Measuring transformers

[Max N]

I am a big fan of Jensen transformers. Dean Jensen and Bill Whitlock are heroes of mine. You can read a bit about the history of Jensen here:https://www.jensen-transformers.com/history/
One of the things I admire about Jensen is the thoroughness of their data sheets. Here's an example:https://www.jensen-transformers.com/wp- ... 1-dmcf.pdf
(Incidentally, if you look at the data in that sheet you will understand why Jensen transformers are so well regarded in the pro field)
One of the nice things about such a comprehensive set of information is that it tells you exactly how to get the best performance from the transformer (and what to avoid)
A few years ago I built a Pete Millett sound card interface, and a few months ago I bought a copy of the audiotester software. To use these you also need a decent sound card, and I decided to go for a Focusrite Scarlett 6i6 2nd gen. I need a project to get myself familiar with these tools, so I thought I would try to measure a transformer and generate all the same comprehensive information that Jensen supply.....
Wish me luck!

[Max N]

Getting ready for the first measurement....

[Max N]

This is the frequency response of the 6i6 in a loop-back test. I guess they give it a soft roll-off to sound better?
I think this means the SNR will be worse outside the audio band?
I wonder if the roll-off could be modified....

[Max N]

The front 2 inputs on the 6i6 have gain controls. I've put some 'calibration' marks on the dials at 1v peak-peak and 2v peak-peak.
With the gain turned all the way down, the 6i6 can handle over 20v pk-pk, which is handy

[Max N]

This is a loop back test showing the noise floor and HD of the 6i6. The 1kHz input is about 1v amplitude (2v pk-pk).

[Max N]

This is my first attempt at measuring the transformer. This is the Andy Grove designed interstage transformer, originally for the 211 push-pull project (which I do still intend to finish one day....). I have since used them to drive other output stages, but without ever really understanding how to get the most out of them.....
They are 1+1:1+1, bifilar wound. There's quite a lot of mains-induced noise at 50Hz and harmonics, which I don't remember being a problem in the past....

[Max N]

Hmmmm, I was 'baking' some E810F and 5687 using a benchtop power supply while running the test...

[Max N]

Things are a bit better with the bench psu off, but mains hum is still there

[Max N]

Now I've earthed the frame of the transformer, the 50Hz hum pick-up is gone. There's a bit of noise or something at about 30kHz.
I should have said earlier, the reason there are 2 curves now is that one is showing what's going in to the primary, while the other is what's coming out of the secondary.

[IslandPink]

Great work.
Did you take away the soundcard distortion profile from the transformer results ? - only, some of the peaks are lower than the soundcard.

[Nick]

The 30k may well be from the pc that you are using to measure, or any other switching supply in the region.

[Max N]

If I ground either the primary centre tap or the secondary CT, the 30kHz noise is gone. Looking at these traces, I think we can say that this transformer is not picking up any measurable noise, down to about 130dB below a 1v signal. Since these ITs are intended to drive output valve grids, they will be carrying at least several volts of signal. This seems like quite an impressive result for unshielded transformers?
I guess it's because the windings are bifilar and/or very well balanced, so any noise signal induced in the windings will be common-mode and not seen by the balanced input of the soundcard. If I use well-balanced pairs of valves then I should see the same common-mode rejection in-circuit?

[Max N]

First attempt at frequency response and phase. Note that the roll-off in the response of the 6i6 isn't a problem here, because audiotester is using one input channel to measure the input signal (reference channel) and the other to measure the output.
I'm not certain that I have the two channels the right way round, so the phase lag might actually be a lead....
In any case the departure from linear phase (DLP) is less than 2 degrees at 4Hz.
I need to measure below 4Hz using a lower sample rate, so I'll do that tomorrow if I get a chance. Obviously with a 196kHz soundcard I can't measure above 98kHz...
But within the constraints of what I can measure, this looks pretty good so far.

[Max N]

Great work.
Did you take away the soundcard distortion profile from the transformer results ? - only, some of the peaks are lower than the soundcard.

No I didn't. Good point, I need to figure out why that might be......

[Max N]

The 30k may well be from the pc that you are using to measure, or any other switching supply in the region.

OK, thanks Nick. I might try running the laptop off the battery tomorrow. Would be good to get to the bottom of it. Might become a problem in the future when I'm measuring phono stages
...