Author Topic: Designing a noise gate with the THAT4315 analog engine  (Read 166 times)

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aion

Designing a noise gate with the THAT4315 analog engine
« on: December 06, 2017, 03:26:33 PM »
I'm working on a schematic for a noise gate based on the THAT 4315 analog engine. Here is the draft version, pending a few questions.



This is a combination of a few ideas from the Decimator, but removing the patented "time vector processing" portion of the circuit and replacing it with the 4315's built-in RMS detector, using THAT Corp's Design Note 100 as a reference. So that's what the input buffer and loop are doing, allowing you to pass the dry guitar signal through as a control and but then apply the noise gate to the signal farther down the effects chain just like the Decimator.

THAT's Design Note 100 uses the 4301 chip, which is designed to run off a bipolar supply. I figured bipolar was overkill for a guitar noise gate, so I'm working through the process of adapting the THAT circuit for the 4315, which runs on much lower current and is designed for single-supply operations. It's half the cost of the 4301 and would require dozens fewer external parts, so it came out looking pretty attractive.

In creating the schematic, some questions came up, mainly about the release/hold section following the RMS detector. (the center-right portion of the schematic)

1) In converting it from bipolar to single supply, I wasn't sure which components were going to ground because it's the halfway point of +V and -V, and which components were going to ground because it's ground. I know the + input of the op-amps should be connected to Vref, but it's R15, D3-D5 and C4 that I'm not sure about, the ones highlighted in red. Should any of these be going to Vref instead?

2) Along those lines, do any DC-blocking capacitors need to be added anywhere in the release/hold section? It's not passing any audio, just a control signal to the EC- pin of the 4315, but I wasn't sure if it would cause issues for the opamps to be direct-coupled.

3) The "release" and "hold" pots are specified as 20k (I'm assuming this is linear). I'd like to use the more common 25k for the pot value. Do any of the surrounding resistors need to be changed if the pot value is increased?

4) There are a few very precise resistor values being used, e.g. 28.8k for R15 and 2.49k for R12. What would be the implications of substituting these for more common values like 27k or 2.4k?

5) The non-linear capacitor (NLC) coming off pin 4 of the RMS detector... I understand the basics of how this works (from studying Design Note 3, last two pages), but with the values given in the THAT circuit, I'm calculating a time constant of 10ms for slow mode and 0.5ms for fast mode. This seems pretty short, but this is without taking into account the impact of the hold/release circuit. Are these good values to pair with the adjustable hold and release?

There is a Marshall amp, the JVM410HJS, which uses the 4301 plus a NLC as a noise gate. They use very different values - different timing capacitors, and a much lower clipping threshold with the 1N4148's instead of LEDs - and I'm wondering if theirs is better-tuned for guitar. I've attached the NLC portion of the schematic from THAT's noise gate as well as the Marshall side by side for comparison. But again, wasn't sure if THAT's values are preferable when used in conjunction with the release and hold circuit.



Thanks in advance for any help you guys can offer! I'd love to get a nice DIY-friendly noise gate designed that can compete with the Decimator for features, since I know that particular circuit has had a checkered history in the DIY world because of the patents.
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EBK

Re: Designing a noise gate with the THAT4315 analog engine
« Reply #1 on: December 06, 2017, 03:57:27 PM »
Quote
1) In converting it from bipolar to single supply, I wasn't sure which components were going to ground because it's the halfway point of +V and -V, and which components were going to ground because it's ground. I know the + input of the op-amps should be connected to Vref, but it's R15, D3-D5 and C4 that I'm not sure about, the ones highlighted in red. Should any of these be going to Vref instead?
R15 to ground.  Others to Vref.

Quote
2) Along those lines, do any DC-blocking capacitors need to be added anywhere in the release/hold section? It's not passing any audio, just a control signal to the EC- pin of the 4315, but I wasn't sure if it would cause issues for the opamps to be direct-coupled.
Don't block the DC.  You can essentially think of that side chain as developing a slowly-varying control signal, which is essentially DC....  (I'm way oversimplifying here.  I'll stop before I get into trouble with the wise elders....)
« Last Edit: December 06, 2017, 04:03:09 PM by EBK »
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EBK

Re: Designing a noise gate with the THAT4315 analog engine
« Reply #2 on: December 06, 2017, 04:29:58 PM »
Regarding the NLC, I have build Pedal Design Five from this page:
http://www.thatcorp.com/pedals/index.html

It's a compressor rather than a noise gate, but the NLC in that circuit works very well with guitar (the whole circuit does, actually), if that is at all helpful to you.
« Last Edit: December 06, 2017, 04:38:44 PM by EBK »
No affiliations. If I glowingly mention specific merchants or products, it is because I like them without having to be paid to like them.

aion

Re: Designing a noise gate with the THAT4315 analog engine
« Reply #3 on: December 06, 2017, 10:00:44 PM »
Regarding the NLC, I have build Pedal Design Five from this page:
http://www.thatcorp.com/pedals/index.html

It's a compressor rather than a noise gate, but the NLC in that circuit works very well with guitar (the whole circuit does, actually), if that is at all helpful to you.

Yes, thanks! One thing I noticed from digging through sample circuits was that all of the compressor/limiter applications seem to set C3 higher than C5 (referencing the component from my schematic) whereas the two noise gates I saw both gave C5 the larger value. Is this just due to the inverted functionality of a compressor/limiter vs. a noise gate? i.e. a compressor reduces gain when the signal is high, while a noise gate reduces gain when the signal is low.
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