{"id":176,"date":"2012-03-11T21:00:42","date_gmt":"2012-03-11T21:00:42","guid":{"rendered":"http:\/\/stompville.co.uk\/?p=176"},"modified":"2020-10-11T15:41:27","modified_gmt":"2020-10-11T15:41:27","slug":"piezo-di","status":"publish","type":"post","link":"https:\/\/stompville.co.uk\/?p=176","title":{"rendered":"Altoids piezo DI box"},"content":{"rendered":"

There is an update<\/strong> to this article here<\/a>.<\/p>\n

As I mentioned in my post on the vagaries of phantom power<\/a>, I have been looking at designs for a compact self-contained box to connect an upright bass to the PA.\u00a0 The bass pick-up in question is a piezo jobbie integrated into the bridge like the Fishman BP-100<\/a>.\u00a0 Needless to say Fishman recommends a suitable pre-amp if using a cable longer than 10 feet (3m).\u00a0 Since starting the project, I mentioned it to another friend who has purchased an inexpensive pickup for her violin – similar to the one pictured below.<\/p>\n

\"Violin<\/p>\n

The tone\/volume unit is passive – there is no amplifier and no battery – so again, major tone-suckage has been experienced.<\/p>\n

Whilst trawling the net looking for inspiration, I came across Scott Helmke’s “Mint Box Buffer<\/a>” project and the Altoids Piezo DI box was born.\u00a0 Let’s cut straight to the chase and show a photo of the finished item:<\/p>\n

\"Altoids<\/a>Not much to see from the outside.\u00a0 There’s a 3.5mm jack for the input, an illuminated switch to select battery or phantom power and a 6.35mm (TRS) jack for the balanced\/unbalanced output.\u00a0 Let’s have a look inside:<\/p>\n

\"Internal<\/a><\/p>\n

There’s a single-sided PCB with a JFET buffer and an op-amp balanced driver.\u00a0 The unit may be powered from battery or phantom power.\u00a0 For best performance the unit should be connected to to a P48 (i.e. 48Vd.c.) phantom-powered microphone input on a mixer but if necessary\u00a0 it can be battery powered and plugged into a balanced or unbalanced line input.\u00a0 The reason there is a 3.5mm jack (rather than 1\/4″) on the input is that players with small acoustic instruments (such as violin, mandolin or ukulele) will appreciate the smaller jack plug on the end of their passive pickup.\u00a0 In order to use the unit with a microphone input a special output cable will need to be made with a 3 pole TRS plug on one end and a male 3-pin XLR on the other end.\u00a0 Assuming the player keeps the DI box about their person, the special cable will only need to be about 2m or 6 feet long as – once it gets to the floor – it can be plugged into a conventional mic lead of any suitable length.<\/p>\n

Here’s the schematic:<\/p>\n

\"Altoids<\/a><\/p>\n

C1 is for RF immunity.\u00a0 R1 and C2 form a low-pass filter with a corner frequency of 34kHz again to (hopefully) eliminate RF interference.\u00a0 C3 blocks DC from the instrument.\u00a0 The input impedance is essentially set by bias resistor R2 at 10M ohm.\u00a0 Q1 forms a source follower with slightly less than unity voltage gain.\u00a0 Q2 forms a constant current source to bias Q1.\u00a0 R3 provides negative feedback to Q2 to improve its temperature stability and reduce the value of the constant current from Q2’s Idss to some smaller value (dependant on the specific JFET used for Q2).\u00a0 U1 forms a balanced driver.\u00a0 There is provision to add gain at the non-inverting stage of U1.<\/p>\n

For no (unity) gain (recommended):<\/p>\n