Technical Input Information - Apogee AD-1000 Operating Manual

Portable reference analog to digital conversion system

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Table Of Contents

Table of Contents

Input Impedance

The AD-1000 is set up to easily match and interface to most any analog audio input. The AD-1000 input assumes

the source impedance (from the unit driving the AD-1000) will be less than 600

put impedances of 50 to 100 . In the very unlikely event of using higher source impedances such as from some

consumer and musical instrument sources, the frequency response at the very highest frequencies will increas-

ingly be rolled off gradually with increasing source impedance. Professional audio equipment output impedance

is almost always less than 100 .

Each side of a balanced input encounters a RF filter and then what appears to be a purely resistive input to

ground of >5k . This makes the input impedance greater than 10k for balanced inputs and greater than >5k

for unbalanced inputs. This is considered to be a bridging input because it sits like a bridge over the output dri-

ving it without any significant loading effect. This permits any low impedance output to drive many such bridg-

ing inputs.

Balanced or Unbalanced Inputs

The AD-1000 input will accept both balanced and unbalanced inputs. No internal jumper selectors are required.

The maximum input level is +28 dBu balanced and +24 dBu unbalanced. This is the level which will produce a

full scale digital output with the multi-turn CAL pots in their minimum position.

DC Removal

The input to the AD-1000 is direct coupled in the Line +4 and –10 CAL positions: no capacitors are in the sig-

nal path. This means the DC inputs will be passed through the direct coupled stages all the way to the conver-

sion stage. The AD-1000 has very effective DC servos to remove both common mode and differential DC inputs

well beyond the specified 50mV DC common mode and 50mV differential. The servos remove any residual DC

from the digital output to beyond the 16th LSB level to deliver a totally centered digital output. This eliminates

the need with most converters to trim for DC removal or run through an external high pass filter which, more

often than not, compromises the sonic integrity of the analog input. The Apogee approach goes back to basics

for a simple method of extracting DC without the attendant number-crunching of a digital high pass filter.

Common Mode Rejection

Common Mode Rejection is a measure of how well an input ignores interference picked up on pin 2 and 3

together. The AD-1000 discrete input stage features excellent common mode rejection. With a well-balanced

input, the common mode rejection at 100 Hz is typically better than 110 dB and at 10 KHz is typically better

than 80 dB. (Reference Settings: +15 dBu input signal and –0.5dB f.s. digital audio output.) This common mode

performance assures quiet performance in the presence of external noise and interference.

Total Harmonic Distortion and Noise

Technically, analog circuitry is often judged by how little distortion and noise it adds to an input. In reality, this

is a good indication, but not the entire story. At Apogee, we trust listening tests over measurement. The AD-

1000 gives the best of both worlds: it sounds great and measures great! The entire analog section of the AD-

1000 typically has total harmonic distortion plus noise performance better than 0.001% with Soft Limit switched

out. (Soft Limit only affects the distortion over the last

2dB.)

Chassis Grounding

A switch located on the top rear of the AD-1000 allows

the chassis and audio grounds to be connected togeth-

er. The default position is pointing toward the front of

the unit, which isolates the chassis and audio grounds. If

you want the grounds connected to remove a ground-

ing difficulty, moving the switch to the back position will

accomplish this. An LED will light to indicate that you

are operating in a non-standard mode.

AD-1000 Operating Manual