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Old 17th February 2016, 05:32 AM   #1
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Richard Murdey
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Default RJM Audio Crystal P jfet phono preamplifier | development thread

This seems to work out not too bad. Gain is touch and go, but noise and distortion figures are respectable. RIAA accuracy is excellent (+/- 0.2 dB 20-20k with the standard values shown.)

Power supply must be very (very!) low noise. Fortunately the current draw is only 10 mA per channel so it should be do-able. I'll be looking into that in a bit.
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Old 17th February 2016, 04:43 PM   #2
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Consider the shunt regulator used in the Hi-Octane phone pre-amp. Single voltage output, needs 8V above the regulated output voltage.
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Old 17th February 2016, 11:43 PM   #3
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The high octane phono preamp

Got it. Thanks.

I was thinking the voltages were too high to use op amps as the control amplifier, but the schematic shows there are some tricks that can be applied. It worries me a little, though: The voltages are enough to zap the IC if anything goes wrong, even momentarily.

Other options are the Salas shunt, or sourcing a high voltage op amp, or building a control/error amp from discrete transistors

Given the low noise requirements at relatively low frequencies, the usual capacitance multipliers are ruled out - some sort of active (feedback) control will be needed, whether shunt or series.
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Old 18th February 2016, 02:03 AM   #4
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Currently my "object of interest": p-mosfet based shunt regulator, based on the PSU for the Salas phono stage. I simplified it a little replacing the current source FETs with resistors. At 135 dB rejection ratio it's good enough as it is.

C1 10uF. This gives a turn on time around 1-2 seconds. Turn off is clean with no spikes. The bias V2 will be replaced with a Zener or LED arrangement. Vin 52 Vin is the rectified, filtered DC from 2x18 VAC secondaries connected in series. Telama 60074 for example.

Gate resistors omitted for simulation simplicity. Rsense is just for bookkeeping so I can see how much current is flowing through the shunt arm.
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Last edited by rjm; 18th February 2016 at 02:07 AM.
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Old 18th February 2016, 07:37 AM   #5
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"Crystal M" regulator stage completed as a functioning simulation.

The diode D1 is critical to protect the gate of M1 during shutdown. I also added emitter degeneration R1 R9 ... this degrades the performance but makes the output less critical on the individual device characteristics of Q1 and Q2. Gate resistors R4 and R8 also added for RF stability, though they don't do anything in simulation.

For the bias voltage of the current source MOSFET (M1) I opted to go with transistor-based feedback (Q2/R3) rather than using a Zener or LED to set the bias. I thought this was safer and smarter as it is self-regulating.

The ripple rejection hits -100 dB over the audio bandwidth.
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Old 18th February 2016, 08:26 AM   #6
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One small addition of 22 ohm resistor R10 on the source of M2. This damps out overshoot when the circuit responds to a quickly varying current load. It's very well-behaved now, but unfortunately ripple rejection drops to 72 dB.
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Old 18th February 2016, 01:30 PM   #7
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.. and something of how the regulator would integrate with the filter stages before and after it, with outputs to both stages of each channel.
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File Type: png Crystal M full response.png (14.2 KB, 228 views)
File Type: png Crystal M full transformer.png (57.1 KB, 131 views)
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Last edited by rjm; 18th February 2016 at 01:45 PM.
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Old 19th February 2016, 12:17 AM   #8
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The whole enchilada, as it were.

The output power supply ripple is -110 dB at 120 Hz (around -140 dB input referred). Shorting over R10 would reduce that further to below the noise baseline, but I don't think that will be necessary in practice.
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Last edited by rjm; 19th February 2016 at 12:24 AM.
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Old 24th February 2016, 01:44 PM   #9
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First ideas about the printed circuit board layout. This is what you might call "pre-alpha". Just to get some ideas about how the components might fit together and how much board area things are going to need.

The voltage regulator isn't included yet. I have to decide on whether it will be included or separate, or whether two channels or dual mono.

While clearly the layout could be shrunk considerably from its present state, board area isn't so expensive that it is strictly requires, and larger component spacing makes it easier to build build and modify.
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Old 29th February 2016, 05:43 AM   #10
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The choice of capacitor for the RIAA network (0.1 and 0.033 uF) and interstage coupling (0.1 uF) is very important, both for response accuracy and board space considerations.

I'm very excited about the Vishay MKP1837 metallized polypro film, box type, which are compact, relatively economical, and available in 2.5% and 1% tolerances.

http://www.vishay.com/docs/26017/mkp1837.pdf

With these caps, it should be possible to put a single Crystal channel with the voltage regulator components on a single 8x10 circuit board.

Without them, there is little option but to split the circuit with amplifier and power supply on separate boards.

The power supply can be made quite compact. A "pre-alpha" board layout is shown below. The phono stage will cohabit the board in the unused space at right.
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Last edited by rjm; 29th February 2016 at 05:49 AM.
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