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If I put my notes here, I might be able to find them again later!
The Lab Circuit ideas and innovations, not yet part of a full project.
Old

Understanding the J113 JFET

Posted 14th April 2016 at 01:08 PM by rjm (RJM Audio Blog)
Updated 15th April 2016 at 11:31 PM by rjm

"To fight the bug, we must understand the bug."
--SKY MARSHALL TEHAT MERU

The J113 datasheet (Fairchild) tells you the following important information,

1. The (absolute) maximum gate-source / gate-drain voltage is 35 V.

2. The gate-source cut-off voltage (V_gs0) varies between -0.5 and -3 V.

3. The minimum zero-gate voltage drain current (I_dss) is 2 mA.

4. (from Fig. 11) the transconductance for I_ds 1-10 mA is about 10 mS largely independent of V_gs0.

5. (from Fig. 14) the voltage noise rises at low frequency and decreases with drain current, but is about 2-4 nV/sqrtHz over most of the audio bandwidth.

*****

I bought 200 J113 off eBay, but my measurements were set back after I realized my test rig was oscillating. Fixed that, and can now say a few things in addition to the datasheet.

The first is that the transfer curve (measured at RT, V_ds 10 V) does...
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Old

Supermatched JFETs

Posted 29th March 2016 at 05:10 AM by rjm (RJM Audio Blog)
Updated 18th April 2016 at 11:52 PM by rjm

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...
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Old

Matching JFETs

Posted 8th March 2016 at 12:29 PM by rjm (RJM Audio Blog)
Updated 6th May 2016 at 09:27 AM by rjm

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...
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Old

CrystalFET Phono Stage

Posted 3rd March 2016 at 04:55 AM by rjm (RJM Audio Blog)
Updated 22nd July 2016 at 10:52 PM by rjm

Development thread 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.
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Old

Voltage Regulators for Line Level Audio. Part 11 : The Crystal M Shunt

Posted 20th February 2016 at 12:49 AM by rjm (RJM Audio Blog)
Updated 22nd February 2016 at 08:35 AM by rjm

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...
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Old

LTSpice filter simulation masterclass: 0 to -100dB in five easy steps!

Posted 18th February 2016 at 11:14 PM by rjm (RJM Audio Blog)
Updated 7th April 2016 at 06:58 AM by rjm

I've never put everything into a single LTSpice worksheet like this before: I find it fascinating. You can really pull apart a circuit to see what makes it tick, before solder ever hits the iron.

Power supply ripple, frequency response, gain, and crosstalk can be established. You can look at turn on and turn off transients, inrush currents, and conductance angle, and check peak currents in the filter capacitors. It's all there if you care to peek in and poke around.

I'm such a huge fan of LTSpice...

The only problem, really, is it is too perfect: all devices are perfectly matched, every part value is exact, and the temperature is always 25 C. Ground loops, wiring inductance, and thermal runaway do not exist. So no, of course there are no guarantees - but as a tool to get you 90% of the way there with the minimum of fuss and bother it is truly indispensable.

Actually I find the more experience you have the more useful LTSpice...
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Old

jFET passive phono preamplifier circuit idea

Posted 16th February 2016 at 01:27 PM by rjm (RJM Audio Blog)
Updated 16th February 2016 at 11:51 PM by rjm

There are various tricks, like parallel input devices and active current sources, that I have avoided here in the interests of simplicity. If you want to go down that road, you can get an idea where it leads, here. Instead, the circuit below is basically a JFET version of my old 6DJ8 amp, here. A single JFET was getting me nowhere in terms of output impedance - around 10kohms! - so I moved to a compound stage buffering each amplifier with a source follower.

Noise and distortion figures look okay. The gain is only 30 dB. A bit low. The main trick is the PSRR, which is awful. The two stage circuit actually amplifies the power supply noise onto the output. So considerable effort must be put into the power supply regulation and filtering. I note that this is pretty much par for the course with this circuit topology where resistors are used instead of current sources on the JFET drains.

The circuit below leaves out the usual RC filter inserted between the power supply...
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Old

Passive RIAA equalization network response calculator

Posted 16th February 2016 at 08:21 AM by rjm (RJM Audio Blog)
Updated 16th February 2016 at 11:47 PM by rjm

This Excel (2013) worksheet will help you fine tune the values of the resistors and capacitors used in the passive RIAA network found in any number of two stage tube, op amp, and FET phono stage circuits.

Excel handles complex numbers well enough now that this job isn't particularly difficult, though for simplicity the DC blocking cap (Cc) is left out of the calculation.
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Old

jFET phono stage | harmonic cancellation jujutsu

Posted 15th February 2016 at 06:45 AM by rjm (RJM Audio Blog)
Updated 19th February 2016 at 10:59 PM by rjm

There are lots of phono stage circuits floating around based on two jFET amplifier stages and a passive RIAA network. I'm not sure who did what first, but there's the Boozehound, LePacific, and of course Salas versions.

Setting aside concerns about the ripple rejection**, today I'd just like to focus on the distortion and noise of the circuit itself. The passive RIAA stage is a large obstacle. It attenuates the signal substantially at all but treble frequencies, and it generally presents a large series impedance - both of which tends to increase circuit noise.

The jFET themselves meanwhile are a fine balance between low current, low noise operation with high distortion, or running at high current, high noise, with low distortion. Circuit gain must be paid for meanwhile with added distortion since the two stage design struggles to manage 40 dB.

After spending some time in LTSpice with this, I realized it was the proverbial rock and hard place...
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Old

Headphone amplifier ground. Where to make the chassis connection?

Posted 2nd February 2016 at 07:11 AM by rjm (RJM Audio Blog)
Updated 24th February 2016 at 01:02 AM by rjm

The discussion thread at the headphone forum is here, but I wanted to throw out the problem to the general blog-reading community here at diyaudio to see if anyone can nail this.

The earthed chassis (light blue) must connect to the circuit common i.e. "ground" (pale green). I do not know where the best place on the circuit ground is to tie that connection.

Suggestions please!

(COM and GND are completely equivalent pads on the circuit board, while IN- and OUT- also pads on the board but physically further away on the ground plane.)

****

Answer: as long as it connects at one point only, or the same point of both channels, it doesn't seem to matter at all. I have it connected at the ground tab of the headphone jack and that seems to be as good as anywhere.

****

The noise was in fact magnetic interference emanating from the transformers. Grounding layout changes / electrostatic...
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