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Old 30th September 2008, 03:16 AM   #31
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Attached is the Gain-Phase plot for both channels. They match to within 0.2 dB at 1 kHz. It would be possible to tweak them to exactitude by trimming the load on one of the SRPP stages a bit.
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Old 30th September 2008, 05:04 AM   #32
Salas is online now Salas  Greece
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Very nice solutions. Now the SRPP loses its Achilles heel. Should perform at its best. More fets are more fets though, and the sonic tests will let you know what weighs more in this circuit. Please let us know more when practically ready and testing. Best of luck with this sophisticated and most original phono!
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Old 30th September 2008, 06:29 AM   #33
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Hi wrenchone

Why do you use 16k2 ohm as input impedance?
The latest schematic don't have any 100pF at input. Enough capacitance from cable?
Maybe noob questions but my MM cartridges want 47k load .

Apart from my qustions. Looks like a "want to build unit"
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Old 30th September 2008, 06:57 AM   #34
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The 16k input resistor is optimized to damp out the ultrasonic peak in the response of my Grado gold cartridge. I didn't add any input capacitor for the reason you suggest - that there is already some from the input cables. The Grado cartridges are relatively insensitive in that regard - the data sheet does not even bother to specify an optimum load capacitance. There will also be Miller capacitance from the input JFET. The PN4303 is a reall lightweight in that regard, having an input capacitance of 6 pF, and a reverse transfer capacitance of 3 pF.

The PN4303 is only in current production from Fairchild, as far as I can tell. I've been using NOS devices from National, who divested their small signal lineup a long time ago. I wish I could get some of the Vishay/Siliconix parts, but they discontinued the 4303 a couple of years ago, and have recently killed just about all their JFET line. The problem with the Fairchild version of the part is that the data sheet rather grudgingly admits that the part is a JFET and has three legs, and that's about all. Noise is not mentioned. They may work just fine, but they may also hiss like vipers. Having said that, I have some of the recent Fairchild parts, and will try them when I get a chance. I also scored some 2N4303s from Amelco (the orignal version of this part) - real museum pieces in a TO-106 package. I'm really interested to see how they work out.

A reasonable substitute for the 4303 may be the J202 - the specs are similar, and the noise is ok, though not as low as the PN4303.
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Old 30th September 2008, 03:04 PM   #35
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OK
The problem with this design and allmost everyone else is that I can't get hands on the components you suggest. Everyone want something at the edge or may I say difficult and "hard to do design".

If I just try 2SK170GR or 2SK117 which I easily can get?
The aftersought shimmering is gone? Distortion figures bad?
Schematic looks good of your design.

About noisy FET's I remember my younger days when we copied the layout of a MXR phase 90 pedal (effectpedal for guitars)
Sometimes it went for a while but eventually it got noisy from some of the FET's. I even did a similar effect from the magazine Practical electronics. Beautiful sound.... until the FET's started to bring in "rain and wind". I think I used the workhorse BF245 in both designs.
I understand you have to handpick FET's more troughly then other transistors?
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Old 30th September 2008, 05:06 PM   #36
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What about simulating a version with easy to get 2N5459 and see if it works at least adequately, listing the pros and cons of such a choice? Maybe some resistors will need optimization.
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Old 3rd October 2008, 03:00 AM   #37
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For MM cartridges, FETs with about 1-1.5 mS transconductance are about right., and even these are a little hot... 2SK170s have way, way too much gain for the application,. Just for grins, I tried a pair in simulation, and the high end starts sagging at about 3 kHz (plus the gain is way too much, but not enough for a single stage). You want to spread out the gain between two stages anyway - its a compromise between distortion and dynamic range.

Appropriate FETS - PN/2N 4303, J202. Both of these have good drain voltage rating (40V) and the right gain. The 2N5457 and 2N5458 will work, but as their breakdown voltage is only 25V, the dynamic range will suffer.

If you insist on using something like a 2SK170, pass on this circuit and try something based on a simple common emitter amp, like the Pacific. One useful frill ifor the Pacific is adding a cascode to reduce the capacitive load on the cartrige from Mr. Miller. Another frill is adding a source follower to the front end so that you have a low (and more or less invariant) impedance driving the passive RIAA network a source follower after the output stage adds peace of mind for driving output cables. . Check my thread "Open Loop Follies" for more on this, as well as the "All American" thread for another approach.
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Old 3rd October 2008, 03:11 AM   #38
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What about this one? Good to drive down to 40k line in preamp loads, or it can take another stage, a simple source follower in its tail for more demanding loads. Can tap off from added source follower's input and have 2 outputs. One buffered, one non.
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Old 3rd October 2008, 07:29 AM   #39
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Quote:
Originally posted by wrenchone

If you insist on using something like a 2SK170, pass on this circuit and try something based on a simple common emitter amp, like the Pacific. One useful frill ifor the Pacific is adding a cascode to reduce the capacitive load on the cartrige from Mr. Miller. Another frill is adding a source follower to the front end so that you have a low (and more or less invariant) impedance driving the passive RIAA network a source follower after the output stage adds peace of mind for driving output cables. . Check my thread "Open Loop Follies" for more on this, as well as the "All American" thread for another approach.
P.S.

The above circuit I posted ties with your quoted paragraph, that I assume was answering to Radioman62 wanting to use 2SKs. I did not post it as having anything to do with your SRPP concept. Its an example of what you say in that paragraph I hope.
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Old 3rd October 2008, 02:22 PM   #40
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Quote:
Originally posted by salas
What about this one? Good to drive down to 40k line in preamp loads, or it can take another stage, a simple source follower in its tail for more demanding loads. Can tap off from added source follower's input and have 2 outputs. One buffered, one non.

That looks like something to try before going for the more complex amplifiers.
Hmm.. If I only had some more sparetime
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