Update: Fully built and tested Altoids Piezo Preamps are available for sale in the shop here.
I’ve been taking my Altoids piezo DI box to open mic nights and have had some interest and feedback. People seem to like the concept but are less impressed with the balanced output and phantom power – which is fair enough given that the mixer is invariably on the stage and the cables are relatively short. Consequently I have produced a simpler version of the unit and (as a result of ditching the op-amp) reduced the power consumption to about 0.3mA and increased battery life to 300+ hours.
Here’s the schematic:
So (compared with the Altoids Piezo DI) the balanced output has gone and there’s a TC54 low-battery monitor circuit which switches on the LED when the battery voltage falls to 7.3V. The battery is connected when a plug is inserted in the input socket.
“Standard” PCB 1/4″ jack sockets are readily available from many suppliers. However, the height above the board is a little high for an Altoids box so I have allowed for low-profile jack sockets. There are two versions of the PCB design.
- The “C” version accepts Cliff S1/BBB CL1115 low-profile sockets. These are available from CPC in the UK. This PCB design also allows for “standard” size 1/4″ jacks.
- The “J” version accepts Jalco JS-5077 low-profile sockets. These are available from Rapid Electronics in the UK and are significantly more expensive than the Cliff jacks.
Here’s the C version in the tin:
and the J version unboxed:
There is a piece of cardboard stuck to the bottom of the Altoids tin with double-sided sticky tape. The PCB is then stuck to the cardboard using double-sided sticky foam pads. This allows for the nuts be be left off the jacks so the lid fits better.
Here are the PCB layouts:
and the foil patterns (300dpi):
Finally, here are the words for the instruction manual. You will need to put a sticker on the underside indicating the input and output jacks. In all the above pictures/graphics, the input is the lower jack.
Altoids Piezo Preamp
1. Connect the input to an instrument. The input has been optimised for passive piezo transducers (passive pickups) but the pre-amp is suitable for acoustic, electric and bass guitars also. The pre-amp can be used anytime you otherwise get a poor signal (loss of high-frequencies, very low signal level) on connecting the instrument direct to the mixer.
2. Connect the output to a mic/line input on the mixer. The pre-amp has an unbalanced output at quite a low level. Connection to a stereo line input on the mixer is not recommended as there may not be enough gain available at the mixer.
3. The unit requires a 9V PP3/6LR61/1604A alkaline battery. To access the battery remove the screw holding the lid closed. The unit is powered-up whenever a plug is inserted in the input socket so disconnect the input when the pre-amp is not in use. Power consumption is very low (approx 150uA) so battery life is in excess of 300 hours.
4. The unit operates at very low power and requires a few seconds to “warm up”. Usually you will not notice this effect.
5. The LED indicates low battery voltage. The LED lights up when the battery voltage falls to 7.3V. The unit will continue working until the voltage falls to about 4V so if you notice the low-battery light during a performance do not be concerned about imminent battery failure.