Posted 28th July 2016 at 03:15 AM byrjm (RJM Audio Blog)
Updated 5th August 2016 at 02:14 PM byrjm(add measurement data)
This is my build log for relatively basic line preamplifier based on rev. 30f boards of the Sapphire3 headphone amplifier. I modified the circuit to run at lower currents (about 10 mA output bias) and adjusted the gain settings to 10/16 dB.
It is built in a Hammond 1550 cast auminum chassis, with an external Plitron 160VA 2x12VAC rectified power supply. The volume control is a 50k Goldpoint V24 stepped attenuator, while the RCA jacks are rhodium plated from Oyaide. The feature set is limited to two switchable line inputs and an output mute.
Chassis Layout Notes
Audio components are conventionally designed as rack-mounted equipment with all controls on the front panel and all connectors on the rear panel. To try and keep internal cabling to a minimum I'm modelling my preamp more like a recording console with both the controls and I/O on the top plate.
Posted 28th July 2016 at 02:05 AM byrjm (RJM Audio Blog)
Updated 21st August 2016 at 11:49 PM byrjm(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.
Posted 24th July 2016 at 01:11 AM byrjm (RJM Audio Blog)
Updated 26th July 2016 at 11:45 PM byrjm
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
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.
* 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....
Posted 17th June 2016 at 01:44 PM byrjm (RJM Audio Blog)
Updated 20th June 2016 at 08:37 AM byrjm
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 26th February 2016 at 11:11 AM byrjm (RJM Audio Blog)
Updated 27th February 2016 at 12:18 AM byrjm
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.
Posted 20th February 2016 at 12:49 AM byrjm (RJM Audio Blog)
Updated 22nd February 2016 at 08:35 AM byrjm
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.
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...
Posted 18th February 2016 at 11:14 PM byrjm (RJM Audio Blog)
Updated 7th April 2016 at 06:58 AM byrjm
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...