SmartPhone DI with Phantom Power

I frequently use my phone to record video for my blog and for my music projects…but I have been frequently frustrated by the limitations of the internal microphone embedded in the phone. So I created this circuit to provide a means to use a high quality Dynamic or condenser studio microphone with my phone. The circuit operates from a rechargeable  9 volt battery and plugs into the standard 1/8 inch four ring audio connector. It provides phantom power at 28 volts…which works fine …you typically don’t need 48 volts It utilizes a SSM2019 balanced microphone preamp IC, operating from a dual polarity charge pump power circuit at +-5V. At the output you need to provide a 2.2k resistive signature such that the pone can detect the microphone connection. The circuit also employs my simple momentary latching switch circuit which I often use and is described here at circuitsalad. Everything about the circuit is straightforward but the phantom power over voltage protection circuitry merits discussion. Basically, the isolation capacitors C5 and C6 are slowly charged up by the phantom supply but depending on what is plugged and unplugged can be be discharged very rapidly into the preamp input circuitry. This will be destructive and requires circuitry to shunt this energy to ground and dissipate it. This is accomplished by means of TVS1 and TVS2…which are prepackaged back to back zener diode. However the issue arises that simply using large zeners or transorbs is not a good idea because they have large reverse bias junction capacitance…which creates distortion as it modulates. To prevent this, I chose a very tiny 9 volt transorb. The one I chose (DF2B6.8ACT.L3F) has only a few pF of capacitance and is very small(402) surface mount package. The device works great but can only sink 1 amp peak. This further requires series resistors R11 and R16 to help limit the surge current to less than 1 amp at 28 volt. R1, R2, R4, and R5 must all be precisely matched in order to maintain circuit balance. R4 and R5 are required to provide a absolute ground reference such that the output does not float to some common mode DC value above 0 volts. The gain of the preamp is -6dB -20dB and is adjustable by means of R10. The reason for the negative gain is the use of an attenuation network that also provides the 2.2K resistive signature for microphone detection. The attenuation is required because of the inherent gain of the phone circuitry, which is easily overloaded.


As seen in the pictures below, the circuit board fits in a compact 3D printed enclosure. The battery sits above the circuit board and is enclosed by a a top cover. I use a rechargeable 9 volt because the device draws about 30mA in total.t I included a 2.5mm circular power plug as a charging jack such that the battery can be charged without removal from a generic 9 volt battery charger.

Picture of home etched Circuit Board:


Circuit Board mounted in 3D printed Case:




Link to Demo:



My Hell Dice Pedal Board Design

I have been performing way more than I use to! I play club gigs twice a week or more these days…and as a result, my relationship with my rig has really changed. Which is to say, my perspective on what I actually use and what features matter most, has evolved. My conclusions are these: I want small, light weight rugged equipment…I don’t want too many knobs or complexity. I like to use a few basic settings and sounds and that is it.  So I have designed a compact amplifier and set of small 1590a form factor pedals to create a complete amplifier/pedal board rig that weighs a couple of pounds and is about 12 inches long and 4 inches wide. It includes: My 100 watt stomp amp with an auxiliary 9 volt output, a high performance PT2399 type delay, a simple but really nice sounding LDR based envelope filter,  A very pleasing two stage LDR phase shift Vibrato and a hex inverter based overdrive with slightly different approach than the typical design.

My New Compact Pedal Board

pedal board2

As an aside all of these pedals are made with machined(not die cast) Hammond sized aluminum enclosures from Ebay (alpinetech), which I etched and anodized. You cannot properly anodize die cast enclosures because of other ingredients that are mixed with the aluminum…so you have to get CNC milled enclosures if you want to anodize the enclosures.  I discussed my home anodizing process here: Anodizing discussion

More Pictures:funkyfilt

vibrato board

delay board

Links to Schematics and Design Notes:

A few notes: Many of the part choices are  not transistors, op amps and voltage regulators I use.. The LDR based effects will need tweaking based on the output efficiency of the LED/photo-resistor combo or if you use a commercial opto-coupler instead. The vibrato uses my pic based LFO…but it can easily be replaced with other LFO circuits. The delay uses a 8 pole switch cap filter IC but if desired, this can be replaced by using the unused op amp on the PT2399 IC as a low pass filter.

Compact Stomp Amp:

Hell Dice Delay:

Hell Dice Vibrato:

Hell Dice Overdrive:

Hell Dice Funky Filter:

Pedal Board Demos From Live Show:

Demo of Delay and Vibrato:

Demo of amp clean no effects:

Demo of Envelope Filter(two minutes in):