# Strange Guitar Noise Gate Pedal Circuit DIY

#### Willen

Joined Nov 13, 2015
315
Hi All,
First time I knew that there are gate pedals to eliminate irritating hums in guitar. Then I searched and found only a simple DIY design. I tried to simulate it. But I am getting nothing noticeable. Simulation shows the circuit eliminates everything around 95%. It should eliminate very low audio (hum hiss) and should allow higher level guitar play. I uploaded the LTSpice file too. Not getting BS170 I used 2N7002 to simulate. Can you check and tweak the circuit for the best result please!

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#### MrSalts

Joined Apr 2, 2020
523
You have several capacitors with no path to ground and, after a few fractions of a second to a few seconds, your caps will charge and hold the (because of diodes not allowing discharge of the caps, you are essentially making sample-and-hold circuits).

#### crutschow

Joined Mar 14, 2008
27,697
What, exactly, is the circuit supposed to do?

#### MisterBill2

Joined Jan 23, 2018
9,235
It looks like the circuit is supposed to bypass AC signals when the input amplitude is below some level, by the FET being biased into conduction until the input exceeds some level. So actually it is a noise clamper circuit, with the clamp level set near zero.
If the only purpose is to reduce the mains frequency signal amplitude, it would be far wiser to invest in proper shielded wire and correct connections and eliminate the hum entry into the circuit. There are also some passive circuits for cancelling hum that I have used in the past to null out the hum picked up from phone lines. Those simple circuits worked quite well, but required adjustment for each line used.

#### MisterBill2

Joined Jan 23, 2018
9,235
By the way, adding a pedal with the gate circuit will not solve the hum problem. It may hide the hum when not playing but it will not get rid of the hum. To eliminate the hum the source of hum entry must be eliminated.

#### MisterBill2

Joined Jan 23, 2018
9,235
But since the TS has not asked for help in hum elimination none will be offered.

#### Willen

Joined Nov 13, 2015
315
Hi, I am a crazy hobbyist so I love to do small DIY projects. The circuit in #1 made me so curious. The attached (1Meg Pot modified) simulation worked. It eliminates below than 20mV and passes above than 20mV (level adjustable). I hope this low amplitude cuts off the hums and hiss. I don't know the output amplitude of electric guitar. I want to experiment that circuit too.

In the case of "elimination of hum", a friend bought a Chinese version of Fender Electric Guitar at $100 with a$40 amp. The guitar looks and sounds good but the amp plays huge hum. When I touch the strings, or ground of output plug, hum stops. When I touch strings, it makes a great pop sound. YouTube shows the little complicated shielding the internal parts of guitar (I don't think I can get such shielding foil here). I also guess the problem in the amp too. While not plugging the guitar, when I touch the screw of the amp, it makes hum and pop. I don't know where is the actual problem. This situation makes me to search about gate circuit. Fixing the source of hum would be the best way to me. I found a magical video of reducing hum by a device HumX :
I don't know how the device works.

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#### MisterBill2

Joined Jan 23, 2018
9,235
The way that the circuit works is that it has an FET with the gate biased into allowing fairly low drain to source conduction by R2. An input signal is amplified by Q1 and fed to a voltage doubler, C1,D1,D2, and C5. The resulting positive voltage makes Q2 conduct, which removes the positive gate bias from the FET, M1. This allows the signal to reach the output. That is how the gate works. It shunts the signal to common until the amplitude is above some minimum level.