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rjm rjm is offline

Richard Murdey

diyAudio Member

About Me

  • About rjm
    Canadian citizen, Japanese resident.
    Audio Circuitry
    Research Scientist
    Real Name
    Richard Murdey
  • Signature
    RJM Audio ( / G+)


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Blog - RJM Audio Blog
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General Information
  • Last Activity: Today 12:46 AM
  • Join Date: 2nd May 2004


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View rjm's BlogRecent Entries
Latest Blog Entry

Posted 13th May 2016 at 08:22 AM by rjm Comments 0
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...

Posted 14th April 2016 at 01:08 PM by rjm Comments 0
Posted in The Lab
"To fight the bug, we must understand the bug."

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...

Posted 29th March 2016 at 05:10 AM by rjm Comments 5
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...

Posted 8th March 2016 at 12:29 PM by rjm Comments 6
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...

Posted 3rd March 2016 at 04:55 AM by rjm Comments 6
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.
Recent Comments
Please use ver. 1.2....
Posted 5th April 2016 at 01:57 AM by rjm rjm is offline
Thank you very much!...
Posted 5th April 2016 at 12:22 AM by BaffleChallenged BaffleChallenged is offline
Thanks for your input....
Posted 4th April 2016 at 11:58 PM by rjm rjm is offline
PM sent!
Posted 4th April 2016 at 11:35 PM by rjm rjm is offline
How / where can I order...
Posted 4th April 2016 at 05:08 PM by richandy richandy is offline
Hide this!Advertise here!

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