ATI M100 User Manual

M100

MICROPHONE AMPLIFIER

OPERATING AND MAINTENANCE MANUAL

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GENERAL

The ULTIMIKE M100 Microphone Amplifier is designed specifically to provide the ultimate in no-compromise audio performance. A rugged, small AC line powered amplifier, it may be placed close to the mike on the stage, in the pulpit or in the overhead lighting structures to shorten sensitive low-level mike lines. Replacing your long runs of low-level mike cable with high level feeds will greatly reduce your susceptibility to RF, hash and hum pickup. In addition, the M100 is itself very well protected from both radiated and conducted interference and has unmatched broadband common made rejection to cancel microphone hum and RF pickup. A unique distortion-free transformer coupled output provides total isolation of ground loops and ground voltage differentials while simultaneously providing a low driving impedance to minimize high frequency rolloffs due to excess cable capacitance.

To allow you to install the M100 close to the mike when necessary in what may be a remote or poorly accessible location, we have designed the preamp to accept a very wide range of input levels (up to 0 dBu) without input overload and we have incorporated a switchable slow acting (but very low distortion) 24 dB limiter to turn down the output level pot when you can’t reach it.

In addition to all these modern conveniences, the ULTIMIKE is a superb performer with ruler flat response, no ringing, excellent CMR, exceptionally low input noise, and trivial distortion.

The ULTIMIKE amplifier is packed in a rugged, compact steel enclosure. All controls and connecters are recessed for protection. Identification markings are printed on the protected reverse side of a heavy duty polycarbonate overlay. All power supply components are well shielded by the steel chassis.

Input mike level audio to the ULTIMIKE must run a gauntlet of ferrite suppressors, bypass capacitors, DC blocking capacitors, and protection diodes designed to block the trash, and transistors (Q1 and Q2) in an instrumentation amplifier configuration provide a very low noise input gain stage. The balanced feedback network R12, R13, R20, and R 21 is driven from the outputs of A1A and A2A to eliminate any input loading that might degrade common mode hum rejection and to allow use of low valued feedback resistors for minim noise generation. Dual constant current stages (Q3 and Q4) sink DC emitter current from Q1 and Q2 and allow complete servo cancellation of input offsets via integrator A3A to maximize input stage headroom at A2B. A differential amplifier and limiter are formed by A1BA2B. Common mode input signals are nulled out by R27 and an LDR (light dependant resistor) in A2B’s feedback loop reduces the input stage gain in response to limiter levels. Even without limiting, the input stage will handle input levels up to 0 dBm (gain – HI switch) or +20 dBm (gain –20 dB).

A3B forms a variable gain stage with a smooth logarithmic adjustment range of +37 to –19 dB. R30, the level potentiometer, is screwdriver-adjustable through the front of the amplifier. When rack mounted, the level adjustment is accessible through the panel.

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A4A makes a switchable, unity gain high pass filter with two-pole response of -3dB at 150 Hz and rolling off 40 dB at 15 Hz. The filter is switched in or out of the circuit with S4.

The output line driver A4B and output transformer T2 provide 16 dB of gain and the total isolation, faraday shielding, RF immunity and ease of use of a true transformer coupled balanced output. A unique feedback technique totally avoids the transformer’s characteristic limitations of high distortion, poor response and hum pickup. Typical output distortion measurements made at both peak (+22 dBm) and nominal (+4 dBm) levels barely exceed generator residuals from 20 to 20,000 Hz. Hum pickup from the power supply is well below the noise level and low frequency response is greatly extended. The output is protected from short circuits but will drive over one-half mile of shielded cable with less than 1 dB of signal rolloff at 20,000 Hz.

We deliberately omit the output connector pin 1 ground lift jumper (W4). If you insist on setting up loops by grounding your cable shields at both ends, you can connect the shield to the output connector plug shell.

A5A and A5B make an active full wave rectifier and filter to drive the limiter. R5 sets a limiting threshold corresponding to an output level of +14 dBm. The limiter has a soft knee and a gentle slope of about 4:1 so that continued peaks greater that 10 dB above the nominal output will cause a slow reduction in gain. If the limiter switch is off, the front yellow LED will light on sustained peaks to tell you to use the limiter or to reduce the gain. With the limiter on, an occasional flash will indicate normal operation but sustained illumination is an indication that you are reaching the end of the gain reduction range.

Q5 is a constant current driver for the limiter LDR, which can be controlled by an external 10 Kohm potentiometer plugged into the phono jack type connector. The remote gain control has an adjustment range of 0 to –15 dB and does not interact with the limiter except that both share the 24 dB total gain reduction range of the LDR. The remote gain control is DC operated and can be located a considerable distance from the M100 without noise pickup.

A5C and A5D form a 40 kHz DC-to-DC converter to efficiently generate the +48 VDC microphone phantom power.

Your M100 operates from an internal, double-filtered bipolar 16 VDC supply, which uses a couple of unique regulated devices called zener diodes. In contrast to fancy three terminal regulators, these devices will live through power line trash and transients and simultaneously protect your valuable circuitry. The supply is designed for minimum susceptibility to power line conducted RF and hash with bypassed ferrite beads, RC primary side bypasses and a mu-metal shielded, non-concentric wound, semi-toroidal power transformer. A large board mounted 1/2A AC line fuse protects your M100 but can make taking off the cover with the AC line still connected into a very exciting adventure.

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