RelaiXedPassive -- audio attenuator


This RelaixedPassive is the latest 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, and it does not add any distortion to the audio signal. This attenuator design inherits important concepts of my earlier RelaiXed preamplifier designs:

The avoidance of clicks in the audio signal is the major improvement with respect to my original attenuator design of several years back, and distinguishes this design from relay-attenuators from some other sources. Note that a soft mechanical clicking noise is still emitted by the relays themselves.

This attenuator specifically targets the DIY high-end audio hobbyist, and aims at integration in the same cabinet with some power amplifier of your 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 remote control for the power switch, including soft-start functionality. Through a heavy power relay and tough power resistors, the soft-start mode supports the safe switching of large toroid transformers and/or large capacitor banks in your power supply. This avoids the need for the typical thermistors, that are used to reduce rush-in currents for larger transformers. The on-board small transformer for stand-by power, and the fuses for the control and the mains power output, simplify the wiring in your amplifier chassis. If you do not trust this mains power switching so nearby to the precious audio signals, the board allows you separate it into two parts: the audio attenuator versus the power supply and power switching. After separation, each board still provides 4 mounting holes, and 4 low-voltage wires are needed for their interconnection.

The main difference with the RelaiXed preamp is of course its passive nature, and the use of standard (single-ended, "RCA" cinch connected) audio as opposed to a balanced (XLR) design. Furthermore, this passive attenuator does not offer left-right balance control, which is available in the full Relaixed preamp.

The resistors in the audio path are relatively large Vishay/Dale "MMB0207" professional MELF resistors. These are very good resistors for high-end audio, obviously surpassing the cheap smaller SMD resistors. Their quality can be seen for instance from their 'current noise' specification, rivalled by hardly any alternatives:

Design and build information

The revision shown here is new as of July 2016, and has some small updates with respect to the 2014 design. The current version has a smaller front control PCB and a modified stand-by power-supply. The audio section and relay control remained unmodified. The design information of the earlier version is still available.

For documentation, you can look into:

Module availability

Upon request (by email) I am happy to send you the PCB together with a preprogrammed microcontroller. So, the PIC18F25J50 microcontroller will contain its firmware image, and you do not need some programmer device. This design revision aimed to improve manufacturability for automated assembly. Offering this design as a pre-assembled module saves you from manual SMD component ordering and soldering, and allows me to offer the module with some guarantees on proper operation. So for now, I intend to offer this module in several variations. These options might still change in the near future:

Option What you receive Guarantee Cost of module Mailing cost Europe/World
1. PCB with everything mounted and tested, module is ready to use. You need to specify a few customization preferences with the order. Yes € 220 €18 / €28
2. PCB with the microcontroller and all SMD components mounted and tested. Also included is a strip of 4-wire colored flatcable as seen on the photo at the top of this page. You should yourself buy and solder a small set of through-hole components. This provides you with some configuration flexibility. Yes €150 €13 / €18
3. Bare PCB, only provided with a pre-soldered microcontroller. You need to buy yourself a full set of components according to the above component list. Obviously, you might assemble your own variation, including choosing a different attenuator impedance level. No €50 €13 / €18

These prices include Dutch sales tax (VAT). The mailing price includes a track-and-trace and a sign-on-delivery mailing service.
Some further explanation on these three options:

1. ready-to-use

For this first option, you need to specify your preference for:

  1. The primary voltage for the power supply: 110-120Vac versus 220-240Vac
  2. The color of the 7-segment LED display: red versus blue
  3. Mount the 4x2 cinch ("RCA") block with audio input connectors Yes or No. Using this PCB-mounted connector is very convenient, however the connector quality is only reasonably good. Alternatively, you can buy high-end single-RCA connectors of your own choice for panel mounting, and wire these to the PCB. The product is delivered with a 2-year guarantee to cover normal operation, not for failures due to wrong wriring.

2. SMD mounted and tested

For this ordering option, you can choose yourself above preferences by acquiring the proper component selection. In general, you should still buy: the rotary switch, the pair of LED displays, the input RCA (cinch) connector block, and components for the power supply section of the board. The standard component list without the already pre-mounted SMDs is also at Octopart. You could use this Relaixed Passive even without any further component on the power-supply section of the board, if you do not need the power-switching relay capabilities and provide some external 5Vdc standby power. This option 2 is also appropriate for a multi-channel setup, where you can connect 2 or 3 attenuator boards to a single front controller. In such a set-up, all attenuator boards will operate in tandem mode, following the same controller state. Sending this board with SMDs only, allows me to send it in a flat package, which saves some mailing cost.

3. Bare PCB with microcontroller

If you choose the bare-PCB option, you would need to buy the components as listed above. For the tiny and cheap SMD thingies, I would recommend to buy a few extra: that saves you from trouble in case you would sneeze and launch them in unknown space. Clearly, you would also have the freedom to modify the resistance-level of the attunator to a higher or lower value then the default 27Kohm input resistance. To calculate other resistor values, my online attenuator calculator can be used. In general, you should be careful with selecting high resistor values: lower values tend to provide a more transparent sound. As I have no control on your assembly, I cannot give guarantees on the product. However, I can give some online assistance if you have trouble with the assembly and testing of the board. If you remain unsuccesfull, I can offer some repair service.

Multi-channel attenuation: multi-board configuration

The design does support larger multi-channel multi-board configurations, where a single front unit controls several audio/relay boards. In a multi-board set-up, only one of the boards needs to have the power supply. Such set-up can be used for different applications:

Technical specifications

Output audio connectivity: One stereo audio signal through a 4-pin screw connector
Input audio connectivity: Four stereo inputs, typically through an on-board 4x2 cinch (RCA) connector block
Input resistance: 27Kohm (can be reconfigured to any other value)
Output resistance: 0 to 13Kohm, varies with the volume selection
Frequency bandwidth: DC to 150kHz (-3dB) at 10dB attenuation,
Left/Right channel separation: 96dB at 1 kHz and 10dB attenuation
Input select attenuation: 123dB from neighbouring input at 1kHz and 10dB attenuation
Power consumption during standby: < 0.1 W
Power consumption in operation: < 2 W
Maximum power switch to external load: 16A, 240Vac
Maximum energy to absorb in soft-start: 1000 Joules
Size of main PCB: 150x74mm. Height above PCB surface: 27mm with RCA block, 22mm without
Size of front PCB: 26x74mm. (For more sizes see the schematics document)

Recent extensions

New:Support was added for a 16x2 character OLED display, as alternative for the standard 2-digit 7-segment display. This particular display from ebay was selected. Obviously, such OLED displays have a great contrast. Its i2c interface is simply connected in parallel on the 4-wire communication bus between the front- and the main- (relay) PCB. To use this display, you would need to update the firmware inside the PIC microcontroller to -at least- the 20160910 revision, which can be downloaded from the project sourceforge site. This display seems to have issues in obtaining a reliable communication. See how to handle that in the User Manual.

Thanks for your interest,
Jos van Eijndhoven

Please note that although I (aim to) create professional-quality audio devices, these developments are just hobby for me, next to a very busy job. These web pages result in many emails to me, which I cannot always answer quickly. So, please, have some patience with me. I always try to answer within a week, otherwise feel free to ask again!
December 3, 2016