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Old 11th January 2010, 08:52 PM   #2281
mige0 is offline mige0  Austria
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I very much would appreciate some one to prove – either right or wrong – what I found strange when measuring such line transformers some years ago.


I digged up the following plots showing different distortion figures of the same line transformer with just different source impedance due to no / big / small capacitor:


Click the image to open in full size.

driven without capacitor (-58dB distortion)


Click the image to open in full size.

driven with 100uF capacitor (-57dB distortion)


Click the image to open in full size.

driven with 10uF capacitor (-49dB distortion)

###########
Measurements were taken at output of small (matchbox size) 1:1 no-name transformer @
0.76 Vrms
source impedance ~ 50 Ohm from SSM2142
Load ~ 25 kOhm from SSM2143
0 dB on plots equals ~760 mVrms (wrong scaling on plots !)
############


Strange thing I found here is, that distortion seems to substantially increase with rising source impedance ???


Above would indicate that we should better be pretty carefully with the source impedance (the lower the better) ?
Any comments ?

Michael

Last edited by mige0; 11th January 2010 at 09:18 PM.
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Old 11th January 2010, 09:14 PM   #2282
mige0 is offline mige0  Austria
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Quote:
Originally Posted by panomaniac View Post
I'm going to try to measure the output impedance of the AKM chip today. I'll let you know what I find.
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Old 11th January 2010, 10:06 PM   #2283
Pano is offline Pano  United States
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Well I was not really able to determine the output impedance of the AKM 4393 in the usual way. I used a 5K pot as a load slowly reducing the resistance. Hoped to find the 1/2 voltage point.

But did not. Voltage stays constant until the load drops to about 80 ohms, then it clips badly. Lower than that results in the waveform going nuts. Maybe short circuit protection in the chip. I tested at 40Hz, 1Khz and 6.3Khz all -9dBFS with exactly the same results.

So I'm not sure what the actual output impedance is, but maybe we can call it 100 ohms?
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Old 11th January 2010, 11:44 PM   #2284
Pano is offline Pano  United States
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Come to think of it, I may try to measure AC current into a 1K load. Might be able to figure the output impedance from that.
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Old 12th January 2010, 12:41 AM   #2285
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Hi Pano

Quote:
Originally Posted by panomaniac View Post
Well I was not really able to determine the output impedance of the AKM 4393 in the usual way. I used a 5K pot as a load slowly reducing the resistance. Hoped to find the 1/2 voltage point.

But did not. Voltage stays constant until the load drops to about 80 ohms, then it clips badly. Lower than that results in the waveform going nuts. Maybe short circuit protection in the chip. I tested at 40Hz, 1Khz and 6.3Khz all -9dBFS with exactly the same results.

So I'm not sure what the actual output impedance is, but maybe we can call it 100 ohms?
What you are seeing when looking at the outputs of a voltage output DAC are the outputs of a pair of OPAMPs with feedback. I suspect that the output impedance is very low, less than .1 ohm. The feedback around the I-V conversion in the DAC keeps the output voltage constant until you hit the current limit of the output stage. Looking at the AKM4393 DAC they list the max output current at 3.5ma. They also list 1K ohm as the min load for a DC load and 600 ohms for a min AC load. I'm assuming that they mean 1K to ground per phase or 600 ohms between the phases.

Were you connecting the load between the output phases or to ground?

The way that most of the voltage out DACs have a DC coupled load makes one wonder about if the output stage is running class A? Also makes one wonder if the DC load resistors should remain even if you are driving the primary of a transformer to keep the bias current up in the output stage. On your many different transformer output stages have you left the DC load in place or removed them?

More questions than answers...

Gary
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Old 12th January 2010, 12:50 AM   #2286
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Quote:
Originally Posted by mige0 View Post
I very much would appreciate some one to prove either right or wrong what I found strange when measuring such line transformers some years ago.

snip

###########
Measurements were taken at output of small (matchbox size) 1:1 no-name transformer @
0.76 Vrms
source impedance ~ 50 Ohm from SSM2142
Load ~ 25 kOhm from SSM2143
0 dB on plots equals ~760 mVrms (wrong scaling on plots !)
############


Strange thing I found here is, that distortion seems to substantially increase with rising source impedance ???


Above would indicate that we should better be pretty carefully with the source impedance (the lower the better) ?
Any comments ?

Michael
Hi Michael,

What you are seeing is normal. Almost all transformers will have increasing distortion with increasing drive impedance. If you carefully look at specifications provided by Lundahl, Sowter, and others you will note that the source impedance will be as low as they can get it to show low distortion. Also note that for showing the best frequency response the source impedance may be different than the impedance used for lowest distortion. In cases where a transformer has a resonance at the high end of the frequency range the HF peak will be worse with low source impedance.

Gary
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Old 12th January 2010, 01:40 AM   #2287
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Quote:
Originally Posted by mige0 View Post
I'd be highly interested ! I guess your offer would be gorgeous not only for me but also provide valid data in the context of that very thread.

Thanks a lot for all the effort - color scheme and information content is fine for me - also stimulus level and source impedance all makes sense only for comparison anyway.

Bottom line of your plots show that 3rd order dominates by far the overall picture of that Lundahl transformer - almost not to distinguish from THD at frequencies < 1kHz (2nd order takes over past ~ 1kHz or so).
Second interesting fact I was not aware of is, that there seems to be a distinct frequency of lowest distortion at ~ 4kHz in this case.

What interests me most is, if substantially (> 10-20dB) different performance can be awaited from different makes - what you think?


Michael
Quote:
Originally Posted by panomaniac View Post
Thanks for the test, Gary. I'd love to see some more. My thoughts on the test.
  • 2V RMS max. That's the highest level of most DACs. Might even drop down 6dB for a more typical level. Not sure.
  • Driving with 300 ohms seems good, unless someone can tell us otherwise. And I would say into a 20K load.
  • Cursor at 1Khz
  • 800x600 is quite legible.

An impedance plot might also be nice. I found a lot of difference in the transformers I measured. Different values, different kinks and wiggles. That should show up in the output, tho - right?

I did have a hard time interpreting this graph, as I am more used to seeing single or dual tones with the resulting harmonic fall out. Looking at your graph I think I see that H3 is prominent, and that the curves for the odd order harmonics are smoother than the even order harmonics. In fact I don't understand why the even order plots are so wiggly. Any ideas?
What would the plot look like driving a 20K resistor?
I agree that this format is more difficult to read than the spectrum analyzer output but it contains much more data in a single chart. It would take many charts from the spectrum analyzer to come close to the detail with frequency as a single swept chart. I'm not sure what caused the bumpy even harmonics but I'll see if anything jumps out...

Taking the ideas presented so far and doing some more thinking (danger! danger!) I'm leaning towards 2 voltages- 2Vrms and .2Vrms for testing the transformers. 20Vrms would be nice for transformers that are for interstage use in tube amps, but I don't have a buffer that can do that (yet). For impedance I would like to test at 100 ohms, 300 ohms, and 1K (maybe 3K for tube interstage transformers). For a load on the secondary I think that 20K would be a good compromise

I'll see if I have any tests that will measure the impedance automatically. Too many runs to do by hand.

At this point we are at 5 or 6 (7 or 8 if impedance is done) charts for each transformer. The test only takes 30 seconds to run so it won't be too bad, just lots of images to keep track of. All the data about the test configuration can be included in the text string at the bottom of the image.

To do the different input impedance checks I'll have to modify the output resistors in the M-Audio card in my lab computer. The stock breakout resistors are 150 ohms each I have some 27.4 ohm 1206 SMT resistors I'll drop into the circuit and check out. If all is good I'll setup a 2 pole switch to add the other resistors in line for the higher impedance settings.

Onto another measurement project...

I picked up one of my brother's spare stock Behringer DCX2496 crossovers last weekend. I'm planning on comparing his stock unit to my unit with the alternate input circuit I've been using for a couple of years. I was using 1:1 output transformers directly driven from the DAC for awhile until I acquired a Crown comtech 800 amp for sub duties. After that I was running the DAC directly to both the main and sub amps as they have balanced inputs and did not care about the DC offset.

It will be interesting to see how they compare for noise and distortion when measured in the same test setup.

Gary
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Old 12th January 2010, 04:01 AM   #2288
Pano is offline Pano  United States
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Hey Gary, I believe you are right about the opamp outputs of the chip. I'll try to measure current into 1K tomorrow. It may or may not tell me anything.

My tests were all done in differential mode - so the resistor was between the +/- pins and not to ground. The way a transformer would be connected. I do always leave a resistive load across the DAC output, more to flatten the impedance than any thing else. But that's not a DC load. Might have to try that.

I have done impedance measurements on a number of transformers. I just use the WT3 woofer tester. It maxes out at 10K, so I have to run the tests at a 9K ohms secondary load or less. That still shows a great deal of difference between transformers. I can supply these graphs if needed.
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Old 12th January 2010, 05:04 AM   #2289
Davey is offline Davey  United States
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To measure the output resistance attach a 1k ohm resistor between either output and ground. Sine wave output and take a voltage measurement loaded. Remove resistor and take another voltage measurement unloaded.

Rout = 1000 times (Vunloaded - Vloaded)/Vloaded.

Cheers,

Dave.
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Old 12th January 2010, 05:52 AM   #2290
AR2 is offline AR2  United States
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Hi Guys,

This is awesome that this thread again is pleasure to read. Doing measurements on transformers are music to my ears. This has been my question on transformers for a while. Gary, I cannot wait to see your results.

Thank you Michael, Pano and Gary
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