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.
@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.
From the ad copy for the kit:
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.Fromel Electronics said:
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
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.
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.
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
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.
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.
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
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.
Test Results
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.
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.
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.
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.
> 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.)
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.)
Thanks PRR. A few minutes ago it dawned upon me to look at the schematic for how the opamps were centered. I usually operate somewhere between Einstein and an imbecile, and very infrequently, at either end of the scale.
I have been in a small room with a 200w Marshall and double stacks, and as you said, nobody cared about hiss or much of anything else, except girls, loud rock music, and where the reefer was coming from....
Why are some of my posts double spacing?
I have been in a small room with a 200w Marshall and double stacks, and as you said, nobody cared about hiss or much of anything else, except girls, loud rock music, and where the reefer was coming from....
Why are some of my posts double spacing?
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I don't know why, but that has been happening to all my posts on diyAudio for some months now, after a forum software upgrade here.
I hit the "Preview Post" button so that I can see what my eventual post will look like, then I delete all the unwanted double spaces by hand, to clean it up before I post.
Sometimes I have to do this multiple times as the forum software appears to spastically re-introduces double spaced lines, apparently randomly.
It's quite annoying, but software bugs have been eroding our quality of life for decades now, so there's nothing new here. C'est la vie.
FWIW, I'm using Firefox on Linux, though I can't imagine why that should cause double-spaces to appear. (I've been Linux-only since 2001, and been on a dozen different forums since then, with no problems anywhere but here, and that only in the last few months.)
P.S. I had to preview, search out, and destroy unnecessary blank lines in this post six separate times to get rid of all of them.
-Gnobuddy
Agreed, yet there are many who love it. As usual, Tuchman's Law probably applies here. I found out before my report above, that the EQ knob "MID" also increases hiss, and additionally, increases volume on all strings. I'm guessing MID controls the harmonics in the upper ranges, but I'm not sure yet. There are articles and opinions in the forums discussing setting this box up properly--a "sweet spot" so to speak.
When compared with a box having only two controls--Level (Vol) and Dist--say "Jack and Jill", We now have not only them, but Moe, Larry, Curly, and Shemp as the EQ. This is one of the areas the Fromel kit purports to improve. Whether it does or simply moves the trash to another bag is yet to be realized by me.
An ABC comparison including the "Monte Allums Sustania" mod YouTube
I'm inclined to try replacing the first JFET with the opamp, with a slight gain, around 4 as suggested previously (not unity as erroneously stated), and reducing the second stage's gain to compensate. It is cheap, requires not too much "buzzard hunching" over the bench, and is easily reversible.
....I hit the "Preview Post" button so that I can see what my eventual post will look like, then I delete all the unwanted double spaces by hand, to clean it up before I post.
P.S. I had to preview, search out, and destroy unnecessary blank lines in this post six separate times to get rid of all of them.
-Gnobuddy
I'm on Firefox/Windows and it worked with preview correction with one shot. So far, Firefox is the common denominator. I will try Edge on my other machine some day.
I think I read about the Phillips DNL in an old electronics magazine article which I read in the early '80s. Some library research turned up a schematic, so I designed my own PCB on graph paper, drew it on copper-clad board with an old ball-point pen filled with household enamel paint, etched the PCB, drilled all the holes with my hand-drill, built the circuit, and used it to quiet down the cassette tape-deck I had also built.
I had the same problem, too.
So I designed my own circuit which did the opposite of the Phillips DNL - it boosted the treble, but only when there was already a strong signal, so you didn't notice the extra hiss! A crude treble-only expander of sorts.It worked, but my circuit design had flaws - sometimes you could hear the treble boost cutting in or out, and I never managed to engineer that problem completely out of existence. It didn't help that I didn't have an oscilloscope, or even a volt-ohm-meter at the time.
I am very thankful for that, too!Luckily I do not need any magnetic tapes any more
But old damaged cassettes were good for other things. The tape made nice streamers and tails for kites - it had lots of air drag, but low weight, exactly what you need to stabilize a kite without weighing it down!
And then there was the spider-ballooning experiment. Spiders can spin a loop of thread, and use it to magically float away into the sky on the lightest of breezes. They have been captured at high altitudes by research aircraft, so spiders may be the only wingless animals that intentionally and routinely manage to fly long distances at high altitudes.
At any rate, when I first heard about this, I wanted to replicate what the spiders do, but of course I couldn't make spider-silk. What other material could I use, that was extremely thin and light? Of course! So I tried to make a loop of cassette tape lift into the air and float away on the breeze.
It never worked, though - the tape-loop always fell to the ground. Something was missing, and it was only rather recently that I found out what: it turns out that the spiders wait until there is a thunderstorm nearby, which generates an intense electric field from the ground up into the sky. That's when the spiders do a hand-stand and spin their loop of silk - and it is actually electrostatic forces from the spider-silk interacting with the electric field that lifts the spiders into the air! Absolutely amazing!
It is thought that the spiders can sense the electric field because it pulls on the fine filaments covering their bodies. If you've ever moved your arm near an old CRT TV and felt the pull on your body-hair, you'll know exactly how this works.
So I don't miss audio cassettes for their intended function - but I am a little sad that other young children won't be able to come up with all the other fun uses for the tape that I discovered for myself when i was a boy.
-Gnobuddy
LOL! Nice turn of phrase there.
I also "buzzard hunch" over my acoustic guitar for a few hours a week. A terrible habit that has been hard to overcome, and one that has actually caused physical injury to more than a few guitar players. Upper back, shoulder, and neck pain and injuries are not that uncommon among acoustic guitar players. Electric guitars are less damaging, because the bodies are thinner and don't force you to round over and around the guitar so much.
(That said, the Les Paul copy I played last night felt like a bowling-ball around my neck. Geez, these are heavy guitars!)
Re the MT-2 tone control circuit, you could simulate it in LTSpice and find out for yourself exactly what the stock circuit, and the modded circuit, do. LTSpice is yet another inducement to "buzzard hunch" at your desk, and it can suck up a lot of hours of your time, but it is a very useful tool as well.
-Gnobuddy
DON'T use a bipolar such as the 49710---it will be noisier than need be! So will the TL071. Best bet is an FET-type---an OPA1642 or 1652 will do nicely!
Yes, more options for sure. MT-2 does what it does and just to compare that the basic distorter stage is about the same as an MXR Dist+. It's what goes on pre and post distortion that add EQ boosts on specific frequencies. The pre adds a mid bump, and the post is what's sometime referred to as a "suspension bridge EQ" two pass band peaks (Low and Hi) with a scooped out middle. I reckon that just lowering the gain on those pre/post stages would tame a lot of it, keeping the gyrators "as is" to retain the EQ shape.
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