ATI Technologies M1000 Operating And Maintenance Manual

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M1000

DUAL MICROPHONE AMPLIFIER

OPERATING AND MAINTENANCE MANUAL

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DESCRIPTION

This section is strictly written to boost the designer’s ego by letting him tell you of some of the clever things we have done inside the MicroAmp. It may be skipped by anyone having anything better to do.

For many years engineers (ourselves included) have been using operational amplifier ICs in audio circuitry. These ICs, developed primarily for the instrumentation market, had many shortcomings when used in high quality audio applications. The lack of audio related specifications, crossover distortion, high noise for low source impedances, limited output capability and limited gain bandwidth product forced many compromises when used in high gain audio systems. High ratio input transformers, output boost stages and multiple stage designs were all used to compensate for op amp deficiencies and in turn added additional response and distortion problems of their own.

Walter Jung in a definitive series of articles' analyzed and defined slew rate induced distortion mechanisms, tested many commonly available ICs and correlated various distortion tests with subjective (listening) criteria. A very significant result of his efforts was the identification of an IC originally developed for the European professional audio market which has almost ideal characteristics for audio use and in particular provides a high rate capability of 13 volts /microsecond, virtually eliminating slew induced Transient Intermodulation Distortion. By contrast the old standard 741-op amp has a slew rate of only .6V/microsecond.

This op amp is now available from several U.S. Manufacturers in single and dual versions and forms the basis for our MicroAmp designs. This chip incorporates a low noise input stage designed for excellent noise performance even with low source impedances thus eliminating the necessity for touchy 1:20 input step up transformers in the M1000. High output capable of driving 600 ohm loads directly to +22dBm with total freedom from crossover distortion, high inherent linearity, 100 dB open loop gain and 50Mhz gain bandwidth product make this an ideal device for highest quality audio. The absence of Transient Intermodulation Distortion may be detected in the smooth effortless high frequency output capability, the absence of the harsh, raspy sound typical of IC amplifiers driven to full output at high frequencies and the freedom from increasing harmonic distortion vs. frequency. As used in the MicroAmp a minimum of 40 dB of loop gain is available for 100:1 distortion reduction even at 20 kHz.

The high output swing capability allows the amplifier to use a simple, low distortion, flat response 1:1 output transformer. ATI provides three models of each MicroAmp; Transformer outputs (with faraday interwinding shields) for high RF field environments, a balanced differential output stage version (bridge circuit) for low RF environments where balanced output lines are required for hum rejection in long runs and the minor performance degradations of an output

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transformer are not desirable and a simpler, lower cost single ended (unbalanced) output stage version with full +22dBm, 600 ohm drive capability for use in low RF environments for short line runs.

Inaudible crosstalk characteristics allow use of the M1000 for either Stereo or Dual Monaural applications. A dual concentric gain control is provided to facilitate either mode of operation. Only minimal coordination is requited to grasp both concentric knobs simultaneously for stereo use.

Our power supply incorporates a couple of unique regulating devices called zener diodes. In contrast to most fancy three terminal IC regulators, these devices will live through line transients and simultaneously protect your expensive circuitry.

INSTALLATION

Your MicroAmp may be desk mounted on its non-slip suction cup feet and left out for everyone to see since it's so pretty. You can stack the amplifiers on top of each other. We don't recommend stacking more than 30 high on a desk since it is difficult to reach the top.

Rack mounting kits are available for your amplifier. P/N 20021-501 mounts a single MicroAmp in 1-3/4" of vertical rack space. P/N 20024-501 mounts two amps side-by-side in 1-3/4 inches.

MicroAmp mounting kit P/N 20090-501 allows a MicroAmp to be mounted under a desktop or behind a cut out.

WIRING

Our insurance company insists on three wire grounded plugs. The third wire ground can cause a ground loop with your station ground. If you are sure your station ground will provide adequate protection to personnel in case of an AC line short to the chassis, a 3 to 2 AC line adapter can be used to isolate the power line ground. We recommend that the adapter be removed and the power line ground reconnected prior to any service work requiring removal of the station ground connection from the MicroAmp chassis.

The four inch copper strap which you are of course using for your station ground is not going to fit around the #6 chassis ground screw on the MicroAmp rear panel. Run strap to within a few inches of the chassis and jump to the chassis ground with shield braid.

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Microphone inputs and outputs use XLR type connectors and require mating plugs. The applicable input plugs are Switchcraft A3M or equivalent. The output plugs are Switchcraft A3F or equivalent. Mike wiring is conventional, hi (in phase) to pin 2, Lo (out of phase) to Pin 3, Shield to pin 1. Either 150 ohm or 50-ohm microphones can be connected to the MicroAmp without modification.

The MicroAmp M1000 will accept microphone outputs up to 125 mVrms (-14dBm available power/150 ohms) without distortion.

Output wiring is indicated on the rear panel label. HI outputs are in phase with each other and with the input signal.

Transformer Output (M1000-1, L1000-1) may have either HI or LO outputs grounded (but not both).

CAUTION: Balanced differential output (M1000-2, L1000-2) has active drivers for both HI and LO outputs terminals. DO NOT GROUND either HI or LO terminals. To drive an unbalanced (one side grounded) Load, connect it between HI and GND terminals and let the LO terminal float. Two separate 600 ohm unbalanced loads can be driven from each output without interaction by connecting one between HI and GND and the other from LO to GND. The two loads thus driven will be out of phase with each other. Individual grounds are provided for CH l/left and CH 2/right outputs.

ADJUSTMENTS

The MicroAmp has sufficient gain and output capability to overdrive some console inputs particularly those providing preamps for high-level input channels. A -50dBm available power microphone output (2.5 mVrms) will provide +8dBm output with the gain pot set to two o'clock position. 12 o'clock is approximately OdBm and 10 o'clock is -10dBm output. Operation with the MicroAmp pot set below 10 o'clock position degrades noise performance and a fixed attenuator pad should be placed between the MicroAmp output and the Console input to allow MicroAmp operation at around 2 o'clock.

MAINTENANCE NOTES

Power supply voltages are + and - 18 VDC nominal, dropping to +/16 VDC under full output. Maximum allowable voltages are +/- 22 VDC (limited by IC). If zeners are replaced, remove Al, A2, and A3 ICs and check output voltage before plugging ICs back into the circuit. Remove power when inserting or removing ICs.

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IC output DC voltages (no signal conditions) should measure 0 VDC +/- .5 VDC. Significant deviation indicates IC or circuit problem. Measurable DC difference between +/- amplifier inputs (other than due to meter loading) indicates IC failure.

Five to ten years hence, replace aluminum electrolytics in power supply.

Keep 250 W soldering guns out of the MicroAmp.

If hit by lightning, replace Al through A3.

MODIFICATIONS

230 VAC OPERATION

Your amp is wired for 115 VAC 50/60 Hz operations unless otherwise requested at the time of ordering. It can be modified for 230 VAC use by removing the Power transformer primary jumpers W1 and W3 and inserting a jumper in the W2 holes.

150-OHM OUTPUT IMPEDANCE

Transformer output M1000s are converted to drive 150 ohm audio lines by removing jumpers W4 and W5 located adjacent to output transformers T3 and T2 respectively. Insert two new jumpers for each output transformer between the original jumper terminals and the empty terminals immediately adjacent to and in line with the original terminals in order to parallel the secondary windings.

Balanced differential output and single ended output will drive 150 ohm loads without modification but will current limit before full supply swing is reached.

INPUT CENTER TAP

The center taps of the input transformer primary are available on the PC board next to the transformers and are labeled CT. They may be grounded by adding jumpers between each CT point and the adjacent empty holes.

INCREASED GAIN

Additional gain can be provided for low output boom mikes by raising feedback resistors R21 and R40 (M1000-1, -3) from 220K to 680K ohms (10 dB increase). The M1000-2 requires that R21 and R40 be increased to 330 K ohms and also R45 and R49 be increased to 820 K ohms for a 10 dB gain increase.

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SPECIFICATION

GAIN 71dB minimum

MAXIMUM INPUT Microphone: -16dBm, 125mVrms

MAXIMUM OUTPUT +22dBm/600 ohms (M1000-1, -1P).

+24dBm/600 ohm (M1000-2, -2P)

NOISE OUTPUT 124dBm EIN with 150 ohm source and 20kHz

measurement bandwith. Yields 74dB S/N ratio with -50dBm microphone input.

INPUT IMPEDANCE Microphone: 150 ohm source, 2500 ohm input

transformer isolated

DISTORTION .1% maximum THD at +20dBm output and

input levels from -52 to -18dBm, 30-20,000Hz.

SLEW RATE 13 Volts per microsecond for minimum TIM

CROSSTALK -70dB maximum at 10kHz.

HUM REJECTION 80dB for common mode hum.

PHANTOM POWER +48VDC at 4MA through 3,300 ohms and

center tapped input transformer. Switch controlled.

POWER 115/230VAC +/- 10%, 47-63Hz, 10VA.

DIMENSIONS in. (mm) 8.5 (216) W x 1.75 (44.5) H x 7 (178)

D, 4lbs, ( .176Kg).

MOUNTING Suction feet for non-slip desk mounting. Rack

mount kits; 20021-501 mounts a single centered unit, 20024-501 mounts two units side by side.

CONNECTORS XLR type in and out

WARRANTY Limited, One Year Warranty

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