Building a high quality gainclone toroid coupling transformer with "tubeness" winding

[ionomolo]

Since many of us have already walked the long hard road to the perfect coupling cap, it's a pleasant idea to start trying out with transformer coupled gainclones, as these, in opposite to coupling caps, can be diyed!

I'm planning to build a toroidal input coupling transformer (1:0.7 to 1:3, prefering 1:1) and face many choices:

· Which core material, target inductance, number of windings and size would be ok for a 1 Vrms output signal amplitude?

· I have many computer smps toroid cores, would these give good audio quality?

· I plan to add a tubeness winding to create a constant field flowing trough the core and add 2nd and 3rd harmonic (Yes, true tubeness as real tube amplifiers have transformers with constant dc bias and this does contribute to it's sound signature). This winding would be loaded with a ccs (Niceness knob), any suggestions regarding number of turns and current? Near-saturation behaviour is very material-dependant, any advise?

· Would a faraday shield (putting the toroid inside a metal candy box) and additional mu-metal shielding arround be enough to keep it dead quiet? Should the faraday shield be connected to power ground?

Thanks. I will post sonic feedback when completed.

[BWRX]

This is definitely an interesting topic for discussion.

You will probably need small diameter wire and many turns to make an input transformer. Small diameter wire, many turns, and toroid cores are not the most DIY friendly combination but it could be done if you have a lot of patience! I'm not sure what kind of core material you'd want to use but it should be suitable for use in the 20Hz-20kHz range (I'm sure you already knew that!). Different core material will give different saturation characteristics as you said. Iron cores will provide a nice rounded characteristic as saturation is approached.

You could add foil shields between windings and external mu metal shielding but why bother if you're just experimenting. It would be beneficial to use them in a "final" version if/when you get to that stage

[ionomolo]

Thanks for the reply, getting down to 20 Hz means winding a lot, and i've noticed the problem with small cores . Thanks for your interest, it will help me in keeping windinding.

My first calculations give 870 turns (aprox) for a -3dB point of 20 Hz on a 47K load (this seems a sensible value for the input impedance in a power amplifier) using with a radius of 1.5 cm and 0.5 cm^2 section. Sounds reasonable?

Brian, can I PM you for a suggestion?

[BWRX]

Of course you can send me an email. Whether or not I may be of any help is another matter! I'm no expert on magnetics and it has been a while since I did any magnetic circuit analysis.

[hermanv]

By SMPS do you mean switch mode power supply? Those are almost always high hysteresis materials and are not suitable for a linear transformer.

You probably want tape wound silicon steel or powdered iron cores. Many people sell ready made toroids, some specially for audio amplifiers.

[ionomolo]

Thanks, i will try to get a better core before doing the 870 x2 turns. The computer power supplies i've seen come with two kinds of cores: ferrites and dull, green-painted ones. May these be powdered iron cores? Not all of them where used as transformers, some were chokes.

The idea is not to buy it done, but to try different arrangements and load impedances plus the third winding to slightly saturate the core.

[Boris_The_Blade]

Powdered iron cores usually have really low mu, and hence low AL, so they aren't really suitable for audio frequencies. Try to find some small E-I laminations for this type of job.

[Conrad Hoffman]

The entry barriers here are pretty high. Having hand wound more than my share of toroids, I wouldn't want to do 800+ turns without a winding machine. And toroid winding machines are hard to come by outside commercial transformer companies. IMO, toroids are no magic answer to transformer design, and you'll have more properties to play with if you go with a conventional laminated design. Another big problem is finding anyone who will sell small quantities of the very high grade steel laminations you'll want. It might be worth looking at commercial line and MC transformers, and seeing if there's enough room to add a winding. There is nothing SMPS related that's useful for wide band designs. One interesting idea is the use of stacked toroids of different types to extend the range, but I don't know if it applies to audio frequencies.

[Boris_The_Blade]

Originally posted by Conrad Hoffman
The entry barriers here are pretty high.

Agreed, you might have much more success looking at cinemag, lundahl, edcor, or ebay for vintage types.

[ionomolo]

I know that it's a hard job, and that maybe isn't worth the effort, but i'm interested in trying it.

Any more advise in core materials? Could a larger toroid be used here?

I know that the devotion that toroids have in PSU transformers is quite exagerate, but since it seems that they do give a wider bandwith (A reason not to use them in power supply units) maybe it's easier to get good results using them in audio.

I belive that the core materials in the SMPS psu could be anything from good to horribe, because they are very different from one model to another.

Is there any way to check the frequency response of a core other than winding some turns and checking HF performance? Will this give a good idea about the quality of the core? Do you know anywhere to buy good ones?

I feel the toroid approach to be more straightworward as it does not need to be glued.