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Old 27th August 2004, 10:57 PM   #1
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Question Lightweight toroidal transformer?

Hi Folks,

Iím putting together a lightweight bass (guitar/upright) amplifier. Itís got to be lightweight. To keep the weight down the speaker box is terribly inefficient. So I hope to use a high powered lightweight amp such as a ZAPpulse 2.2 SE to compensate.

The cookbook for the ZAPpulse 2.1 states:
ďFor normal use one 500 VA transformer is enough to drive 5 channels of 200 Watts RMS (total 1000 Watts Audio Power) because there is no thermal loss in the amplifiers,ÖĒ

So what I wonder is, I only want to drive one of these modules to 200W, would I be foolish to think that I could drive it with around a 150VA transformer (If thereís such a thing)

I need something lightweight, and adding 14lbís for a transformer isnít the right solution.

Thanks for any help,
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Old 28th August 2004, 03:12 AM   #2
Eva is offline Eva  Spain
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Think about using an SMPS. Designing and building a SMPS from scratch is not a trivial task but modifiying a standard AT or ATX PC power supply to output something like +-50V regulated and current limited to +-4A [up to 400W continuous] may be technically affordable

What power rails are required?

My first experiment in that area was to modify an old worthless 200W AT PSU to provide 6..15V adjustable, regulated and current limited to about 25A. It worked fine and I used it as a bench power supply for some time [mostly to test car audio gear]

What I did was essentially removing everything but the required components for the supply to provide a single output [+12V] slighty modifiying the feedback network to sense only the +12V output and adding a potentiometer here to adjust the output reference. All protections except main current limiting were removed and power good checking was also removed. Current capability was about 15A [250W] before current limiting

Then I dediced to rewind the main transformer to optimice turn ratios and wire thcknesses and to design a new output inductor and increase the current rating of the output diodes and the current limit thresold toghether with some minor modifications to the control circuit and the primary base drive circuit. I got 25A current capability [375W] before current limiting

After some time of use I dediced to start redesigning the entire unit and now I'm using a prototype that provides 72A continuous [about 1000W], but I had to increment somewhat the complexity and I spent aobut 5 months to get it working fine

In comparison, each of the first modifications were accomplished in a couple of days so with some SMPS understanding it's not hard to modify an AT PC PSU for your needs

Warning : If you decide to experiment in that area it's almost mandatory to get a mains isolation transformer, a variac and a >20Mhz oscilloscope. Never experiment with mains live circuits. Never apply full mains voltage to the circuit you have just built or modified. Never assume the circuit is working properly until the oscilloscope shows proper waveforms
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Old 28th August 2004, 05:02 PM   #3
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Thanks Eva,

I appreciate this, and it looks very interesting if I had the time and knowledge, but for now I'm looking for an off the shelf solutions as much as possible.

The max rail is 60-0-60, but giving it less gives you less power.
I've searched around and found some COTS SMPS for around $50 USD at 48Vdc @ 150watts. I can't find enough info if these have floating grounds and would work, but the price is right.

Other's I've found are much higher in price.

Ideally, just a proper sized toroidal for my needs would be best, but I suspect the claims of the ZAPpulse may need more than what's indicated.

thanks,
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