Original design noise gate pedal using surface mount chip parts Original design noise gate pedal using surface mount chip parts

Original design noise gate pedal using surface mount chip parts

Originally designed circuit

I designed a noise gate pedal, one of the pedals I haven't made yet. The noise gate requires a circuit (VCA = Voltage Controlled Amplifier) that automatically opens when there is a signal and closes when there is no signal. This time, I tried using the simplest J-FET.

I often see circuit diagrams made by someone, but I have never thought about it myself. This is the first time that I made it myself by referring to the circuit diagrams of various pedals.

Signal processing block diagram

I used a JFET to switch the signal on and off. The operating principle of the JFET is very simple, and the voltage applied to the GATE can control the current flowing between the SOURCE and DRAIN. In other words, when a high voltage is applied to the GATE, the SOURCE-DRAIN will conduct, and when the GATE voltage is lowered, the SOURCE-DRAIN will close.

Therefore, the level of the input signal is converted to a voltage, and this is put into the GATE of the JFET, so that a circuit that turns on the switch only when the signal is input can be created.

This time, I used a JFET called "2SK30A", which is often used in audio circuits because of its good sound quality. This FET is used in the electronic switch of the BOSS's pedals.

I wrote the block diagram. The circuit that converts the level of the input signal to voltage is a simple one that uses diodes, resistors, and capacitors. The principle is to store the input signal in a capacitor and convert it to a DC voltage. At this time, the current of the input signal is one-way due to the diode, and the electric charge is accumulated in the capacitor only when the input signal becomes higher than the electric charge of the capacitor. A large resistor is attached to the capacitor, and the accumulated charge is slowly discharged to GND. As a result, the signal turns off smoothly as it attenuates.

In order to turn on the FET fully, it is necessary to apply the same level of voltage as the power supply voltage, so the DC level is raised by the DC operational amplifier.

DC AMP signal image driving FET

DC AMP to raise the detected level

Since the charge (DC voltage level) stored in the capacitor is small to give to the GATE of the FET, it is necessary to amplify this voltage level so that the FET can be controlled. The DC level is amplified simply by using an operational amplifier.

At this time, note that general operational amplifiers (4558, etc.) have a phenomenon called "output inversion". This is a phenomenon in which the output reverses to the voltage in the opposite direction when an input near or exceeds the negative power supply (-V) or positive power supply (+ V) is given. Such an operational amplifier cannot be used to amplify the DC level between GND and the power supply voltage like this time.

For DC AMP, it is necessary to use a "single power supply operational amplifier" that does not cause output inversion even if a voltage close to GND is input. I used the typical "LM358" for this purpose.

General operational amplifier that causes "output inversion" that cannot be used for DC amplifiers

Prototype on breadboard

I bought a breadboard for the first time to check the operation of the circuit and adjust the constants. This is for building circuits by inserting component leads into holes in the board for electronic work and prototyping. I decided the constants while checking the signal condition with a portable digital oscilloscope purchased for $50.

However, breadboards do not have a shielding effect because the parts are exposed. If you actually connect the output to an amplifier and make a sound, the noise will be large. I think it would be improved if the circuit was actually placed on a universal board or printed circuit board and put in an aluminum enclosure, but the noise level cannot be evaluated on a breadboard.

The quality of a noise gate depends on whether the gate closes smoothly as the sound of the guitar decays. Bad reviews of commercial noise gates include "sudden cuts" and "unnatural sound attenuation". This adjustment is difficult because it is a simple circuit. I managed to adjust the constants so that it worked.

Mini size pedal using surface mount chip parts

Design surface mount PCB

This time, as with the previous "Suhr Riot Clone" production, I created a PCB on which surface mount chip components can be mounted. I designed a double-sided PCB using "KiCAD" and placed an order with "Fusion PCB" in China, which manufactures PCBs in less than two weeks. Since I received a $5 coupon, the production cost was zero this time as well, and I only had to pay the DHL shipping fee ($15).

Board designed with KiCAD (pattern diagram, 3D image)

Purchase of chip parts

There are few stores that sell surface-mounted chip parts for personal work, and there is a minimum purchase unit of 20 to 100 pieces, so it is difficult to purchase cheaply. I bought them separately for each type of parts from the four shops I usually use.

面実装チップ部品エフェクタ 共立エレショップさんから届いた部品

Board arrival, component mounting started

Purchase a workbench with a magnifying glass

The board I ordered has arrived from Fusion PCB. It took 10 days from ordering. I immediately started mounting the parts. When I made Riot last time, the surface mounting chip parts were so small that it was difficult for me with presbyopia to mount them, so I bought a workbench with a magnifying glass this time. I think the magnification is less than double. Even when I looked closer to the board through the lens, it was easy for me with presbyopia to focus, and the lens was large enough to be seen with both eyes, so it was comfortable.

I steadily attached the parts with tweezers and completed the mounting in about 3 hours.

Check operation

This time, before incorporating it into the enclosure, I checked the operation with only the board. As usual, I generated a sine wave with free software on my PC and input the signal from the audio i/o (steinburg UR22mkII) to the board.

Circuit diagram error

I tried plugging in the power supply, but unfortunately it didn't work and there was no signal at the output. The green LED indicating the GATE status was not lit at all.

I followed the circuit. The signal came to the input (pin 3) of the first stage operational amplifier (LM358) that creates the level signal, but the signal did not come out from the output (pin 1). The output was equal to the power voltage.

When I reviewed the circuit diagram, the connection destination of the operational amplifier (-) was wrongly set to GND. Probably, it seems that I made a mistake when I finished the design on the breadboard and wrote the circuit diagram. I reconnected this line to the correct + 4V (reference voltage).

Fine-tune the constant while turning the pod

After correcting the above wiring mistake, I was able to confirm the same operation as the breadboard. There was no noise I was worried about on the breadboard and the SN was good.

This pedal has two pods, a "SENSE" that adjusts the sensitivity of the guitar input and a "DECAY" that determines the length of time the gate closes. For "SENSE", the gain was insufficient for a single coil with a small output, so the constant was adjusted so that the gain of the first stage operational amplifier (LM358) would increase.

Drilling of enclosure

For this enclosure, I used the same HAMMOND 1590A that I used when I made Riot. The hole positions of the switch, jacks, and pots are the same as Riot. Since the LEDs are directly attached to the board, they were processed while paying attention to the position.

Placement of parts

The placement of the parts inside the small enclosure assumed the cross section below. Since the LED is directly attached to the board, holes are drilled while paying attention to the position. The PCB is fixed by sticking 2 mm thick cardboard on the inside of the case.

Sound samples

You can listen to 4 samples. The same phrase will be played twice. The noise gate is OFF first, and then the noise gate is ON.
In the case of the third long tone phrase, it seems that the sound is cut off in the middle of attenuation, but I think that there is no problem when using it for live performances.
It's completely silent while you're not playing, so you'll hear effects such as delay and reverb clearly, as in the fourth sample.

When using a high gain distortion like Riot with a single coil, it was difficult to use because of noise, but when this noise gate is inserted, the distortion sound is cut out, it sounds like a lump, it becomes more powerful, and the volume I feel like it has gone up.

Sample: Short cut sample, DECAY longer (MP3)

Sample: Code phrase sample (MP3)

Sample: Solo, long tone. (MP3)

Sample: Solo. With a repeating long delay (MP3)

The recording environment is:

Fender Stratocaster (rear single) → Self-made noise gate → Self-made Riot → MOOER Preamp006 (Fender blues deluxe clean, cabinet ON) → UR22mkII → DAW (Studio One)

Completion of the original design noise gate!

The color of the enclosure this time is "silver hammer tone" with a silver mesh coating. I put the stickers of the knob name and the initial mark of "N".