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Pedal Power

Choir members singing in the park with microphones, speakers, piano and pedalling

So that we can operate at outdoor events without a power supply, and for an extra bit of audience participation, we have developed pedal power. This was initially for the piano but now also for sound reinforcement and public address. The equipment consists of one bicycle, a stand with a generator, an amplifier with a power output for the piano, and an optional mixer if microphones or other sound sources are needed.

If other community groups want to borrow this equipment, please get in touch. Below is a technical description for those who are interested.

The Generator

Scooter motor attached to bicycle trainer

The generator is a 250W 24V permanent magnet scooter motor as recommended by magnificentrevolution.org. It is more compact and simpler to use than the obvious choice, a car alternator, but produces an output voltage proportional to speed, i.e. it has no regulation. It is mounted to the frame of a Cat-Eye training stand kindly donated by a member of Merton Cycling Campaign. The stand has a roller which is held against the back wheel of the bicycle by a spring, and originally drove magnetic and fan brakes.

The bicycle is standard and merely has to have its front wheel removed to fit into the stand. The stand was missing some adaptors which means that the back wheel has to have a particular style of quick-release skewer for it to fit.

Novel use for a shaver adaptor
The three-LED voltage indicator

Whoever is pedalling is guided by a simple “speedometer”. Yellow, white and red LEDs indicate under, correct and over-voltage respectively, and have a slow transition to give warning of incorrect pedalling speed. They are ultrabright to be viewable in sunlight. In practice it is very easy to maintain the correct speed.

The Amplifier

Amplifier: Input sockets, volume control, mains input and switch, piano power output and status LEDs
Front view of amplifier

The generator connects to the amplifier box. This contains a class-D stereo amplifier which offers high efficiency (and hence only needs the box for a heatsink) and high quality. The “Amp 4” kit operates from a single supply of about 18 to 32V so is ideal for powering directly from the generator for maximum efficiency. It has had minor modifications to allow its output relay to operate over the full voltage range, to reduce thumps by delaying the closing of the relay on power-up, and to reduce the effect of the shared earth when connecting straight to the piano by optionally “quasi balancing” its inputs.

Speaker outputs, generator power input (with 12V output for “speedometer”) and “quasi balanced” switch
Rear view of amplifier

The amplifier inputs are on two phono sockets (isolated from the chassis in “quasi balanced” mode). Its outputs are on two 4-pole Speakon sockets wired so that stereo or mono cables can be connected. (The amplifier uses the bridged configuration so speaker grounds are kept isolated.) There is one green and two red status LEDs, which indicate power, distortion, over-temperature and under-voltage by various combinations.

top: mains power supply; middle: amplifier; bottom right: DC-DC converter and power mode/overvoltage relay
Internal view of amplifier

The amplifier box also contains a PTN78060W DC-DC converter which provides 12V at 3A from 15 to 36V at over 90% efficiency. This is fed to an output socket to power our Roland EP880 stage piano.

Because the converter is not isolated, the signal and power earth are shared. This causes a varying voltage (due to the varying current drawn by the piano through the resistance of its power lead) to be superimposed on the signal, resulting in nasty buzzing noises. The “quasi balanced” mode, where the amplifier's input earth reference is no longer the amplifier's earth but an approximation of the piano's earth (via the piano signal cables), reduces this markedly.

The final ingredient in the box is the guts of a Dell laptop power supply to allow the amplifier to be mains-powered. A relay switches between the two power sources, and also serves as an over-voltage cutout when using generator power. Short-circuit protection has been disabled on the power supply as the large reservoir capacitors in the amplifier were causing it to trip. Most of the capacitors on the power supply output have been removed to prevent them welding the relay contacts together as they discharge into the amplifier capacitors as the relay closes.

The speakers are Studiospares Forte which were recommended as a compromise between size, efficiency (93dB @ 1W @ 1m) and sound quality. They are “installation speakers” which do not have a mounting hole for a stand. Instead, pieces of aluminium tube have been bolted to their mounting brackets to allow them to be stand-mounted. The speakers, brackets, cables (10m and 15m), amplifier and various leads all fit into a large wheeled duffle-bag.

The Mixer

Partial view of mixer and piano with picnicers behind
We don't need no stinkin' pylons
Pedaller with pylon in the background

A more sophisticated setup can be achieved by adding an audio mixer. This is a Behringer 1204FX which has four mic and two stereo inputs, and has been modified to operate from the generator. A Traco TEL 15-2422 15W isolated DC-DC converter has been added to produce +/-12V from 18-36V for the analogue electronics. The original supply voltage is +/-15V but the converter was bought second-hand with a considerable saving and the lower voltage just results in a little less headroom. The digital effects unit in the mixer requires +5V and this is handily supplied by a TEL 2-2411 2W converter which was included with the other device. The three power rails are simply wired in parallel with the outputs of the mixer's built-in mains power supply; the current drawn by the unused outputs when operating in either mode is small.

No special provision has been made for phantom power when operating from the generator. There is simply a diode from the +12V supply which results in about 10V (as opposed to 48V when mains-operated) when it has passed through the soft-start circuit. This is sufficient to power AKG C1000 condenser microphones which are used with the unit.

Using isolated DC-DC converters means that there is no shared power and signal earth between the mixer and the amplifier. When the mixer is connected to the amplifier, however, their signal earths are joined so there is still a shared earth with the piano when it is feeding the mixer. This is overcome by making a balanced connection between the piano and the mixer's stereo input, using mono jack plugs at the piano end feeding tip and ring of stereo jack plugs at the mixer end.

All photos: Matthew Marks