Denecke AD-20

ULTRA LOW NOISE MICROPHONE PREAMP AND A/D CONVERTER 

The Decode AD-20 is a high quality battery operated microphone preamp coupled with a low noise 20 bit Analog to Digital Converter. The S/PDIF outputs are optical and coaxial and can drive standard consumer DAT machines MiniDisc Recorders or computers that accept digital audio I/O cards, such as the VXPocket v2.  The main use for the AD-20 is location recording.  Most low cost DAT machines and MiniDisc Recorders have noisy front ends and a good preamp with a higher bit converter can improve the signal to noise by up to 15dB.

If your recording to a portable MD recorder you might need a cable from Sony to convert from standard optical toslink to the Sony mini optical/line input.  This cable is supplied with both the Sony R700 and 900's sold at this site.  Both coax and optical digital outputs run at the same time so you can connect more than one recorder to the AD-20 if you wish.

FEATURES 

      Long battery life of 8 to 10 hours from one 9 volt battery.

      Low noise 125 dB EIN unweighted at max gain (+45dB)

      Dynamic range at minimum gain (+17dB) is 97dB unweighted

      Individual gain pots for left and right.

      Low battery indicator LED

      External 9 volt input for studio work.

      Optional line input cables with 22dB pads (10K input Z)

SPECIFICATIONS

     Size: 3" x 5.75" x 1.25"

     Power: 5.5v to 25v at 50 ma

     Weight: Approximately 1 lbs.

     Input: Balanced 3 pin XLR at 10K

     Output: S/PDIF optical and coaxial (on 1/8th inch mini-jack).  20 bit 44.1kHz S/PDIF signal.

The A/D Converter:
 

The digital output on the AD-20 is a 20 bit 44.1 kHz S/PDIF signal.  Because the dynamic range of the AD-20's A/D is about 103dB A-weighted or 98dB unweighted this equates to about 16.5 bits of resolution before the natural noise of the A/D can be heard.  Because the natural noise floor of the system hangs right on the 17th bit, the AD-20 also works great as a 16 bit recording device without the need for additional dither (noise) to be added to the signal.  You might say the AD-20 has its own natural dither.  Had the S/N of the AD-20 been beyond the 17 bits then additional dither noise would need to be added at the 17th bit to avoid excessive quantization noise.  Of course using a recorder or computer capable of capturing 20 or 24 bits will result in even greater dynamic range from your AD-20.  If you have the gain knobs adjusted to a very high gain level some of the mic preamp noise will also be heard at the bottom of the dynamic range.

The Microphone Pre-Amp:
 

The AD-20 offers true balanced mic level inputs on two XLR connectors.  In order to connect unbalanced mics to the AD-20 you should use a cable that will properly balance the signal (usually this involves a small resistor/pad between ground and the 2nd leg of the balanced input that isn't being used).  The mic preamp has a low noise 129dB EIN A-weighted input and distortion is a mere .0015% THD+N at minimum gain.  The individual gain pots can be used to adjust the gain on either channel.  The gain of the pre-amp is hard to measure because there is no analog out (just digital).  At the lowest gain it takes a -5dBu signal to reach 0dB 'clipping' on the digital input.  At the highest gain setting it takes a -46dBu signal to reach 0dB 'clipping' on the digital input.  This gives the AD-20 a range of about 41dB.