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AMZ Opamp Module

Direct Box, Buffer and Booster pcb
© 2016 Jack Orman

This is a pre-assembled dual opamp module that can be used to build a buffer, booster or direct box. It has a clean, transparent sound and can be used in numerous ways on your pedalboard or even used to mod existing effect pedals. As can be seen in the photo, the board is very small and about as wide as a 9v battery.

The circuit has two op amp gain stages. It uses the NE5532A dual op amp and can be adjustd for gains from 1 to 11 (0db to 21db).

It is simple to hook up with only 4 wires needed in the basic buffer configuration. The outputs can also be wired for a balanced output which can be used to drive long cables or go directly into DAW interfaces and mixers (direct box use).

Power supply protection and filtering has been provided on board.

The input impedance is 1M without the optional pulldown resistor. Low end response easily covers the bass guitar range too.

A 12 volt power supply is recommended though the board will work on 9v to 18v. Current requirements are low (about 7 or 8 ma.). Absolute maximum input supply voltage is 24v.

Order the pre-assembled Opamp PCB from the catalog page.

The 8-sided pads on the pcb are for wires to connect to the external parts. The input pad at the bottom left is for the signal from the input jack or footswitch. There are two output pads near the bottom right of the pcb and they can be used individually, or connected to a stereo jack for balanced line drive. The ground connection (Gnd) should be wired to the power jack ground or connected to the ground lug on either the input or output jack.

R1 is for an optional pulldown resistor, and is used to help prevent pops if the buffer is wired to a bypass switch. A 2.2M resistor with 1/8w rating is recommended. If you are not using a bypass footswitch, do not add R8.

The opamp board can be powered by 18v if required, with no changes needed.

6 Examples Uses for the Buffer Board

  • 1. Standalone Buffer
  • 2. Buffer the input of a wah or fuzz
  • 3. Buffer the wah/fuzz output
  • 4. Convert True Bypass to Buffered Bypass
  • 5. Guitar Direct Box
  • 6. Dual Booster

1. Standalone Buffer

This is one of the most common uses. The buffer is installed in a small box that can be placed anywhere that it is needed on the pedalboard. This is the most flexible configuration. The buffer will mount easily in a tiny 1590A box along with the LED, input, output and power jacks. The P1 potentiometer is not installed when the outputs are used as buffers. Out1 is non-inverting and Out2 is inverting. The impedance of each output is 100 ohms. You can use both outputs, or only one and leave the other output unconnected with no problems.

The module is ideal as a buffer just as suppplied, however, if you would like to add a bit of insurance to prevent stray interference from getting in, a resistor can be added to ground the floating input where the gain pot is normally installed. This mod raises the output levels slightly above unity gain so be aware of that. All that is required is to add a 100k resistor from pad 1 to pad 3 where the potentiometer (P1) would be installed. The center pad 2 is left unconnected, though actually pads 1 and 2 are already connected by board traces. A 1/4 or 1/8 watt resistor is adequate and either carbon or metal film can be used.

2. Signal Booster

Another common use would be to connect a potentiometer to the P1 pads and use the module as a signal booster. A 10k linear taper pot is suitable but a reverse audio taper will give a smoother response across the range of adjustment. Small Bear Electronics sells a 16mm version that is ideal, though it is stocked at other pedal parts suppliers too.

The gain is adjustable from 1.9 to 11 with a 10k pot, which is 5.6db to 21db. A larger value potentiometer will allow less gain on the low settings but the max gain is still limited to 11. For example, a 50k pot has a gain range of 1.2 to 11 (1.6db to 21db).
There is also a 16mm right-angle mount potentiometer with straight pins that allows mounting directly to the pcb. The holes on the board are sized and spaced to accept this mounting configuration, which simplifies mounting the pcb since it is supported by the pins.

The pins of the pot are inserted from the back of the pcb and they are soldered to the pads on the top side. Make sure that there is a space betwee the metal case of the pot and the pcb so it does not cause a short circuit.

Small Bear Electronics carries an Alpha pot in this mounting style with reverse audio taper in both 10k and 50k values.

3. Direct Box

The AMZ Opamp module is designed so that it can also function as a unbalanced-to-balanced line converter suitable for use as a direct box. Use a stereo jack for the output and send output 1 to the tip connection, out 2 to the ring and ground to the sleeve (or body). A stereo cable is used to connect the direct box to the recording interface and 1/4" to XLR cables are available from Monoprice and other suppliers.

4. Buffer the input of a wah or fuzz

Another common use would be to install the buffer inside a wah or fuzz pedal as shown here. Many of these pedals have low impedance input and the buffer will convert the input to high impedance and extend the range of frequency response.

The AMZ pre-assembled opamp module is small enough that it can conveniently fit into a small corner inside the pedal.

Use only one of the output pads (do not parallel them) so that the build-out resistance of the buffer is high enough to prevent unwanted interactions with the wah or fuzz.

Power and ground connections to the buffer are required but not shown in this wiring diagram.

Used in this manner, the pedal is still true bypass since the buffer is only active when the effect is switched on. (LED wiring on the switch is not shown.)

5. Convert True Bypass to Buffered Bypass

This method is similar to the Boss use of buffers in their pedals. The input to the effect pcb is buffered and the the bypass is also buffered, since the buffer pcb is always in the signal chain.

Because the buffer is serving a double function, this is one of the most common mods for pedals.

Power and ground connections to the buffer are required but not shown in this wiring diagram.

Used in this manner, the pedal output is now buffered and it is no longer true bypass. R1 is not required. (LED wiring on the switch is not shown.)

6. Loop Bypass

I have never seen this connection in a pedal but it is interesting nonetheless. The buffer is inserted in the true bypass loop and converts the pedal to buffered bypass. The interesting feature is that the bypass is buffered but the pedal input is not.

This allows one pedal to have a dual function. The effect circuit (pcb) drives the signal chain when it is active, but the buffer drives the signal path when the effect is bypassed. Since the effect pcb is isolated from the buffer, it performs as it always has, but when the footswitch is toggled, the pedal becomes a standalone buffer.

Power and ground connections to the buffer are required but not shown in this wiring diagram.

R1 might be required. LED wiring on the switch is not shown.

More Uses and Ordering Info

Since the buffer board is tiny and requires little power, it can be retrofitted into almost any existing pedal to provide proper interfacing with guitars, basses, and oher pedals.

Another use would be as a conditioning input for projects that are not designed for use with guitars, and which might have low impedance inputs. Class D amp modules would be an example, or even microprocessor analog inputs.

The board isn't much larger than a postage stamp and can be mounted to the side of the enclosure with double sided tape or sticky pads. It could even be stuck to the side of the 3PDT footswitch to keep it out of the way.

Order the pre-assembled Buffer PCB from the catalog page.

Drill Template

This is the drilling template for the module when using the pcb mounted potentiometer. The plus sign to the right of the opamp is the center of the shaft of the gain pot. The graphic is 300dpi and can be printed on clear transparency film or even paper.

Position the printout and use a sharp nail or punch to mark the place to drill the mounting hole for the pot.

Frequency Response and Distortion

I measured the frequency response of the opamp buffer module and it is flat from 20 Hz to 20,000 Hz and only down about 1db at 10 Hz. This easily covers the full range of both the guitar and bass spectrums.

A look at the harmonics produced by the module shows that it is very clean. The only harmonic of any prominence is the third (at 600 Hz) and it is -100db below the fundamental 200 Hz audio, which demonstrates that the module is quite clean (low distortion). The measurements were made when powered by a 18v power supply.

These measurements were made with a bare pcb connected to the test computer with alligator clips. The performance will be even better when mounted in an enclosure with shorter connecting wires.

AMZ Opamp Module Frequency Response

AMZ Opamp Module FFT

Order the pre-assembled Dual Opamp PCB from the catalog page.

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