MXR Smart Gate -> Boss NS-2 style noise suppressor

[Hevimies]

I was wondering, would it be possible to make an effects loop to MXR Smart Gate schematic (I don't have noise gate yet, but I'm thinking of building one) like in Boss NS-2?

[dave h.]

or heres a better question, that i was wondering a few days ago:

when you hook up the FX loop, does it mean that its only reducing noise on the loop, or just the input, or both (reduction comes after return jack) or is it just an extra feature, since you probably always want a NR pedal on?

on the topic of noise gates, i have no idea how they work, but is it possible that you could build an pedal that would work only when the input signal is UNDER the gate level? sort of like an amplitude crossover, or something.

[Paul Perry (Frostwave)]

is it possible that you could build an pedal that would work only when the input signal is UNDER the gate level? sort of like an amplitude crossover, or something.

Certainly.
A noise gate has an envelope follower (that is, a circuit that looks at the average level of the signal over a short period) and compares it wiht a set level. When the level is below the reference, it triggers some logic & turns the gain to nothing. It isn't difficult to modify a circuit to work the opposite way.
(there are more complex noise gates that don't suddenly chop the gain, but rather decrease the gain when the signal is below a certain point. It would be difficult to modify one of these, but any 'simple' stompbox noise gate will probably be the gate type).

And another thing that is possible, is to switch between two sources depending on which is louder at the time.

[dave h.]

yeah thats kind of what i wanted to do, only not REALLY two different sources. the signal above the gate would be fuzzed up, out of phase with itself, and volume slightly decreased, and the signal below the gate would be really really really really fuzzed up, loud and feedbacky. so you get awful sounding feedback and humming and radio signals and such between chords.

it would be cool to hook up two CD's to a device like you were talking about, and listen to them "battle."

[StephenGiles]

Do you have a link to the MXR Smart Gate schematic?
Stephen

[Hevimies]

Do you have a link to the MXR Smart Gate schematic?

It's at tonepad, www.tonepad.com

Damn you, Dave H, you hijacked my thread!  

[Fp-www.Tonepad.com]

I haven't seen the smart gate schematic, but my guess is that it's the same as the mxr noise gate available at my site.

There is a mod which involves a cap change in order to mute only the high frequencies... it's the 10uF cap going to Q3.

Changing it to 0.015 will cut mid and high freq only.
Changing it to 0.005 will cut only high freq.

[Hevimies]

Yeah, it's the same...

That mod would be nice too... Maybe a switch to toggle between original and mod caps.

[Mark Hammer]

1) That is an older MXR noise gate shown at Tonepad under the "Calatte" name.  The Smart Gate is a more recent device of Dunlop/MXR origin.

2) The whole deal with noise gates is the extent to which they can tell the difference between music/signal and non-signal.  Gating is easy; knowing WHEN to gate is hard.  The simplest means for assuring that the gate is only opened for music is to set the gate threshold high.  The problem is that it doesn't let subtlety come through and chops off the beginnings of notes is service of keeping the output scrupulously clean.

The problem that also accompanies gates is that it is easiest to tell the difference between signal and noise at the outset (i.e., immediately after the guitar jack).  After 4 or 5 pedals in a row, the cumulative hum and hiss starts to verge into signal levels similar to note tails (i.e., late note decay).  If you stick the gate at the front, you have no impact on the audio crap accumulated afterwards in the chain.  If you stick it at the end, you can eliminate all the cumulative crap, but the difficulty in discriminating signal from noise at that point costs a hefty price.  Most players will either sacrifice playing style to be able to use a higher gate-on threshold, or else bite their lip and accept a lower threshold and less noise reduction than the probably need just to retain playing dynamics.

The glorious compromise is to use a loop, take the sidechain feed from the guitar, and stick the gate it controls at the end of the loop.  In other words, such a gate looks for the difference between note and noise at that point when it is easiest to detect, but USES that information to impose gating and noise reduction AFTER all the noise has been accumulated.

In principal, this could be done with almost any gate, as long as you know how to insert a send/receive loop between the input buffer and wherever the gating action occurs.  Looking at the Callate layout/schem, my best guess is to interrupt the signal between the .01uf cap and the pair of 1Meg resistors that centre the input to IC1b, and feed that to a send jack.  The return jack then comes back through a .01uf cap to the pair of 1Meg resistors and IC1b.  That would be the "leanest" approach with that particular circuit.  Naturally, the gate would be used like a loop selector pedal right after the guitar, except that the loop would always be "on".

The other possible change that comes to mind is that the 500k pot might have to be altered for greater usability.  As is, it is intended to use over a very wide range of noise floors.  By inserting the gate and envelope follower immediately after the guitar, you may find that you only need a very small portion of that control range.  The solution would be to stick a fixed resistor in series with the input of the 500k pot to mimic one turned down a bit, or alternatively to increase the 680R resistor in the ground leg of IC1a to 820R or 1k to reduce gain a bit.  Both would do the trick.

[Fp-www.Tonepad.com]

The whole point of the loop is to extract the envelope while it's in it's purest (?) form and use that envelope to control the gating of the modified return signal.

Q1 splits the signal to the envelope detector and to the attenuating circuit. Looking at the circuit, by inserting a serial effect loop between Q1Emiter and the 22k resistor going to the two caps, you'd be able to extract the envelope from the guitar before it becomes 'dirty' and use that to gate the return.

All you'd need is a capacitor to couple the effect send. It would probably be a good idea to add an input buffer, but that just complicates things.

[R.G.]

The logical extension is to make a note processor pedal set that does note processing in a side chain analogous to the sidechains within a pedal.

You'd use one pedal immediately after the guitar to extract stuff about the signal: envelope, inverse envelope, start point, stop point, signal level steps, things like the ones from "Guitar Note Processing" (GEO, March 2, 2001).  The box would be optimized to do a good job of picking out key features of the note string it was seeing, and putting out useful control signals. A really useful signal is a variable-delayed note end signal; it ticks not at the end of the actual note, but some delay time later. REALLY usefull for a noise gate not chopping off that last bit of a note.

This box would offer buffered and non-buffered outs, tuner out, all the miscellanous junk that you want the first box in a chain to have.

Other boxes in the set do the other stuff. The obvious first one you laid out is the noise gate, which goes 'way down at the last pedal in the chain, just before the amp. Feed it from the "gate" signal and it lets through signal only when there's a note present, as determined by the Note Extractor box.

Other useful boxes: Voltage controlled Amplitude, Voltage controlled phase, voltage controlled delay, voltage controlled filter. You can feed them combinations of envelope, inverted envelope, and RC delayed gate to do things like attack delay (VCA plus RC delayed gate), funk filter (envelope plus VCF), Note Shortener (VCA plus one shot driven from Note Start signal); the list goes on, the only trouble being how to combine them in useful ways.

Dang. Now I gotta go design some more...

[dosmun]

Isn't the Smart Gate the same circuit that was originally the Scholz Smart Gate?

[Mark Hammer]

The logical extension is to make a note processor pedal set that does note processing in a side chain analogous to the sidechains within a pedal......This box would offer buffered and non-buffered outs, tuner out, all the miscellanous junk that you want the first box in a chain to have...... the list goes on, the only trouble being how to combine them in useful ways.

Assuming one wants the most accurate and reliable dynamic information to work with, the first thing in line ought to be the thing extracting envelopes (although sometimes I like some dynamic perversion along the way to mess with the envelope extraction, but that's just me).  The challenge is that not all sidechain-controlled devices want or need or benefit from the same type of envelope signal, or even the same level.  Consequently, what you sort of need is a splitter as the first stage, so that the signal can be sent to multiple stations to extract an envelope in the way most appropriate for the effect in question.  For example, the time parameters for a limiter will be different than for a swept filter or envelope-controlled flange, and different again for a noise gate or ducker.  Each such device ought really to have its own personal envelope extractor with time constants (and possibly control over them) suited to the function, in addition to output levels suitable to the sort of control element to be, well, controlled.  What you need for an LED may be different than what you need for an OTA or FET, etc.

PLEASE NOTE that "noise suppressors" are generally somewhat different than "noise gates".  The latter simply attenuates the entire spectrum by the same amount every time, so long as the input signal is above a given threshold.  The latter adjusts both the level and the high frequency content in a more articulate manner.  The basic idea is that signal can easily mask noise when the signal is loud enough, but noise becomes more obvious as the signal declines in level.  By employing a sliding lowpass filter that trims treble during quieter passages, in tandem with a downward expansion that reduces the amplitude of softer passages even more as they get softer, the user gets to retain note onset and offset dynamics reasonably well while keeping noise well in the background at those points where you might notice it.

Dang. Now I gotta go design some more...

Don't we all, my friend, don't we all. :wink:

[dave h.]

Do you have a link to the MXR Smart Gate schematic?

It's at tonepad, www.tonepad.com

Damn you, Dave H, you hijacked my thread!  

sorry.
i think we both got all our questions answered though?
this place is very helpful.

[william]

The input of the NS-2 goes into a buffer and then is split into two parts, the send jack and the envelope follower of the gate.  The Send jack then goes to the return jack.  The return jack is switchable so that when no cable is inserted the jack is bypassed and the signal goes through the gate.  When you insert a cable into the return jack, it cuts the direct connection from the send jack, and you get only the return signal.  It then goes to a Voltage controlled op-amp which does the actuall gating, then a buffer and finally out the pedal.  The Envelope follower is seperate from the loop send and return, so only the original guitar signal affects the envelope follower.  

I think with the mxr Noise gate, if you put the send jack between the .01uF capacitor and the 22k resistor, and then the normally closed return jack right after that. You mught need to put some kind of buffer in there.

Its a good thing to note that you shouldn't use delay's in the loop.  The gate will cut the delay.  Time based effects like reverb and delay should go after the Noise gate.

[Hevimies]

sorry.
i think we both got all our questions answered though?
this place is very helpful.

I was just kidding...
And yeah, I've learned a lot about pedals and electronics from this forum.

Thank you all guys very much for the replies!