Noise Gate, based on the Engineer's Thumb Redux Circuit.

[jhsa]

Hey guys, I have been wanting to build some noise gate since a while. Tried a few schematics I have found online but didn't like them much. I didn't try all of them of course. 
One of the problems that is quite common with the circuits I've tried is that they normally stutter when turning the gate On.
I have seen a video of the Engineer's Thumb Redux compressor, which is also based on the Engineer's Thumb Compressor  which I have already built

https://www.diystompboxes.com/smfforum/index.php?topic=97123.0

The difference is that the Redux version also adds a Noise Gate to the original circuit.
The E.T. Compressor is based on an LM13700 IC, and the Redux version also  uses it for the noise gate.

I have tried to make the Noise Gate part work without the compressor circuit but I was having some problems. What made me stop using the LM13700 chip was that when the gate was closed there was always some sound, and my knowledge about how this IC works is not enough to debug this problem. It seems that many of us actually don't fully understand how it works because I couldn't find answers anywhere. 
I have never studied electronics and all I know I have learn by reading and experimenting.

Anyway, I have decided to use a LED and LDR as the Gate instead.
Here is where I'm at. It seems to be working fine but I am sure it will need some tweaking.
I also added an extra input to get the guitar signal from the beginning of the effects chain. A switch selects the gate trigger signal from the beginning of the effects chain, or from the noise gate input itself.

Please feel free to breadboard it and let us know how it worked for you.
All credits go to the creator of the ET Redux circuit, as most of the circuit is his. I think his name is "Jonny Reckless".
Perhaps the electronic Gods on this forum can help us improve this circuit further...   Have fun!!

EDIT: I have used a TC1044S for the charge pump.

[Kevin Mitchell]

What made me stop using the LM13700 chip was that when the gate was closed there was always some sound, and my knowledge about how this IC works is not enough to debug this problem. It seems that many of us actually don't fully understand how it works because I couldn't find answers anywhere. 

OTA based amplifier designs are pretty common. Once you generate a current to drive the amplifier bias pin (via an envelope circuit) it's a pretty straight forward compressor design in theory. Even the datasheets have examples that would lend to half of a comp circuit. That's one reason why I like the ET so much - it's super easy to understand.
For more info on OTA comps, check out ElectroSmash's page on the Dyna Comp.

Also, nice contribution! I'm sure someone will have some fun with it.

[Mark Hammer]

I like my schematic drawings to be more compact, so that when I print them out on a page, they are easier to read in fit-to-page mode.  So here is my adaptation of jhsa's original drawing.  I hope you don't mind.

[jhsa]

Cool, thanks, but the schematic I have posted unfortunately has a mistake that will not let it work.. Do you guys want to try to spot it before I correct it?

[jhsa]

OTA based amplifier designs are pretty common. Once you generate a current to drive the amplifier bias pin (via an envelope circuit) it's a pretty straight forward compressor design in theory. Even the datasheets have examples that would lend to half of a comp circuit. That's one reason why I like the ET so much - it's super easy to understand.
For more info on OTA comps, check out ElectroSmash's page on the Dyna Comp.

Also, nice contribution! I'm sure someone will have some fun with it.

Thanks. I hope someone finds it useful and test it also, as soon as I correct the mistake I have just saw on the schematics
The OTA works well as part of the compressor circuit. But the noise gate needs to actually MUTE the sound completely, and probably due to my lack of knowledge I couldn't make it mute the sound completely when the gate was ON.
Another problem I had was "Thump" noises when the gate switched, if I tried to increase the current on the control pins.
The LDR doesn't make any noises and it cuts the sound completely, as long as I isolate it well from sunlight of course. I made a little video showing the effects of a not well isolated LDR/LED combo.. It wouldn't happen when placed inside an enclosure of course, so nothing to worry about too much

https://www.youtube.com/watch?v=oHKTc-FTLFQ

[jhsa]

Updated Schematics on the first post as former version had mistakes. I hope it is fine now.
Sorry about that.

[jhsa]

I am still waiting for the PCB I have designed for the schematic posted on the first post, but I desperately need a noise gate on my gig yesterday as I was playing in an extremely noisy place, which is beside an electric train track. One day before the gig, I have decided to mess with that one knob Noise Gate circuit we can find online, as it is quite simple and small, and it would fit on a little piece of stripboard. But this is what I ended up doing, because that transistor circuit didn't work very well, and it was cutting way to early. I know, I know, it is not as simple as the transistor circuit  Breadboarded it and it did work quite well after some testing, So, I have built it as you see on the pictures. It is somewhow based on both the transistor circuit, and the schematic I posted above, which is my main gate project, 😉 and I installed it inside a buffer's enclosure, after the buffer circuit.
I kept the audio part of the One Knob NG and used the gate circuit of my other project, except that I used 2 Mosfet transistors instead of the Led/LDR combo.  Saved 2 Opamps 😉
The charge pump is an ICL7660S. I am not using the Attack and Release pots for simplicity, and do not know if they would react well, but I have left them on the schematic in case I want to mess with them later. I didn't test this circuit with the pots 😉
My gig went well, and as soon as I stopped the guitar strings, I had complete silence. And it allowed me to let the thinner strings to ring until the noise sound was louder than them if I wanted. If I stopped the strings vibrating, it did mute the sound. I really liked that, and I was really surprised how well it performed.
Here are some pictures in case you want to try it.   

[jhsa]

Finally the PCBs arrived 
Here is a little video showing the Noise Gate after I have just finished building the PCB I think I am quite happy with it 

https://www.youtube.com/watch?v=xXp2bpY6UxI

I looks like I cannot edit the first post, so I will have to post the new schematic here. It's a shame because modifying the first post would prevent people from building some circuit that is not the latest and has probably mistakes.

[jhsa]

Noise gate built, installed on my pedalboard (don't know how I managed to find space for it) and working. Been testing it, and made a little video.
One thing I have noticed is that with this noisy guitar I do lose some sustain, as the noise floor is quite high. I believe it should be fine with a less noisy guitar. actually I think there is something here in my house causing the noise. I take this guitar to my band rehearsal, and it is much quieter there..

https://www.youtube.com/watch?v=qzSfFmVYW6I