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CrystalFET 2 MM passive phono preamplifier with JFET amplification

Posted 21st April 2017 at 01:05 AM by rjm
Updated 21st April 2017 at 04:44 AM by rjm (revised to 2.0g (capacitors C9 and C10 misslabelled))

In hindsight the original CrystalFET suffered from overreach. To obtain the 55 dB midband gain required for an MC phono stage lots of other things I might have wanted to build into the design like degenerative feedback were discarded.

Degenerative feedback as it applies to JFET amplifiers is the practice of leaving the source resistor partially or completely unbypassed. It reduces the voltage gain, but linearizes the dynamic operation of the amplifier - coercing the gain to the ratio of the source and drain resistors rather than leaving it defined by the JFET transconductance. The circuit gain can then be fixed by the designer without worrying so much about matching devices.

The CrystalFET circuit is a traditional two stage passive photo preamplifier design, the only novelty here is the addition of a second JFET configured as a source follower to each amplifier stage. To obtain above 50 dB gain no degenerative feedback could be used, and the JFETs had to be not just extremely well-matched but also cherry-picked from a very narrow range of properties so as to maximize the voltage gain. By re-branding the circuit as a moving magnet only, roughly 6 dB of open loop gain is available in each stage to be re-purposed as degenerative feedback. That doesn't sound like a lot, but it makes a big difference. We go from having to cherry pick exact devices and still not having enough gain to being able to use many different model of JFET with confidence. Only a nominal matching/binning is required, and, worse case, a small adjustment of the drain resistances. Most JFETs with a pinch off voltage -Vgs0 of about 1.5 V will be suitable: J113, 2N5484 and 2N5485 for example. J113 is optimal.

So that's the CrystalFET 2 circuit in a nutshell: 1/3 of the source resistance is left unbypassed for about 6 dB of degenerative feedback. The JFETs run at about 2-3 mA and there are many suitable types which can be used. The total gain is about 38 dB typically, so it will only work with high output MC, MM, and step up transformers and head amps. It retains the original's source followers which lower the output impedance of each stage to just over 100 ohms. This allows a lower impedance RIAA network to be used for reduced noise, and also means the output impedance of the phono stage is usefully low.

From a board level, I have decided to scrap the voltage regulator and punt that job over to you. A 35 V regulated supply is required, the current draw is under 25 mA. Anything will work, batteries, pi filters, shunts etc, but ripple and noise needs to be very low as the circuit itself has no PSRR to speak of (it's actually negative at low frequencies). The upside is the board is now much more open, with more room for boutique capacitors etc. The board was designed for Nichicon FG 'muse' series, though of course larger capacitance values of regular FW KW series can be substituted.

Gerber files attached, so if you want to give this a shot and get some boards made at pcbway etc. be my guest. [order as 100x80mm 2 layer, default] House rules apply: don't pass them off as your own work, and don't use them for commercial gain.
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Attached Files
File Type: zip Crystal-20g.zip (63.3 KB, 52 views)
File Type: zip pcb-crystal-20g-bom.zip (220.3 KB, 52 views)
File Type: asc Crystal 20.asc (5.2 KB, 102 views)
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