back to back diode on signal path on BOSS HM-2 and AmpTweaker Tight Metal

[Agung Kurniawan]

Hi guys, its me again.





I have no idea about those diode on the signal path but on Tight Metal they work like a noise gate that shorting the circuit(?)
anyway, can you guys explain to me please

[dschwartz]

Yes, it is a crude noise gate. Signal below the diodes forward voltage will shut up..very effective to reduce the noise floor, at the cost of crossover distortion.

[Agung Kurniawan]

Yes, it is a crude noise gate. Signal below the diodes forward voltage will shut up..very effective to reduce the noise floor, at the cost of crossover distortion.

so if I using 4148 it will shut when the signal below 0.7v right?

[dschwartz]

Yes, it is a crude noise gate. Signal below the diodes forward voltage will shut up..very effective to reduce the noise floor, at the cost of crossover distortion.

so if I using 4148 it will shut when the signal below 0.7v right?

Yes

[Mark Hammer]

Yes, it is a crude noise gate. Signal below the diodes forward voltage will shut up..very effective to reduce the noise floor, at the cost of crossover distortion.

It's difficult to know whether the noise-control function was the intent, and the crossover distortion merely a perk, or whether the intent was to produce some crossover distortion, and the gating function was a perk.  I'm leaning toards the latter, though. Certainly one does not see such application of diodes in other high-gain pedals from Boss, even though they could have just as easily stuck them in to control hiss.  When I experimented with crossover distortion in the "Contrafrizz" ( https://www.youtube.com/watch?v=eQItWIof9mM ), I found it added some nice textures, especially on the bridge pickup (which is what I'd expect "metal" players to play mostly).  So my sense is that this application of the diodes was intended to create the sound of the HM-2.

[Quackzed]

It does make it pretty unique, it's a bit opposite in some ways to regular clipping as the 'crossover' sound tends to be more noticable as the note dies away, or when you play softer, and when you play loud its often hidden beneath normal peak distortion and isn't really very obvious.

[Mark Hammer]

In many respects, crossover distortion  of guitar notes is a wee bit like pulse-width modulation.  After all, the signal has to swing wider than the forward voltage of the back-to-back pair to simply be audible.  Pick softly, and more of the signal falls below that forward voltage.  Pick harder and more of the signal makes it through.  Combined with clipping to chop the tops off, crossover distortion chops the sloped sides of the wave off, yielding something more closely approximating a square wave.  In theory, at least.

[Quackzed]

yeah, definately. with a good strong signal, you get some very squared sounds, for me thats the good bit. the other side of the coin is when the signal is faidng out, you get more and more of the zero volt flat line with just the signal peaks making it over the diodes (what i tend to think of as crossover sounding) which can be very nasty and buzzy...
!!! you know you just inspired an idea  !!! 
i just read that sketch of yours with the envelope follower controlling a phaser /tremolo. What about envelope controlling some crossover diodes?!?
so they only cut out low signal stuff (square up the sides) with strong signals, via envelope follower, but are blended out for weak signals (the nasty buzzy crossover side of the coin)...
basically keep the pwm like side of 'em and blend out the ugly side of 'em... though i guess you'd lose alot of the noise gate perks...  is that crazy? that'd be a DYNAMIC circuit, from cleanish to VERY squared... you'd have some normal diodes to ground for the crossover diode side as well...

ok, sorry for derailing a bit there...    I can get carried away....

[Kennt82]

  I had the same questions when I was building the Tight Metal. The designer previously used the same gate on some Peavey amps. In researching it, I found it's called a "CORING CIRCUIT" and is used for low level noise reduction after noisy preamps. 

  A high gain modern metal pedal, of course has more than just a little noise. The one on the Tight Metal is adjustable. I jumpered that part on my HM-2, but there was no difference. Even though it's associated with extreme metal, it's not insanely high gain on it's own, IMO. 

   It may be worth it to create a layout with various diodes and pots to find the sweet spots on
individual noisy distortion circuits.  I even tried this on my unusably hissy Box of Metal build and it works, though not as smoothly as I'd like. The Tight Metal gate works very well with the Triple Wreck. Experiment with placement.

And some light reading from 10 years ago (reply#4):
http://www.diystompboxes.com/smfforum/index.php?topic=54854.0

[Mark Hammer]

And that's why I suspect the series diode pair was a tonal choice, rather than a noise-control choice, by Boss, when it comes to the HM-2.  They have no shortage of distortion and overdrive pedals designed prior to and after the HM-2, where they adopt many of the same strategies for a number of things.  And many of those pedals have more than enough gain to be concerned about potential hiss.  But the HM-2 is the only one I'm aware of using the series diodes.  Of course, just because Boss may have used series diodes for tone, rather than noise-control, does not mean other companies did not use series diodes for noise-control, as opposed to tone.