Noise Supressor

[Astronaurt]

Hey there folks, I'm looking to find some Schematics or reading or anything on building a Noise Supressor circuit. I've been dissatisfied with conventional Noise gates and how much they garble up the audio signal. I've tried looking for the boss NS-2 schematic, but I can't seem to find it anywhere. So I'd like to find or design an analog circuit that acts as a compressor, but doing Gain reduction BELOW the threshold setting rather than above it; hence, significantly lowering the volume of noise without being as tone-destructive. I'm not sure how complicated this project would be tho, I'd really like to hear what other people have thought on this subject! and anywhere I could get pointed to for more info would be highly appreciated. Thanks.

[Mark Hammer]

You mean this one?:  http://img.photobucket.com/albums/v517/uncle_boko/NS2.gif

What you want, actually, is an SSM2166 chip.  The chip provides both downward expansion (noise suppression) and compression on the same chip.  Pricey, as chips go, but it requires very few external components to make an extremely effective signal processor for a total cost which is shockingly economical for what you get.  Highly recommended.

[Astronaurt]

Awesome! This stuff looks like exactly what I was looking for. Thanks a ton Mark!  Just from reading the datasheet on the SMM2166, I'm kinda struck by all the applications I could use it in. Not only as a guitar noise supressor, but I've also been thinking about making a Microphone Pre-amp for Vocals/Harmonica, and this would go with that perfectly. Well, now I gotta put yet another project on the pile of unfinished projects. lulz.

[Mark Hammer]

A number of us here, including myself, have made compressor pedals based on the chip, and its a winner.  A compressor that doesn't seem to lose the attack or last fading embers of a sustained note, but is dead quiet when you stop, is a joy to use.

The chip was intended for use within a dedicated "channel strip", where a variety of mic functions could be implemented.

[amptramp]

I have an ancient RCA A-33 12-tube radio that has an interesting circuit called "magic monitor".  This circuit was updated from octal tubes to 7-pin miniature tubes in the VRA123 and VRA141 radio-phono consoles.  The final version used a 6AV6 and a 6BA6 tube and was mounted on a separate chassis that had its own filament transformer but took the 200 VDC B+ from the radio power supply.  In operation, there are two signal paths.  The first goes from the crystal phono cartridge through a 1 megohm resistor in parallel with a 330 pF capacitor to the selector switch which goes to a 500K volume control.  A second path goes from the phono cartridge to a 6AV6 amplifier.  The output of the amplifier is coupled through a 180 pF capacitor to one of the diode sections of the tube operating as a shunt detector.  This detected signal goes through a two-section R-C filter using identical 470 K and 5 nF devices to a 3.3 megohm resistor connected to the 6BA6 grid.  There is a 1000 pF capacitor from the grid to the selector switch and a 27 pF capacitor from the selector switch to the plate.  It is connected as a reactance amplifier so that as the rectified signal increases in value, the 6BA6 is driven further into cutoff.  This turns the 6BA6 into a variable capacitor with a high capacitance at low signal levels and a minimum capacitance at high signal levels.

In operation, the magic monitor reactance tube acts as a capacitor connected to the top of the volume control and fed by a high impedance source.  At low signal levels, it imposes a high capacitance across the volume control.  At high signal levels, the capacitance is reduced to a negligible value.  It acts as a voltage-controlled variable cutoff filter that turns down the treble as the signal gets lower.  This is a lot smoother than the obvious cutoff of a noise gate.

This can be updated to FET's and diodes to provide the same variable cutoff lowpass filter.  In fact, the circuit would be similar to some auto-wah circuits with changes in rectifier attack / decay time constants, which can be implemented as variable front-panel controls.  The second transistor in a wah circuit is also used as a variable capacitor, but in a bandpass rather than a lowpass filter.  This offers an alternative to specialized semiconductor devices that may not remain in production.

[MikeH]

Awesome! This stuff looks like exactly what I was looking for. Thanks a ton Mark!  Just from reading the datasheet on the SMM2166, I'm kinda struck by all the applications I could use it in. Not only as a guitar noise supressor, but I've also been thinking about making a Microphone Pre-amp for Vocals/Harmonica, and this would go with that perfectly. Well, now I gotta put yet another project on the pile of unfinished projects. lulz.

FYI - this chip/design works well for simple low-level noise from mild to medium gain OD or compression, but if you want something to clamp off high gain distortion feedback, you will most likely find it to be inadequate.  I think the noise reduction component is really mostly there to quell noise brought on by the compressor function.  That said, it makes a really great compressor.

[Astronaurt]

I've been scanning the usual online sites, but no one seems to have any more SMM2166's.  is there any chance you'd know anyone whos got a stock of 'em Mr. Hammer? 

[derevaun]

You might be able to score a couple surface mount specimens from AD's samples program.

[MikeH]

I've been scanning the usual online sites, but no one seems to have any more SMM2166's.  is there any chance you'd know anyone whos got a stock of 'em Mr. Hammer? 

They're kind of hard to come by.  You could try posting a "WTB" add in the for sale forum.

[Ben N]

http://www.smallbearelec.com/Detail.bok?no=234

$8.75

[Mark Hammer]

Like I say, pricey for a chip, but don't let the $8.75 scare you off.  You add another $6 worth of passive components, pots and knobs, and you have yourself one helluva compressor and noise-control system for under $15.  I think that's a pretty good deal.

[drhulsey]

I have searched here and Googled for a schem/layout for a SMM2166 based compressor without any luck.
Any sources that anyone knows of?

[Mark Hammer]

Matt Armstrong posted one here just a few months ago.  Perhaps he'll pop in and remind us of the link.

Alternatively, there is the Q&D Compressor at AMZ.

[derevaun]

I had this thread bookmarked: http://www.diystompboxes.com/smfforum/index.php?topic=87300.0

[Mike Burgundy]

Yup, great stuff for the money.
Attack mod is great.
My only (tiny) gripe with this chip is that it's somewhat temperature depandant - the settings change depending on ambient temperature. Our rehearsal studio varies from 5-10 degrees centigrade in winter (when we're just in and starting to warm up) to 35+ in high summer (when we're just in and play a bit before begging for a beer). I've noticed it needs some tweaking (mostly recovery gain - it gets louder when it's warmer) but aside from that, audio does not suffer at all. It's the only thing that's consistently been on my pedalboard for years now.

[drhulsey]

Thanks for the pointer!