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Reducing Fuzz Box Noise - Boss MT-2
Reducing Fuzz Box Noise - Boss MT-2
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Old 16th September 2019, 02:14 AM   #1
ITPhoenix is offline ITPhoenix  United States
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Default Reducing Fuzz Box Noise - Boss MT-2

Hello everybody. This particular box is somewhat noisy in its stock form. My intuition says a large part is coming from the first stage. I see a 1M elevating resistor before the JFET gate. Would this add significant noise to the circuit, even though it is not directly in the signal path?
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Old 16th September 2019, 03:51 AM   #2
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by ITPhoenix View Post
My intuition says a large part is coming from the first stage.
Fuzz-boxes have a lot of gain in them, so it's not entirely unexpected that they might hiss a lot. This one is the infamous "metal zone", i.e., it is designed to have an entirely ridiculous excess of voltage gain. Hiss is almost inevitable.

In a well-designed audio amp, the first stage invariably generates most of the noise, simply because the first stage has voltage gain, and so the signal level is much bigger from that stage onwards. That means the stronger signal swamps out noise from subsequent stages.

Unfortunately, this circuit violates that design principle - the input stage is a JFET source-follower, with a voltage gain of slightly less than unity (1). So this stage adds noise, without making the signal any bigger - a bad idea, particularly when followed by a ton of gain from subsequent stages!

Guitars need an input impedance of around 1 megohm from the circuit they're plugged into. This was not available with the bipolar transistor op-amp used in the MT-2. The poor design with the unity-gain input stage was most likely driven by the unavailability of a low-noise, FET-input op-amp when this pedal was designed. Instead, they used a JFET to get the required input impedance, but made the bad decision to set it up for unity voltage gain.

Speaking of op-amps, what sort of op-amp is in the circuit? Is it a 4558? That's not what the schematic shows, but online, I found other owners of the product who found 4558 op-amps in the pedal. (Edit: apparently it's a version of the op-amp with 8 pins in a single row, which is going to make op-amps swaps almost impossible; you'll need some sort of adaptor board to go from a standard 8-pin DIL to an 8-pin SIL layout.)

The 4558 dates from the dawn of audio-op amps, and it's noisier than some later op-amps. You might get a slight improvement in noise by swapping to more modern op-amps instead. But de-soldering and removing op-amps from a small PCB with narrow, delicate copper tracks is a quite a delicate operation, and might kill your pedal stone dead if anything goes wrong during the process. (And any audible reduction in noise will be slight at best.)
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Originally Posted by ITPhoenix View Post
I see a 1M elevating resistor before the JFET gate. Would this add significant noise to the circuit, even though it is not directly in the signal path?
That resistor will add noise when the input is open-circuit. But as soon as you connect a signal source to the input, the source impedance of that signal appears in parallel with the 1M, lowering it. If there is another stomp-box in the chain before the fuzz box, the impedance might be lowered a lot (but the noise of that preceding stomp-box is now likely to dominate.)

What happens to the noise level if you insert a 1/4" mono plug wired to short-circuit the input? This will bypass all noise from that 1M resistor, so what remains will be noise from the input JFET and the subsequent op-amps.

Rather than fiddle with the delicate innards of the MT-2, I would try using a very quiet (low noise) clean gain pedal between guitar and MT2. If you can get a signal gain of even a few dB ahead of the MT2, from a more modern pedal with better noise performance of its own, that should cut down the noise from the MT-2 itself (you can turn the output level down more, cutting the noise with it.)

But I have a suspicion you're always going to have to live with noise when you use a metal distortion pedal - it's the nature of the beast. Which is why noise-gates were invented, and are invariably used by guitarists who play metal and other genres that use extremely high-gain signal chains.


-Gnobuddy

Last edited by Gnobuddy; 16th September 2019 at 03:56 AM. Reason: Original op-amp has single in-line layout!
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Old 16th September 2019, 04:53 AM   #3
ITPhoenix is offline ITPhoenix  United States
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Good stuff to think about.

"What happens to the noise level if you insert a 1/4" mono plug wired to short-circuit the input? This will bypass all noise from that 1M resistor, so what remains will be noise from the input JFET and the subsequent op-amps."

This I will try. There is a slight increase in hiss when I plug the guitar in. I don't remember which volume setting increased it, though. I do know it has crappy, carbon pots in it.

The opamps are NJM4558L and I am restricting any noise work to the first stage.

Ultra low-noise JFETs are almost impossible to find in small quantities, like the 2SK184. I found the LSK389 (a dual) on eBay for reasonable price.

I agree with the noise gate idea, but I would rather do without it, and I will not be using all the gain available from this particular box. The Jensen C10Q speaker is not high gain friendly so this is adding to the problem, although they sound very sweet without the fuzz box.

Fromel Electronics makes a kit that improves the MT-2 in several ways and is cheap. It consists of film caps in lieu of the ceramics, MOSFETs to replace the clipping diodes, and EQ mods that make the unwieldy unit more managable. They do not mention noise reduction, but if I can get a good sound without too much drive, it may ameliorate the hiss due to the lower settings.

Since the JFET is nothing but a buffer, your idea to replace that with an opamp sounds like the best idea. There are some out there with a noise figure that of a 1k resistor. I would replace the 10k input resistor with a metal foil type, and add a 800-1k just before the opamp input.

Thanks again for the good ideas.
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Old 16th September 2019, 06:54 AM   #4
voltwide is offline voltwide  Ireland
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Low noise guitar pu amplification requires low noise CURRENT input devices. So you may forget any op-amp with bipolar input that offer low VOLTAGE noise but high noise current. I fiddled around extensively with these things so I can say the best bet here is a TL071 op-amp with a gain of 3-5V/V. There is no further room of improvement using discrete low noise JFETs.

@Gnobuddy: I know and accept your bias against hard rock guitars. And I do not accept noisy stuff as well. So I built a "hot" pre-amp with excessive gain - and extreme low noise. Maybe later on I will deliver some sound sample...
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Old 16th September 2019, 03:14 PM   #5
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by voltwide View Post
Low noise guitar pu amplification requires low noise CURRENT input devices. So you may forget any op-amp with bipolar input that offer low VOLTAGE noise but high noise current.
Agreed. The venerable 4558 actually isn't bad when it comes to voltage noise! Unfortunately that doesn't help - for the input stage, we need an op-amp with a good noise figure when the source impedance is tens to hundreds of kilo-ohms, because of the nature of the electric guitar.
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@Gnobuddy: I know and accept your bias against hard rock guitars.
I guess you could say that as soon as you develop preferences for any type of music, you have a bias against the types you don't like as much.

In my case, I enjoy the sound of chords with some colour in them - major sevenths, major sixths, suspended seconds and fourths, and so on. These simply don't work with heavily distorted amps, and the two-note "power chords" that are the staple of more distorted genres don't hold my interest for long.
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So I built a "hot" pre-amp with excessive gain - and extreme low noise. Maybe later on I will deliver some sound sample...
I look forward to it!


-Gnobuddy
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Old 16th September 2019, 03:51 PM   #6
voltwide is offline voltwide  Ireland
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I agree that heavy distortion works best with single notes, or 2-note power chords, more notes end up in mudd.
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Old 16th September 2019, 04:17 PM   #7
ITPhoenix is offline ITPhoenix  United States
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Default Opamp selection

How about the LME49710? Does anyone think this source is a good risk?
2pcs LME49710HA ADAPTER TO-99 | eBay
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File Type: pdf lme49710 .pdf (1.31 MB, 1 views)
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Old 16th September 2019, 06:55 PM   #8
voltwide is offline voltwide  Ireland
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The setup while recording the sound clip was this one:
Download link:

Dropbox - crossroads_TASCAM_0110.mp3 - Simplify your life
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Last edited by voltwide; 16th September 2019 at 07:04 PM.
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Old 16th September 2019, 08:04 PM   #9
ITPhoenix is offline ITPhoenix  United States
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Default Sound Clip Evaluation

Well, it sounds dead quiet. You recommended the TL071 op-amp with a gain of 3-5V/V. This is too much boost to place as the first stage of the MT-2; it will have to be attenuated to unity gain. Is it necessary to have this gain to keep the noise low?

Oh, skip the question about the Chinese TO-99 supplier (bad idea). If I go with the LME49710, the SOIC package will be mounted on a PDIP adapter. This is small and makes attaching the wires easy.
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Old 16th September 2019, 10:32 PM   #10
voltwide is offline voltwide  Ireland
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In case unity gain is your limit you have to go for it. In that case you can leave the JFET source follower, no point to replace it.

Some voltage amplification in the input stage normally helps reducing noise contribution of the following stages.
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