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Old

Just the buffer, ma'am.

Posted 28th July 2016 at 02:05 AM by rjm (RJM Audio Blog)
Updated 27th December 2016 at 11:46 PM by rjm (added BOM)

I've had quite a few requests for the bboard buffer circuit without the built-in regulators, so here is a bboard 2.1 standalone 2-layer board, measuring 5x8 cm. Gerber files attached in zip file.

It is designed for +/-12 V rails, but the circuit will work with anything from +/-5 V to +/-18 V. A regulated power supply is recommended.

This is a line buffer. It intended to drive cables, not headphones.

Available for $15/pair shipped. Several people have asked me about kits. I figured the BOM was so basic it wouldn't be necessary but I can send you the boards with the parts to populate them for $50/shipped. You will still need to provide the power supply.

BOM attached. (updated to 21f3 Unity B) [As a transitional step this board is being merged with Project Unity.] B-board 2.1 became the Unity B circuit, the older board is just missing the ability to convert to the Unity B H headphone driver.
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File Type: asc bboard buffer 21.asc (4.6 KB, 184 views)
File Type: zip buffer-21f (274x).zip (78.5 KB, 80 views)
File Type: zip pcb-bb-21f3-unityB-bom.zip (88.5 KB, 22 views)
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Old

Sapphire Line Preamplifier

Posted 24th July 2016 at 01:11 AM by rjm (RJM Audio Blog)
Updated 26th July 2016 at 11:45 PM by rjm

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
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File Type: asc Sapphire Line 10d.asc (10.1 KB, 170 views)
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Old

INs and OUTs of the Asus Xonar Essence STX PCIe computer sound card

Posted 26th June 2016 at 08:12 AM by rjm (RJM Audio Blog)
Updated 18th July 2016 at 03:42 AM by rjm

Asus Xonar Essence STX, Audacity 2.1.2, VLC, Windows 10 [and DigiOnSound6 Express for 24 bit recording]

Purpose

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 signal peak and noise baseline levels in the FFT spectra.

Summary

* 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 recording line input 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 needed 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....
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Attached Files
File Type: zip RMAA report Xonar STX loop back.zip (29.2 KB, 35 views)
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Old

VSPSX phono stage design idea

Posted 24th June 2016 at 12:48 AM by rjm (RJM Audio Blog)
Updated 23rd February 2017 at 07:00 AM by rjm

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.
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File Type: asc VSPSX-10a.asc (13.4 KB, 41 views)
File Type: zip pcb-vspsx-10a4-bom.zip (168.8 KB, 2 views)
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Old

Voltage Regulators for Line Level Audio. Part 12 : The Foursquare Shunt-Source

Posted 17th June 2016 at 01:44 PM by rjm (RJM Audio Blog)
Updated 20th December 2016 at 11:01 AM by rjm

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.

[developed further under the S-Reg name, with pcb layout.]
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File Type: asc foursquare shunt regulator full.asc (4.6 KB, 126 views)
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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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File Type: zip pcb-crystal-14e-bom.zip (246.7 KB, 40 views)
File Type: asc Crystal 14.asc (11.2 KB, 114 views)
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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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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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File Type: asc Crystal M.asc (2.9 KB, 695 views)
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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

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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