Spectrotek Services  Rife Plasma System Components

SPA4 Signal Processor and Amplifier

Version 2.0 now shipping.

Information current as of .3 December 2017

Works with a variety of different signal generators or computer audio.

( Click on the picture for a larger view )

The SPA4 Integrated Audio Signal Processor and Amplifier

The 500 watt SPA4 signal processor and amplifier is available assembled and tested, complete with a high-efficiency fan-cooled heat sink assembly.

The price of the SPA4 assembled and tested with the HS2 heat sink and fan is $350.00

(Kit version also available, less HS2 and fan.)

The SPA4 amplifier has been designed to provide researchers with an integrated audio signal processor and high power amplifier system capable of accepting an input signal from multiple frequency sources while providing enough output power at 3.1 MHz to operate large plasma tubes.

The SPA4 accepts inputs from a wide range of signal sources, including laptop and desktop computers having a line level output or headphone jack, CD players, DVD players, MP3 players, Most cell phones, the UDB1108S, the Spooky2, the GB-4000, the F-165, F-175 and other F-series units, the PROGEN II 4050, or any other frequency generator that is capable of producing a square, sine or triangle wave output or a 5 volt modulated 3.1 MHz signal. The SPA4 is fitted with the Quick Change option, so the power amplifier MOSFET may be easily and rapidly replaced should the need arise - no soldering is required.

Standard carrier frequency of 3.1 MHz is supplied unless otherwise specified at time of order.

The powerful but quiet cooling fan mounted on the rugged HS2 heat sink allows the PA2 to be operated at full power for as long as the researcher desires. A separate power supply for the cooling fan is not needed, because the PA2 provides power for the fan.

The outout amplifier MOSFET in the SPA4 is connected to the circuit board with clamping screws, so that it may be easily and rapidly replaced should the need arise - no soldering is required.

The SPA4 has been designed so that its output matches a standard 50-ohm load impedance, making it possible to connect the output of the SPA4 to a conventional antenna tuner or to the our LC31 coupler system.


Please download and read the operating manual for the SPA4 before placing your order.
You may also download the assembly manual for the SPA4 kit version.
Order the SPA4 here.


  • This is a professionally manufactured etched printed circuit board incorporating 3-ounce tin-plated copper traces, and plated-through holes. All parts are high-quality through-hole components.
  • Resistors are high stability carbon or metal film, 5% tolerance.
  • The SPA4 amplifier circuit board is available assembled and fully tested, or as a do-it-yourself kit. Bare circuit boards are also available.
  • Screw terminals are provided for all connections to the SPA4 - there are no wires to solder!
  • 1-year warranty against manufacturing defects.
  • Lead-free construction for ROHS compliance.

 SPA4 Technical Specifications:

DC Power Supply Input:

  • SPA4 Logic circuits +19 to +22 volts DC filtered and regulated, at 0.75 Ampere.

  • SPA4 RF output amplifier +12 to +190 volts DC maximum, filtered and regulated at 2.5 amperes maximum, the actual current required varies with power output and modulation duty cycle. 

Carrier Frequency:

  • 3.1 (standard) or 3.3 MHz. Frequency to be specified at time of order. The carrier frequency may be changed by the user by replacing the plug-in oscillator module with a module of a different frequency.

  • Custom frequencies in the range of 2.6 MHz to 3.7 MHz are available on special order. Just email us with your frequency requirements.

RF Power Output:

  • The SPA4 will produce up to 500 watts peak power, or 250 watts average power as measured across a 50-ohm load when the carrier is modulated by a 50% duty cycle square wave. 

Input Audio Signal Waveform and Frequency Range:

  • Sine, triangle, or square wave from a computer or various signal and frequency generators.

  • Frequency range when using INPUT A, X2 mode: 10 to 30,000 Hz (30 KHz.)

  • Frequency range when using INPUT A, X1 mode: 6 to 200,000 Hz (200 KHz.)

  • Frequency range when using INPUT B: 1 to 400,000 Hz (400 KHz.)

  • Direct input modulated 3.1 MHz carrier using INPUT B. 

Modulation Modes - INPUT A:

  • X1 Mode - The modulation frequency equals the input audio frequency using sine, triangle, or square waves.

  • X2 Mode - The modulation frequency equals TWICE the input audio frequency using sine or triangle waves only.

Modulation Frequency Ranges – Input A, X1 Mode:

Input Audio Frequency Range in X1 Mode, @ 50 - 7200 mV p/p Input Level with a Sine or Triangle Wave Input Signal:

  • Low - 6 Hz.

  • High - 200,000.

  • This will square wave modulate the carrier over the range of 4 to 200,000 Hz.

  • Usable frequency range from 4 to 400,000 Hz with increased audio input level.

  • This will square wave modulate the carrier over the range of 4 to 400,000 Hz. 

Modulation Frequency with a Square Wave input signal of 50 - 7200 mV p/p:

Note: Square wave audio is usable in X1 Mode only, and the duty cycle follows the duty cycle of the input signal to the SPA4.

  • Low - 75 Hz.

  • High - 200,000 Hz.

  • This will square wave modulate the carrier over the range of 75 to 200,000 Hz. 

Modulation Frequency Ranges – Input A, X2 Mode:

Input Audio Frequency Range in X2 Mode @ 40 - 7200 mV p/p input level with a Sine or Triangle Wave Input Signal:

  • Low - 40 Hz.

  • High - 30,000 Hz at maximum audio gain, or 60,000 Hz at minimum audio gain.

  • This will square wave modulate the carrier over the range of 80 to 60,000 Hz or 80 to 120,000 Hz.

NOTE: In the X2 Mode, when using a sine or triangle wave audio input, the output of the Controller Board will maintain a 50% duty cycle ratio to within 10% or less across the input frequency range of 40 to 30,000 Hz at maximum audio gain, or 40 to 60,000 Hz at minimum audio gain. This allows unattended operation of the equipment without the need for constant audio gain adjustments when using an audio sweep signal generated by a computer sound card. It is necessary to maintain low source impedance and a constant voltage as the audio frequency changes during the sweep. 

Modulation Modes - INPUT B:

  • Square wave modulation of the internally generated 3.1 MHz carrier by a square, sine or triangle wave, TTL level input signal or,

  • Amplification of an externally modulated, 3.1 MHz TTL level input signal. 

Operation of the SPA4 Input B with a square wave modulation signal:

 A minimum of 2.25 volts-peak-to-peak is required to trigger the SPA4. Ideally, the drive signal should be a square wave TTL signal, which has a voltage swing of 0 to +5 Volts. 

Input B Modulation Duty Cycle vs. Frequency with a square wave modulation signal: 

For a duty cycle of 1% to 50%:          1 Hz to 400 KHz

For a duty cycle of 1% to 89%:          2 Hz to 400 KHz

For a duty cycle of 1% to 98%:          3 Hz to 400 KHz

For a duty cycle of 1% to 99%:          4 Hz to 400 KHz 

Operation of the SPA4 Input B with a sine wave modulation signal: 

A minimum of 2.5 volts-peak-to-peak is required to trigger the SPA4 when using sine waves. The maximum allowable sine wave voltage input should be limited to no more than 7.0 volts-peak-to-peak to prevent possible damage to the input circuit of the SPA4. When using sine waves, the duty cycle of the modulated output of the SPA4 will be limited to the range of 20% to 58%. 

Input B Modulation Duty Cycle percent vs. voltage with a sine wave modulation signal: 

2.5 VPP for 20% duty cycle (Minimum duty cycle % possible with sine waves).

3.12 VPP for 37% duty cycle (for Maximum Sidebands).

5.0 VPP for 50% duty cycle (for Normal Operation).

5.58 VPP for 58% duty cycle (Clipping of input signal occurs above this level).

The maximum recommended modulation frequency when using the SPA4 is 400 KHz. 

CAUTION: When the modulation frequency begins to exceed about 400 KHz, excessive RF voltages will be developed in the SPA4’s tank circuit and the LC31 coupler. These voltages may cause the STW20NK50Z MOSFET in the SPA4 to fail.


SPA4 Output Waveforms at a Power Level of 300 Watts

The left side photos are of the SPA4 output waveform across a 50-ohm dummy load.

The right side photos are of the SPA4 output waveform across a Cheb 8-inch Phanotron tube.

The SPA4 was coupled to the Phanotron tube using the 3.1 MHz link coil coupler.

The carrier frequency was 3.1 MHz. The modulation duty cycle was approximately 50%.

Modulation frequency 20 Hz.


Modulation frequency 2 KHz.


Modulation frequency 20 KHz.


Modulation frequency 200 KHz.


RADIO  FREQUENCY  WARNING  NOTICE

        The SPA4 contains a high-frequency switch mode power supply module designed to furnish a high voltage alternating current at a frequency of approximately 3.1 MHz across a 50 ohm resistive load impedance.         If the SPA4 is installed incorrectly or used improperly, it is capable of causing severe radio frequency interference. To prevent this from occurring, please observe the following warnings:

        The SPA4 is to be used as a research device only, or as part of a complete system to drive a plasma tube.

        Do not touch, or allow other persons to touch, any of the internal circuitry of the SPA4 while power is applied to the SPA4.

     Dangerous RF voltage will be present when the SPA4 is operating. Contact with this RF voltage may cause painful radio frequency burns.

        The SPA4 is not intended to be used for any form of radio transmission in any manner whatsoever.

     The SPA4 is not intended to be connected to an antenna or to any radiating element or to be used for any form of radio communications in any manner whatsoever.

        The SPA4 is designed solely to be used as a source of power to operate a plasma tube.

        All electrical connections to the output terminals of the SPA4 are to be made by the use of properly shielded 50-ohm coaxial cable capable of handling at least 500 watts at 3.1 MHz.

        All connections to the SPA4 are to be made in such a manner as to minimize any RF radiation from the connecting wires.

        The operating frequency range of the SPA4 has been restricted to a 1 MHz portion of the spectrum centered at 3.1 MHz.

        Use of the SPA4 may be regulated by your local authorities. You are responsible for complying with local regulations and laws.


Go to our home page.

This web site and all contents including pictures, text and diagrams is Copyright 2012 - 2018 by Ralph M Hartwell.