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Old 10th September 2005, 03:01 AM   #1
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Default JFET SRPP RIAA Preamp

Attached is a schematic showing a JFET SRPP RIAA preamp using the PN4303 JFET , which is available form Goldmine Electronics and from Mouser. Distortion was (according to simulation) lower for the 4303 than for higher gain devices like the 2SK170, PN4393, or J110, which was the rationale for chosing a MOR device like the 4303. The 2N5459 might also be suitable for this application, but the breakdown voltage is marginal at 25V. FETs were selected in pairs for equal VGS at 2ma drain current using a homebrew matching jig. The circuitry at the right is a discrete shunt regulator used to knock down a 40V unregulated power supply to +30V. Gain is 38 dB each side at 1kHz into a 20k load, with 0.1dB gain matching between channels. I intend to use this preamp to replace a modified Pacifiic RIAA already used in a test setup in my living room (subject of a previous thread about 6 months ago) The Pacific was (and still is) pretty satisfactory working with a Kenwood KD-2055 turntable with a Grado Gold cartridge. However, the high level distortion (as simulated ) for the SRPP should be better than for the Pacific. I am eager to make the replacement to see if I notice any difference. When I replaced the stock Nikko preamp with my current setup, I immediately noticed the change. I would never go back to the Nikko.

As a side note, yes, I'm aware that Elektor has published an SRPP circuit, but I haven't seen it, as I didn't fork out the money to download it. People who have seen the Elektor circuit are free to comment on the similarities/differences. I will follow with a picture of the completed board when I can charge the batteries for my camera...
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Old 10th September 2005, 07:00 PM   #2
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Here's a pictrue of the board. The blobs of hot melt are for strain relief. If there is interest, I'll post some pics of the gain-phase plots. Oh yeah, in the current incarnation of this preamp, C9 and C10 are 1uF, not 3uF.
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Old 11th September 2005, 05:24 AM   #3
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Richard,

Looks really good. But no cap on the output of shunt reg? I'm curious if it sounds better without C12 loading input. Bet the treble is improved. Also, C9 does not have to be that big, though we all understand the reality of having certain parts already in the bin.

What is output impedance? Might have a little trouble driving difficult loads, idling at just 2mA.

jh
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Old 11th September 2005, 10:58 AM   #4
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I haven't checked the output impedance yet, but in my simulations, the output stage was definitely happier with a 20k load than with no load in terms of distortion (not surprising with a SRPP). The gain and phase were measured with 20k load using a HP4194A analyzer. I usually like about 40dB gain at 1 kHz for a MM RIAA preamp, but 38 dB was not too far off the money. The RIAA preamp will be driving a 20k pot - not too challenging a load. There is a unity gain line amp in my test preamp box between the pot and the outputs that go out to the cold, cruel world. That one has a much higher idle current (~10ma or so). As for C9 and C10 - the 1M resistors folllowing C9 and C10 are part of the RIAA equalization, like it or not, so I wanted the capacitive reactance of C9 and C10 very much out of the way at low frequency. I also had the caps in my parts bin - Roederstein MKP1839s. (I wish I had more of them). The input capacitors in the schematic are place holders. I'll have to figure out what works best with my Grado Gold. Since it has about 1/10 the inductance and lower resistance than the average MM cartridge, load capacitance doesn't have such a big effect.
Oh yeah - there are some 1uF bypass caps on the 30V rails that aren't shown in the schematic. If I have a little time, I can run a gain phase plot on the regulator before I plunk the whole mess in the box. I was so happy to get the gain equal between channels Friday evening that I carted the preamp home right away with the intention of trying it out .
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Old 12th September 2005, 06:44 AM   #5
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Well, I replaced the modified Pacific that resided in my preamp test bed for about 8 months with the SRPP and it's - different. I'll have to wait 'til I get a good night's sleep so I can do more critical listening, but I would preliminarily call it a bit more "delicate" in its treatment of classical music. There is no residual noise coming from the preamp at normal listening levels, despite the use of cheap "MOR" grade JFETs in its construction. A tentative thumbs up so far.
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Old 13th September 2005, 06:09 AM   #6
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From the way this preamp sounds, I'll have to improve my speakers before I can take its full measure. It handled Muhal Richard Abram's "View from Within" LP with aplomb,with its wide range of honks, blurts, delicate hand-struck pecussion and crisp vibes. The clarinet in the London Stereo Treasury recording of the Mozart Clarinet concerto may also be sounding a bit more true to life. It sounds like I ought to look at borrowing a better pair of speakers for auditioning this thing, as I won't be building new ones in a hurry.
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Old 14th September 2005, 06:37 AM   #7
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Attached is the revised (more complete) circuit, showing all gate stopper resistors and +30V bypass caps. The load resistors at the inputs are currently 47.5k, but will be changed to 16.2k, based on simulation of the resonance of the cartridge inductance with the stray and lumped load capacitance. Simulation shows that this is sufficient damping to get rid of a catridge resonance near 100kHz. The Grado Gold has an inductance of 45 millihenries and a resistance of about 475 ohms, which accounts fror the HF resonant peak.
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Old 15th September 2005, 08:06 AM   #8
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For those interested here is the Gain-Phase plot of both channels at 1/2mV input, 20k load performed using an HP4194A analyzer.
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Old 16th September 2005, 03:37 AM   #9
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The attached schematics show basic gain cell circuit topologies I've tried or am planning to try. All are simple circuits using only N-channel JFETs with no global feedback. Circuit 1 is a simple common source amplifier with cascode loading driving a source follower output stage for low output impedance. This circuit was described in a thread called "open loop follies pt.1" from a few months ago.

Circuit 2 shows the SRPP gain cell used in the preamp described in this thread. The parts count is much lower, and you get gain and reasonably low output impedance for driving a passive RIAA network or volume pot. The SRPP also allows one to use non-exotic JFETs and get pretty reasonable results. I'll be living with this circuit for a while, and then go to circuit (3).

Circuit (3) is very similar to (1), except for the addition of a JFET current source dumping into the gain FET (Q1). The rationale behind this attempt is to be able to use a high IDSS JFET like the J110 and still be able to simultaneously program a reasonable gain and center the drain voltage near 1/2 the supply voltage for optimum dynamic range. Running the gain FET at high idle current is also meant to reduce the distortion at high signal levels, like in the second stage of a passive equalized RIAA preamp. The simualtions of this circuit look promising, though the SRPP may have lower distortion with fewer parts. Even if this is the case, I'll go ahead and try (3) just to see if it has a different sonic palette.

This all started because I was very curious as to how a simple open loop (no global feedback) preamp would sound. Answer - pretty *&##@%$ good. Also, I was a bit annoyed with some pundits in another thread (actually, one i particular) that insisted you needed an exotic high-gm Japanese JFET for decent sound. I started in (1) with the 2SK170, one of those exotic FETs, then designed it out of the circuit to see what sort of performance could be had with "proletarian" part types readily available in the U.S. I'm pretty pleased so far with the sound of an SRPP circuit using a humble PN4303. Circuit (3) is all built up and ready to go. I'll try it and describe the results in another thread after I've lived with the SRPP preamp for a few months.
This is the last I will post on this thread unless someone else has some comments or questions.
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Old 16th September 2005, 03:56 AM   #10
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I like circuit #1, think it will sound better than #2. I bet #1 has lower distortion, #2 with plenty of good 2nd order, but maybe too much.

Not sure if #3 will work. The cascode works by feeding the upper transistor with a current source (signal). In this case, you've taken the current away with the extra fet. Won't this remove dc bias current from output? Could go very nonlinear.

jh
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