High Gain Amplifier

High Gain
Amplifier

High
Gain
Amplifier

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This project is a High Gain general purpose amplifier. It may be used in conjunction with a shotgun mike or other source where a high gain with low background noise is required. The schematics provided are broken down into three separate boards with a fourth which shows how the boards are connected to complete the unit. A modular approach is taken to better allow for later revision if that is desired. One such revision might be to replace the general purpose bipolar transistor Preamplifier board with a true differential input. That adaptation would depend on the type of mike or pickup you are utilizing in your application.

Each module is comprised of one printed circuit board associated with that particular function. A component layout is shown for each module where it is applicable. The boards and their description follow:

This is the Bipolar Transistor Preamp. Common transistors are used to minimize costs. These may not exhibit the lowest noise level possible, but their performance is adequate for most general purpose work. An RF filter is included on the input to prevent unwanted intrusion of stray radio signals.

The second schematic is of the gain stages. Also included here are two filters. The first is a parametric equalizer which provides a notch that may be adjusted to optimize the signal being amplified. This comes in very useful in EVP recording since it can attenuate signals outside the speech bands while providing a boost to those that you are trying to capture. A second filter, called Low Cut, reduces low frequency components. This is intended to cut out wind noises or other rumbles that may be heard outside in an uncontrolled environment. The Filter / Gain circuit provides two audio outputs. One goes to the Relay Driver; the other to the level control, then on to the Headphone Driver stages.

This is the Relay Driver and Headphone Output schematic. Two functions are provided for. The first is to allow the amplifier to activate an optional device should a signal be present. For instance this will allow any recorder with a start / stop switch option to be turned into a voice activated recorder. The amplifier provides a relay with a set of normally open dry contacts that accommodate this operation.

The other major function is to boost the level of the signal to a point where it can drive a set of headphones. A complimentary pair of bipolar transistors provide impedance matching as well as 50 mw of power. A separate output with limiting resistor allows the output of the amplifier to also feed a line level input of a recorder. Using the system in this manner permits real time monitoring for EVPs or other similar applications. A "Voice Emphasis" filter is included to bring out the fricatives associated with speech if that is needed.

Finally, this drawing shows the connections between the individual boards. The connections to various switches and controls are also depicted. Placement of the boards within the enclosure is not extremely critical; However leads should be kept as short as possible, especially around the preamp and low level areas.

These amplifiers are also available as a completed and tested assembly. The price is $260. If you would like more information or to place an order, E-Mail Me for details.

The High Gain Audio Amplifier

The Amplifier is shown along with a horn type microphone and the Dynamic EM Field wand. Plans for building the wand are provided elsewhere on this website. Full coverage headphones are recommended for use with this amplifier.

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