Simple noise gate  [documentation]

Original effects with schematics, layouts and instructions, freely contributed by members or found in publications. Cannot be used for commercial purposes without the consent of the owners of the copyright.
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bmxguitarsbmx
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Post by bmxguitarsbmx »

Manfred,
Great work man. That totally makes sense. In other words, the circuit doesn't half or full wave rectify the signal to extract the envelope, it level shifts and then integrates it to its average value. I think that solves it!

Mathes, give us a challenge next time. That was too easy ;P

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Post by bmxguitarsbmx »

rcustoms wrote: 21 Sep 2022, 03:47 can work with any P-channel silicon field-effect transistors?
I'm not so sure. Mathes may have gotten even more lucky than he realizes by choosing the J175.

P Channel jfets conduct when the gate is positive to the source. At rest this circuit is biased to be "off" at some Gate voltage that is positive compared to the Source and is in the "MUTE" state. I didn't know this region existed. Apparently it does, because when signal is present, the level shifting creates a negative average voltage and lowers the Gate to Source voltage to the point where it conducts and "UNMUTES" the JFET. Vgs is still positive, but in "UNMUTE" mode, it is actually less positive than "MUTE".

Do Channel Jfets turn off at some larger, positive Vgs? This circuit sure seems to tell you that they do.

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Post by Matthes »

bmxguitarsbmx wrote: 21 Sep 2022, 04:19 Mathes, give us a challenge next time. That was too easy ;P
Haha, yes, I'll do my best.
bmxguitarsbmx wrote: 21 Sep 2022, 04:40 I'm not so sure. Mathes may have gotten even more lucky than he realizes by choosing the J175.
That is true, I didn't chose that one consciously, it was by chance.
And then I simply tweaked the other component values to make it work.

Thank you all for spending time and analysing the circuit!
It is really exciting for me to read all your comments and learn a bit more about these things.
I will definitely spend more time in this awesome forum in the future!

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Post by Manfred »

I ran another simulation, measuring the drain-source resistance with the virtual ohmmeter.
There were no resistance changes depending on the input voltage and the setting of the threshold potentiometer.
I have no idea why the circuit works.
Maybe the component parameters of the simulation do not capture all the details, but actually the simulation so far matched the values of the real circuit well.
Can you measure the drain-source resistance with and without signal at the input?

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Post by Ben N »

bmxguitarsbmx wrote: 19 Sep 2022, 15:43 The Key input/output allows the muting to happen in the effects loop, so that it silences the most noise. But, the sensing of signal/noise happens at the Key Input as it has the least processing and cleanest signal to trigger a gate.

The mute jFet is ground referenced through the "KEY OUT" ground connection, which then would be connected to the send and return grounds of the amplifiers effects loop (or through the input and output of your distortion box). Doing it this way prevents ground loops. The mute Jfet does need to be ground referenced to work, as Manfred points out. Switching jacks could be used to pass the input from the "KEY IN" to the "SIG IN" jack when not trying to key an amp or distortion box, like Ichabod is proposing.
The "Key" and associated circuitry is what is usually referred to as the sidechain.

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Post by Matthes »

Ben N wrote: 21 Sep 2022, 09:48 The "Key" and associated circuitry is what is usually referred to as the sidechain.
Yes, you are right. I called it "Key" because I was using a Fortin Zuul before (they also calle it Key input).
But I guess the term "sidechain" would be more appropriate.

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Post by bmxguitarsbmx »

As per Manfred's idea above, I hooked up a J175 with the Drain referenced to ground with a 470k resistor and measured the resistance between Drain and Source. This is just one Jfet, but it should give the idea on what the range of voltage for mute/unmute is. It helped me to do this simple little test to visualize it.

V on Gate: Drain and Source resistance:
0V 86 ohms
1.5V. 290 ohms
2.3V. 1.2kOhms
3.65V 940kOhms
4.66V. No Current flows on 20Mohm setting

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Post by Manfred »

As per Manfred's idea above, I hooked up a J175 with the Drain referenced to ground with a 470k resistor and measured the resistance between Drain and Source. This is just one Jfet, but it should give the idea on what the range of voltage for mute/unmute is. It helped me to do this simple little test to visualize it.
I also mean a measurement in the circuit, once with and once without guitar signal at the input, to see how the JFET works without refrence to the source or to the drain.

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Post by bmxguitarsbmx »

Manfred wrote: 22 Sep 2022, 10:25 I also mean a measurement in the circuit, once with and once without guitar signal at the input, to see how the JFET works without refrence to the source or to the drain.
I'm not sure what you mean by "...without reference to to the source or drain."

There will need to be a ground reference on either the Source or Drain to work. The connection to ground would be provided by the guitar amplifier's ground system that connects back to the the noise gates ground through the guitar amp input jack.

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Post by bmxguitarsbmx »

Ben N wrote: 21 Sep 2022, 09:48 The "Key" and associated circuitry is what is usually referred to as the sidechain.
Could we re-label all the jacks of this noise gate using "side chain" terminology? We could ditch the "key in/key out" terminology and it would be an improvement.

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Post by Manfred »

bmxguitarsbmx wrote: 22 Sep 2022, 15:45
Manfred wrote: 22 Sep 2022, 10:25 I also mean a measurement in the circuit, once with and once without guitar signal at the input, to see how the JFET works without refrence to the source or to the drain.
I'm not sure what you mean by "...without reference to to the source or drain."

There will need to be a ground reference on either the Source or Drain to work. The connection to ground would be provided by the guitar amplifier's ground system that connects back to the the noise gates ground through the guitar amp input jack.
Thanks for the confirmation that is also what I thought.
Such things often come to mind when walking through the forest with our dog.

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Post by Matthes »

bmxguitarsbmx wrote: 22 Sep 2022, 15:50 Could we re-label all the jacks of this noise gate using "side chain" terminology? We could ditch the "key in/key out" terminology and it would be an improvement.
Yes, I agree that that would be an improvement.
I will be out without internet access for the next days, but I will upload a revised version after the weekend.
Let me know if there is anything else that should be changed.

By the way, yesterday I played a live show using the gate (fast & loud Metal, rhythm and lead).
I payed extra attention on its performance and it worked like charm, I am really happy with it.

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Post by Ben N »

Thanks for the report, Matthes. Did you only use it with your effects loop, or have you also tried it just with effects ahead of the amp?

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Post by Matthes »

Here the revised schematic with "KEY" in/output renamed to "SIDECHAIN":
SimpleGate_v2.png

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Post by Matthes »

Ben N wrote: 23 Sep 2022, 11:56 Thanks for the report, Matthes. Did you only use it with your effects loop, or have you also tried it just with effects ahead of the amp?
I did not try this in a "real band situation" but as quick test at moderate volume with an overdrive pedal.
This configuration also seems to work well.
And last weekend I played two live shows with a wireless guitar system, it worked as well as with cable.

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Post by AxelKay »

The D0, is that a 1N4001 or 1N4007 perhaps?

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Post by bmxguitarsbmx »

AxelKay wrote: 06 Dec 2022, 17:26 The D0, is that a 1N4001 or 1N4007 perhaps?
Both are fine. It is for power supply reverse polarity protection. A lot of pedal builders like to use Schottky rectifiers, like the 1n5819, for even lower voltage drop. (or, for even more headroom, you can omit it if you promise never to connect the power supply up backwards).

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Post by AxelKay »

bmxguitarsbmx wrote: 07 Dec 2022, 17:47
AxelKay wrote: 06 Dec 2022, 17:26 The D0, is that a 1N4001 or 1N4007 perhaps?
Both are fine. It is for power supply reverse polarity protection. A lot of pedal builders like to use Schottky rectifiers, like the 1n5819, for even lower voltage drop. (or, for even more headroom, you can omit it if you promise never to connect the power supply up backwards).
Makes sense. So even a more common 1n5817 should do the same (?)

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Post by bmxguitarsbmx »

1n5817 will be great!

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Post by dylan159 »

Hi, a few comments based on the latest schematic:
  • The audio range input impedance about 320k. Not too bad, I think you said it's fine for you. Other than the buffer (I advise against sharing a 072 with the sidechain since there will be crosstalk), ditching the input pulldown and having it 1 or 2.2M total is a good compromise, in parallel with the amp or or pedal input.
  • C3/R5 highpass the sidechain output, while C4 adds treble emphasis. Personally, I'd think having as much of the fundamental as possible to keep it from cutting out would be desirable, but you can tell me more about your choices here.
  • Is it a rectifier? Is it not? Turns out it depends, the 3904 is cut off and can discharge the capacitor only on positive input peaks with low threshold, while it's linear with threshold high. The 3904 is operated in reverse beta at that, but the difference is subtle, just a slightly slower capacitor discharge (we're splitting milliseconds though). Honestly, a diode would work too, with the op amp output sinking the current.
  • The threshold control sets the gate bias together with a convoluted path to ground through three resistors (not a critique, just an observation while trying to analyze the circuit). It also influences the recovery time together with the switch. The threshold is varied by simultaneously adjusting the gate bias down and the voltage gain of the 3904, if seen as grounded emitter amplifier. Vref is pulled down by the 3904, but I guess it's not too bad since the input amplifier doesn't need to be linear anyway.
  • As mentioned by others, pulldowns and coupling capacitors on the signal path would be good to have, although there are probably some resistors pulling to ground on the other ends. What if instead, at that point, one biases drain and source to Vref and the gate to ground, allowing the use of a N-channel jfet? This while keeping the series jfet wiring and passive signal path, which is fine for fx loop uses.
These were first-glance observations, so I hope I'm not too far off and they're still useful.
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