Transformer power rating

ndle higher audio power as on average it's more than 10x the frequency and the core size needed is much less (that's one reason SMPSU uses 25KHz to 2M Hz, to use a small transformer). A toroidal transformer needs well matched bias currents, avoid them."

The higher frequency will mean it can handle more power before saturation, but the problem is that the lamentations will be too thick and eddy current s will over heat the thing.

Audiophoolery.

John :-#(#

handle higher audio power as on average it's more than 10x the frequency an d the core size needed is much less (that's one reason SMPSU uses 25KHz to

2MHz, to use a small transformer). A toroidal transformer needs well matche d bias currents, avoid them."

, but the problem is that the lamentations will be too thick and eddy curre nts will over heat the thing.

AIUI the power limit at 50/60Hz is down to copper loss & core loss, of whic h copper loss dominates. Going up in frequency does not change copper loss

- not over the audio band anyway. Core losses rise but not greatly over the audio band. Thus I'd expect little change in power rating for audio versus mains use.

OTOH reusing a mains transformer for audio output is liable to cause lament ation at higher frequencies due to phase shift causing instability.

NT

A mains transformer can handle higher voltage at higher frequency up to a reasonable limit. Beyond that reasonable limit it will not produce what you want. It will never be able to conduct more current for which it was designed.

Try to run a mains transformer (loaded) at 25kHz to 2MHz and see what happens. Leakage inductance will eat your lunch.

Toroidals can be designed for many applications. It is a mistake to categorize them as you have.

A second problem is the winding may have too many turns, certainly that's true thinking of smps use. High winding inductance with high capacitances will cause severe suffering at higher frequencies.

--
 Thanks, 
    - Win

handle higher audio power as on average it's more than 10x the frequency an d the core size needed is much less (that's one reason SMPSU uses 25KHz to

2MHz, to use a small transformer). A toroidal transformer needs well matche d bias currents, avoid them."

yes, not something that will be made use of when used as an af output trans former though

and since the highest currents in af are normally low freq, it won't really do more volts either, so no more VA.

I haven't categorised them as anything. The metal strip wound ones do lack core gaps though.

NT

Indeed. DAMHIKT.

NT

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n handle higher audio power as on average it's more than 10x the frequency and the core size needed is much less (that's one reason SMPSU uses 25KHz t o 2MHz, to use a small transformer). A toroidal transformer needs well matc hed bias currents, avoid them."

** It's bunkum.

** Neither, it due to core saturation effects setting in if the supply volt age is higher or frequency lower than specified. Saturation causes a massiv e and non-linear increase in primary current.

nd

** The rated current of the winding does not change, but using a higher fre quency allows a proportionally higher voltage to be applied. The main restr iction to this is insulation failure. A frequency plus voltage increase of about 2 or 3 times it the limit. ** Core losses diminish with rising frequency, since the core is magnetised less and less.

In general, mains and audio (tube output) transformers are similar size for the same power rating. High quality tube output transformers are larger as they are rated to work saturation free down to 20 or 30 Hz.

Also, toroidal mains transformers CAN be used for tube output if care is ta ken to keep DC balance within tolerance. A number of guitar amps use toroi dals for mains and output.

However, typical E-core mains transformers have very poor high frequency re sponse and cannot be so used. E-core audio transformers have layered windin gs to overcome this.

.... Phil

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** One increases the frequency and voltage in proportion to get higher power throughput - so the number of turns used is just right.

** Unlike SMPS transformers, Audio (tube output) transformers are *wide band* devices that operate with *flat response* from 20Hz to 60kHz or more.

Core size sets the low frequency power handling and the number of turns depends on the supply voltage being used. By layering primary and secondary windings, leakage inductance is reduced to trivial levels.

..... Phil

they can, but one would not recommend it. I did so many years ago.

NT

The skin depth for copper at 20 kHz is 0.5 mm, so with conductor diameter greater than 1 mm, the resistive skin losses will increase. This is an issue for at least for output transformer secondary windings..

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y

** As a teenager, I once used a mains transformer from a tube radio for aud io P-P output using a pair of 6V6s. The tranny's secondary was 275-0-275 pl us 5V and 6.3V heater windings - connected in series. With an 8 ohm load, f requency response was falling above a few kHz and no NFB could be applied.

.... Phil

Although if you do it with a toroidal transformer, you get away with quite a lot of bandwidth. Winding stackup matters!

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: https://www.seventransistorlabs.com/

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udio P-P output using a pair of 6V6s. The tranny's secondary was 275-0-275 plus 5V and 6.3V heater windings - connected in series. With an 8 ohm load, frequency response was falling above a few kHz and no NFB could be applied .

I was luckier. The 50 watter was ok with nfb, and the little single ended 3 watter sounded fine without nfb around the transformer.

NT