RelaiXed -- Balanced pre-amplifier

RelaiXed is a high-end audio preamplifier for the DIY audio hobbyists, featuring:

• Balanced audio input and output connections, next to a few 'single ended' (cinch) input channels
• Volume control in 1dB steps through a sequence of resistors and relays, for best sound quality.
• Operable through IR remote control

Its design is first published in the Dutch Elektor Audio Special 2 of December 2007. This project was a cooperation between Sander Sassen and myself, Jos van Eijndhoven. An initial small set of prototype PCBs was quickly gone. The current version was slightly updated after doing various measurements and listening tests.

Characteristics

• Audio volume control through relays and resistors provides a very good audio quality. The combination of good-quality resistors and relays ensures a consistent behavior and extremely long lifetime. It is amazing to hear the more clear and open sound in comparison with a conventional potentiometer that uses a sliding contact over a resistive layer. These relays-and-resistors can even compare favourably with high-end audio rotary switches for volume control. (Observed by earlier users of my passive attenuator.) I am aware that using solid-state volume control ICs would have resulted in a smaller and cheaper design, however I don't trust the sound quality of those.
• The design offers six stereo audio input channels to select from. The available PCB allows print-mounted XLR connectors for the balanced input and output channels. When building the preamp, one can choose a configuration with 3 single-ended (cinch) input channels and 3 balanced (XLR) input channels, or, alternativley, 4 single-ended and 2 balanced channels. The output signal is always balanced: the design will perform single-ended-to-balanced conversion for the inputs that need this.
• This design employs a new generation of high-end audio opamps to buffer the output signal and add some amplification. The design includes a high-quality very-low-noise power supply stabilization for these audio opamps: That seems to positively influence their output sound. The design was made for (tested with) the LM4562 opamps. Alternatively, other pin compatible types can be used, such as the -also very good- OPA2134.
• The preamplifier has a remote control, through a conventional IR reception. It can receive (decode) the Philips RC5 and RC6 protocols, as well as the Sony SIRC protocols. Within these protocols, the preamp can 'learn' your favorite buttons and device setting, to properly coexists with other devices. Normally, a rotary switch -with push button function- is also connected for direct manual control.
• The design employs two microcontrollers: one operates the small front panel with the IR receiver and LED display. The second one operates the relay board. When there is no user activity, the one on the relay board switches itself off, including its internal clock signal. That prevents any high-frequency digital signal interaction (parasitic EMI coupling) with the audio circuit.
• Next to the normal audio control (volume up/down, input channel selection, audio mute), the preamp also provides an output signal to connect a power relay. The IR remote power on/off function activates this relay, and can be used to switch other (audio) equipment next to its own analog power supply. In 'power off' mode, the preamp digital supply and front-panel microcontroller remain active to receive later a wake-up command. In 'power off' mode, the preamp power consumption is very low: between 1W and 0.1W, depending on the quality of your tiny digital power-supply transfomer. Note that the mechanical clicking of relays is well audible when you adjust your volume. If you want true silent behavior, a relays-based design is probably not your preference.
• Optionally, the relay board can be built to handle 6 cinch-inputs only, by omitting a few componentents. Also, the relay board can be configured to handle 4 independent, non-balanced, audio channels. This can be useful for multi-channel audio enthousiasts. Furthermore, multiple relay boards can be controlled with a single front-panel interface, giving even more multi-channel options.

Building RelaiXed

The design uses easily available components, that you can buy at your favourite shop. (Sorting out piles of components is not the way I prefer to spend my audio hobby time.) To help you to easily build your own preamp, I have gathered the following documents:

• The schematic drawings and PCB layout: pdf document.
• The corresponding component table with types, numbers, and shop ordering info, version as of May. 30: pdf document, or xls spreadsheet.
• The RelaiXed building and use guide, version as of May 12th, 2008: pdf document (mainly updated the test description).
• A collection of datasheets of used components: zip document
  (Large doc, just as background info, probably you won't need this.)

You can see a few more photos from building my own RelaiXed in my RelaiXed Picasa album.
(For reference, these are the December 2007 version design files according to the Elektor publication: schematics and PCB, component table, guide.)

A few difficult things can be obtained from me:

• A pair of PIC16F819-I/P microcontrollers preprogrammed with the RelaiXed firmware, for €20,=
• The PCB,140x291mm, double-sided, professionally made with text annotation and gold-finishing of solder areas for €65,=. (top view, bottom view. I will seperate the main PCB from the front-mount PCB for you.)
• A set of Dale RN60-series (0.5W, 1%, milspec) resistors for all audio signal resistors in this design (resistors labelled R*L and R*R, total 64 pieces), for €25,=
• A set of 4x LM4562 opamps in DIL-8 housing, for €30,=

Thanks for your interest and trust in me,
Jos van Eijndhoven
May 18, 2008

Jos van Eijndhoven
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