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Old 28th November 2010, 10:54 PM   #111
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Thanks for the "help". You might as well have cited the Pacific Ocean. For those who really want to know, this is a link to a Vishay app note on jfet current sources that cites the Vgs ratio rule. Of course the same stuff will apply for the SRPP. http://www.vishay.com/docs/70596/70596.pdf

Last edited by wrenchone; 28th November 2010 at 10:58 PM. Reason: url correction
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Old 29th November 2010, 12:33 AM   #112
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After a little work, I found that you can satisfactorily cascode the PN4393 with the PN4391. It took a little more bias current in the 4393. The circuit is shown here.

BTW, taking into account the 2:1 Vgs guideline, I took a look at the 2SK117, which is similar in gm to the PN4393, but has a lower Vgsoff. They also were cascoded very satisfactorily by the PN4391. One thing I noticed is that there is a sweet spot for the bottom fet drain current in the case of the 2SK117. When increased above that point, the distortion starts to rise. Below that current, the distortion also rises. Whether this is the real deal or a Spice artifact remains to be seen. I didn't try to carry the simulation as far for the the PN4393 circuit - it may do the same thing.

One thing one needs to remember is that these are just simulations and should be used as indicators only, pointing the way to promising circuits that still need to be built and tested.
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Old 29th November 2010, 12:36 AM   #113
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Here are the simulation results.
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Old 29th November 2010, 02:18 AM   #114
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The Vishay data sheet states the PN JFETs as 3nVrtHz. There is 5.25nVrtHz in the very first SRPP branch, it roughly looks like. I would use K117 as the basement part so to save some noise.
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Old 29th November 2010, 03:20 AM   #115
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I already have a board stuffed with PN4393s in the front end. It'll stay that way for a while, at least until I can hear how it sounds. A few more parts and some power leads, and it'll get power up tomorrow. The 4393s front end amps are uncascoded in this version, as I didn't even start to simulate cascoded SRPP stages until a day or two ago. The second stage uses 2SK30A, though it could have as well used the PN4303.

Interesting point - using a cascoded SRPP gives the lowly 2N5457 series a new lease on life, as the cascode allows them to run off of a fairly high VCC without fear.

Meanwhile, the noise issues in my living room get spookier. It appears that I have very little AC line immunity, as I can hear the noise from turning off my CD player in the speakers. This may be traceable back to the brute-force supply I'm using for my outboard preamp supply, an unregulated open frame supply manufactured by Acme/Standard Power. The filter capacitor on this supply is huge, and it's followed by a capacitance multiplier, then another capacitor, so nothing should get through by that route. However, I think I may have leakage/noise isolation issues with the transformer in that supply.I may try replacing it. I used it for convenience's sake, as it was hanging around - maybe a mistake.
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Old 29th November 2010, 03:31 AM   #116
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Will be nice to see your latest proto and learn how you liked it. Noise in phono stages is not only a matter of hiss. That varies with speaker sensitivity and total system SNR. Its it cloaks resolution after a point. I am not demanding on that matter in the greater scheme of things, there are much more strict goals set by others, but I would roughly say 2nVrtHz for MM and 1nVrtHz for MC are ballparks for modern designs. Also the PSU noise can easily creep in with such designs and takes some local filtering care if its enough.
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Old 29th November 2010, 03:43 AM   #117
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I think I won't get to the bottom of any RIAA noise issues until I get all the snarky system noise issues fixed. It's a bitch when an amp is quiet as a mouse at work and hums in your living room, which is where you wanted it to sound its best all along....

I do have one of my prototypes in a diecast box at work, waiting until I have some spare time to wrestle with the AP analyzer there. I'm also trying to debug a couple of tube amps, though.
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Old 29th November 2010, 03:53 AM   #118
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When system noise trouble shooting will be over, I can assure you will be surprised of how much music is hidden after comparing a 6nVrtHz MM phono with a 2nVrtHz one for instance. Such things shine by their absence in my experience. One thing at a time though.
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Old 29th November 2010, 05:19 AM   #119
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Well, the front end of my current RIAA preamp (the one actually sitting in my system - I have 2-3 others lying about waiting their turn) uses a cascoded 2SK170, and though it sounds very nice, I'm not getting much low-noise joy therefrom. I have to solve my power supply/grounding issues, and I suspect a rewiring of my turntable and tonearm is also in order.
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Old 29th November 2010, 07:26 PM   #120
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This is a pic of the "lemon-lime" SRPP board. A schematic will follow when the thing is functioning to my satisfaction.
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