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SM7UYJ 29th March 2011 08:44 AM

DIY bass transformer for ESLs ?
 
I'm doing some experiments with standars toroid transformers for my full range ESL project.

The different transformers I have tried are:
  • Core Windings Ratio
  • 4x60VA 2x12/240V 1:80 (in theory)
  • 2x80VA 6V/240V 1:80
  • 2x200VA 12/240V 1:40
  • 2x400VA 5V/240V 1:100


Using an old LM3875 amp, 1 Ohm in series with the primary - 9 Ohm paralell(or none) and driving my ES bass panel 250x1500mm d/s 4mm C=600pF.

I get distorsion when I crank the volume up and the transformers start to sing - core saturation ?

Has any one any sugestions/tips on how to best make a DIY transformer capable off 20Hz ?

I'll be crossing over to my mid/high-panel at abt. 2-300Hz, so it "only" has to handle 20-200Hz! :)

Roger

chinsettawong 29th March 2011 12:43 PM

With 4 mm spacer, I think you need a lot more turn ratio of the transformer. You might also need a lot more amp power than what your LM3875 can do too. Your amp might be clipping at that volume level, no?

Wachara C.

SM7UYJ 29th March 2011 01:00 PM

The probe to my scope decided to die last night, så I have to get a new one - after that I'll do some more measurements!

But I dont think its the amp thats clipping as my lower ratio transformers can be cranked higher :confused:

Roger

chinsettawong 29th March 2011 01:26 PM

Hi Roger,

Have you tried higher bias voltage - something like 7,000 - 8,000V? :)

Wachara C.

SM7UYJ 29th March 2011 01:42 PM

Hi Wachara,

Yes, but I backed down to 5.5kV that I use now as I got some hiss with higher EHT!

The hiss could just be "contaminated" spacers from the build process that needed to be "burned clean" - I'll try to raise the EHT a bit!

Roger

bolserst 29th March 2011 02:50 PM

Quote:

Originally Posted by SM7UYJ (Post 2520206)
I'm doing some experiments with standars toroid transformers for my full range ESL project.

The different transformers I have tried are:
  • Core Windings Ratio
  • 4x60VA 2x12/240V 1:80 (in theory)
  • 2x80VA 6V/240V 1:80
  • 2x200VA 12/240V 1:40
  • 2x400VA 5V/240V 1:100


Using an old LM3875 amp, 1 Ohm in series with the primary - 9 Ohm paralell(or none) and driving my ES bass panel 250x1500mm d/s 4mm C=600pF.

I get distorsion when I crank the volume up and the transformers start to sing - core saturation ?

Has any one any sugestions/tips on how to best make a DIY transformer capable off 20Hz ?


240V toroidal power transformers work great for driving ESL at frequencies > 300Hz.
They do not work well for use as bass transformers unless you use many of them and hook the driven low voltage windings in series or choose transformers with higher secondary voltage windings.
Here is why.

Power transformers are designed with the core operating just below saturation(Bmax ~ 10,000).
This is done for reasons of economy, not using more iron or copper than necessary to get the job done.

Bmax = E x 10^8 / (4.44 x f x N x A)

Bmax = Peak AC flux density in core (Gauss)
f = frequency
N = number of primary turns
A = core area cm^2

Note that Bmax is inversely proportional to the number of primary turns.
Typical Bmax for modern transformer iron is about 12,000 gauss at the onset of saturation

So, no matter what VA rating, the number of turns in the 240VAC winding is chosen such that Bmax ~ 10,000 for 50Hz – 60Hz.
What this means is that if you have a 6V/240V transformer and you drive the 6V winding with a 60Hz audio tone,
the transformer core will start saturating at about 20% (12,000/10,000) above 6Vrms = 7.2Vrms.
At 20Hz, the situation is even worse. Saturation will start at 7.2Vrms x (20/60) = 2.4Vrms.

For hybrid use, these transformers work fine because at 300Hz saturation won’t start occurring until 36Vrms which is more than the output of a 100 W – 150W amplifier.

SM7UYJ 30th March 2011 09:50 AM

Bolserst: Depressing news, but what I love about this forum - Thank you for sharing your knowledge and saving me lots off frustrating hours in the work shop! :)

So if I get things right, stripping my 400VA toroid from its original secondary winding - add more turns to the primary (my "ES secondary") lets me lower the usable frequency before saturation accurs?


Roger

bolserst 30th March 2011 02:41 PM

Quote:

Originally Posted by SM7UYJ (Post 2521449)
Bolserst: Depressing news, but what I love about this forum - Thank you for sharing your knowledge and saving me lots off frustrating hours in the work shop! :)

So if I get things right, stripping my 400VA toroid from its original secondary winding - add more turns to the primary (my "ES secondary") lets me lower the usable frequency before saturation accurs?

It gets confusing talking about primary and secondary of transformers when they mean one thing when the transformer is used as a power transformer and another when used as and ESL step-up transformer.

Winding naming convention:
For power transformer use, lets call the windings by their voltage ratings.
Your 400VA transformer has a 240V winding and a 5V winding.
For ESL use lets call the windings by:
HV=High voltage winding connected to stators
LV=Low voltage winding connected to the driving amplifier.

First, understand that saturation of the core is determined by the winding that is being driven. For power transformer use it is determined by the voltage applied to the 240V winding. For ESL use, it is determined by the voltage applied to the LV winding. So, no matter how many turns you add to the HV winding, the core will saturate at the same input voltage if the number of turns in the LV winding remains the same.

Your question restated:
What I think you are saying is: you want to remove the 5V winding and add more turns to the 240V winding. This would be your HV winding. You would then need to wind on a LV winding to connect your amplifier to. If you increased the number of turns in the LV winding by the same ratio as you increased the number of turns in the HV winding you would still have the same step-up ratio (50:1) and you will have increased the allowable input voltage before the core starts to saturate.

An example:
A typical 400VA toroidal power transformer is wound at about 0.4Volt/turn.
Your transformer may be different, but it will be pretty close.

So, your 240V winding has about 240/0.4 = 600 turns.
And, the 5V winding has about 12 turns.

Lets say you wanted to be able to drive the transformers at 30Hz with a 100Watt amp.
Input to the LV winding would be about 30Vrms so the required turns for the LV winding is roughly 12turns * [ 30V/(5V*1.2) * (60Hz/30Hz)] = 120 turns.
So you need to increase the number of turns in the LV winding by a ratio of 10 from the original 12 in the 5V winding to 120.
To keep the 50:1 step-up ratio you would need to increase the number of turns of the 240V winding by the same ratio, from 600 to 6000.
Adding 5400 turns of wire to a toroidal is not easy to do.

If you chose a full power bandwidth limit of 60Hz instead of 30Hz your LV winding would need to have 60 turns and your HV winding 3000.

geraldfryjr 30th March 2011 03:12 PM

Hi,Steve, very well put.
For those very reasons one needs to use a rather large core to get to 20hz. jer

SM7UYJ 30th March 2011 03:56 PM

bolserst:

I may have totaly confused my measurements, but as I recal I got about 1V/turn on my 400VA trafo!? :confused:

With 12 turns on my LV side I get a transformation ratio off abt. 1:20.

This should make things a bit easier as to ad windings to the HV side, but I definitly get what you are saying!

I'm beginning to think that my setup perhaps is good enough as it is!

The bass panel plays with good output from ~30Hz and I get distorsion only when I crank my small 60W amp up to 75%.

Roger


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