Audio Sniffer

About Audio Sniffer

The Audio Sniffer is a high quality headphone amplifier for debugging audio electronics. It has a gain from 1x to 100x (or 1x to 500x depending upon which potentiometer you use); and with an optional input attenuator, this can go down to 0.1x. It has an input impedance of 1Mohm, which means it will not affect your circuit, and can handle over-voltage without damaging the Audio sniffer, or blowing out your eardrums. Its easy to use, with just a headphone jack and volume knob, and runs off a battery, which is automatically disconnected when you unplug your headphones.

Because of the high input impedance and large gains, its difficult to make a low noise, low distortion headphone amplifier at a low cost. The Audio Sniffer makes a good balance between peformance and price, with -120dB noise and -75dB distortion at a gain of 1, and -85dB noise and -60dB distortion at a gain of 100. At a gain of 500, this figure goes down to -80dB noise and -45dB distortion, which is the lower end of usability.

The advantages of the Audio Sniffer over an oscilloscope are two-fold. First, its 1/100 the cost, but almost as importantly, it has finer depth resolution. It is difficult to see small amounts of noise and distortion on an oscilloscope, especially a digital one which usually has 10b resolution. The human ear can detect low level distrubances that are next to impossible to see.

Finally, the Audio Sniffer has a 20Hz to 20kHz frequency response (the upper end is actually much higher at lower gains), so you won't have to worry about the DC bias being off, or missing some important aspect of your signal.

Modifications, Additions, Enhancements

• Gain Potentiometer - the gain potentiometer can be changed out for either 100k or 500k. The larger value gives more gain, but less resolution and more noise.
• Input Attenuator - the optional scope probe allows the Audio Sniffer to handle voltages up to 60Vpp, which can come in handy when working on modular synths, which can have +/-15V rails (30Vpp). The scope probe also has a switch on it, which allows for regular 1:1 operation, so signals as small as 10mV can be heard.
• Volume Adjust - if the Audio Sniffer isn't loud enough for you, just swap out R5 and R6 with smaller values (100ohms would be a good starting point).

Documentation

Files: Version 1.0

• board image board eagle file

• schematic image schematic eagle file

• frontpanel image filefrontpanel eagle file

• library eagle file

• board gerber files

• frontpanel gerber files

• parts listing ODSparts listing PDF