ALOT of Coupling Transformers?

"Barry Reise"

** You post makes very little sense.

You must specify the frequencies and power level involved and if you need isolation between the input and output.

Or better still, say what you are trying to do.

... Phil

sparse information indeed. By neglecting to talk about specifics, or application you, you have completely 'not allowed' imaginative solutions, like "active" coupling transformers that are 1/4 inch cube and go down below 0.001Hz, but using the givens...

the max turns ratio you will need to match worst case 16 ohm to 100 ohm is 2.5:1 which is not too uneven.

The 'gotcha' is going ot be that low cutoff frequency. 10Hz not so bad, but even going down to 1 Hz, you will need 1.25 to 2.5H on the 16 ohm side. That can be a lot of turns depending. Going down to 0.1 Hz just makes it 10 times worse.

Years ago I made a very small transformer that had 100 turns on the core and measured 1H, but that was a SMALL core!

Just to operate down to 1 Hz, you have to get that 2.5 H inductance, yet keep the Rdc below 2 ohms, not an easy fete.and you still haven't said how much volt-seconds you're trying to hold off. You could easily end up with a 100 pound transformer, barely meeting your "... like at DC (via DMM)..." requirement.

*IF* you have absolutely no DC current in any winding, and *IF* you are somewhere between 1 to 5 Vrms; I'd recommend you look into the cores made of amorphous material. . These are often wound onto a torroid core, so they're even more fun to wind. *IF* you don't need over 50Vrms isolation; you can easily wind the torroid in a single pass using bifilar or trifilar wire sets with the wires of 'proper' size and 'matched' for balance.

I've wrapped single torroids using a bobbin made of a 1 foot long 1/8 inch wooden dowel with all the wire you need on the bobbin. If that's too fat to fit through the center hole, make the bobbin longer until it all fits. If you need more copper crossection than 18 Awg, I'd use multiple strands, rather than going to 14, or 12 Awg, simply too hard to work with. ...and don't splice in the middle!

More complete Primary specs: Zp matching to Zs over what bandwidth how much power thru Secondarily important specs: EMC /shielding /field containment DC bias current? Isolation breakdown voltage? Isolation impedance? DC will probably be gigohms, but what about measured at 1kHz, 100kHz, 1MHz? Noise?

NOW you can talk about how to make the transformer.- select core shape/ material, find core cross section, wire size, performance predictions a lot more than is possible from your description.

The 'gotcha' is going ot be that low cutoff frequency. 10Hz not so bad, but even going down to 1 Hz,

** The OP asked for an "audio" transformer - so the lowest frequency is 20Hz.

Let him say if it is otherwise in his app.

... Phil

If 2:1 ratio is ok, which would give you around 64ohms secondary, and litle or no isolation is needed, a mains transformer with the mains winding removed might work, with CT secondary

NT

What are you trying to do with coupling transformers that cannot be better done with op-amps?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Thank you for your reply.

The "DC" referred to measurement of the winding resistance via DMM, not an impedance meter. IOW it was not the lower freq limit.

Some "audio" transformer will do 10Hz, but 20Hz would be OK. Top end

2KHz. 10W power rating. Apart form the 16R pri and 50/100R sec, that's all that matters.

I am really looking for a supplier who has a wide range these things off-the-shelf. If not, I would need to source a type of core I can wind myself without industrial equipment.

The suggestion to use the secondary of a mains transformer is something I had looked into. But a center-tap is not going to provide complete isolation. And completely split secondary windings are usually equal resistance, ie 12-0-12V. This does not provide the desired ratio.

Barry

"Barry Reise"

** The 953-0398 toroidal tranny from Element 14 is all you need.

Twin 115 volt primaries and twin 25 volt secondaries with a 50VA rating at

50 Hz - wire the primaries in parallel and the secondaries in series to get 115V to 50V.

Scale that down to 20 Hz and the relevant voltages change to 46V to 20V.

For 10 watts at 16 ohms you need 12.6 volts - so 20 gives a nice margin.

A cheaper alternative is the similar E core 178-0860 - this will be less good at high frequencies and draw far more magnetising current.

You pays your money and takes your choice ....

... Phil

Keyword is 'might work', because many of those AC mains transformers have their performance peak at 50/60Hz and purposely reduce higher frequencies. Worse, they're usually higher coercivity material [since with AC mains the 'signal' is always there] which means that if used for audio the transformer disappears at low signal levels, nothing to 'activate' the magnetic properties of the core and thus it's practically acting like an air core and not much signal goes through..

s

Thanks Phil, I've GOT to do a better job of reading. then I would have caught the OP's follow-on contradictory statement, "I am using the low end of the band, so these resistances would be more like at DC (via DMM) than the usual 1KHz."

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What is the EXACT URL for "...953-0398 toroidal tranny from Element

14..." ?

I got zilch both at Element14 and using google

"Robert Macy"

Keyword is 'might work', because many of those AC mains transformers have their performance peak at 50/60Hz and purposely reduce higher frequencies.

** Most are good to a few kHz and toroidal types work well out to 50kHz or more.

Worse, they're usually higher coercivity material [since with AC mains the 'signal' is always there] which means that if used for audio the transformer disappears at low signal levels,

** Absurd nonsense.

... Phil

Or

avoids linewrap.

Grant.

"Robert Macy"

What is the EXACT URL for "...953-0398 toroidal tranny from Element

14..." ?

** It's Farnell under a new name.

... Phil

OK, they're on my Christmas list.

And why did they ever change their name from Far--nell to El--e--ment--four--teen? I thought electronics guys were smart.

Barry

or just leave the primary on, if its all sufficiently insulated

NT

r

Why do you say "** Absurd nonsense." ?? Even new materials in audio transformers have some coercivity to them and require a certain amount of 'energizing' to 'wake up' the magnetic material.

To demonstrate to yourself this statement is true, make the following test with an old analog storage scope using very slow horizontal sweep, or do X-Y plot of input/output transfer function on a regular scope: Drive transformer using an audio source at 300Hz. Vary the amplitude of the source uniformly in time. Watch the signal that gets through the transformer. As you turn the amplitude down, you will find a 'nonlinear' transfer function as the coercivity of the transformer comes into play. Essentially, coercivity is like something that must be 'overcome' by the input signal, else the transformer acts almost like an AIR core, the metal is just not there.

When I was first starting out as an Engineer, this phenomenon caught me way off guard. I was using a Stancor 1:1 600 ohm telephone coupling transformer, which usuallly has over 1 H core. At very low signal the low end of the pass band simply disappeared, implying core inductance of less than 0.1H ??!! Luckily, my mentor used to own [as CTO] a transformer company and educated me on the basics of real transformer design/operation.

thanks search for element part number came up: Multicomp MCTA050/25 but at least they post a data sheet.

"Robert Macy" "Phil Allison" "Robert Macy"

Why do you say "** Absurd nonsense." ??

** Because no such effect exists.

Consider that the laminated cores used in the vast majority of AC power transformers and audio output transformers the same - ie silicon steel, grain oriented or possibly not.

Toroidal AC power transformers all use grain oriented silicon steel, the exact same cores are used for audio applications.

You need to test some REAL transformers if you still think everyone is wrong.

... Phil

Only if you're operating in the low frequency limit where inductance is critical. Generally, where mu_r is over a thousand under all conditions, it's high enough not to notice.

To be precise, coercivity isn't the property that produces this effect, because coercivity is measured with a fully saturating waveform. Small signal effects occur in their own (very small) hysteresis loops. However, the physical mechanism ("sticky" magnetic domains) is the same.

As for frequency response, Phil is correct about toroidials -- they are good enough to have been used in some fairly involved tube amplifiers. The leakage is small enough that lots of NFB can be used (cathode feedback, global, etc.) while maintaining the high bandwidth (~50kHz).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms