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Old 25th January 2014, 06:59 PM   #1
mogliaa is offline mogliaa  United Kingdom
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Default Measuring CCS impedance

Hi All,
I've been playing with a test setup similar to Gary Pimm's shown here: CCS performance measurments .

When I tested a CCS I've got good results but was limited by the noise floor of my soundcard adapter. Here is a summary of my tests: More on CCS | Bartola Valves

Click the image to open in full size.

Actually the 1Meg resistor is only 47k to speed up charging time of the coupling capacitor

I built recently a 60dB differential preamplifier which I used with success in measuring complex impedances. However, when I try to use same setup is not possible. The CCS will develop a voltage across the 10ohm sensing resistor which obviously is way too much for a 60dB preamp and it saturates the stage.
I may be blind and not see an obvious way of fixing this, but how can I use my differential preamplifier to capture the low level signal without getting full clip at the output? I thought about returning the CCS DC before the sensing resistor but this puts the HV supply plus the 4K7 resistor in parallel with the CCS and will distort the measurement of the impedance as the CCS Zo is in parallel with the 4K7 resistor. I thought about adding an extra CCS between the power supply and the CCS DUT, but this again puts two CCS in parallel when measuring impedance.

Am I missing something really obvious here?
Thanks for the help
Ale
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Old 25th January 2014, 07:04 PM   #2
Elvee is offline Elvee  Belgium
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Why not use an AC coupling, just like for the stimulus?
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Old 26th January 2014, 08:29 AM   #3
mogliaa is offline mogliaa  United Kingdom
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I wanted to avoid a reactive component straight into the measuring point. At least I'd need a 10uF connected between R1 and the positive input of the differential preamplifier. Also a 10K shunt resistor will be needed between the + AND - inputs of the differential preamplifier to allow the coupling capacitor to charge. Should settle within a second and shouldn't impact much the measurement as being in parallel with the 10ohm resistor in AC.
In theory should work but wouldn't be the most accurate way of doing this...
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Old 26th January 2014, 09:57 AM   #4
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Mr. Jung explained the test procedure here.
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Old 26th January 2014, 10:07 AM   #5
mogliaa is offline mogliaa  United Kingdom
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Thanks Jazz. Definitely something on these lines is the way to go. I think my Rigol waveform generator will disconnect the output if a DC current goes through it, So I suspect it won't work by stacking it to the DC supply. I may need an alternative workaround to this...
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Old 26th January 2014, 04:35 PM   #6
tvrgeek is offline tvrgeek  United States
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When I modeled every 2-device topology I could think of, a feature became clear you won't see on the bench. The stiffness varies with frequency. So before doing the bench verification, I recommend doing a investigation in SPICE. On the bench, do what Jung said.

I settled on using feedback pairs. With good devise selection, they have less drop so better for VAS, and are "good enough", far better than a LED/BJT but not the super stiff JFET/BJT cascode. They hold up past 20K pretty well, but a self biased FET is the champ for that.

After about a year of fiddling, I concluded that one has to make a real amp with a socket to swap then in and out as what the models tell you may not correlate with sound. Is consistent better than absolute stiffness? If your rails are regulated, is a resistor sonically better?
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Old 26th January 2014, 05:19 PM   #7
RNMarsh is offline RNMarsh  United States
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That reminds me --- back in the 1970-ish era, there were current diodes like 1N5314 etc. I used them because the higher voltage ones (50v I think) were made with cascoded FETs for higher Zo. simple and small and just a drop-in part.
They are still around, I'm sure.

Thx-RNMarsh
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Old 26th January 2014, 05:58 PM   #8
tvrgeek is offline tvrgeek  United States
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If I am not mistaken, it was Nelson Pass that said stiff CCS's measure great and sound bad. I wonder if it is the change across the BW?

1N5314 $5.70 at Mouser. Much thanks for the tip.
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Old 26th January 2014, 05:59 PM   #9
tvrgeek is offline tvrgeek  United States
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http://www.centralsemi.com/PDFs/prod...N5283-5314.pdf
really low current though.
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Old 27th January 2014, 02:10 PM   #10
DF96 is offline DF96  England
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Unless the CCS impedance is very high, its linearity matters too. Simulation may be helpful here too.
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