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If I put my notes here, I might be able to find them again later!
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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 27th April 2016 at 10:29 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 29th April 2016 at 02:10 AM 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.

Black boards pictured are the original rev. 1.1a prototype, which I ended up using for mc operation. There was a connectivity error in the schematic used to make the boards, so the layout was redone as rev. 1.2a.

1.2a are for mc operation only, about 55-56 dB. I'm giving these away for $5 for one pair, see here. [only four sets left!] Rev. 1.2a is up and working. No abnormalities, near-perfect agreement with the LTSpice simulation.

Last, rev. 1.3c features switchable 35/56 dB gain for moving magnet and moving coil cartridges, and adds a jumper for the regulator "boost" feature. The basic circuit hasn't changed, just fixes and refinements of the concept. Pictured below.
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Old

Chromecast Audio Output Noise

Posted 26th February 2016 at 11:11 AM by rjm (RJM Audio Blog)
Updated 27th February 2016 at 12:18 AM by rjm

Measured at 24/96 with my Asus Xonar STX soundcard (~ -147 dB noise floor)

The Chromecast Audio output noise powered with the included USB wall wart supply is -130 dB at 1 kHz, rising gradually at lower frequencies and showing some switching power supply noise peaks at 4763 Hz and higher multiples, never exceeding about -120 dB.

This is respectable performance given its price point.
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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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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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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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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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