Dual ChannelEVP Mixer
Projects ----------------- EVamP 2 EVP Preselector |
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COMMENT??? Got A Question??? A Click on the Wizard opens Technical questions or just want make |
EVamP 2 is a two channel mixer board with special filtering to emphasize voice frequencies
while keeping unwanted noise to a minimum. Several features make this an ideal system to use in
the capture of EVP. The mixer has two identical channels which provide high gain and bandpass
filtering. Each has a bass and treble control to boost or cut the high and low frequencies. In addition,
an emphasis control permits a 20 db boost which allows the "S" sounds to be heard more clearly.
A bandpass control is available which improves the vocal frequencies while suppressing those not generally a part of speech.
Each channel has both a line level and mike input which are switchable. This allows for the system to
both capture EVP and provide a means of analyzing recordings in the field. The line level inputs
are intended for playback from the recorder if desired to sample your recordings. The mike inputs
include phantom power to permit the use of electret elements without requiring separate batteries.
There are two primary output modes. A dual channel line level output is intended to feed a separate
recorder to save any captured sounds. Again control allows this to be adjusted to prevent overdriving
the device. A separate headphone output is available which allows realtime monitoring of the signal.
Gain is adjusted independently of the line level output. The headphones can also be used to hear
playback from your recorder if desired. A phasing switch can put the left and right side either
in phase with one another or 180 degrees out. This sometimes aids in hearing faint sounds
embedded in background noise.
A switch on the front panel allows you to choose whether channel "A" feeds both sides, channel "B"
feeds both sides, "A" and "B" are combined to a mono signal, or "A" and "B" are treated as a stereo
(two channel) signal. While some controversy exists regarding its use, a white noise generator is
provided. Two controls allow the level of noise injected into each channel to be set. Power for the
system comes from built-in rechargeable batteries and its own AC supply. Simply plugging the unit
in will charge the batteries, and you can use it while charging is taking place. It has full power
redundancy and will continue to operate normally on battery without interruption should AC power
fail. Finally, the unit has remote start capabilities. If your recorder has a start / stop capability, the
EVamP 2 unit can activate that, making the entire system voice activated if you so desire.
The schematics and board layout drawings below will provide you with the basics to build your own
EVamP 2. It should be pointed out this is a fairly complex project, although not one that would
require any special consideration besides that normally given when handling ICs and static sensitive
components. Fully assembled and tested circuit boards are available from
Brown Electronics Labs by special order. Click the Wizard above for more information.
Input Preamp and Compensation Board.
(2 required, one for each input channel.)
Output Board and Drivers
White Noise Generator
This board is optional. You may omit this if you choose not to include this feature in your mixer. If you do not include this, simply ignore all references to wiring this in the final assembly.
Internal Wiring and Hook-up.
Once you have stuffed your circuit boards you are ready to
assemble the amplifier itself. It is recommended that it be placed in a steel box to provide shielding
against unwanted outside interference. While it is not extremely critical, wire runs should be kept as
short as possible and proper grounding techniques employed. This attention to detail improves the
overall operation of the unit. The wiring between the boards and jacks is shown below.
Completed EVamP 2 Mixer / Amplifier
