Integrating noise-gate circuit

[BeëlzeM]

Hey guys,

I've got this schematic:
http://img407.imageshack.us/img407/4...ereduction.jpg

The first part of the circuit is a boost circuit, something like a Tubescreamer or an SD-1 or whatever. But then those two lines from the boost circuit go into the second part of the schematic which is a noise-gate. It's that THAT4301 chip. The gate is supposed to kick in when not playing but it lifts off when you're playing. It grounds the signal the moment you touch the strings but it can feedback for a while, the kind of feedback you want to have.

In the upper right corner of the schematic are two LEDs, a red and a green one. The green one lights up when you're playing (noise-gate off), and the red one lights up when you stop playing (gate kicks in).

The pot VR7a just controls the gate. VR8a controls the amount of boost by the overdrive circuit.

I can't really figure out the noise-reduction circuit. I want to take the clipping overdrive circuit out of it all and just have the noise-gate left. I want to have one input and one output obviously for a guitar signal to go through.

Any help on this would be great

[Steve Conner]

Hi,

The circuit is a little confusing because there are two inputs to the noise gate. The plain guitar signal, before the distortion stage, is fed to the gate chip's level detector. This is the path to pin 1 via R30, C20.

The distorted signal is then fed to the actual gate part of the chip via C1, R5 into pin 17.

So, my first line of attack would be to unhook C1 from IC2b's output, and hook it up to IC3a's output instead, so that both the level detector and the gate get the same signal. Then you can get rid of IC2 and all the parts that go with it.

This circuit also seems to include some EQ. TR1 and TR4 are gyrators of the kind used in graphic EQs.

[BeëlzeM]

Hey, thanks for the reply. So this would be pretty much it?:
http://img266.imageshack.us/img266/9...eduction2x.jpg

I wasnt sure if I could feed the signal into both the level detector and the gate at once in this circuit.

Yeah there was some EQ'ing in the OD circuit but I wanted to take that out anyway.

If this should work than I think I'm trying this one...

Cheers

[BeëlzeM]

In addition to my post above (I'm still not sure if my second schematic is correct):

Are there ways to reduce noise with a simple circuit? I've seen circuits like these to reduce noise in some signals. Circuits like these for example:



Or this:




These should reduce noise for about 35 dB between 100 Hz and 20 kHz. Would anything like this work?

Jerry from FJA has something like this and I was wondering how he's doing that:
http://www.youtube.com/watch?v=UxCeN4zKq48

[Steve Conner]

Your above circuit seems correct to me, though I was hoping for a second opinion from someone else.

The two circuits you just posted don't seem to have anything to do with audio noise reduction. The second one is a capacitance multiplier to remove noise from DC voltages, and I've no idea what the first one is.

Peavey used to use a pair of back-to-back diodes in the signal path of some high-gain amps as a kind of ghetto "noise gate". I bet the FJA mod is something like this. The 5150 achieved a similar effect by just biasing the power tubes very cold.

[BeëlzeM]

Your above circuit seems correct to me, though I was hoping for a second opinion from someone else.

The two circuits you just posted don't seem to have anything to do with audio noise reduction. The second one is a regulator to remove noise from DC voltages, and I've no idea what the first one is.

Alright, just forget about the noise reduction circuits in my last post. Was just a shot in the dark actually. Weren't those Peavey noise filters just 2 diodes in series with a resistor in parallel? I'm curious how that FJA circuit works.

Thanks for helping me out on the first circuit though, if it looks fine I think I'm trying that. Any more opinions?

[Gregg]

You didn't mention where do you want the NG integrated but I'll assume you meant in a tube amp. If this is your case it's not difficult to do and after you're done there will be only one additional LDR in the sound path.
I don't know how effective is Peavey's reverse didodes NG but many people say it sucks.

[BeëlzeM]

You didn't mention where do you want the NG integrated but I'll assume you meant in a tube amp. If this is your case it's not difficult to do and after you're done there will be only one additional LDR in the sound path.
I don't know how effective is Peavey's reverse didodes NG but many people say it sucks.

Yeah, in a tube amp. Just between the input jack and the preamp I think. What do you mean with an additional LDR? The schematic isn't using any LDR's right? Or am I misunderstanding you?

I haven't really heard good things about the Peavey noise filtering, either. There are ways to filter out the noise by a good amount, have no idea where to look for basically.

Oh, by the way. Would this circuit alter tone significantly?

[Gregg]

Yeah, in a tube amp. Just between the input jack and the preamp I think. What do you mean with an additional LDR? The schematic isn't using any LDR's right? Or am I misunderstanding you?

I don't think placing a NG between the input jack and preamp is a good idea. It should be after the preamp especially if it's higher gain preamp.
The gating element in the schematic I meant is an LDR which forms a voltage divider with a resistor thus cutting the signal out. I've used this many times and even a high gain amp becomes dead quiet.

[BeëlzeM]

What if there are no +/- 15v regulators inside the amp? I assume I have to build a regulator circuit in the amp as well. Should I take that from the PT? Is the reverb driver transformer any good for this? It's a modded JCM 800 Marshall btw. But I'm also looking at building this inside of a 5150 in the future, just experimenting first though. I'm not doing it just yet, my 800 has some other problems that I need to take care of first, but I just want to get this clear for myself.

[Gregg]

What if there are no +/- 15v regulators inside the amp? I assume I have to build a regulator circuit in the amp as well. Should I take that from the PT? Is the reverb driver transformer any good for this? It's a modded JCM 800 Marshall btw. But I'm also looking at building this inside of a 5150 in the future, just experimenting first though. I'm not doing it just yet, my 800 has some other problems that I need to take care of first, but I just want to get this clear for myself.

I don't know what kind of power supply you have in your JCM800 but you don't need +/-15V to run the NG. I guess you can rectify the filament supply and add a charge pump to create negative voltage. This NG will run on +/-6V without any problems. In 5150 you already have the voltages you need.

[BeëlzeM]

http://img6.imageshack.us/img6/1781/...mpowerpsdc.jpg

That's my powersupply. I just need to take one 6.3v lead from the filaments supply and invert the other with something like this? Any help on how to wire that up would be cool. Will that have any consequences for my filaments supply?

[Gregg]

Actually you may not need the charge pump but you can use voltage doubler instead. Check out how it's done in Mesa Mark IV.
You won't need so big value capacitors because it will draw much less current.

[BeëlzeM]

Actually you may not need the charge pump but you can use voltage doubler instead. Check out how it's done in Mesa Mark IV.
You won't need so big value capacitors because it will draw much less current.

I'm not sure how I would do that. How do I get that from the filaments supply, and where is the negative voltage?

I know this is a way to do it. But I can't get this from the filaments supply, can I?
http://img13.imageshack.us/img13/922...regulatorl.jpg

The voltage doubler will get me 12.6 volts on each lead from the filaments supply, right? From which 6.3 is for the powertube heaters and then I have another 6.3 left for the noise gate circuit. How would I wire what to the filaments supply? Any directions? I will still need to create the negative voltage, right?

[BeëlzeM]

So I have these two 6.3 volt filament supply leads from my power tranny going to the heaters of the powertubes.

How do I wire these, and what additional circuit do I need to power that noisegate circuit that runs on +6.3v and -6.3v.

I think I need to double the voltage on one filament supply lead for the positive voltage. And double the second filament supply lead and invert the doubled signal with a voltage inverter.

Am I right? How would I do that?