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Old 26th August 2012, 10:23 PM   #301
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The j175 is not that noisy, if you get a good batch. If they are too noisy, change manufacturers.
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Old 26th August 2012, 10:30 PM   #302
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Originally Posted by coluke View Post
@dirk: nice job


L.
Gee thanks. Considering my lack of experience, that's a fine compliment.
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Old 26th August 2012, 10:53 PM   #303
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Originally Posted by john curl View Post
If you can't get the parts, find a good IC.
For the record, I did a simple Google search for two complementary jfets, the J113 and the J175. I found both of them for sale under $0.50 each. You just have to buy a bunch of them and select them out. If you cannot invest $100 for this, then buy an IC. It will give you better performance.
Yeah, unfortunately you're right. An LME49710 + LME49600 will do this job just fine and take up less space, but it's just not fun or a challenge.
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Old 26th August 2012, 11:22 PM   #304
Bonsai is offline Bonsai  Taiwan
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Originally Posted by john curl View Post
This is a recommended alternative to a complementary differential fet design. This design was used in several professional mixers as well as in three 30ips master analog recorders.
sorry about the bad copy, the original schematic goes back to about 1977.
I quite like this, but the FET output stage is a bit idiosyncratic for my liking. You could use a FET source follower with a BIP current source load for true class A output? No need to skimp on the supply current - this is DIY.
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Old 26th August 2012, 11:25 PM   #305
Bonsai is offline Bonsai  Taiwan
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Originally Posted by dirkwright View Post
I juiced up the whole thing and distortion is now 0.0005% @ 20kHz into 30 ohms @ 1.1Vrms. DC offset on the output is -1mV.

I have since added Miller capacitors (@10pF each) to get rid of a 15dB spike @ 10MHz. Distortion is now 0.0006% under the above conditions. S/N is 120dB @ 10kHz.

Unfortunately, large heatsinks are required on the outputs.

Finally, something I understand.
Nice!
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Old 26th August 2012, 11:36 PM   #306
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For the sake of circuitry discussions, this one from John Linsley Hood I found in my archive.


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File Type: gif JLH JFET Gain Block.gif (8.3 KB, 475 views)
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Last edited by EUVL; 26th August 2012 at 11:57 PM.
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Old 27th August 2012, 12:56 AM   #307
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Originally Posted by EUVL View Post
For the sake of circuitry discussions, this one from John Linsley Hood I found in my archive.


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OH! I'm going to try a variation on that! thanky thanky!
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Old 27th August 2012, 02:00 AM   #308
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Originally Posted by john curl View Post
Yes, these are the devices. Many manufacturers still make them. However, they have a LARGE Idss spread, so only dumb luck will match the P's to the N's, but that is not very important, within limits. Just good N to N, and P to P is the most critical.
Would you elaborate ?
It would seem to me N to P would be more important
than N to N and/or P to P ?
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Old 27th August 2012, 03:20 AM   #309
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Originally Posted by bcarso View Post
At least, if the two output devices are reasonably well-matched, and perhaps with some ballasting R in the sources, the quiescent bias can be not too unstable.

Not enough. Thermal dependence of current of 1'st stage (diff-pair + current source in the tail) is amplified by the 2'nd stage, and thermal dependence of it's current _added_ to it.
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Old 27th August 2012, 05:36 AM   #310
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Originally Posted by john curl View Post
This is a recommended alternative to a complementary differential fet design. This design was used in several professional mixers as well as in three 30ips master analog recorders.
sorry about the bad copy, the original schematic goes back to about 1977.
I think I'd start with this one, with updated JFETs. The output stage can use a J202 (maybe with a 2n5461 as the p-ch compliment), perhaps with cascoded drains to increase the rail voltage capability, a la Burson HD/OPA-Earth.

The input stage can be bipolar (2sc1845, etc.) or cascoded 2sk170. Also worth looking at is the Toshiba 2sk241 small-signal depletion-mode MOSFET for the input stage - low Vdsmax makes a cascode mandatory.

Last edited by linuxguru; 27th August 2012 at 06:05 AM. Reason: correction
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