohms speaker is specified as impedance, not DC resistance), then you will figure the core size and material, turn ratios.
I use custom made Plitron transformer per our product specification requirement, then they will submit us the prototype.

[Home_Command_Center]

Re: Custom Transformer Construction?

I see on the Plitron site they have some stuff that looks close to what I need, any info about the general cost of an impedance matching transformer of the magnitude I'm looking for? they don't seem to want to talk about costs...

Thanks for the pointer though, I like what I'm seeing so far from them...

[edit] I sent their tech department an e-mail, we'll see what comes back...

[budm]

Re: Custom Transformer Construction?

You can E-mail them, one-off is REAL expensive.

[Home_Command_Center]

Re: Custom Transformer Construction?

yeah, hopefully they will have something off the shelf that will do the job.
if it works well and is affordable I may wind up getting a bunch of them for future projects.
seems like most of the ones they have are more extreme than what I need, like for electrostatic speakers with ohm loads that can drop below 1ohm at times, and then big transformers for the output channels of tube amps.
maybe I'll get lucky though.

[Home_Command_Center]

Re: Custom Transformer Construction?

I haven't heard anything back, so I'm going down another road for this project...