Simplistic NJFET RIAA - Page 5 - diyAudio
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Old 3rd September 2008, 10:10 PM   #41
Salas is offline Salas  Greece
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For the time being I am dropping R17 just in case since at least in another simulator and NJFET modeling it showed some peaking. I am not after absolute 0.1dB actual RIAA conformity, because neither the vinyl records nor the best monitor speakers can follow that. Seen any other aberrations in your simulation? Or you just simulated the RIAA only out of my circuit, under low Zo input and high Zin load? Because the response of my circuit is rather limited due to Miller and no GNFB or local NFB. I still wonder how high it peaked.

Thank you for double checking. Appreciated.
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Old 3rd September 2008, 10:38 PM   #42
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Don't you Helene's ever go to bed?

Sims can vary because of our different models, cable capacitance and sloppy wiring etc., etc. but it looks like your RIAA is +/- 0.15dB from 20 Hz to 20kHz -- pretty darn good. R17 plays a pretty important role in the ultrasonic range --

I put 2 bode plotters on -- one with the standard RIAA, the other with the enhanced RIAA -- note the difference in vertical scales.


Click the image to open in full size.

Parenthetically, the difference between Standard and Enhanced RIAA Curves is about 6.9 dB at 100kHz -- but we aren't going to hear that high anyway -- we just want to eliminate the possibility of problems down the food chain:


Jack
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Old 3rd September 2008, 10:53 PM   #43
Salas is offline Salas  Greece
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OK, no R17 to be sure.

And yes! The night is young!
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Old 4th September 2008, 08:35 PM   #44
Salas is offline Salas  Greece
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Default Final Theoretical Schematic

So to sum up the circuit idea and cross checks up to now, the final schematic to be made for real life test is in the attachment. I have checked, and It can work fine with 36V B+ too, using 4X9V or 3X12V batteries in series, instead of a regulated PSU.
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Old 4th September 2008, 08:59 PM   #45
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Very nice Salas , 73db are perfect for low gain mc ......
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Old 5th September 2008, 03:34 AM   #46
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I had a look at the Sears phono preamp schematic - comments.

I tried a JFET SRPP passive equalization phono stage a while back (search threads here for "JFET SRPP"). Distortion performance (simulated) was impressive for only 4 devices/channel. I used the PN4303, which has fairly low gm (needed to keep the gain from going through the roof without totally strangling the FET) and is a moderately low noise device (good for MM, not MC). I ran into problems with variable gain on the first stage due to the variable load of the RIAA equalization network vs. frequency and finite/relatively high output impedance of the first stage SRPP, and canned the project in favor of another approach. It didn't occur to me to use a higher impedance RIAA network - duh! This will breathe new life into the first stage. The second stage had issues due to relatively high output impedance. I would not use Sear's solution of a common source second stage due to high distortion (0.3-0.5%, though it's overwhelmingly 2nd harmonic). Using an SRPP second stage, loading it with a fixed resistor, followed by a current source loaded source follower dramatically drops the simulated distortion (0.02 to 0.06%, depending on how you bias the stage and how much load you use).

These sorts of simulation results may make me break out the old SRPP RIAA board and try it again after a little surgery.
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Old 5th September 2008, 05:18 AM   #47
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If you take a look at Pass DIY's Pearl Phono from years ago, it is essentially the same topology as that in Post #1, wiith some refinements.

And you can replace the 2SK389 in the Pearl with a pair of matched 2SK170s, without saying.


Patrick
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Old 5th September 2008, 05:30 AM   #48
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I'm pretty familiar with the Pearl - I wanted to try another design that didn't require the 2SK170 or 2SK389. The SRPP topology wrings surprisingly good performance out of a relatively proletarian JFET like the PN4303. With the perspective from a couple of years of "fallow time" and inspiration from the Sears preamp, I might be able to cajole the SRPP circuit into a very decent showing.
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Old 5th September 2008, 07:56 AM   #49
Salas is offline Salas  Greece
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Default Note on JFETS used

With 10mA Idss we get 70dB gain in my circuit. With 12mA Idss G=73dB. Even with median 8mA Idss BLs we hit 68dB. Just keep your strongest matched pair for the first stage. Matching must be used for same stages between channels so to keep the gain together for L&R. Also 500R trimmers can be used instead of 220R so to trim best for drain voltage and consistency between channels for that.
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Old 5th September 2008, 08:10 AM   #50
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Why do you want all that gain? Are you trying to run with a moving coil cartridge? One of the reasons I went with the PN4303 was to have a resonalbe stage gain. The gm is olly about 1.5-2k mmoos, about 1/10 the value for a 2SK170.
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