S-reg voltage shunt-source regulator for line-level audio circuits.

This is designed to accept rectified 12x2 VAC input, producing +/-12 V output split rails.

Features soft-start and over-current protection, standard configurations for load currents up to 50 mA. High performance: 80 dB ripple rejection up to 100 kHz.

update: BOM uploaded
update: web page (stub)

The time of year where I like to draw up some plans for the next 12 months.

This year’s output looked something as follows,

• Lowering the noise floor by shielding the power transformers of my Sapphire3 headphone amp (and eventually replacing them with Triad VPM shielded toroidal medical transfomers)
  
• Developing the CrystalFET phono stage.
  
• Developing the the VSPSX and bboard 2 to the point of getting the boards fabbed.
  
• Tweaking the Sapphire3 headphone amp slightly to use as a preamplifier.

At the end of all this I find myself sitting on four different voltage regulator circuits, several variants of the transistor output diamond buffer, five phono stages variants, and a couple of nebulous ideas about developing a discrete voltage gain amplifier.

I'm considering how to package this all up in such a way as to best appeal to diyaudio builders of widely varying application needs and skill levels while keeping a simple...

16:9 4K

rendered from VSPSX phono stage Eagle .brd output

Although the original Sapphire headphone amp can be configured as a line stage, or use as-is as a line stage, I've gone ahead and made a new circuit variant with a new set of boards.

The Sapphire Line (in development) combines the shunt-series regulator, bboard 2.0 buffer and an op amp voltage gain stage. Same basic idea as the Sapphire of course, but with a much less beefy output stage so the low noise regulator can be added and everything still fits on the board.

rev 10e - now with support for 2520 op amp modules

The original VSPS is getting up in years, and could do with a bit of an update.

The VSPSX adds the foursquare shunt-source voltage regulator and bboard v2.1 output buffer.

Development thread here.

[updated attached content to rev. 1.0a4]

Two mistakes in previous BOM:

C4 was a 5mm lead spacing Wima MKP but the board spacing is only 2.5mm. The part is changed to Wima MKS with 2.5mm lead spacing.

R22,23 was incorrectly given as 47.5k, it should have been 4.7k and has been since updated to 10k. This is not a critical value, the circuit will work with any of the resistances.

[further work]

The boards have been revised to 1.2a. The new design is posted to the development thread.

A little bit of the sapphire headphone amp output stage, and a little bit of the LSK489 application note of all things (scroll down to Fig 10.).

Yes, this is probably the third "version 2" of this line buffer I've posted... I keep tossing it out and starting over. This variant looks pretty good: though the transistor count is a little high for such a basic function the performance is definitely there.

The main innovation re. the sapphire circuit is to replace the bias set resistors with diodes made out of the Vbe of transistors Q9 and Q10. This generates more voltage than is ideal, but can be handled by using largish values for the emitter resistors R13 and R14. Since this is a line stage buffer and not a headphone amplifier the output impedance of about 30 ohms and the limited output current swing are not critical flaws. It will drive 600 ohms at 0 dB with 0.001% THD. The whole circuit draws just 150 mW. The input impedance is a very high ~15 Mohms...

Truth be told, for a self-biased jfet audio circuit like the CrystalFET the main reason we need to used matched jfets is to ensure that the signal gain is the same in both channels. The operating point of the amplifier stage (the voltages and currents) can be allowed to vary a little so long as the transconductance, g_m is the same, as this is directly proportional to the open loop voltage gain, A, as

A = g_m R_l (transconductance x load resistance)

Now, yes, ideally you would find two jfets with identical saturation current and pinch off voltages, ensuring not just the same gain but also the same operating point. In practice though you are usually binning parts that are close to each other based on some reference parameter like the pinch off voltage (V_gs0) that you hope closely correlates with the signal gain. This is not quite as good though as the calculating the actual transconductance of the particular device in the circuit it is to be used in. And since...

with only two resistors, a 9 V battery, and a voltmeter...

The current-voltage relationship for a jfet device is approximately a quadratic expression defined by just two parameters, the saturation current, I_dss, and the pinch-off voltage, which I'll call V_gs0.

I = I_dss (1-V/V_gs0)^2

In principle, therefore, to characterize the device all we need is two data points (I1, V1) and (I2, V2) to solve the expression above for I_dss and V_gs0. We don't need to measure I_dss or V_gs0 directly.

All you need to do is connect the jfet device-under-test (DUT) as shown, and measure the voltages across two different source resistances. That's it. The excel worksheet computes the I_dss and V_gs0 values for you (or you can do it by hand, the formulas are provided.)

The math is a bit messy, but if you can solve a quadratic expression it's easy enough.

*****

Note: I found it was important to include...

Development thread here. Web page here.

CrystalFET is a J113 jfet-based two-stage phono preamp, with passive equalization and on-board MOSFET-based shunt voltage regulator.

Jfets Q1 and Q3 should be matched between channels, and for best results the value of drain resistors R2 and R9 should be selected based on the jfet pinch off voltage of Q1 and Q3 respectively.

rev. 1.4e (final version) removes some unneeded resistors and tidies up the board layout a little. The resistors have been renumbered.

A while back I did a series of blogs on voltage regulators. Back with a new entry today: The Crystal M, configured here for 40 V DC output and a 25 mA load.

The circuit is based on two p-channel MOSFETs, the top one is a constant current source, the bottom one a constant voltage source. As the load current changes, the voltage source adjusts its current to balance.

It's lifted directly on the Salas shunt design (as reworked by me for my own jFET phono stage), but the circuit can also be considered a distant, DC-coupled relative of the Zen amp.

I trick, I discovered, to getting it to work nicely - the attached screencap shows it well-behaved while handling a full-swing output current pulse - is the source resistor R10. This resistance dials-down the current gain of the MOSFET, damping out the overshoot.

The ripple rejection is about 70 dB over the audio bandwidth. The output impedance is about 0.05 ohms over the same frequency...