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george a 18th August 2011 12:43 PM

opinions - transformer cores for ribbons
 
Hi All

I'm looking for suitable cores to build some impedence matching trannies for some ribbon speaker (down to approx 500Hz I hope.)

Can anybody offer an educated opinion on these ?

Push pull single end Audio output transformer core kit1 | eBay

I'm also looking for AMCC alternatives with a reasonable price into Australia (postage seems to be a killer -what a silly place to put a country..)
AMCC = amorphous C-core as per RAAL

Also also , Has anybody had any decent success using toroidal powr transformers as audio transformers? (say 240v step down to 9v)

Many thanks in advance

George

geraldfryjr 18th August 2011 05:20 PM

Yes,We have had Great Sucsess using the modern day toroidal power transformers for audio.
Mostly lately in step-up,I had started to embark in a step down design but other projects are in the way at the moment.

Using the standard 240V primary winding will allow you to get down to 15Hz at 60v.
You shoudn't have any problems there.

Just remember that if you are driving a ribbon that the secondary turns must have a very low DC resistance,less than that of the ribbon this can be difficult but not imposible.
Do not confuse this with Impedence ratio.

And, The secondary winding must cover the entire surface area of the toroid completely to keep the losses from leakage inductance on the low side of things.

All of the same rules apply in step down as in step up used for esl's.
Your High Frequency response will be determined by the leakage inductance and any stray capacitance + any other load capactitances.
In this case as you are not driving an esl (pure capacitance) driver,the transformer resonance should be rather high.

I Have tested a few different ones that I have around and tested to be good and flat up to as much as 200khz and as far as 1Mhz (the limit of my audio generator) with peaks due to the self resonance of the transformer itself in the 300Khz to 900Khz range.
This has been documented in another thread.

I had taken one of my transformers that has two 120v windings and and had connected it it to an amplifier and a fullrange speaker at a 1:1 ratio just to see if I could hear any major differences through it.

There was only one that was annoying.

It was at the extreme low end that had a slight distortion and was expected probably due to core saturation other than that it was transparent to me as far as what I heard from the mid bass (100Hz) on up,this is a simple test that you can try yourself as well.

The bass wasn't severly bad or anything but I could tell it was there and this could probably be rememdied if I had a much larger core and more primary turns as this is what determines the lowest frequecy of operation of a transformer.

My cores are in the 200 watt range maybe a little more as it varies abit depending on how the manufacture rates them.
Mine are 1.75"H X 4"D with the stock windings.


I hope this helps you and gives you some idea's. :)

jer

george a 19th August 2011 09:30 AM

Hi Jer

Many thanks for the great trouble you've gone to in your response.
I will investigate the toroidal transformer option as part my my ribbon re-build

- currently in pieces due to accident when i was carrying 1 of the 2 30' ribbons across the room , a bit to close to the other ribbon assembly- the neo magnets caught sight of each other with a resulting mess - i"m glad my fingers weren't in the way!.
so a total rebuild is required.
However - you make an interesting comment re having the secondary winding cover all of the toroid core , all the discussions i have seen and photos only have very few windings - 3 -or so on the secondary , with no chance of giving much cover at all to the core - .....or do we use foil.....?

Thanks
George

bolserst 19th August 2011 01:53 PM

Quote:

Originally Posted by george a (Post 2677999)
- you make an interesting comment re having the secondary winding cover all of the toroid core , all the discussions i have seen and photos only have very few windings - 3 -or so on the secondary , with no chance of giving much cover at all to the core - .....or do we use foil.....?

Yes, foils helps with complete coverage.
If you need a secondary with very few turns, try using several foil secondaries of the same number of turns spaced around the toroid and then hooked in parallel. This will lower the DC resistance of the secondary as well as the leakage inductance.

LineArray 22nd August 2011 03:59 PM

Quote:

Originally Posted by bolserst (Post 2678205)
...
If you need a secondary with very few turns, try using several foil secondaries of the same number of turns spaced around the toroid and then hooked in parallel. This will lower the DC resistance of the secondary as well as the leakage inductance.

Yes, i had good results making a secondary this way ....

my "paralleled secondaries" had a single turn, so not even insulation
was necessary. But that depends on primary/secondary turn ratio and
primary inductance to be achieved for lower cutoff.


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