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Cal Weldon 17th April 2011 10:50 PM

Woofer magnet and plate amp trafo
Hi, I need an answer from you smart guys. I am designing a new mini boombox and wish to mount the woofer on the bottom and the plate amp on the back. Do I have to be concerned about how close the woofer magnet is to the plate amp transformer (or any part of the plate amp for that matter)? I would like to have them about 1 inch apart if possible but it can be increased a bit. How far away is minimum or does it matter at all? It's 6.5" woofer with a "normal" sized ceramic magnet and the plate amp iron powers 30 watts.


Andrew Eckhardt 18th April 2011 12:14 AM

I'm ignoring your first sentence in order to make this reply.

Having the woofer magnet structure an inch away wont make any difference atall. The transformer circuit is closed and the magnet will interact mainly with leakage flux. Actually anything leaving the speaker motor is also leakage flux. If you put the magnet In the transfomer magnetic circuit, then you'd have a problem.

Cal Weldon 18th April 2011 01:02 AM

Thank you Andrew. I also forgot to mention the woofer has a bucking magnet so that might reduce any concern I might have had even further?

Andrew Eckhardt 18th April 2011 01:09 AM

It's hard to say how "clean" the cancellation is at an inch and at whatever angle, but there really isn't any concern either way. The effect either on the transformer or speaker would be very tiny, maybe even difficult to measure. Even the force of attraction at one inch might be what, grams? Definitely don't worry.

kevinkr 18th April 2011 01:13 AM

There is another concern and that is the proximity of the woofer wiring and voice coil to the transformer particularly if said transformer is an EI type and leaks appreciable field. When the amp is running the amplifier's low source impedance will complete an electrical circuit comprising the VC and wiring that the power transformer can induce an EMF in - this will result in hum.

Surprisingly low levels of hum/buzz in an efficient powered woofer can be hard to eradicate particularly in the case of the 2nd and 3rd harmonic of the line frequency which is often seen in surprising quantities in the leakage flux of mediocre power transformers.

Just to be safe I would connect everything up before building the box and place in the approximate relationship they would have in the box. Measure the voltage across the VC with the woofer close and far from that transformer, and see if it varies significantly. Check to see how dressing the woofer wires affects this issue as well. You can listen as well but this may not be a reliable indicator outside of the box.

Long ago I helped design a powered loudspeaker system, and the power transformer lived in a cup about 6 inches from the woofer - took a lot of work to get the magnetic coupling from the transformer down to acceptable levels. (We succeeded ultimately)

Andrew Eckhardt 18th April 2011 01:21 AM

I'd say induced leakage field from the transformer into input circuitry would be a bigger problem, and still small if the signal wiring/routing done right.

In other words, where the woofer magnet is in relation to the power transformer hardly comes into play. The speaker leads are outside the feedback loop so there isn't much effect at that point, although twisting them automatically is always a good plan. The relationship of signal wiring to power transformer is pretty much already a fixed condition in a plate amp.

It would be much easier to force a current through a loaded voice coil from the front of the cone. I think I've done that before with a soldering gun. It seems like it would be very hard to do that from the back of a fairly good sized motor.

Cal Weldon 18th April 2011 04:13 PM

Thanks, I will follow the advice. I can space them as much as 2" and will twist the leads. And yes, I will connect all before enclosing them, as always. ;)

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