RelaiXed2 -- DIY balanced pre-amplifier

Introduction

The Relaixed2 is an audio pre-amplifier design, created for easy re-builing by DIY audio enthousiasts. Although inspired by the original RelaiXed design, it has been thoroughly updated in almost all aspects. It targets the high-end audio segment, and is characterized by:

• The design provides a balanced (XLR) stereo output, and 6 selectable inputs of which 2 (or 3) are balanced inputs and 4 (or 3) are single-ended (cinch) inputs. Balanced cabling towards the power amplifier in principle provides cleaner audio signal transfer.
• Audio volume control and stereo balance control is implemented through a series of relays, 6 per channel, providing 1dB volume steps with high quality and reliability, beyond what can be obtained with other types of volume controllers.
• Besides through a single front knob, the pre-amplifier can be remotely controlled through IR signals. It supports multiple IR protocols, to provide large freedom in remote controller selection.
• The hardware design entails a larger PCB containing the audio processing, the power supplies, some digital decoding, and the on-board XLR connectors, and is to be placed against the back-plate of the pre-amplifier chassis. Next to that, a small front-PCB contains the microcontroller, a 2-digit LED display, and the IR receiver, and is to be located against the front-plate of the pre-amplifier.
• The software ('firmware') of the embedded microcontroller can be easily updated through a USB connector. All embedded software as well as the PC interface software is made public available.
• Upon request I provide the PCBs to other audio hobbyists, as well as the required pre-programmed microcontroller. These PCBs are being made with first-grade industrial quality, two-layer through-hole, and gold finishing. For all other components I provide a component table with order-numbers for a well-known internet shop. Note that I will not freely provide the PCB design files.

With respect to the earlier Relaixed design, the main improvements are a strong reduction in noise level, an overall improved audio sonic quality, eliminating a 'pop' in the audio output on power-up, as well as suppressing relay switch transients in the audio output.

Design information

The design is documented with an extensive set of on-line documents and fotos:

• The hardware design files with schematics and PCB layout with sizes: design_20120128.pdf.
• Pictures of the produced PCBs, taken by my flatbed scanner:
  Relay-PCB top view Front-PCB, display and inside view
• First fotos from the boards with mounted components: Front-PCB Relay-PCB
• The User Manual 2012-03-31, with basic performance measurements, build instructions and fotos, and operating guide.
• The HW Design Manual, with motivation and explanation on the schematcs, and more detailled measurements, indented to be readable for persons with some electronics background.
• Component list and prices: Component table. NEW from Mar 29: Added component type and price info from Mouser.
• A zip file with datasheets of all electronic components used in the design.
• A SW Design manual explaining the organization of and build process for the embedded software as well as for the PC-side interface software.
• The online software project, that provides all source code in a revision control system (svn), also in a web-browsable format. If you want to contribute to improving/extending this software, please conact me.

PCB and microcontroller availability

Upon request (by email) I can send PCBs and preprogrammed microcontrollers.
NEW from Apr 25: with a new set of PCBs made, I can do a slight price reduction. A pair of PCBs now costs €72,=. The preprogrammed PIC18F24J50-I/P microcontroller costs €8,=. Packaging and mailing with a 'sign-on-receipt' service costs €10,= throughout Europe, and €18,= elsewhere.

Additionally, there is some support for creating a RelaiXed cabinet.

Design background

Obviously, this single page is not suited to explain all of the design background. Nevertheless I would like to mention some of my ideas that guided this design. In the past I did design and listening tests on vacuum tube amplifiers. That background and experience did lead me to:

• Do not use more semiconductors in the audio path then needed. More semiconductors easily make the sound less open and transparent.
• For those semiconductors that remain, use quality types that are designed for high bandwidth. I suspect the non-linear (parasitic) capacitances inside the semiconductors. In devices that support high-frequency operation these capacitances have lower (less disturbing) values. Otherwise, the nonlinearity of these capacitances can be reduced by operating the design at relatively high supply voltages.
• Always choose good quality components! Nevertheless, I often find the advertised/dedicated 'high end audio' components over-priced, and instead look for first-grade professional (industrial) types.
  In general it is good to look for low-noise components. Low noise often correlates with good production quality. The resistor type and brand in this design is chosen for its excellent noise specification of 0.01uV/Volt, at a very afforable price (compared 'high end audio' resistors).
• The relais + resistor attenuator avoids the weak contacts of a sliding potentio-meter, and avoids the single-chip digital controlled integrated attenuators (as I distrust most semiconductors).
• The on-board XLR connectors provide a short and good-quality connection to the audio electronics, without you having to worry about wiring audio inside your chassis. No audio signals are routed to the front (display) module. The digital electronics on the main (relay-) PCB are 'silent' during normal use: the digital signals do not switch state, there is not a 'life' clock signal. This prevents interference from the digital side into your precious analog audio signals. Only during volume- or channel-changes there is a moment of digital activity. The digital circuit is strongly separated from the audio, there is not even a common ground signal.
• Good-quality components and tools lead to more satisfaction in this nice hobby and to reliable devices with a long lifetime expectation. There is already too much rubbish being made...

One warning: when turning the volume, one can clearly hear the mechanical clicking sound of the relays. If that would really disturb you, relay-based attenuators are not your thing...

NEW from Mar 31: A wikipedia page that describes the theory and formulas behind the attenuator.

Foreseen extensions

I still plan some extension that are not yet part of the current design release. This is among others:

• Support for an attached D/A-converter module, to be integrated with the relaiXed design in a shared chassis. A multi-input DA would be controlled seamlessly by the RelaiXed input selector. (Done in my Jan. 6 release!)
• Some extra firmware control to choose and fix relative volumes per input channel. This way, switching between input channels would retain a more uniform sound level.
• Support to drive multiple relay boards from a single front-PCB controller with per-board relative volume control. This targets the volume control for multi-channel (surround) audio systems as well as volume control for people that employ separate power amplifiers per speaker (-driver) unit, and want to do volume control behind their (digital) cross-over filters. (The current version does support multiple relay boards on a single front-panel, but all boards will behave identically.)
• Support a separate (alpha-numeric or graphic) LCD display as alternative to the basic 2-digit LED display. When connected through I2C, this would require a software extension only...

Thanks for your interest,
Jos van Eijndhoven

Apr. 25, 2012