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Old 11th November 2012, 11:21 AM   #2051
KSTR is offline KSTR  Germany
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^
Placeholders for short/jumper from VAS to OPS and a trimmer for Iq.
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Old 11th November 2012, 09:45 PM   #2052
RNMarsh is offline RNMarsh  United States
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Ok. I'm back in the land of OZ (USA). Got pcb? Got prototype ? It's been 5 weeks, gentle men. I've been all over the place and did a lot in 5 weeks. Whats the progress on the opa in that time? Thx-RNM
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Old 11th November 2012, 10:29 PM   #2053
fas42 is offline fas42  Australia
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Originally Posted by RNMarsh View Post
Whats the progress on the opa in that time? Thx-RNM
Well, we reckon its backside looks pretty clean ...

Frank
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Old 11th November 2012, 10:54 PM   #2054
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Quote:
Originally Posted by RNMarsh View Post
Ok. I'm back in the land of OZ (USA). Got pcb? Got prototype ? It's been 5 weeks, gentle men. I've been all over the place and did a lot in 5 weeks. Whats the progress on the opa in that time? Thx-RNM
This is a hobby project, we spend what time we can spare on it. Hopefully it's not like Gibbs' boat in the basement.
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Old 11th November 2012, 11:18 PM   #2055
RNMarsh is offline RNMarsh  United States
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I guess guilt doesnt work. How about - have to finish it becuase you are not retired yet? ["Retirement - never having to finish anything because everyday is Saturday"]

Would rude work? I'll try that next. :-)
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Old 11th November 2012, 11:28 PM   #2056
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Would rude work? I'll try that next. :-)
Nope, rude will just entice a response in like kind. I would hope the process has almost as much value as the outcome. There have been lots of recommendations for good discrete devices and I hope folks are picking up some nuggets from the design process.
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Old 11th November 2012, 11:34 PM   #2057
mlloyd1 is offline mlloyd1  United States
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scott:

us folks are indeed.

thanks,
mlloyd1

[QUOTE=scott wurcer ... I hope folks are picking up some nuggets from the design process.[/QUOTE]
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Old 11th November 2012, 11:54 PM   #2058
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thanks,
mlloyd1
Always welcome my friend, we should do an SY home invasion sometime. My last visit was ruined by Sandy.
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Old 12th November 2012, 05:48 AM   #2059
RNMarsh is offline RNMarsh  United States
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I am enjoying the info and sim and all, also. FYI [Just pushing - gently - to keep it going at more than a snail's pace.]

Signed - impatient for more.

Last edited by RNMarsh; 12th November 2012 at 05:53 AM. Reason: impatient for more
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Old 12th November 2012, 10:40 AM   #2060
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Default JFET 990

I'm posting what is very near a final[*] design for a JFET 990. Such beasts have appeared in the past so what's different?
  • According to LTspice, it should have distortion into 600R up to 20kHz at SW-OPA levels. ie much better than JE-990
  • one less amplifying stage so should (??) be more stable.
  • Needs less Iq than SW-OPA for low distortion into 15R
  • +/- 60V/us slew
The i/p stage is BF862 with J111 cascodes as per SW-OPA. This is the bit I'm most suspicious off as I have relatively little experience with FET i/ps. I'm indebted to fas42 for his more accurate BF862 model but there doesn't seem to be much difference using that or any of the other FET models from various gurus.

Guru Wurcer's recommendations on FET matching etc for SW-OPA are applicable.

Q2 is shown at the +ve rail cos I suspect it could be part of a dual diode with D3.

D1 is to prevent one manifestation of latching on overload.

This is actually a 2 stage amp. The i/p & current mirror are the Transconductance Stage.

C4 turns the VAS & Class B o/p into the single integrator which sets the OL bandwidth etc. This is Cherry's solution.

The input to the integrator is very sensitive and anything connected to it must be clean. Q16's Cob is unfortunately not clean as it's modulated by Vcb. The Q15 cascode takes it out of the picture.

In the original 990, the emitter follower feeding the VAS does something similar. Unfortunately, that topology requires a diode across the Miller capacitor to avoid latching which re-introduces the non-linearity.

Keeping the bases of Q10/15 at Hi Z (not connecting anything there)
  • avoids the evil Class B distortions enumerated by Self with his Lo Z drive and critical Iq settings
  • maximises slew as nothing slows it down at that point
  • reduces xover distortion to the minimum even with small Iq. eg This circuit has less distortion into 15R with Iq 7mA than SW-OPA with 10mA

But the Cherry capacitor C4 is referenced to the +ve rail which reduces +ve rail PSR. The 'cure' is injecting an identical compensating current via C5 into the current mirror. I've used this dodge in a Jurassic low feedback PA and so does Guru Wurcer in the AD797 datasheet.

Using TI-an1516.pdf methods

CMRR 126dB to 20kHz
-ve PSR 125dB to 20kHz
+ve PSR 129dB falling to 93dB@20kHz

C4/5 should be 15p for 10x & greater gain
47p for 3x
100p for 1x

Need to do more work on stability with reactive loads but preliminary sims show similar behaviour to AD797. R18=10R on the output seems to work well but further optimisation and better recommendations are possible.

Q9 allows Iq to be varied over a wide range and should be strapped & superglued to Q10/15.

Re-arrangement of the current sources is to avoid latching. C6 is there for the same reason rather than to improve PSR

Do I believe this SPICE sh*t?

As apart from the 15R results, the distortion products are mostly around -100dB, it is likely that this and SW-OPA will be as critical as AD797 and OPA627 for layout & decoupling. In many situations, layout & decoupling is the MOST important factor in noise & distortion.

But most of the improvements have been in the direction I expect from previous experience so yes, I believe it may have similar distortion to SW-OPA. Not so sure about CMR & PSR though.

More to come.

[*] in as much as anything which hasn't been built can be called final
Attached Images
File Type: gif 990-3rl11nov12.gif (50.3 KB, 418 views)
Attached Files
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Last edited by kgrlee; 12th November 2012 at 10:56 AM.
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