New at runoffgroove.com: AZABACHE

[stm]

Hi guys,

We are proud to announce a major revisit of the Professor Tweed which we've named AZABACHE.

Project information here: http://runoffgroove.com/azabache.html

Enjoy!

[azrael]

Cool! I'm always stoked for a new ROG release.

gate clippers, a la the Ginger, again?
I appreciate the idea of staying away from JFET hard clipping, but doesn't this change the tone in a different way? Or is it designed where each stage is supposed to clip it to those levels, and that's how you want it to sound?

[stm]

Azrael, the clipping voltage of the diodes happen to be slightly less than the voltage required to make a J201 clip in the common source configuration without source bypass capacitor.  This means the three J201 stages amplify but are not really overdriven.  It's the diodes who actually produce the overdrive.  Nevertheless each J201 stage does introduce a nice amount of 2nd harmonic, so the overall sound is rich in both odd and even harmonics.  As the gain is turned up, one, two or the three J201 stages reach the point where the diodes are overdriven, so the harmonic content increases gradually as the signal gets hotter.  Having said that, the clipping characteristic of the 1N4148 or 1N914 diodes is softer than a JFET that hits the supply rails, so the usual nasty fizziness is avoided.  A key aspect of the sound in this circuit lies in distributing gain and clipping among several low-gain stages, each with some high-frequency filtering, instead of having a single high-gain clipping stage. This arrangement proved to cleanup very well when the guitar volume knob is turned down.

[Quackzed]

i like that approach as it is much more consistent, as the diodes also prevent anything above their conduction threshold from hitting the next stage.
you can get some gain variation from the fets but your still hitting the diodes either way...

[azrael]

Yeah, there's a little bit of gain variation due to possibly different drain resistances, but it should still be pretty close to what they're aiming for, I would think.

stm: great explanation. Have you guys ever thought about writing an article on different EQ circuits? You guys do an meticulous job of creating useful controls, and I've liked some of the older active EQs you have posted on here before.

[lopsided]

nice, I am interested!
have drawn myself a quick vero layout today, but it will take me some time to actual test it.

[Toney]

Thanks Runofgroove!
Always a pleasure to see a new design or revision from you guys.

[Mark Hammer]

1)  Thanks for the new project.

2) I see that, apart from the 220N "bright" cap, there are no other source-to-ground caps anywere else along the way.  Just a bunch of 1k5 resistors.  What about sticking a 22-47uf cap to ground from one or more source pins with a small-value resistor (e.g., 470R) for just a bit more oomph?  Depending on the speaker characteristics, perhaps increasing the 150pf cap to ground to 220pf or even 270pf might be called for, since the extra boost will extract more grit from the diodes.

[B Tremblay]

I see that, apart from the 220N "bright" cap, there are no other source-to-ground caps anywere else along the way.  Just a bunch of 1k5 resistors.  What about sticking a 22-47uf cap to ground from one or more source pins with a small-value resistor (e.g., 470R) for just a bit more oomph?  Depending on the speaker characteristics, perhaps increasing the 150pf cap to ground to 220pf or even 270pf might be called for, since the extra boost will extract more grit from the diodes.

During development, we had only two gain stages after the tonestack and they did have source bypass caps.  We found that this produced clipping we disliked and to make up for the gain that was lost when those caps were removed, a third moderate gain stage was added.

EDIT:  Removal of the source bypass caps was also prompted by our conclusion that the gain felt a bit too high for this project's target sound.

[Mark Hammer]

Fair enough.  Not like five JFETs with 1k5 resistors and 4 pair of diodes (though I suspect that first pair of LEDs is largely for "signal outliers", rather than anything one would normally run into) is destined to deliver clean tone anyway!  

What does it sound like when those diode pairs are lifted?  Might make a useful mod to use a DPDT toggle to lift two pair.  Or does that mess up something else I didn't realize you had already anticipated?

[B Tremblay]

Without the diode pair, hard clipping ensues and there is an undesirable fizziness to note decay.

[stm]

Thanks for your comments Mark.

Yes, the LEDs in the first stage are indeed for "signal outliers".  For instance, when I use the bridge humbucker pickup in my guitar and play hard the peaks reach easily 5 to 6 Vpp, so the LEDs do actually make some difference by avoiding the 2N5457 getting into hard clipping.  Also, the LEDs make the circuit "booster friendly".

As for the gain, I believe this pedal as presented is even gainier than the real deal (a stock amp of the types used as an inspiration for the target sound), however this fact is not fairly represented by the sound samples included with the project.  If you want harder clipping you may try lifting the diodes in one or two stages, however as Brian mentioned, low to medium overdrive tones will suffer, as well as guitar volume cleanup, which are two characteristics where this pedal outstands from the rest (in my personal opinion).

[Mark Hammer]

Interesting, and useful, info.  Also shows how much thought was put into this.  Thanks.

Would reduction of gain in any of the JFET stages be more helpful in constraining peaks than the various diode pairs?  For example, a 2k pot in series with the 1K5 resistor?

[Ben N]

I always liked the idea of having one tweedish "amp sim" overdrive on board to just leave on and control with guitar vol, along with a a more middy, TS-ish one for boost & drive, especially when using a larger amp. I have used a TSF, a DOD FX-50 and a OCD-ish circuit to cover the first requirement, but this looks like it may be an improvement on all of them. Brian, Sebastian, you guys never disappoint.

[joegagan]

nice work! congrats on another detailed and well thought out project.

[GGBB]

Pardon my ignorance on these matters (understanding design from schematics), but is this strictly an overdrive pedal or is it a pre-amp?  I've been considering my options for going amp-less lately but without going the modelling route as I want to keep my pedals (my sound!).  I plan on using a red box for cab sim, and was looking at the sansamp stuff for the pre-amp.  Does this fit the bill?

Edit:  I also see that the bright and scoop controls could easily be replaced by 1M pots.  Would that be an effective mod (in other words would pots give enough range of variation to be more useful than just on-off switches).  If so, would linear taper work fine?

[Mark Hammer]

As something intended to come close to capturing the sound of tweed amps, it's both!  Remember, those ones go up to 12.   

Most of the various FETs-for-tubes emulators are intended to nail some of the character of various amps when they overdrive.  At least the front end.  Missing is the tonal impact of the power stage, and whatever speakers come with the "real thing", and in some instances that can be the key to what a particular player likes about that amp, or a critical aspect of its signature sound.  That's no criticism whatsoever.  A 59 Les Paul body with Chinese ceramic mag pickups won't sound like a 59 LP, and a 100W Marshall head played thruogh an array of 16 six-inch speakers won't sound like a Marshall,  either.

If your goal is to provide a cleanish boost, with that many cascaded stages, and a 9V supply voltage, you may have to run the Gain pot on the low side to achieve pristine tones; which may be well short of what's needed to drive a power amp (though again, I defer to Ben and Sebastian when it comes to what sorts of clean output level this thing is capable of).

Whether something like this could provide the degree of control that a full-fledged pre-amp might would depend on your needs as a player.  I often play through a tweed Princeton that has a volume and tone control and nothing more, so as simple as this design's set of controls are, it's still a big step up from there.  If you need more control than that, however, you may lean towards one of the other ROG designs that include T/M/B tonestacks.

[GGBB]

Thanks Mark.  Sounds like this won't quite do it.  I am looking for primarily clean sounds, but more than just boost.  Essentially I want to have just pedals and cab sim going directly to a mixing board, so I need something to give me the amp tone.  I won't need to drive a power amp+cab, and I don't play with the tone controls once I dial in what I like, but I do switch between clean and lead/overdrive channels although I could easily lose the lead/overdrive.  I have a typical setup right now - guitar->pedals->combo amp with more pedals in fx loop->mic->PA - but I'd like to try dropping the amp since we (church) now use one of those headphone monitoring systems where I can control my own mix.  I don't like the headphones, but if I want to hear the keys and vocals I have to wear them, which muffles the sound from my amp anyway.  Plus, it's just a 30W 1x12 tube combo, so why does it have to be so heavy!!!

[Ben N]

As Mark said, you should take a look at all the offerings at runoffgroove--you may indeed find something that fits the bill, especially if you pair it with a cab sim (like their Condor), even if this one does not meet your needs.

[Mark Hammer]

A few points should be added.

One of the great things about the ROG circuits is that they generally don't rely on diodes to produce clipping.  Most of them can be adapted to higher supply voltages for greater headroom.  You just need to retweak the trimpots to "center" the signal appropriately.

I think it is also important to note that while a great many vintage amps (that the ROG circuits try to capture the vibe of) are known and sought out for the overdrive tones, with few exceptions they WERE able to operate cleanly for much of their range.  And heck, even the stiff upper lip Silverface Twin would bark a little if you antagonized it, so expect a little clipping with anything you build.  The trick will be to have that bark come in at the point where you want it to, rather than too soon.

And since the topic has been raised, do any of you thousands out there have anything to say about running ROG circuits offer supply voltages higher than 9V?  I know that some of the Shaka Braddah versions from Aron and Doug were run as high as 33V.  What happens to an English Channel or Matchbox at 18V or 27V?