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bee696 31st July 2006 03:55 AM

Suggestion on the input transformer for tube preamp
I should use a 600:600 or 10K:10K input transformer? they're both 1:1 transformers, but what are the differences? thz!

BHD 1st August 2006 06:54 PM

I've been wondering about this myself.

Not to derail your thread, but I've been considering experimenting with transformer coupling and phase splitting. Edcor has a line of really inexpensive interstage transformers with combinations of 150, 600 and 10K ohms. Some are 1:1 and others are not. Here's the link:

I want to build a PP amp using an input transformer as phase splitter, coupled to the two triode sections of a 12SL7, this would then be transformer coupled to a 12SX7, which would then transformer coupled to a pair of UL or triode strapped EL34 tubes and a 6.6K output transformer.

So, the amp would be transformer coupled and differential from input to output. The tubes mentioned would be used because I have them on hand.

I have done some searching on the forums, but I'm still having a bit of trouble wrapping my smooshy grey matter around some of these concepts.

The subject is discussed in these threads, I hope that some people can shed some light on the subject for those of us that are having some trouble with these.

I think one of my main questions is why you would pick one of those interstage transformers over another, 10K/600, 600/150, etc... for a given application - and if not, in what application would you use the other values?

CijTele52 2nd August 2006 03:56 AM

If you source's output impedance is low enough to drive 600ohm load, you may use 600:600 because it will pick up less hum. If you have high impedance output (like tube output), 10K:10K should be your choice.

NickC 2nd August 2006 04:43 AM

For input trans, some caveats.
They picked up easily, proper placement is a must. Worst problem you will know about the problem after putting it into a cassing. On bread board and prototype somehow the problems is not so apparent perhaps of the extra spave.
Input trans ratios are like you say 1:1 or sometimes used as a stepup like Sakuma designs.

For 1;1 you can get away with 10K input impedance, it is low but it is fine with valve amps. You will notice a drop in volume due to the impedance but there is no signs of drop in dynamics. I personnely used a 600:600 in my preamp it is really great. However because i used a TVC before my preamp it is okay to drive 600ohms however i took the preamp or used it differently there will place some strain on the cdp or phonopreamp. So with 600 ohms mobility of u bring to friends place or swaping equipment is not advisable.
Plus point 600 ohms like mention has lower noise pickup than 10K as it has lower impedance. Noise find it harder to induce to a higher impedance.

With all the bad points and caveats, input trans is like a magic bean. Get it right and you will the benefits of wider soundstage, more open sounding and very more fluid sounding.

Hope this helps

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