Audio Volume Relay Attenuator with IR Control

resistor&relay close-up This page shows my Audio Volume control circuit and implementation. The circuit provides both audio volume and input channel selection. A stepwise volume control is implemented with a set of small relays and resistors. In a high-end audio system, a noticable sound improvement over potentiomeneters can be obtained, also over 'audio grade' potentiometers. Clearly, the IR remote control provides convenience over solutions with stepped attenuator rotary switches. The sealed relays will maintain contact quality over a practically endless lifetime.
(For my overview on alternatives and theory look here.)

Message as of December 2007: My PCBs and component sets were sold-out since September 2007. Due to the enthousiastic requests I received from you, I finally decided to create a new set of PCBs. These PCBs, related component sets, and slightly updated documentation will become available end of January 2008. The updates entail: a) New firmware inherited from my 'RelaiXed' pre-amplifier, supporting now also the Sony IR protocol, and shut-down of the microcontroller on the relay-PCB when there is no user interaction. b) An optional transistor to drive an external power-switch relay came on the PCB.

The circuit has the following features/characteristics:

Compact and simple circuit: just 6 tiny relays implement a 64-step logarithmic stereo attenuator. The 64 steps of 1.1dB together span a 70dB audio attenuation range.
An IR receiver allows remote control. The IR 'eye' and a numeric display are on a small front-mount printed circuit board, separate from the relay board. The device can learn and store your favorite key codes. The controller currently understands the popular Philips 'RC5' protocol, as well as the newer 'RC6' protocol. This provides interoperability with all 'generic' controllers.
The PCB supports audio input channel selection next to volume control. Four extra relays on the PCB allow selection between four audio input channels. Together this is fine to implement a compact high-quality passive pre-amp, but of course allows a combination with your favourite active buffer or DIY integrated amplifier. Finally, there is a 'power on/off' output signal, allowing to remotely control the power of other equipment through some external relay.
The display PCB can drive more than one relay PCB. Extra relay PCBs allow a multi-channel input select and volume control, for 5.1 or 7.1 sound, or for stereo audio with balanced (XLR) connections. The 'serial control' from the display to the relay boards reliably sends the new relay state, not just increments.
The circuit operates from an AC powersupply voltage of 6.0 to 7.5 volts. For tube amplifier builders: a spare 6.3V transformer winding will work fine.
An optional rotary switch can be mounted on the display PCB for manual control.

The circuit implementation targets high-end audio applications, with small but high-quality relays, and PCB space for somewhat larger audio-grade resistors. In particular, it is designed as appropriate solution for tube-amplifier enthousiasts, who prefer to have no semiconductors in their signal path. However, very nice sonic results have also been obtained with driving modern class-D amplifier modules.
The circuit does not create clicks/glitches in the audio output signal while making volume steps, due to carefully switching relays 'off' before others 'on'. However, you can clearly hear the mechanical activity of the relays. If you don't like that, relays are not your solution.

Description

The empty PCB. The left section will be cut-off, separating the front-mountable display PCB from the relay PCB. The PCB is dual-layer with top-side text annotation, and has a gold finishing for the solder contacts. The overall (joined) size is 100mm high, 110mm long.

Each PCB section has a surface-mount PIC 16F819 microcontroller. The display PCB controller handles the IR signal decoding and 7-segment display control, the relay PCB controller decodes the serial commands from the display device and drives the relays. Upon request I can distribute the PCB in this state with the firmware programmed into the microcontrollers (if the demand is not overwhelming :-).

The fully mounted 'relais' PCB. The tiny G6K-2P relays behave fine for audio. The diodes, voltage regulator IC, and elco provide a 5V supply voltage. My default audio resistors are currently high-quality Vishay-Dale 'RN60D' resistors, which are commonly appreciated for high-end audio application, with values to emulate a 47K potmeter. Before, I also used good-quality 1W Vishay-Beyslag 'MBE'resistors, which have a light-blue color, and are fine for most applications. I still have some of these to replace a 100K or a 22K potmeter. At the right side, the audio I/O can be wired. At the left side, the PCB has a 2-pin connector for its 6Vac power input, and a 5-pin connector for the display PCB and an optional extra power-switch relay.

The mounted front PCB. The two-digit 7-segment display gives feedback on the current volume setting and input selection. The (optional) rotary switch is for manual volume up/down. It is non-blocking (can be turned around-and-around), with 30 light clicks per full turn. Furthermore, this switch also provides a push-button action, such that every press selects the next input channel. You find the black IR sensor mounted just right from the center. Clearly, the wires between the display PCB and the relais PCB do not carry any audio signals.

Audio attenuation as function of relay setting with standard E-12 series resistor values.

Input and output resistance as function of switch setting. By using the 'most significant' bit switch at the end of the sequence, a reduced output resistance is obtained for low volume settings. This might help to reduce hum pickup.

Full schematics of the design, in 2 pages: the relay pcb and the display pcb. Note that the circuit consists of three sections that are totally isolated from each other, each having separate grounds: 1) The control with the microcontrollers, display, power supply, and relay coils, 2) Left channel audio resistors and relay contacts, and 3)Right channel audio resistors and relay contacts. This allows your choice of grounding scheme when building your system.

User guide version as of October 2006, in pdf. (I still need to do a few minor updates to this text to reflect the new (gold) PCB version.) It contains assembly instructions, and how to learn the device to recognise the button codes of your IR remote handheld. Recognisable buttons (IR commands) are volume up and down, mute, input channel up and down, numeric selection of input channel, and power off. Clearly, remote power on/off only works for other equipment, when this controller remains powered. If the controller itself is switched off, it will remember its last settings (volume and input channel) in embedded EPROM, and will load those settings again when powered on.

Datasheets of all the components that I used in a zip archive. You might look in here to check pin connections, or check compatibility with other part types. (Well, to save on my webspace, all component datasheets except for the PIC16F819-I/SO microcontroller which you can easily find at the Microchip website.)

Example of the attenuator mounted in a small industry-standard Eurocard casing. The case top cover and its transaprent front are removed for the photo. Note that this photo is taken from an earlier verion of the design, which required many wires between its display and relay board. (The new design uses only 3 wires.) This example relies on an external adapter for its power supply.

Upon request I can provide you (interested hobbyist) with parts for this design. The PCB with a complete set of components for a stereo setup costs €150,= (including relais, rotary switch, display, but excluding audio cinch connectors and housing). An extra slave board to support balanced audio or multi-channel will be €100,=. I would pre-mount both (SMD type) microcontrollers and program them with my firmware. All other components are provided as is, for traditional through-hole mounting.

Best whishes for a happy and musical 2008 !!

The design information presented on this website (schematics or firmware), cannot be used for commercial purpose without explicit permission.
Jos van Eijndhoven, December 29, 2007
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	The empty PCB. The left section will be cut-off, separating the front-mountable display PCB from the relay PCB. The PCB is dual-layer with top-side text annotation, and has a gold finishing for the solder contacts. The overall (joined) size is 100mm high, 110mm long.
	Each PCB section has a surface-mount PIC 16F819 microcontroller. The display PCB controller handles the IR signal decoding and 7-segment display control, the relay PCB controller decodes the serial commands from the display device and drives the relays. Upon request I can distribute the PCB in this state with the firmware programmed into the microcontrollers (if the demand is not overwhelming :-).
	The fully mounted 'relais' PCB. The tiny G6K-2P relays behave fine for audio. The diodes, voltage regulator IC, and elco provide a 5V supply voltage. My default audio resistors are currently high-quality Vishay-Dale 'RN60D' resistors, which are commonly appreciated for high-end audio application, with values to emulate a 47K potmeter. Before, I also used good-quality 1W Vishay-Beyslag 'MBE'resistors, which have a light-blue color, and are fine for most applications. I still have some of these to replace a 100K or a 22K potmeter. At the right side, the audio I/O can be wired. At the left side, the PCB has a 2-pin connector for its 6Vac power input, and a 5-pin connector for the display PCB and an optional extra power-switch relay.
	The mounted front PCB. The two-digit 7-segment display gives feedback on the current volume setting and input selection. The (optional) rotary switch is for manual volume up/down. It is non-blocking (can be turned around-and-around), with 30 light clicks per full turn. Furthermore, this switch also provides a push-button action, such that every press selects the next input channel. You find the black IR sensor mounted just right from the center. Clearly, the wires between the display PCB and the relais PCB do not carry any audio signals.
	Audio attenuation as function of relay setting with standard E-12 series resistor values.
	Input and output resistance as function of switch setting. By using the 'most significant' bit switch at the end of the sequence, a reduced output resistance is obtained for low volume settings. This might help to reduce hum pickup.
	Full schematics of the design, in 2 pages: the relay pcb and the display pcb. Note that the circuit consists of three sections that are totally isolated from each other, each having separate grounds: 1) The control with the microcontrollers, display, power supply, and relay coils, 2) Left channel audio resistors and relay contacts, and 3)Right channel audio resistors and relay contacts. This allows your choice of grounding scheme when building your system.
	User guide version as of October 2006, in pdf. (I still need to do a few minor updates to this text to reflect the new (gold) PCB version.) It contains assembly instructions, and how to learn the device to recognise the button codes of your IR remote handheld. Recognisable buttons (IR commands) are volume up and down, mute, input channel up and down, numeric selection of input channel, and power off. Clearly, remote power on/off only works for other equipment, when this controller remains powered. If the controller itself is switched off, it will remember its last settings (volume and input channel) in embedded EPROM, and will load those settings again when powered on.
	Datasheets of all the components that I used in a zip archive. You might look in here to check pin connections, or check compatibility with other part types. (Well, to save on my webspace, all component datasheets except for the PIC16F819-I/SO microcontroller which you can easily find at the Microchip website.)
	Example of the attenuator mounted in a small industry-standard Eurocard casing. The case top cover and its transaprent front are removed for the photo. Note that this photo is taken from an earlier verion of the design, which required many wires between its display and relay board. (The new design uses only 3 wires.) This example relies on an external adapter for its power supply.