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.

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

PCBs are out of stock since 2018, not made anymore (by me)
The PCB availability as mentioned in the paragraphs below is NOT valid anymore! Basically because I currently lack time for this project...
For any DIY audio hobbyist who wants to build this design and have PCBs fabricated, the Gerber PCB design files are provided in this zip archive.
NEW: If you want to make your own modifications to the schematics or PCB, I hereby provide the complete design source archive, refreshed for use in KiCad version 5!

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. If you order more then one unit, I can offer 30% reduction on the price for the extra (2nd and further) units.

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.

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...

The audio opamp selection

Although this RelaiXed-SMD design mostly relies on compact SMD components, conventional DIL-8 sockets are used for its main audio opamps. This allows for a wide selection of available (dual-) opamps, and an easy swap if you want to do a later upgrade or just test some alternatives. The internet is full of stories on good audio opamps. Be aware that, first, there is not just a good (or best) opamp: the suitability of a specific opamp highly depends on the surrounding circuit. And, second, modern audio opamps tend to develop in the direction of low noise and low distortion, but definitely also towards a very high bandwidth. Since I personally distrust semiconductors in audio for their non-linear (parasitic) capacitances, high bandwidth seems a promising direction for me...

The circuit uses 4 such dual opamps: one each for left and right audio, where each half of the dual opamp is used for the + and - signal of the balanced audio, and one each for the positive and negative power supply stabilization, where the halves are used for independent left and right channel voltage stabilization.

Mounting SMD components

Mounting the components on this 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. It also shows the 4 sockets for the 4 dual opamps: 2 for the power supplies and 2 for the audio itself. The photo might help you with your own assembly: Click for a real hi-res picture!

Front control module

Next to the main (audio and powersupply) PCB of above photo, the design uses a small front-side PCB with 2-digit display, the infrared receiver, and the user control knob. This PCB also contains the microcontroller for the digital control of the main PCB. These PCBs are connected through 4 wires. Obviously, audio signals are not passed to this front PCB. This PCB is actually identical to the one used for the 'classic'(non-SMD) version of the Relaixed, so it uses mostly through-hole components. This board also holds a USB connector which can be used to upload new firmware into the microcontroller. That connector is located on the back side and not visible on this photo:

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, while using a LME49720 opamp, 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

Sept. 28, 2019