Extend freq audio xfmr

The low frequency performance of an audio transformer is maintained by using a mass of high-permeability iron. To get higher frequency performance, one uses thinner iron plates with insulating spacers (just oxide layer on the plates, really) to prevent internal eddy currents, or for very high frequency one can use powdered-iron or ceramic (ferrite) cores.

But a ferrite that performs well into the RF range is more expensive (and larger) than laminated-iron for a given power level. And this is an OUTPUT transformer, power level is high. If you can use biamplification (split off the woofer or subwoofer to a separate output circuit) the low frequencies can be handled with iron core, and high frequencies with whatever is appropriate. If it all has to go through a single core, you might need to get a custom-designed unit, or use something meant for switchmode power supplies (and the specs won't include '8 ohm').

"whit3rd" Conrad Ferrer

** That is totally wrong.

High frequency roll off in a wide band audio transformer is controlled by the winding method - NOT the damn core material !!

A toroidal type or an E or C core shape where the primary and secondary windings are multi-layered and interleaved will have wider bandwidth due to reduced LEAKAGE INDUCTANCE .

........ Phil

"Conrad Ferrer"

** A judiciously chosen capacitor across the load may help extend the response a bit.

You'll need a scope, square wave generator and drive amp to do the job.

....... Phil

You might find this useful:

Paul Mathews

"Paul Mathews"

** I see it is your own article.

On a number of occasions, I have been was called on to diagnose amplifier failures where the PA system installer simply had no understanding of "core saturation " whatever. Nor at the time did a company called QSC.

The handbook for their USA850 and similar models directed owners and installers to do what was almost certain to destroy the amplifier. Three such victims were delivered to me with a " WTF happened ? " complaint.

The output device protection circuit fitted to most QSC amplifiers is not capable of saving the amps if ever driven into a saturating transformer load. The life expectancy reduces to a few seconds in this event.

The eventual fix was to install a 12dB /oct HPF with -3dB POINT @ 100Hz internally in each channel and simultaneously make sure that no sub sonic thumps were generated by the amps under ANY operating condition.

When contacted about the failures, QSC tech support were the OPPOSITE of helpful.

....... Phil

Correct, it's the use of special interleaved winding methods to create low leakage inductance that wins the day, not core materials, at high frequencies. But note, if we were talking resonating inductors and high-frequency energy storage, why then it'd be another matter entirely. Core losses would rule.

To answer Conrad, because it _is_ a leakage-inductance issue, it's hard to externally correct the problem.

You can push it up a bit by boosting the level of the high frequency signals going into the transformer. For normal sound there is very little power above a few KHz so adding a boost starting at 25KHz won't be a problem.

Are you driving the transformer from a low impedance such as a normal audio power amplifier or a higher impedance? If the drive impedance is low, the cut off will be where the leakage inductance as the same impedance as the load. If your drive signal has a higher impedance, you are likely to see a different cut off.

Forgive the plug that follows, for what is quite possibly the lowest leakage inductance PA transformer available commercially. It can be seen here, accurately specified and conservatively rated:

Unlike many PA transformers, this one was NOT designed according to power transformer design rules. The balance of copper and core losses and the winding design were entirely driven by audio fidelity and power delivery considerations, including tolerance for amplifier output DC offset. (Power transformer design rules place the emphasis on minimizing cost within a temperature rise budget, which results in poor audio performance.) End of plug. (I have no financial interest in the manufacturer, a former client.) Paul Mathews

"Paul Mathews"

** At the specified 20mH, it must be the worst on the planet !! ** 20mH should be 20 uH - no ?

** OK - 500 mohms is about double the usual value, plus the extra turns pushes the saturation frequency lower. However, using a wound toroidal core means that the limit is quite dramatic when it arrives. Still very bad news for many power amplifiers - particularly QSC - that cannot take being effectively dead shorted ,twice each cycle, at some low frequency while delivering high power.

Pffffffftttt.......

....... Phil

maybe you could have two transformers, for different bands, with the primaries and secondaries in series, say a ferrite for high frequency wich becomes very low impededance at low frequency and so has no effect at low frequency where the iron tranformer does it all, and maybe a capacitor accross the low frequency transformer prim and sec.

Colin =^.^=

Negative feedback?

And that's a problem?

Bye. Jasen

Hence the low-cut filter option available for most commercial power amps. The Rane power amps monitor and limit their own volt-seconds output. Paul Mathews

"Paul Mathews" "Phil Allison"

** You really are totally clueless - aren't you ?

You think in dumb clichés and post opinionated drivel.

" > It can be seen here, accurately specified and conservatively rated: >

** 20mH should be 20 uH - no ? "

** Cat got your tongue on this point ???

...... Phil

In fact, it's six of one, half dozen of the other. When the core material is slightly conductive, eddy currents exclude the flux (by Lenz's law) and that means the flux, since it is not in the core, is not constrained to link the primary and secondary windings. The presence of unlinked flux is the definition of leakage inductance.

ing

Thanks for noticing that typo. Yes, 20 microHenry. Font error: should have gone to a symbol. Thanks for the insults, too, I'm honored. Can't go long without a good insult from crazy Phil. Paul Mathews

I suspect a measurable fraction of that 20 uHy might be just in the loop area of the lead wires to the connector, especially if the wires happen to be spread out a bit more than in the pictures. I think that pair should be twisted.

"whit3rd"

** You are an witless ass

- f*ck off .

....... Phil

"John Popelish"

** Can you spell the word " F A N A T I C " for me - John ?

...... Phil

Now that you have given me a hint, sure.

It just seems to me that if you are going to go to all the trouble of building custom magnetics, you also don't encircle the mounting bolt with winding currents, and for impedances as low as 8 ohms at 20 kHz (64 uHy, max) you don't add extra microhenries in series with it by laying open loops of wire in the chassis. Anyways, I wouldn't, so I'm a fanatic.

I'll bet they twisted the pair when they measured the leakage inductance.