Measuring transformers

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SimonC
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Re: Measuring transformers

Post by SimonC » Mon Jul 29, 2019 8:27 am

Max N wrote:
Sat Jul 27, 2019 8:11 pm
Nick wrote:
Sat Jul 27, 2019 6:05 pm
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
...
I've just had a similar problem, the culprit was my lads fancy multi-colour led lit keyboard...

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Mike H
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Re: Measuring transformers

Post by Mike H » Fri Aug 02, 2019 12:19 am

Max N wrote:
Sat Jul 27, 2019 3:54 pm
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....
Looks pretty much like every other transformer I've checked the freq response of (including LTspice models) - the LF roll-off is due to the core being too small at that freq, the HF roll-off is simply because of the rising reactance of the windings, adding series impedance. Also there is inter-winding capacitance, so you can get a low-pass LC filter effect as well. But I see nothing unusual about that plot except that the HF is quite high.
 
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Re: Measuring transformers

Post by Max N » Sun Aug 04, 2019 5:14 pm

Mike H wrote:
Fri Aug 02, 2019 12:19 am
Looks pretty much like every other transformer I've checked the freq response of (including LTspice models) - the LF roll-off is due to the core being too small at that freq, the HF roll-off is simply because of the rising reactance of the windings, adding series impedance. Also there is inter-winding capacitance, so you can get a low-pass LC filter effect as well. But I see nothing unusual about that plot except that the HF is quite high.
Hi Mike
I think the plot you're looking at is the roll-off of my sound card. It was a loop-back test, the transformer wasn't connected.
This was the plot of the transformer - flat from 4Hz to 98kHz. I need to figure out how to extend the measurement below 4Hz to see where the LF limit is.....
Attachments
Andy Grove IT frequency response and phase, no pads.jpg
Andy Grove IT frequency response and phase, no pads.jpg (17.03 KiB) Viewed 288 times

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Nick
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Re: Measuring transformers

Post by Nick » Sun Aug 04, 2019 6:52 pm

You also should drive the transformer into its expected load and from its expected source impedance to see how it will behave in actual use.
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Re: Measuring transformers

Post by Max N » Mon Aug 05, 2019 9:42 am

Nick wrote:
Sun Aug 04, 2019 6:52 pm
You also should drive the transformer into its expected load and from its expected source impedance to see how it will behave in actual use.
Yep, that’s next on the list. (As I said, I hope to measure everything that’s on that Jensen data sheet)

The source impedance of the soundcard is quoted as 94 ohms balanced, and I’ve done a quick test to verify that. So currently my setup is a long way from how I have been using these transformers (mainly driving them with 5687 and similar). One of the motivations for this measurement exercise is to understand how to get better performance from the transformer.
My thoughts are different valves, or parallel valves, or some kind of buffer or follower....
But first I need to establish exactly how low the impedance needs to be - or more accurately, where is a good compromise.....

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Re: Measuring transformers

Post by simon » Mon Aug 05, 2019 1:16 pm

Nice thread Max, looking forward to reading more

Max N
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Re: Measuring transformers

Post by Max N » Tue Aug 06, 2019 1:15 am

simon wrote:
Mon Aug 05, 2019 1:16 pm
Nice thread Max, looking forward to reading more
Cheers Simon, it’s taking me a bit longer than I’d like. Hope to have some more measurements soon.

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Re: Measuring transformers

Post by Max N » Thu Aug 08, 2019 10:47 pm

As Nick suggested, here is some data showing the effect of source impedance. I haven't investigated the effect of load impedance yet, but the 6i6 is quoted as 52kohm which I guess is not too unrepresentative of the grids of the following stage as long as we stay away from A2. I will do some tests later at much lower impedance to simulate driving grid current.

I wanted to extend the test to lower frequencies but I found that I had to drop the sampling rate of the 6i6 down to 48kHz, which then meant an upper frequency of 24kHz. So what I have ended up with is two separate sets of curves - first here is the low frequency results from 1Hz to 20kHz.

The upper family of curves are the amplitude response, using the Y-axis graduated from +5dB to -20dB.
The lower curves are the phase response, using the other Y-axis graduated in degrees.

Red is 0 source impedance (I think the measurement method eliminates the impedance of the 6i6 because of the way the reference signal is derived)
Green is 680 + 680 ohms
Blue is 1200 + 1200 ohms
Magenta is 1800 + 1800 ohms (roughly representative of driving the IT with one half of a 5687 per phase). This looks to be about 1.5dB down at 3Hz
Attachments
Andy Grove IT frequency resp with 0, 680, 1200 1800 source impedance LF.jpg
Last edited by Max N on Thu Aug 08, 2019 10:59 pm, edited 1 time in total.

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Re: Measuring transformers

Post by Max N » Thu Aug 08, 2019 10:56 pm

And this is the higher frequency result, from 70Hz up to 90kHz.
Same colours/impedance as before.
Not especially informative because of the upper frequency limit of the soundcard. The 1k8 + 1k8 (magenta curve) is starting to dip at 90kHz, about -0.5dB compared to the response at 3kHz. I will have to use a signal generator and scope to get a -3dB point for each source impedance.

Next step I will drop the load impedance to simulate A2 operation of the following stage......
Attachments
Andy Grove IT frequency resp with 0, 680, 1200 1800 source impedance HF.jpg

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