WD SUT's

[bob orbell]

Hi all, I am still not completely happy with my SUT's from when Peter C was in charge, so I have done a few tests of my transformers and a Pickering step up amp. there are a few pictures, so follow the posts. Picture 0001 is direct connection from generator to scope, 0002 is the same but in auto mode.

[bob orbell]

Picture 0003 is one transformer, at 1000Hz, picture 0004 is at 10000Hz.

[bob orbell]

Picture 0005 is at 20000Hz, and 0006 is at 50000Hz.

[bob orbell]

Picture 0007 to 0010 are the SS Pickering, and as can be seen, the wave form is very little altered, compared to the transformers.

[bob orbell]

The sound of the Pickering is very very nice, why is the transformer distorting the signal?, any thoughts anyone. BOB

[Richard]

Hi Bob, they look good to me and the distortions are just limitations of bandwidth and phase shift which you'll find on TX coupled gear at high and low frequencies. When/if you set up Phono3S adjustable capacitors you'd use 1KHz SW and tweak for the cleanest corner. Pic 03 shows a little LF phase shift at 1KHz, 04 looks good for 10KHz, 05 shows a little HF roll off at 20KHz and 06 shows more HF roll off at 50KHz. Hopefully John will add/amend my thoughts. Did you check at lower F ?

[bob orbell]

Hi Richard, I have set the trimmer capacitors exactly as you have said using the MM input a long time ago, the problem is, the phono sounds so much nicer with the Pickering SS step up amp than the SUT's, I did ask questions some time ago about changing the WD SUT's for something like Sowter's, and John replied saying that the WD ones are good, I now see that the one's Matthew supply's are different. Also, I can never remember being unhappy with the SUT that WAD supplied, these were 1-10 only. Any thoughts. BOB

[John Caswell]

Hi Bob Richard et al,
There are a few question to be asked here. Where are you looking at the outputs, at the output of the phono stage or outputs from the step-up amp /transformer? I assume that the Pickering StepUp amp is just that, an amplifier with no bandwidth limitations, in which case the results given are just what I would expect, whereas the transformers have bandwidth/phase shift limitations especially with the 10K, 20K and 50K square waves which tend to equate to 100K, 200K and 500K sine waves. FAR too high for a phono stage try a 2-3K square wave (= 20K - 30K sine wave)
Sowter specify a roll of -3db at around 100k for their 8055 transformer but that will depend somewhat on circuit configuration, and looking at the Morgan Jones book, my phono stage results are similar to his.
I would say that generally the pics shown give a very good idea of how good the phono stage is - I feel that you are worrying over very minor problems. I don't feel that changing to another transformer type will help the situation, no matter how much you pay - you are up into the law of diminishing returns

[Richard]

Hi both, yes I just scoped my Lundahls and got very similar results to Bob. The trace was cleaner (perhaps due to less noise in my hook-up with shielded cables) but there are a few cycles of ringing on the leading edge at 1KHz, which develop with increased F to look similar to Bob's at higher F, and a very sloped top as F was reduced down to 20Hz. But music isn't square waves is it Switch to sine and things look much happier with a flat audio bandwidth rolling off 20Hz and 20KHz.

[bob orbell]

Thank's for the reply John, the tests I did were with a 47k load at the outputs of the Pickering and the secondary of the transformers with same 47k load, just for my curiosity, I will try again with the frequency you suggest, I am at this moment in time reading all about audio transformers and take on board your comments. BOB