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Old 23rd September 2009, 05:55 PM   #1491
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Quote:
Originally Posted by PMA View Post
Because there is no global feedback to equalize components tolerances. I have the circuit in MC, so I can see what active part tolerances are resulting in.
Ahh yes of course. So this BT circuit needs carefull matching of the active devices.
Sounds like a pain, but I guess that's the price for no global feedback.

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Old 23rd September 2009, 05:56 PM   #1492
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Because there is no global feedback to equalize components tolerances. I have the circuit in MC, so I can see what active part tolerances are resulting in.
Global feedback does not equalize component tolerances.
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Old 23rd September 2009, 05:56 PM   #1493
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PMA, do you have access to a 'sensitivity matrix' through Spice, or is it buried deep in the program?
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Old 23rd September 2009, 05:57 PM   #1494
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Originally Posted by syn08 View Post
Global feedback does not equalize component tolerances.
Okay, suppress in LG ratio.
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Old 23rd September 2009, 05:59 PM   #1495
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So this BT circuit needs carefull matching of the active devices.

jd
Both matching and selection of proper components - output MOSFETs do make a big difference.
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Old 23rd September 2009, 06:02 PM   #1496
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Originally Posted by john curl View Post
PMA, do you have access to a 'sensitivity matrix' through Spice, or is it buried deep in the program?
I do have an access to sensitivity analysis. But I am sorry, I cannot spend much of my time with this. Or you wanted to hire me?
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Old 23rd September 2009, 06:12 PM   #1497
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Thanks PMA, I just wanted to know if it was accessible to anyone here. I haven't used it since the '60's, but it looked like a useful approach for answering questions such as resistor tolerance vs ? .
Probably easier to cut-and-try.
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Old 23rd September 2009, 06:12 PM   #1498
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Both matching and selection of proper components - output MOSFETs do make a big difference.
Of course. Look at the gain in http://www.diyaudio.com/forums/lounge/146693-john-curls-blowtorch-preamplifier-part-ii-post1930934.html it's proportional with the MOSFET transconductance.

This is just another reason why this design is a PITA. High transconductance MOSFET devices are very hard to exactly match at, say, 1% AND over a wide range of drain current. Not to mention the gain which is also proportional to the load impedance (which is, to add insult to injury, never purely resistive). Not to mention the MOSFET AC parameters matching.

Last edited by syn08; 23rd September 2009 at 06:16 PM.
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Old 23rd September 2009, 06:15 PM   #1499
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What this circuit lacks: FM distortion, higher order harmonic and IM distortion, instability at either low or high frequencies.
Pretty good tradeoff for quality audio.
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Old 23rd September 2009, 06:15 PM   #1500
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Quote:
Originally Posted by syn08 View Post
Fold the circuit up/down. Then fold again left right. You got a circuit with two transistors. The gain is approximatively G=(Rd/Rs)*gm*Ro where Rd=R5/4, Rs=R19/2, Ro=R21/2 and gm=gm4*4 Total is G=gm4*R21*R5/R19.

Done by visual inspection, so hope I didn't miss anything. Add the distortion cancellation common to all matched symmetrical circuits and you got the entire picture. Offset, drift, PSRR, CMRR are all horrible, that's the reason why the gain stage is 25% of the entire design (rest goes to servos, power supplies), you would also need 0.1% matched resistors. You also need a stiff metal box to keep the **** out of the common mode and keep the temperature more or less constant.

A substandard design by any criteria of the 21st century.

P.S. You could of course cascode the JFETs and get some decent PSRR, at the cost of some headroom.
You forgot about non-linear input capacitances of output MOSFETs that cause signal rectification and dependence of their idle currents on frequency, hence dynamic distortions. Nulls will drift with frequency as well, that would be reflected by 2'nd harmonic distortion dependence on frequency. PS sag, ripples, noise, and non-linearity will be injected in sources, so such simplicity requires PS complexity.
I would not call it elegant, Sorry John and Pavel. It is primitive.
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