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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 12:06 PM
  • Join Date: 2nd May 2004


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View rjm's BlogRecent Entries
Latest Blog Entry

Posted 26th June 2016 at 08:12 AM by rjm Comments 0
Asus Xonar Essence STX, Audacity 2.1.2, VLC, Windows 10 [and DigiOnSound6 Express for 24 bit recording]


To confirm the calibration of the sound card input and output gain. Also, to determine the relationship between the signal voltage, the recorded signal amplitude displayed in Audacity, and the magnitude of signal peak and noise baseline in the FFT spectra.


* Setting the volume slider of the device output to 100 gives 1 V rms output for an amplitude 0.5 sine wave.
* Setting the volume slider of the device into to 100 gives records a 1 V rms tone as an amplitude 0.5 sine wave, which is displayed in the frequency spectrum (FFT) as peak of magnitude 0 dB in Audacity when both channels are averaged.
* volume setting 100 for unity gain loopback
* 0.5 amplitude sine wave = 0 dB FFT = 1 V rms
* noise baseline in averaged stereo FFT is 3 dB lower than single channel measurement
* The measured performance...

Posted 24th June 2016 at 12:48 AM by rjm Comments 0
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.

The LTSpice schematic below abbreviates the transformer, rectifiers, and filter capacitors as V1 and V2. Op amp OP27 is a placeholder.

My main concern would be fitting all this stuff on a 80x100 board. That and, well, I'm not totally convinced yet that the complexity vs. reward ratio makes its worthwhile.

Posted 17th June 2016 at 01:44 PM by rjm Comments 0
I'm not totally sure this would work as advertised, but I can't see any obvious reason why it would not...

It's pretty much the same circuit as I used in the CrystalFET, which started out in a previous blog post in the Voltage Regulators for Line Level Audio series, but here I've replaced the MOSFETs with bipolars. It is shown configured to deliver 20 mA @ 12 V, split supply. Enough to power an op amp phono stage for example, or a preamp, or the voltage gain stage of a headphone amplifier.

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...
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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