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Old 27th January 2014, 09:24 PM   #11
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Originally Posted by tvrgeek View Post
If I am not mistaken, it was Nelson Pass that said stiff CCS's measure great and sound bad.
That's why he used a light bulb.
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Old 28th January 2014, 11:28 AM   #12
DF96 is offline DF96  England
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Well-designed circuits measure sufficiently good but, to some, sound bad. Different people draw different conclusions from this.

It seems to me that if you want a CCS then you should have a good one: high impedance, linear impedance, not too frequency dependent apart from the unavoidable stray capacitance. Device parasitic capacitances are the thing to watch, as they are very non-linear.
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Old 28th January 2014, 12:29 PM   #13
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Some thoughts on the depletion MOSFET cascode CCS:

CCS performance measurments
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Old 30th January 2014, 10:02 PM   #14
tvrgeek is offline tvrgeek  United States
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Hmm. Every time I mentioned a depletion mode FET, I got sneered at. They make sense to me.

Anyway the question I keep bring up is which is more important to the sound, maintain a consistent impedance through the audio range, or being as stiff as possible? Where a self-biased JFET is not as stiff as the standard BJT-LED, it has much wider BW. The JFET/BJT cascode is many times stiffer, but falls off even faster. At some point, there is always a diminishing return from every parameter.

So, is the slighter stiffer feedback pair that holds up a little longer SONICALLY better than a single FET? How much better than a resistor is really needed? 5 times? The FET can do that. 1000 times? You need the cascode. A cascode has a larger minimum drop which may cause issues in a VAS, but be fine in the IPS. What happens to all of the choices when at clipping? Some get ugly.

DF is quite correct, if you play with different devices, you can get very different results. For any topology, you need the correct component selection. Green LED's are more linear with temperature than RED, but than again, the CCS should be in a very stable environment by definition so it should not matter.
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Old 31st January 2014, 03:41 PM   #15
DF96 is offline DF96  England
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Quote:
Originally Posted by tvrgeek
Anyway the question I keep bring up is which is more important to the sound, maintain a consistent impedance through the audio range, or being as stiff as possible?
Stray capacitance puts these two objectives at odds with each other. Better to allow for the capacitance, and ensure that it and the CCS impedance are as linear as reasonably possible.

Fortunately, linearity and high impedance are not necessarily enemies as being really good at one means you don't have to be quite so good at the other.
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Old 31st January 2014, 07:07 PM   #16
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Ale: While the "Noise Inspector" is a nice implementation by TI -- the circuit noise with the OPA37 front end is 30nV/RtHz -- you can do better with a single chip design like the SSM2019 or THAT1510 orTHAT1512. Which are differential in, single out. With A=1,000 the SSM2019 is spec'd at 1nV/RtHz. The measurement amplifier I used for the regulator shootout used a pair of back-to-back electrolytics and clamp diodes with the SSM2019 -- (WJ used the SSM2017 in his 1995 articles, but it has been supplanted by the 2019). Response for the SSM2019 was flat from 10Hz to 100kHz.

Next time I will include the clamp diodes, but will also have a switch so that the coupling capacitor can be "pre-charged". (Even with AC Coupling I have blown right through the protection). It will be balanced in-balanced out with BF862 and OPA1632 and a servo.

Last edited by jackinnj; 31st January 2014 at 07:11 PM.
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Old 31st January 2014, 08:19 PM   #17
tvrgeek is offline tvrgeek  United States
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Quote:
Originally Posted by DF96 View Post
Stray capacitance puts these two objectives at odds with each other. Better to allow for the capacitance, and ensure that it and the CCS impedance are as linear as reasonably possible.

Fortunately, linearity and high impedance are not necessarily enemies as being really good at one means you don't have to be quite so good at the other.
I modeled a dozen or so two-device CCSs and I would say this is not so. Yes, the device selection is important as you can make the best circuit poor with a bad choice, but the topology seemed to have a large effect on the impedance over frequency and it was the primary driver of basic stiffness.
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Old 31st January 2014, 10:00 PM   #18
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A- the circuit noise with the OPA37 front end is 30nV/RtHz --
My error, I read the table incorrectly, it is 4nV/RtHz
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Old 1st February 2014, 08:32 AM   #19
mogliaa is offline mogliaa  United Kingdom
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Hi Jack,
Thanks for your suggestion. It is 4nV/SQRT(Hz) as you say. I'm happy with it so far. Never used the SSM2019, THAT1510 orTHAT1512. Did you implement a different circuit topology?
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Old 1st February 2014, 10:54 AM   #20
tvrgeek is offline tvrgeek  United States
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That's why he used a light bulb.
My old Heath signal generator did this as well. Talk about sensitivity to vibration! It did provide mostly thermal compensation pretty cheaply.
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