I had an interesting conversation with Chris the sales manager for Torriod Corporation. A large portion of their business comes from custom transformers. Chris said they are open to supplying small quantities but it only becomes profitable for them (and more affordable for us) if the quantities are in the 5 - 10 range. Their company has an engineer on staff who is knowledgeable in audio...his name is Ming and he sent me an email offering his services. I don't know what specs make for a good transformer for audio...that's where I need your help. I'm thinking of doing a small group buy and see how it goes. They could ship directly from their facility. I have many questions, one of course is at what cost.
It's good to hear about an alternate supply for quality custom transformers. One of the recommendations that I see frequently for audio power transformers is to wind them on a larger core than standard. This helps keep the magnetic field contained within the toroid, especially when the transformer is being used for Class A duty. Of course we also like to see an electrostatic shield between primary and secondary windings plus an outer magnetic shield band. These last two qualities are what make the Antek AS series transformers work well for our application.
The larger core is beneficial; larger means that the core is higher maximum power than needed for the application.
What we want is a lower core excitation: no more than 1 T instead the usual 1.6-1.7 T.
For lower core excitation more primary windings are needed; that's why the larger core is needed to provide the extra winding space.
With the lower core excitation there are further benefits: less inrush current, even less than "normal" strayfields, and quieter in operation especially in case of class A amplifiers with their high current demand.
The electrostatic shield is good when it is a real foil, not just an extra open winding.
When these conditions are met you end up with a very good transformer which can very well do without an outer magnetic shield band.
Ask any manufacturer to specify core excitation when enquiring.
daanve, Yes, I agree what you said about the transformer. We designed all our audio amp power transformer with low flux density. We only use copper foil for electrostatic shield. Since toroidal transformer stray magnetic field is small, most time you do not need magnetic shield band. If the transformer is mounted closed to the high gain input stage, then a magnet shield will be beneficial.
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Some of the amplifiers that are built by forum members have about 20 dB gain in the input stage. Others use unshielded signal transformers as autoformers to get 1:4 or 1:5 gain. In those cases, reducing tray magnetic fields from the power transformer to a minimum would definitely be beneficial.
Besides the already mentioned Gauss band and the electrostatic screen, here are two threads with some interesting thoughts on transformers:
noise from transformer esp. post #3
Silent Transformers in Europe
noise from transformer esp. post #3
Silent Transformers in Europe
In my projects I use the mini dissippante 3u which can only support about 115mm diameter toroid. Torroidy up to 400va work for comparison.
If you are looking for a custom designed transformer to fit in the space, please let me know the transformer spec and dimensions limitations. We can send you the price quotation.
Ming