RelaiXed2 -- Balanced pre-amplifier -- preview

Since announcing that I am busy with a new 'relaiXed2' design, I received quite some questions on what this device will be like. Therefor, on this page, some preview highlights... (See below for schematics, revision 2010-07-21)

The basic ideas behind the original RelaiXed preamplifier are maintained, but in fact, it will be a whole new design. The only thing remaining unmodified is the physical formfactor: PCB sizes and location of mounting holes and audio connectors.
Known issues that are repaired as result of feedback from the earlier relaiXed usage:

• The earlier Relaixed has gone through 3 small variations. Although there were small improvements, unfortunately, the noise level did increase. In the latest version, some persons reported that in their system the noise level did reach an audible level. The new RelaiXed2 will have an extremely low noise level, also far lower than the first version. The extremely low noise noise level is expected to result in a better sound quality.
• A quite bad 'pop' in the output signal on power-up is now suppressed.
• A few persons mentioned soft but audible clicks in the audio output for a few specific volume steps. This is tackled with an improved PCB layout, lower impedance levels, and an optional circuit extension.

Modifications in the analog design

• The design now has a low-impedance attenuator chain in the signal path AFTER the opamp amplification stage. As result, there are none 'high impedance' points in the audio circuit. This reduces output noise to extremely low values, while maintaining a high amplifier input impedance.
• Finally I decided to drop the old 'Dale' resistors. They have been cumbersome to obtain for all the attenuator values, and their last datasheets took back some of the good old specifications on 'current noise'. I now selected the Vishay-Beyschlag 'professional MELF' resistors, series MMB0207. These have among the best noise spec of any metal-film resistor. Their compact size allows a better and more compact layout of the attenuator chain on the PCB which will further help to improve the audio. The '0207' are the largest in their family, which makes them easier to handle for manual soldering, and makes them available in smaller quantities. These resistors are still quite cheap, and show noise figures close to the extremely expensive 'bulk metal foil' resistors.
• The analog power supply stabilization circuit is redesigned to become somewhat simpler with a lower component count, while staying close to the original high-quality specifications. The now adopted zener diodes result in a simpler circuit: their low impedance significantly improves the line regulation of the LM317, as well as reduces the effect of the current noise of the LM317. The zener voltage noise itself is filtered out with an RC-stage. (In general I am not so fond of zeners, and I am aware that LEDs can show lower noise figures, but I chose for the lower component-count and good-enough specifications.)

Modifications in the digital control

• The design now employs only a single microcontroller, situated in the front display print. The relay print has fixed-function logic circuits only. This resolves my older headaches on incompatibilities in the communication between different display versus relay controllers. The relay print now takes commands from a standard 'I2C' 2-wire protocol.
• The left- and right- (balanced) attenuator chains can be controlled individually from each other. This allows balance-control next to volume control. An extended configuration with 3 relay boards attached to a single front display would allow individual volume control on all 6 channels, by using a dedicated I2C address for each board.
• The PIC microcontroller is swapped for a newer type, becoming a 'PIC18F24J50', which features a glueless USB port. See below.

Modifications in the microcontroller firmware

• A USB port on the display-print allows any user to hook-up the microcontroller directly to his PC to install new RelaiXed firmware updates. That is, without needing a dedicated programmer device, and even without needing to install a custom device-driver. From a firmware developer point-of-view, the USB port allows to trace the program execution for debugging purpose.
• The display-print has a 'ICSP' programmer/debugger port for inserting the USB bootloader firmware. This port is separate from the I2C communication port to the relay-PCB, to help code debugging.
• I will make the firmware 'hex' image freely available for download, as well as all the source files! This will enable other enthousiasts to make and publish their own improvements/modifications/extensions to the firmware.

The 2nd draft version of the design is design_20100721.pdf. The first 4 schematic pages are all on the relay-PCB, followed by the (provisional version) PCB layout design. The fifth schematic page is for the small display-PCB, followed by its PCB layout. Publishing this early schematics, before the actual implementation, gives me a chance to receive your feedback....

Thanks for your interest,
Jos van Eijndhoven

July 22th, 2010