FX-1 Special Effects Feedback Processor

The FX-1 is a feedback control device that is designed to expand and enhance the capabilities of commercial special effects processors. All manufacturers claim that their processors are the best on the market, but the truth is, they are all excellent and their minor differences are insignificant. As a musician though, you like to explore new territory and your idea of special effects may be somewhat different than design engineers and company executives.

fx1 picture

If you have ever used a special effects processor, you know how volatile they can be. Just when you need to deepen an effect, you end up with runaway feedback and you must quickly turn down the volume before you blow your speakers. The FX-1 feedback processor controls the overall gain of the commercial processor with just one stepped potentiometer that will allow you to take any effect to the brink and no further. If feedback goes out of control, you need only to adjust the gain control on the FX-1.

Control Chart

fx1 rear front

1. Power indicator LED.

2. Master gain control that in most situations will be the only control used to tame the effects of runaway feedback. A stepped potentiometer is used to assure very accurate adjustments as well as setting references.

3. The phase switch connects the two channels through a non-polar capacitor. This does not exactly produce monophonic sound but instead can produce partial blending effect that can enhance some material depending on what special effects unit you are using. The switch should be tried with all kinds of sound to find out what works best. Plus indicates the capacitor is in circuit.

4. This three-position switch puts the FX-1 into the feedback loop or takes it out. There is a momentary position on the switch which that enables you to engage the FX-1 for only as long as you hold the switch down (When you release the switch it will return to the off position).

5. A wide variety of control devices can be accommodated because of this adjustment potentiometer. Control voltages can range from a couple to 15 volts.

6. Varied voltages levels applied to this input jack enable the FX-1 to determine how much of an effect is used for each and every note of a sequence.

7. An LED indicates the presence of a control voltage applied to the input jack.

8. The crossing switch merely reverses the channels, which may or may not have a dynamic effect, depending on both the material and the type of special effects device you are using.

9. A switch engages or bypasses the high pass filter.

10. Control of the amount of filtering is set by this stepped potentiometer.

Rear Panel Jacks

Four RCA jacks labeled SED, provide access to your Special Effects Device. A 1/4-inch phone jack, labeled control, should be connected to the defeat input on your Special effects Device. The four RCA jacks labeled FX-1 should be connected to the output of your mixer and the effects return inputs. Power is supplied by a 3-volt 200-mA wall unit that plugs into the appropriate power jack.

Preventing Runaway Feedback

When using the FX-1, you should adjust the input with the mix and output controls on your special effects unit to their minimum settings. The gain of the FX-1 should be fully on and a signal and applied to the input. The input should be turned up until the device responds with a blinking LED. Another LED indicates overload and you should follow the manufacturer’s recommendation in their owner’s manual. Now you may set the mix and the output control for the desired effect (You may need to readjust the gain control for the effect you want). If feedback becomes a problem, you can adjust the gain control on the FX-1 to tune it out. With this procedure, you can operate your special effects generator much closer to the verge of runaway feedback and still control it.

feedback control graph

Gated Repetitions

Certain delay settings on commercial devices sound like multiple hits, rather than echo, when gated. This can transform a dull rhythm track into a spectacular performance. When a positive programming pulse is fed to the FX-1, the chosen effect is canceled. A negative state pulse enables the effect. A graph is included to show how the process works. When a repeating effect is chosen and the programming step pulse is low, the repeated pulses are heard. If the pulse step goes high, the repeated pulses can not be heard. Elaborate rhythmic patterns can be generated, with this mode of operation, that cannot be achieved with any other device.

gated repetitions

Frequency Sensitivity

Commercial special effect devices can be more sensitive to some audio frequencies than other and this can interfere with the effect. While you are trying to deepen the effect for one sound, a lower frequency is driving your unit into a state of runaway feedback. A high pass filter is included in the design of the FX-1 for the purpose of cutting the level of low frequency feedback so that high frequencies can receive the emphasis you need. The filter alone can eliminate the cause of runaway feedback in many situations. High pass filtering was chosen because, without exception, low frequencies are responsible for the most troublesome feedback problems. As simple as the single pole filter is, it can tame low frequency feedback problems quite dramatically. By removing low frequencies from the feedback loop only, no tonal imbalance will occur. When low frequencies begin to overload the circuit, turn the filter on and tune them out. If the filter does not improve the situation or makes it worse, just switch it off.

Circuit Analysis

The circuit is remarkably simple, as you can see in the schematic. This is because the FX-1 is basically just a feedback routing device. A signal from your processor is fed to jacks J6 and J7 and is divided between jacks J4 and J5 and the two sections of the dual potentiometer R7. The fixed resistors R5 and R6 take the signal from to your mixer and feed it back to your processor through jacks J8 and J9, while the bulk of the circuit processes the signal.

fx-1 schematic

Switch S3 is a two pole, center off, spring loaded toggle, which will allow you to apply any special effect to a single note or phrase. When the switch is in the center position the unit is off and if pressed down, the effect is engaged only as long as it is held. Upon release, the switch returns to the center off position and if you need continuous operation, just flip the switch to the top position. Despite the simplicity of the filter, it consists of merely a pair of feed-through capacitors, C2 and C3, and dual potentiometer R7 and despite it’s simplicity, it is one of the most dynamic elements of the design. Mainly this is because the filter is in the feedback loop where the amplified signal is reapplied to the input. As you work with the FX-1 you will find that adjustment of the filter can help tame runaway feedback that you can demonstrate by flipping switch S4 on and off.

Light Emitting Diode D1 and resistor R1 function as a pilot light, while LED D2, resistors R2 and R4, along with transistor Q1, function as a control voltage monitor. A three-volt DC 200 ma wall adapter supplies power. Banana jack J11 applies an external control voltage to the two transistors Q1 and Q2. Potentiometer R8 enables the user to adjust the control voltage for the desired effect. Resistor R9 is a current limiting resistor and R10 acts as a reverse bias reference.

Voltage Controlled Effects

Transistor Q2 responds to the control voltage by engaging the defeat control on the special effects processor. This input, on commercial units, was designed to be operated by a simple foot switch, but with the FX-1, there is a whole range of possibilities. Gated repetitions is only one of the effects that can be achieved with voltage control. Multiple time-varied state, complex high-speed modulations, dropout, and pulsed overload compensation, are just a few of the many new things that can be added to the arsenal of sounds.

Hyper Drive

One of the most important previously mentioned effects is the pulsed overload compensation. A very volatile patch can be driven even further into overload if it is pulsed off just before it creates havoc and is then quickly allowed to build up once again. Timing is critical in this kind of effect and the space between pulses must not exceed the time it takes for a sound to go from applied effect to speaker destruction. A block diagram shows how to set up this mode of operation and a graph is provided to show how this is accomplished. This may very well be the most valuable function of the FX-1.

Sequencer Programming

An analog sequencer can actually provide a different amount of the designated effect to each and every note of a sequence. A slow speed can produce interesting and complex rhythmic patterns. If a high rate of speed of note sequence is used, new tonalities can be generated and slight changes in the individual note programming and the rate of speed can produce dynamic changes in the overall tambour of the sound. New sounds can be generated this way and they can

sequencer controlled effect

sampled and stored for future use. A chart, included here, shows an example of how each note of a sequence can receive a different amount of the chosen effect. Much time should be spent exploring the possibilities of this particular mode of operation.

sequencer selected effect


Any type of enclosure can be chosen, however, nineteen-inch rack cabinets are more professional and can be constructed easily and inexpensively.

rack cabinet construction details

Wiring is not critical and is for the most part, point to point. The two transistors are mounted on terminal strips and the filter capacitors are mounted on the terminals of switch S4.

inside rack

Potentiometers R3 and R7 have small increment clicks for more accurate adjustment. Light emitting diode colors in the prototype were chosen to match the colors of knobs and lettering (Red for the power indicator and blue for the control voltage indicator). You may choose any colors you feel are appropriate.

Although the FX-1 can give you greater control over feedback problems, it has limitations, so you should still exercise caution when using the FX-1 in this mode. Do not think that because the circuit is simple, the effect is limited, for if you are looking to create new sounds, the FX-1 can provide for you, a plethora of new sounds and effects. The setup can be seen below.

circuit boards

Patching Diagram

patching diagram

Parts List


  • R1 2.2-K
  • R2 1.5-K
  • R3 100-K Dual Potentiometer
  • R4 1-K
  • R5 10K
  • R6 10K
  • R7 1 Meg Dual Potentiometer
  • R8 100K Potentiometer
  • R9 10-K
  • R10 220K


  • C1 10-mfd
  • C2 220 pf
  • C3 220 pf


  • Q1 2N2222 NPN
  • Q2 2N3565 NPN


  • Red – Power indicator
  • Blue – Control Voltage Indicator


  • S1 SPST toggle
  • S2 DPDT toggle
  • S3 DPDT center off momentary contact toggle
  • S4 DPST toggle


  • J1 Power supply jack
  • J2-J9 RCA jacks
  • J10-Phone Jack
  • J11-Banana Jack


  • 19 inch rack type, made from 1 ½ inch aluminum angle molding


  • Color-coded telephone wire

LEDs or other components may function differently on the unit you build, so feel free to substitute components that make your unit function more to your liking.