Altoids piezo pre-amp

By | March 29, 2012

Update: Fully built and tested Altoids Piezo Preamps are available for sale in the shop here.

Altoids Piezo Preamp

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:

Altoids Piezo Pre 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:

Altoids Piezo Pre C version

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:

Altoids Piezo Pre PCB overlay

and the foil patterns (300dpi):

Altoids Piezo Pre PCB foilFinally, 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.


6 thoughts on “Altoids piezo pre-amp

  1. puresounds

    Hello how do you do.
    I am intresting your piezo pickup project.
    I have made it.
    Because I sometimes act as Sound Enginner for Gypsy Jazz.
    And in Japan 2N5458 is a few available at some small electric shops.
    I try several guiter pick up at Live stage.
    My feeling This is clean and good sounds.
    I use LM833 opamp.
    I think sevral pickup need more input impedance.
    So i try to change bias register R2.
    Best regards.

    1. SmudgerD Post author

      I can’t imagine any pickup would need more than 10M Ohm input impedance and if you increase the value of R2, you just introduce more noise into the system.

      Regarding the 2N5458, you could use 2N5457 or or 2N5484 or PF5102, etc. You only need to change the value of R3 so that there is about 800mV across R3.

  2. barnabywalters

    Hi! I’d like to try using this circuit (or a similar battery powered piezo preamp circuit) as the basis for a multiple channel on-board preamp system for a musical instrument. My plan is to have three channels, each of which is based on the circuit here, all sharing the same power supply and Vbias. Each should have their own volume control, and then there should be a master volume control.

    Do you think this would work simply by using passive components, feeding the output of each preamp through a potentiometer and series resistor, all into a master volume potentiometer? Specifically I’m wondering if just using passive components like this will result in a too high output impedance, and also whether adjusting each channel’s volume control would interfere with the volume or spectrum of the other channels?

    I hope to be able to build this circuit soon and test it out, but am curious if you have any ideas about how best to achieve this. Many thanks for the circuit!

    1. barnabywalters

      I also simulated a version of the circuit with the simplest possible op-amp unity gain non-inverting buffer circuit after the channel and master volume pots, to provide a very low output impedance (with a series resistor on the output for over-current protection). I achieved this with a single-rail power supply by using Vbias to add a DC offset to the combined signal before being buffered by the op amp, and then a capacitor on the output to remove the DC bias. It works in simulation but I’ve not had the chance to test it yet, and it seems too simple — does this sound like something which could perform well in practise?

    2. SmudgerD Post author

      I would leave out D1, D2 and R4 for each channel, then replace R5 with a 100k LOG volume control. From the wiper of the volume control take a 10k resistor and tie it, along with all the other channels to the non-inverting input of an op-amp (e.g. LM833) in voltage follower mode (see the article on the piezo di for ideas on how to power the LM833 from a single supply). Then a.c. couple the output of the LM833 add in D1, D2 and R4 and use a 10k potentiometer in lieu of R5 for the master volume. HTH

      This sounds very much like what you’re doing in your second post. If you want to email your schematic I will be glad to take a look.

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