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The objective is to develop a tool kit for designing studio quality near field audio monitors producing a maximum continuous sound level intensity of 100 dB SPL, corresponding to 0 dBFS, consistent with OSHA 1910.95 for sound workers, at 1 meter, for a program level of 85 dB SPL, consistent with EBU R68-2000, at 1 meter. The -3 dB frequency response of the audio monitors should extend to below 40 Hz., and, above 25 KHz. An input voltage of 0.775 V RMS = 0 dbm should correspond to -15 dBFS = 85 dB SPL, the program level, to within a 6 dB pad for consistency with the other standards, 0 dBm and 0 dBV. The development methodology for the audio monitors will follow the outline of Studio Speaker and consist of 4 designs: a single driver sealed enclosure; a dual driver MTM sealed enclosure; a single driver bridge sealed enclosure; a single driver ported enclosure. Each enclosure will consist of one, (2 for the MTM,) HiVi B4N 4" Aluminum Midbass and Dayton Audio DC28FT-8 1-1/8" Silk Dome Truncated Tweeter. The tape archive, b4n.tar.gz, contains the simulations and printed circuit board(s), (PCB,) development database, (including industry standard Gerber format files, connectivity "rat nests", component vendor and bill of materials, cost estimates, etc.,) to reproduce, enhance, or modify the circuits used in the near field audio monitors. The design environment used is gEDA. The power supplies for all designs are two Lenovo/IBM laptop "bricks" wired in series to produce +/- 20V DC @ 4.5A, (available non-OEM on eBay for under $10 US, each, 50K hours MTBF.) Each PC board contains a +/- 15V regulator, and separate power and signal grounds. The power amplifier is based on the LM3886 "chip amp," and contains a DC servo loop to eliminate the output coupling electrolytic capacitors. All designs use common electronics, (i.e., the same PC boards, or fraction thereof,) and the signal paths use 275V polypropylene capacitors-a substantial cost reduction can be obtained by relaxing this requirement, (the requirement is the prevailing wisdom in the industry.) The signal path parametrics can be altered, (usually,) with only resistor changes on the PC boards-the capacitor values chosen are in the center of the applicable range. The signal conditioning paths use NE5534 Low-Noise High-Speed Audio Operational Amplifiers, (and the designs are compatible with the LT1115 Ultra-Low Noise, Low Distortion, Audio Op Amp, as a direct replacement.) Near Field Audio Monitor CharacteristicsSingle B4N Sealed Studio SpeakerThe single B4N sealed design is appropriate for studio monitors, or PC work station monitors, and has the smallest enclosure of the design suite. Optimal Sealed Enclosure, Vcc = 20V, Vss = -20V, 1 Meter:
Optimal Sealed Enclosure, Vcc = 20 V, Vss = -20 V, 1 Meter, Frequency Compensated:
Dual B4N MTM Sealed Studio SpeakerThe dual B4N MTM sealed design is appropriate for studio monitors, and is the most expensive, (and produces the most accurate sound,) of the design suite. Optimal Sealed Enclosure, Vcc = 20V, Vss = -20V, 1 Meter:
Optimal MTM Enclosure, Vcc = 20 V, Vss = -20 V, 1 Meter, Frequency Compensated:
Single B4N Bridged Sealed Studio SpeakerThe singe B4N bridged sealed design is appropriate for studio monitors where output power is a design requirement; the output power is limited by the speaker driver limitations. Optimal Sealed Enclosure, Vcc = 13V, Vss = -13V, 1 Meter:
Optimal Sealed Enclosure, Vcc = 13 V, Vss = -13 V, 1 Meter, Frequency Compensated:
Single B4N Ported Studio SpeakerThe single B4N ported design is appropriate for cost effective, high quality PC work station monitors, albeit with a larger enclosure volume than the sealed designs. Optimal Ported Enclosure, Vcc = 20V, Vss = -20V, 1 Meter:
Optimal Ported Enclosure, Vcc = 20 V, Vss = -20 V, 1 Meter, Frequency Compensated:
PC BoardsThe PC boards are dual layer, (top and bottom, only,) and use a minimum of 20 mil line widths and spacing, (common in linear designs,) which can be manufactured economically at almost any vendor of choice. The boards are designed to be mounted flat, (with 4 fasteners, one in each corner,) usually along the edge of an aluminum enclosure for attachment of power dissipating components to the chassis/enclosure, which functions as a heat sink for the components.
Figure I is a plot of the sealed enclosure signal conditioning printed circuit board, measuring 6.5" X 3.2", and component BOM, sealed.bom. One is required for each channel, and provides both the high frequency and low frequency crossover paths and signal conditioning.
Figure II is a plot of the ported enclosure signal conditioning printed circuit board, measuring 4.2" X 3.4", and component BOM, ported.bom. One is required for each channel, and provides both the high frequency and low frequency crossover paths and signal conditioning.
Figure III is a plot of the power amplifier printed circuit board, measuring 5.0" X 3.1", and component BOM, PA.bom. One is required for each channel, and provides both the high frequency and low frequency crossover paths to the speaker drivers. SchematicsThe development database for all audio monitors in the suite is in the tape archive, b4n.tar.gz. The development system uses gEDA Electronic Design Automation tools, particularly, Gschem(1), for schematic capture and Spice electronic circuit simulation netlist extraction. Ngspice was used for the Spice circuit simulations. RCS was used for version control in the development. The PC boards were designed using PC Board Circuit Editor from the gEDA suite. There are four designs of audio monitors in the suite, and all share common electronics; the files in each RCS directory under the TLD are linked to common sources, (i.e., a "library,") branching to the different designs-the directory structure follows the design hierarchy for each design. The parametric differences between the designs are calculated using Calc - C-style arbitrary precision calculator. For example:
Figure IV is a plot of the signal conditioning circuit for all sealed enclosures, (the PC board in Figure I was made from this schematic-its actually the second level down in the hierarchy.) Descending into the Linkwitz/BiQuad transform circuit, (block X3,) for example:
Figure V is a plot of the Linkwitz/BiQuad transform circuit for all sealed enclosures. Note the absence of component values. The values are calculated by a Calc script, lt.calc, which is redirected to a file; there is a Spice include directive to include these values into the schematic for simulation. These schematics are used to generate the PC board layout. Note the expediency of changes in parametric values, (i.e., change the Calc scripts, not the schematics.) Additionally, the PC board is routed from the same schematics and values, minimizing the chance of sending the wrong circuit, and/or values, to a vendor. LicenseA license is hereby granted to reproduce this software for personal, non-commercial use. THIS PROGRAM IS PROVIDED "AS IS". THE AUTHOR PROVIDES NO WARRANTIES WHATSOEVER, EXPRESSED OR IMPLIED, INCLUDING WARRANTIES OF MERCHANTABILITY, TITLE, OR FITNESS FOR ANY PARTICULAR PURPOSE. THE AUTHOR DOES NOT WARRANT THAT USE OF THIS PROGRAM DOES NOT INFRINGE THE INTELLECTUAL PROPERTY RIGHTS OF ANY THIRD PARTY IN ANY COUNTRY. So there. Copyright © 1992-2015, John Conover, All Rights Reserved. Comments and/or problem reports should be addressed to:
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