RelaiXedSMD -- DIY balanced pre-amplifier


The Relaixed2 is an audio pre-amplifier design, created for easy re-builing by DIY audio enthousiasts. This page describes its SMD version, which is a more compact implementation as result of selecting mostly SMD-type components. The more compact implementation can free-up cabinet space for integration with a power amplifier. Similar to its standard implementation, it targets the high-end audio segment. It is characterized by:

The RelaiXedSMD design shares with the RelaiXed2 design a special relay driving scheme that supresses audible glitches in the audio outputs that were experienced in other/earlier relay-based designs. New in comparison with the earlier RelaiXed PCB design is the option to mount a PCB-based cinch-connector block, as alternative to the more elaborate wiring of individual panel-mounted cinch connectors.

This is a foto of the volume control with the resistors and relays:

A chain of 6 relays provides 64 volume positions with steps of 1dB. The separate attenuator relays for the left and right channels also provide balance control.

Just behind the 6 relays in above foto is the empty socket to hold the opamps that are used for the audio amplification. The standard DIL-8 socket allows a choice of 'dual opamps'. The standard type for this amplifier is the LME49720. Normally one can apply the DIL-8 version of this opamp. However, for best audio performance, its TO-99 metal-can package is advised: that one is more expensive, but really provides a proper sound improvement. Yes, that is somehow incredible, as its technical specs remain identical... This metal-can version -in my experience- does away with the sound problems that people often affiliate with opamps.

Design information

This SMD design is documented here with a couple of on-line documents and fotos. The user manual of the original RelaiXed2 is not yet cloned and adapted for this design, but is still usefull for background info.

PCB and microcontroller availability

Upon request (by email) I can send PCBs and preprogrammed microcontrollers.
A pair of PCBs now costs 62,=. The preprogrammed PIC18F24J50-I/P microcontroller costs 8,=. Packaging and mailing with a 'sign-on-receipt' service costs 13,= throughout Europe, and 18,= elsewhere.

I do not send out sets of components: you better order these directly from an online shop. A complete set of components costs roughly 200,=. You might contact me for availability and price of a pre-built module, which I could have available sometimes.

NEW from June 29, 2014: The PCBs had been out-of-stock for a few weeks, but a slightly updated PCB is now available again! Small layout changes improve manual solderability, and it is now produced with a thicker-then-default copper layer. The pricing for a single board remains the same. If you order more then one unit, I can offer 30% reduction on the price for the extra (2nd and further) units.

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:

One final 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...

Mounting SMD components

Mounting the components on thios board is best done in an order with the lowest-profile components first. That way, best accessibility for soldering is maintained.
For manual soldering of these small SMD components, there is a standard procedure:

These are a few pictures from such soldering (click for a larger view):

The picture below shows the PCB with lowest profile (mounted first) components. That might help you with your assembly while a real manual is stil absent... (Click for a real hi-res picture!):

Electrical specifications

Bandwidth: DC - 330kHz (limited by input filter)
Signal gain at max. volume, cinch in to balanced out: 1 : 1.55 + 1.55 (is +9.8dB)
Signal gain at max. volume, balanced in to balanced out: 1 + 1 : 2.1 + 2.1 (is +6.5dB)
Signal input impedance: 47 Kohm
Signal output impedance: 400 Ohm
Distortion (1 kHz, 5V in/out, single-ended or balanced): 0.0001%
Output noise density at max. volume, shorted input: -160 dBV/sqrt(Hz) is 10nV/sqrt(Hz)
Output signal/noise ratio in 20kHz audio band, relative to 2Vrms nominal signal: -123 dB
Left-Right channel separation at 1 kHz: -137 dB
Power consumption during standby: 0.6 W
Power consumption at max volume: 9 W

Distortion measurements show extremely low values. This indicates that -next to the opamps- also the other circuit components (resistors, capacitors, wiring) behave good. Around 1kHz, -120dB corresponds to 0.0001% distortion, and is close to the 'straight wire' reference measurement. Above 5kHz the distortion increases normally, except that in true balanced operation a cancelling occurs to maintain a very low distortion level.

Building the relaixedSMD into a cabinet

I built my prototype into the same Hifi2000 slimline cabinet as was done for the earlier RelaiXed2 cabinet. Note the shiny metal-can opamps in line with the relays, with a attached black ribbon cooling fin.

The audio connectors in default backpanel. This panel is made by Schaeffer from this design file:

The chosen power-entry has both a power-switch and a fuse. The RelaiXed is normally not powered-off: in standby it can be powered-up through the remote control.

Thanks for your interest,
Jos van Eijndhoven

June 29, 2014