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Reducing Fuzz Box Noise - Boss MT-2
Reducing Fuzz Box Noise - Boss MT-2
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Old 17th September 2019, 03:08 PM   #21
ITPhoenix is offline ITPhoenix  United States
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Frommel MT-2 kit: Boss MT-2 Metal Zone Mod Kit by Fromel Fromel Electronics

@Gnobuddy:
"The coupling cap isolates the instrument at DC - but the whole point of a coupling cap is that at the frequencies of interest, it couples the input signal through (because its reactance is infinite at DC, but falls as ~1/frequency .)"

I really should have been specific in my reference to noise. It is what I call "idle hiss", i.e., when there is no input signal, or the input is grounded. Once the guitar is playing, the hiss will be buried under the sheer amplitude of speaker output. I heard of some modders purposely adding carbon comp resistors to the second or later stages of tube amps to add a little character.

I will conduct experiments with a grounded input and the effects of different guitar settings probably tonight.

"Remember, people sign up at diyAudio with varying levels of expertise in electronics......So, it's a truism for you (good!) But not necessarily for everyone who posts here."

Thank you for that reminder. I'm a little stressed, being at the lower end of the spectrum myself.
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Old 17th September 2019, 05:37 PM   #22
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by ITPhoenix View Post
From the ad copy for the kit:
Quote:
Originally Posted by Fromel Electronics
...replaces all the cheap electrolytic and tantalum capacitors with high quality audio grade capacitors so your tone is unrestricted and full of power....

...Replace the cheap ceramic caps that sound shrill...
It sounds as though this kit does two things: one, it changes some of the cap values that affect the voicing and EQ controls. This might legitimately provide improved sound, depending on your personal tastes.

Two, it trots out the usual subjective stuff about the "sound" of electrolytic, tantalum (which is a type of electrolytic) and ceramic caps, and implies that these are all caused by these being "cheap", further implying that if you spend more, your caps will sound better.

None of these beliefs have been verified by a statistically significant number of double-blind listening tests under controlled conditions; more importantly, a simple objective experiment shows there are no audible differences.

The experiment: pass the same identical input signal through one ceramic and one "audiophile" cap of the same value; follow with identical resistors, fine-adjusted to produce exactly the same RC time constant and so, phase-shifts; use an instrumentation amp with a very good common mode rejection ratio to subtract one signal from the other, amplify the result, look for an output signal greater than the noise level of the signal.

If you can't find one, that means there is no measurable difference in the signal after passing through the two different types of capacitor. And that's what I found, decades ago, still in my teens, and armed with much better hearing than I have now.

Electric guitar is particularly tricky, because it IS all about subjective assessment - musicians want it to sound a certain way, not measure a certain way. But that leaves the musician vulnerable to charlatans and ignoramuses alike, the ones who believe (or profess to believe) that if you spend $10 on a capacitor instead of $1, it will invariably sound better.

It doesn't help that musicians almost have to be superstitious by the nature of what they do best; if the musician doesn't hear nonexistent angels and see nonexistent rainbows when she plays her instrument, her audience certainly won't see and hear those things!

When trying to make assessments of an electric-guitar circuit, I try to take off my musician hat, and put on my scientist / engineer hat as much as possible, so as to minimize my tendency to fool myself into believing in non-existent things. And then, when I play music for the enjoyment of myself or my friends, I try to take off the engineer hat, and put on the musician one, and try to conjure up those visions of angels and unicorns and rainbows as I play.


-Gnobuddy

Quote:
Originally Posted by ITPhoenix View Post
I really should have been specific in my reference to noise. It is what I call "idle hiss", i.e., when there is no input signal, or the input is grounded.
To be clear: the input coupling cap acts more or less as a short circuit at all frequencies of interest, when the input is shorted to ground, or connected to an audio source (including a guitar), whether or not that audio source is actually producing an active signal.

In other words, this means that as soon as you plug anything with a non-infinite source impedance into the input jack, that 1M JFET gate bias resistor is paralleled by the input source impedance at all audio frequencies of interest - and hiss is reduced as a result.
Quote:
Originally Posted by ITPhoenix View Post
I heard of some modders purposely adding carbon comp resistors to the second or later stages of tube amps to add a little character.
Put on the kettle instead, that way you can make a cup of tea to go with the hissing sound of the steam.

I think the preference for added low-level hiss is one of the major reasons why some people still prefer hissy vinyl records to silent digital audio.
Quote:
Originally Posted by ITPhoenix View Post
I'm a little stressed, being at the lower end of the spectrum myself.
If you work in IT (as your username hints), I can fully understand your being stressed. It's a tough job, and you probably spend much of your working life playing whack-a-mole with one problem after another after another, with no end in sight, ever.


-Gnobuddy
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Old 17th September 2019, 06:31 PM   #23
shanx is offline shanx  Canada
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I think modifying some of the stages that shape the frequency response both pre and post diode clipping could help with bringing down the noise floor. But that would change the character of the metal zone sound...maybe you prefer that sound the way it is..not sure.

There's a big mid freq bump before the distortion stage, and after the diodes a voltage divider with lowpass filtering. Following that there are two gyrators creating frequency response peaks. The higher frequency peak, to me looks like its boosting in a range where lots of the noise will be most noticeable/offensive. The main distortion stage with the diodes following is not much different than a MXR distortion + or similar..it's these big boosts in specific frequency ranges that give the metal zone sound, but likely contriuting to that high apparent noise floor.
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Old 17th September 2019, 06:46 PM   #24
voltwide is online now voltwide  Ireland
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Quote:
Originally Posted by Gnobuddy View Post
It's a very old idea. Nearly fifty years ago, in the early 1970s, Phillips designed and sold their "Dynamic Noise Limiter", a circuit that rolled off treble frequencies, but only when the input signal was below a certain strength. It was intended to reduce playback hiss from the company's audio Compact Cassettes, and it worked quite well. (Then Dolby B came along, and took over much of the market.)
-Gnobuddy
Exactly it was DNL I had in mind, and I did not suspect anyone would remember this ancient system. And like you, I built a DNL unit to reduce tape noise. It worked for me for some time. At the end I felt some treble missing so I had to deal with the noise. Luckily I do not need any magnetic tapes any more
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Old 17th September 2019, 09:18 PM   #25
PRR is offline PRR  United States
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Quote:
Originally Posted by ITPhoenix View Post
....With the guitar plugged in and not outputting anything, the filter cap is isolating the the instrument, so I'm having trouble seeing the parallel resistance in this case................
What filter cap?

C042, a 0.047u coupling cap? That's a real low impedance over most of the audio band. Under 10k impedance 400Hz up.
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Old 17th September 2019, 09:36 PM   #26
ITPhoenix is offline ITPhoenix  United States
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No substantial difference between a grounded input and the guitar, except when the vol pot is turned back from 10, there is a slight increase in hiss, as member PRR alluded. Only conductive plastic pot(s) will cure this.

It just dawned upon me the 1M is not only a signal raiser, but a ground reference as well. In my experience, resistors from ground do not present a noise problem. I was looking at as a voltage imposed across the resistor connected to in JFET gate, assuming it would be significant source of noise.

Excessive noise results by the confluence of the following: High Level, High Distortion, and High Treble on the pedal. High treble on the amp will also increase the noise (obviously).

At unity output gain and conservative Dist and EQ settings, the pedal is not overwhelmingly noisy, so nothing seems to be wrong with it. It might be outrageously noisy using a 100w amp connected to 2-4x12 cabinets, I would imagine.

I will try replacing the JFET with a super quiet opamp, and making the appropriate 2nd stage gain adjustments, and see what happens. Speaking of these adjustments, this pedal has way too much boost. For unity output gain, the Level knob must be around 1-1/2 to 2. I chose the OPA1611 for a variety of reasons and will start with unity gain. A report will follow.

I would like to thank everyone who responded with all those good suggestions and information. If someone would cast an opinion on the following question, this would be most appreciated:

I never tried elevating an opamp, so I'm wondering if I simply connect the 1M to the non-inverting input and nothing more. Or would it be better to set up a 1M/1M divider from 9v to ground? And is such case, would the resistor off the 9v create noise. I read a few design notes that this happens, and 1M foil or wire-wounds are almost impossible to find.
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Old 17th September 2019, 09:44 PM   #27
ITPhoenix is offline ITPhoenix  United States
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Quote:
Originally Posted by PRR View Post
What filter cap?

C042, a 0.047u coupling cap? That's a real low impedance over most of the audio band. Under 10k impedance 400Hz up.
Yes, that's the one, but what if there is no signal? Is it possible the cap is leaking a little like all do, and that is permitting the effective parallel?

I just posted something before your post came in a suggested since the 1M is a ground reference, it would have little to no effect on noise, with or without a signal present.
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Old 17th September 2019, 09:59 PM   #28
Guerilla is offline Guerilla  Denmark
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Sounds good voltwide
Cheers!
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Old 17th September 2019, 10:34 PM   #29
ITPhoenix is offline ITPhoenix  United States
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Quote:
Originally Posted by shanx View Post
I think modifying some of the stages that shape the frequency response both pre and post diode clipping could help with bringing down the noise floor. But that would change the character of the metal zone sound...maybe you prefer that sound the way it is..not sure.

There's a big mid freq bump before the distortion stage, and after the diodes a voltage divider with lowpass filtering. Following that there are two gyrators creating frequency response peaks. The higher frequency peak, to me looks like its boosting in a range where lots of the noise will be most noticeable/offensive. The main distortion stage with the diodes following is not much different than a MXR distortion + or similar..it's these big boosts in specific frequency ranges that give the metal zone sound, but likely contriuting to that high apparent noise floor.
I lack knowledge in the area of complex circuits, but I understand what you're saying. I will hold off on the Frommel kit for now. That kit replaces some or all of the diodes with MOSFETs or something. The MT-2 works fine after fiddling with the settings, and yields more options than the MXR.

Thanks for your insight.
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Old 18th September 2019, 02:12 AM   #30
PRR is offline PRR  United States
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> No substantial difference between a grounded input and the guitar, except when the vol pot is turned back from 10, there is a slight increase in hiss.... Only conductive plastic pot(s) will cure this.

Full-up, a guitar is 10k or so. With 250k Vol pot set half-up, the source impedance is 62k. This is enough to hear.

Conductive Plastic is not a miracle cure. ALL resistors above absolute zero temperature hiss. The baked carbon on standard pots is not especially bad relative to an "ideal" resistor.

> 1M foil or wire-wounds are almost impossible to find.

Foil/wire also not a miracle fix.

> voltage imposed across the resistor connected to in JFET gate, assuming it would be significant source of noise.

Gate and cap DC current is nearly zero.

> It might be outrageously noisy using a 100w amp connected to 2-4x12 cabinets, I would imagine.

"Outraged" is relative. The 100W full-stack would only be used in LARGE rooms for large crowds. (Or in small rooms for fans too crazed to care about a little/lot of hiss.)

> {C042, a 0.047u} Yes, that's the one, but what if there is no signal? Is it possible the cap is leaking a little like all do, and that is permitting the effective parallel?

If the cap is connected to a low-Z source (guitar or short) it will bypass the 1Meg quite well. Does not have to be a signal.

Yes, a cap can leak, erratically, but that is rare.

Half-supply for an opamp can be derived the SAME way Boss did it for the JFETs and also the opamps here. Medium-Z voltage divider and a BIG cap so the divider does not try to feed half the rail crap into the input. (In most of these stages, the crap through the 1Meg will again be bypassed by whatever source it comes from; but rail crap can be bad so you want a clean Vref.)
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