RelaiXedPassive -- audio attenuator


The RelaixedPassive is the 3rd member of my RelaiXed designs. It is an audio volume control and input channel selector with a minimalistic passive design: there are are no active parts in the audio path: no transistors, opamps, tubes. It is a true attenuator. This attenuator design inherits several aspects of my earlier RelaiXed preamplifier designs:

The avoidance of clicks is the major improvement with respect to my original attenuator design of several years back. The main difference with the RelaiXed preamp is of course its passive nature, and the use of standard (single-ended, cinch connected) audio as opposed to a balanced (XLR) design.

This attenuator specifically targets integration in the same cabinet with some power amplifier of your own choice. Integration with a power amplifier avoids longer cinch cables and connectors at the output of the attenuator. Due to the relatively high output resistance of a passive volume control, passive attenuators in general are sensitive to cable quality and cable type. Integration with the power amplifier removes this sensitivity.

To take further benefit from the IR remote control in the amplifier integration, this design comes with a full-fledged power switch control with soft-start functionality. Through a heavy power relay and strong power resistors, the soft-start mode supports the safe switching of large toroid transformers and/or large capacitor banks in your power supply. On-board fuses for the control and the mains power output and the little on-board transformer simplify the wiring in your amplifier chassis.

Design information

Component availability

Upon request (by email) I am happy to send you the pair of PCBs with a preprogrammed microcontroller. These PCBs are empty: you buy your own set of components from the provided table, and do your own assembly. A pair of empty PCBs together with the preprogrammed PIC18F24J50 microcontroller costs €50,= incl. VAT. 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.

Including a bag with a complete set of components (making it into a real kit) is rather cumbersome for me: this component sorting is a somewhat error-prone process for me, and the real online shops are simply better in doing this. If you experience difficulty in obtaining some of the components, I am happy to help you out. A complete set of components costs roughly €100,=, ex. VAT, in Europe. Prices tend to be slightly lower in other continents. Note that I cannot maintain exact prices in my component table, they fluctuate too often, but my table should give a reasonable estimate. Note also that a few components are different for 110-120Vac regions versus 220-240Vac regions.

For this design I intend(ed) to also provide completely built and tested modules (PCBs with all components mounted). My first contact to do production resulted in a price quote which was somewhat higher then I had hoped for, partially due to the relatively small production quantity. Also, I probably need a redesign of the front module for easier automated production. I decided to postpone this production a little while, and first see how you react on this design. In the mean time, I can potentially provide manually built modules, if the counts are rather small. You can contact me about that.

Further design options

Probably most of this text should better move into a real user guide, but for now:

Thanks for your interest,
Jos van Eijndhoven

September 28, 2014