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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

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Old 26th November 2012, 01:14 AM   #2211
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I use to purchase freon by the 50 gallon drum back in the day, but alas that is long since over. I think that they would probably put you in jail if you used freon that way today. This was the blowing agent that we used in the manufacture of polyurethane foam horn lenses. Just watch out on a hot summer day, the boiling point is more than low and it would blow up in your face if you were not careful. I suspect that pure Isopropyl alcohol will do the trick. 111T is what we used to use in vapor degreasers before that was banned also. Seems that most of the best solvents have been outlawed in the US but not in countries like China that got a special exemption for many more years to come.
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Old 26th November 2012, 01:24 AM   #2212
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China that got a special exemption for many more years to come.
An understatement for sure.
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Old 26th November 2012, 03:06 AM   #2213
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I've been busy and missed this, looks like a great idea.
Thanks, Scott. The LTP and VAS are largely your idea, and the output JFET stage is from a posting by John Curl early in this thread. I added the inductor trick, but that's also due to Walt Jung or somebody from AD in the 60's.

I'll just claim credit for cobbling it all together and simulating it in LTSpice with suitable models.
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Old 26th November 2012, 03:37 AM   #2214
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One of Brad's posts on base-current shot-noise being an additional contributor to noise seems to have vanished into the ether.

It was well outside my ken anyway, but speaking of noise performance, there seems to be another Toshiba JFET that looks good for both the upper and lower LTP cascode elements - 2sk184 in TO92s. 50V, 15mS and moderate Idss, with good noise performance down to 0.5 to 1 mA with Rg=10 kohm.

The samples I've measured show 1.5 to 2.5 mA Idss, which is low but usable in the LTP.
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Old 26th November 2012, 04:01 AM   #2215
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Thanks - great, that settles it. I'll go with J111/J174/J175 (all appear to be multiple-sourced, and I have access to quantity of Vishay/Siliconix J175 locally, which makes the Idss selection easier).

<snip>.
I was just looking at the pdf for the Fairchild devices with a hopeful eye. From what I can see they do not seem to be particularly complementary. The curves appear to be rather different, and iirc the N channel appears to have more gain than the Pch?

Is this going to work? Have I missed something?

http://www.fairchildsemi.com/ds/J1/J175.pdf

http://www.fairchildsemi.com/ds/J1/J111.pdf

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Old 26th November 2012, 04:07 AM   #2216
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One of Brad's posts on base-current shot-noise being an additional contributor to noise seems to have vanished into the ether.
I requested that, having inadvertently posted it in this thread. It applied to a different circuit in a different thread.
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Old 26th November 2012, 04:42 AM   #2217
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... they do not seem to be particularly complementary. The curves appear to be rather different, and iirc the N channel appears to have more gain than the Pch?

Is this going to work? ...
It's only a push-pull source follower, so it will work with maybe asymmetric current swings even with mismatched Yfs, etc. Worst case, it may have 10 dB higher THD than the sim suggests, but it will probably be mostly H2, which won't hurt audible sonics.
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Old 26th November 2012, 05:29 AM   #2218
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but it will probably be mostly H2, which won't hurt audible sonics.
Open loop?
Because as soon as you close the loop it isn't H2 anymore.
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Old 26th November 2012, 09:30 AM   #2219
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linux, are you testing overload behaviour? I've only done some skimpy sims but can't seem to stop it from Phase Reversal on +ve overload with diodes in the usual places.

Your circuit has about 15dB more THD than the FET990 or SW-OPA but that's a fair exchange for 3 less active devices.

Won't do 3.162Vp into 15R but that's an unfair headbanging test.
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Old 26th November 2012, 05:31 PM   #2220
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I've only done some skimpy sims but can't seem to stop it from Phase Reversal on ve overload with diodes in the usual places.
Confirmed - the inductor makes the overload behaviour much worse, but it provides higher loop gain in the audio band.

I'll try some of the usual clamps, but it looks like it doesn't like the inputs/output going within 4V or so from the rails. The BJT output stages are much better in this respect.
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