Because they can easily be confusing and are often ambiguous. For example the primary inductance is listed as 15H but this is meaningless unless a frequency is specified. Unlike ferrite, the steel and mumetal used in transformers has a frequency dependent inductance (it is also level dependent).
They say the preamp should have an output impedance of up to 600 ohms yet they measure the frequency response with a 6K4 source impedance.
Despite the ambiguous spec, experience tells me that a transformer with a quoted primary inductance of 15H is most likely to be a 600:600 type. This means when used as an input its secondary load should be about 600 ohms and as you have shown, this is easy to achieve. The real problem though is it this now means your preamp has to be able to drive a 600 ohm load. And most will not.
Cheers
Ian
Here is the notice on their webpage, Ian:
CUSTOM DESIGN SERVICE
After more than 75 years doing custom designs we have a standard range of products for almost every requirement and we are now making a small change to our business model. Our custom service for power output transformers and chokes is withdrawn and instead we will do new designs for any significant gaps in our standard product line.We want to continue our support for enthusiasts and manufacturing customers alike so we will certainly be happy to cooperatively develop new products which have a strong potential to lead to volume production.
Thanks for the recommendations, @OldHector , I was aware of Toroidy, but not Majestic Transformer Co., will investigate.
Hi Euro 21 Thank you for your response. I actually did read the datasheet several times.
The DAC has an output impedance of 121R, and the preamp has an input impedance of 10K, so 600R input impedance would not be sufficiant.
But I will recall my statement that Sowter does not respond. I got a reply from Brian Sowter today, to put a 1K resistor on the secondary, so
I will get around 900R resulting input impedance on the preamp. That is still a little low, but probably the only way to get them to work in my
application.
Hi Ian,
thanks for chiming in, you have helped me before, and I very much appreciate it. The Sowter 3603 is listed as a 600:600R bridging transformer. My source impedance in the test is a 50R generator, and I tested with 1K, and 10K secondary loads. 1K looked superb and 10K looked horrible.
In that the DAC that I want to use the transformers with, has an output impedance of 121R I think a resulting input impedance of 600R to the preamp is to low. Brian Sowter suggested a 1K secondary load which gives me around 900R, but that is still a little low for me.
I could change the I/V resistors on the DAC to a lower value, which will give me a lower output voltage on the DAC, but also a lower output impedance.
On my other DAC i use Sowter 3575 10:10K bridging transformers, and they work fine, without any compensation. And then why no use those again? - because I have a set of 3603 lying on the shelf, and they were not exactly cheap.
I have tried to get my head around audio transformers for some time, and papers from Ian has helped me a lot. But the use of transformers is not common ground for most folks anymore. My experience so far have showed that the output impedance adaption is more important than the input adaption.
Which is pretty much what I would expect.
The problem with output impedance is it gives you absolutely no indication of drive capability which depends primarily on the available current. The vast majority of op amps intended for audio applications will drive 2K but no lower (yes there are ones that will drive a 600 ohm load but I suspect they are not inside any DACs. Also, most op amps are susceptible to instability with a capacitance load so they often incorporate build out resistors of 100 to 200 ohms which tens to dominate their output impedance. So altering I.V resistors may not result in a lower output impedance. Which DAC are you using?
It is likely the more critical of the two.
Cheers
Ian
Ah, that one definitely supersedes the one I saw which said they were just limiting the range of standard transformers they made. Nothing about custom stuff. They made me a custom mic input transformer a few years ago of which I have purchased quite a few. Other people have also bought them to use in their own builds based on my designs. But when someone wanted to by them last month they said they would no longer supply it and they suggested a standard one instead. A great shame.
Cheers
Ian
Somewhat late to the party, but the discontinuation of the custom transformer service is baffling
I got a pair of extra meaty KT88 push-pull units and was happy to pay the extra. Admittedly I have a series of e-mails from Mr Sowter on the design side so perhaps that's what's missing? Easy to keep cloning a popular line, less so to actually engineer something??
I think its a shame: you offer the service because you are competent and price it accordingly! If you can't offer the service...
If anyone is listening to this old thread then - Where to shop instead?
Andy
I got a pair of extra meaty KT88 push-pull units and was happy to pay the extra. Admittedly I have a series of e-mails from Mr Sowter on the design side so perhaps that's what's missing? Easy to keep cloning a popular line, less so to actually engineer something??
I think its a shame: you offer the service because you are competent and price it accordingly! If you can't offer the service...
If anyone is listening to this old thread then - Where to shop instead?
Andy
Shock! Horror!!
In my time in Blighty, (last Millenium) Sowters were still in Ipswich.Yus non transformer afficianadoes will be unaware that Sowter quality was cos they were hand carved from Unobtainium & solid BS by Suffolk virgins. Not sure St. Ives counts
It's like valves from the great Mullard Blackburn factory, hand made by virgins. Never equaled even today.
I think some of us are overly critical of Chinese quality. I'm certain there are young Chinese engineers/makers who want to make the best cars, speakers, transformers bla bla in the World. We could call them Soichiro Honda and Akio Morita for convenience. It's just that we don't know who they are and their companies.
For at least speakers, if I was returning to commercial production, I would go to a particularly factory in Guang Chou, not for the price but for the quality. I tried but failed to get similar quality from our Western factories in da previous Millenium.
My last communication with Ipswich/ Sowter was 20 yrs ago, so times and all of us have moved on.
Try ...........https://transformers.uk.com/... I had several cutom made units, mains and audio done by them and worth asking what old /traditional designs they can make.. Their website claims so. To my surprise I spec´d a Williamson transformer construction for my high power parallel P-P with multiple secondaries. Naturally there is a price, otherwise it´s a fix-finished off the shelf from another vendor.
Bench baron
I ordered probably what was close to the last set of custom transformer designed by Brian. Made me a pair of 5K output transformers and a mains transformer for my A2 801A single-ended amp. He even put the filament windings on the mains with multiple screens, no noise issues whatsoever. Shortly afterward I was told that service was being discontinued.
For custom mains and filament, Toroidy has done a great job for me recently. If you are willing to pay a premium, Monolith Magnetics does very nice custom work.
For custom mains and filament, Toroidy has done a great job for me recently. If you are willing to pay a premium, Monolith Magnetics does very nice custom work.
Shock! Horror!!
...
For at least speakers, if I was returning to commercial production, I would go to a particularly factory in Guang Chou, not for the price but for the quality. I tried but failed to get similar quality from our Western factories in da previous Millenium.
Thank you for this refreshing pov!
I must say i had been quit impressed by some Chinese mic's some 10 years ago: not clones of anything and they sounded really good!
It's sure there is some very talented people in China: how could it be different with so many people. And the same could be said about India too ( we have brilliant example within members of this forum).
Too bad the language barrier is here with Chinese people ( at least for me as expressing myself in English is already a miracle and i don't see how i could acquire Chinese in my lifetime...).
In respect of custom transformers, I have had good service from and can recommend Automatic Electric Europe (here: https://www.aeetransformers.com/).