How much gain is enough??

[GibsonGM]

I've been thinking about boosting the input of my MXR Dist+ with an opamp circuit.  A good idea handed me in here was to use an opamp ckt from the TS in front to drive the 741 in the MXR.  I'm designing something now, and trying to dial in a range of boost.  Not knowing much about how dB and voltage translate into actual sound, does anyone have any suggestion as to what dB level is good for a booster?

I drive the thing sometimes now with a Big Muff set clean, and turn up its level....not great considering the midscoop but fine for testing the concept.  Measuring AC voltages, I get about 200mV output from the MXR when picking low E;  adding the Muff makes that about 500mV.  This is a 2.5 voltage gain, right? = 8dB?  I'm thinking that allowing for a 10dB boost should then be sufficient.  I need to know this so I can trim the filtering section of the TS booster and get it into the right range.
Another thing is that in bypass, the TS booster puts out a 2.5 dB boost (bypassing the 500k pot in feedback loop).  Gotta fix that, lol

Thanks!   

[Mark Hammer]

Keep in mind that boost/gain is mutiplicative.  So, an add-on front end with a gain of 10, followed by a stock Dist+ gain stage set to x30, means the clipping diodes are confronting a signal with a gain of x300.  I might remind you that they normally would encounter a maximum gain of x213. So, a front-end boost of x10, followed by the normal circuit dimed, is a combined gain of x2130.

Bottom line is that it won't take all that much boost on a front end to achieve a wide array of levels and tones.  Gain of 10 ought to be more than enough.  Assuming that your Dist+ circuit is stock, then you not only get the broader array of levels, but if you can get comparable (to the stock Dist+) combined gains without maxing the Dist+ gain control and losing the bottom end, then you have the option of both "thicker" (add-on plus lower gain Dist+ setting) and "thinner" (add-on reduced and Dist+ maxed) sounds.

Certainly one possible mechanism to try is to use a single pot as the ground leg resistor for the two gain sections.  Right now the standard Dist+ has the fixed resistor and gain pot placed between the inverting pin and the .047uf cap to ground.  Flip the cap around so that it goes between the pot and inverting pin, and your gain pot now goes directly to ground.  The advantage here is that the wiper can serve as the ground connection for the normal clipping stage, and the other leg can serve the same function  for the (non-inverting) add-on stage. 

Keep in mind that you are not obliged to use the full pot value for adjusting gain in each stage.  For instance, were one to use a 1meg pot for doing this, would every 10k resistance drop in the ground leg of the clipping stage HAVE to result in 10k of resistance increase on the ground leg of the add-on stage?  Not at all.  You could easily stick a resistor in parallel with that ground leg so that the range of resistance adjustment is much less for that stage.

[sfr]

My Dist+ (with something different diodes in there, but I don't remember what; otherwise I built it "stock".) sounds amazingly good with the front end driven by an LPB.  I believe the LPB is "stock", although considering it's one of my very first builds, there's the chance there are some wrong/different cap values in there.  The max gain on the LPB sounds like just a *wee bit* too much gain.  (So ideally, you should want twice as much )  It's nice that the clipping diodes keep the output pretty constant whether I turn on the front-end boost or not.

[GibsonGM]

That all sounds pretty groovy, guys!  Maybe I'll look at the LPB instead.  I got closer to what I want by cranking the front end with my Big Muff, just that the tone bites by definition.

Mark - sounds like i don't need to worry so much about filtering the boost stage; the Dist+ should go a little trebly when hit with a higher input, naturally?  I'm familiar with how it loses bass as gain increases.  Seems like the place to start is pick a booster, breadboard it, and just go by ear...

[WGTP]

If you use a dual op amp, you can easily build a 10X stage in front of the normal stage.  Or, you can go overboard and build 2 of them and run on into the other. 

[sfr]

Yeah, what about using the Tonepad layout for two microamps, adding the Dist+ clipping diodes to the second one, and chaining them? 

[R.G.]

How much gain is enough???

I've talked to enough guitar players to know that it's enough when
(a) the windows blow out
(b) the shingles on the roof rattle
(c) the speakers catch fire

and

(d) the output tubes melt.

That's finally enough.

[Mark Hammer]

Well yeah, maybe for a blues player, but what about someone who plays metal?

[R.G.]

...um, when the stellar primary of the planetary system undergoes gravitational collapse and blows 85% of its mass off into space in the form of gamma rays, X rays and other radiation and a shell of relativistic plasma?

But only when you really dig in with the pick.

[burnt fingers]

A bigmuff driving a Boss Metal zone with a Shaka to kick in for a solo boost ought to be enough. 

Seriously though, I like the direction you're going here GibsonGM.  I just finished a D+ and have had thoughts of somehow goosing it a little.

I tried a mosfet boost which is ok but it makes the low end a little overbearing.  Maybe a treble booster withe the caps tuened just right.  That would be small enough to stick on a board with the stock circiut.

Scott

[Mark Hammer]

Some of you folks might be pleasantly surprised at what simply dropping the 4k7 resistor going to ground down to 3k9 or 3k3 will do for you.  With 3k3, that's a difference between a gain of 213 vs 304.  Be sure to bump the .047uf cap up to at least .1 or .15.

Note that there is a difference between a pre and post-gain recovery stage.  The clipping diodes will set the maximum output level of the circuit.  So, if your tastes run to germanium diodes, all the pre-gain boosting in the world won't do anything for maximum output level although it WILL add more potential sizzle to what you ARE able to get out of the pedal.  Conversely, a post-clip gain recovery will not add anything in terms of the clipping derived from the pedal itself, though it WILL deliver a much hotter output version of whatever clipping you've produced.  And that, in turn, can push subsequent stages (e.g., your amp) into delivering whatever clipping they do.

In some respects, maybe I was premature in my suggestion of a where-do-you-want-me-to-stick-this-gain control.  Perhaps the place to put it is not as a control to alter the relative gain of a pre-clip boost section and the actual clipping section.  Perhaps the ideal thing is to have a 3-stage circuit, with 3 noninverting stages, and a dual-ground-leg wiring of a gain pot that controls the relative gain of a pre-clip booster vs a post-clip gain recovery stage.  That'd actually be a nice well-rounded circuit if you tack on a tone control of some sort.  Four controls: Fuzz drive, pre-vs-post gain shift, tone, and volume.  Note, as well, that this tyope of circuit would do a nice job compensating for the differential output levels of various diode types.  Heck, you could probably even have Schottky diodes in there and get a decent output level.

[GibsonGM]

Actually, what I have in mind guys, is this:  I love that AC/DCish, Blue Oyster Cult type fat rhythm sound my Les Paul is capable of thru the MXR...but when soloing, I like a bit more treble and scream sound.  Rather than do a 2nd pedal, have to switch 1 off and 1 on, I thought I'd run a boost into the MXR.  This has the added benefit of changing its clipping characteristics, like Mark talks about (makes it trebly & LOUDER!).  I had ideas of just bypassing the gain pot, using a 2nd one to increase it, but really liked what was happening when I put my Big Muff out in front, just didn't care for the midscoop.

Slacker suggested I try a TS-type opamp ckt in front, so I could use a DPDT to switch it out...run the boost in SERIES with the MXR, and have the boost return to unity via the switch (therefore almost no popping/shock to the system so to speak).  My only issue now is to get the thing to go to unity (while maintaining the freq response that seems right) by bypassing a fixed resistor where the 500KA pot currently is in the ckt.  I now see that I probably don't have to worry TOO much about cutting bass when I boost the MXR, since it probably will get lost anyway!  Simpler design issue.

This thread was started to see about how much gain I need to throw at the MXR to get a noticeable increase in volume for leads, but also how that'll relate to the treble increase.  I do plan on having a pot to control the amt of boost, lol.    I'm liking what I'm hearing here!   

[Mark Hammer]

Note that with the stock Dist+, as you increase gain and get that low-end rolloff (below 720hz at max gain), the low end chunk from the guitar has less to say about how fuzzy the tone is.  In a sense, even though one has maxed out the gain, it still isn't really as fuzzy as it could be if the ground leg cap were a larger value and kept all that bass in there to push the diodes harder.

But that circuit packs ALL the gain into the one stage, such that your choice is either to live with the correlation between gain and bandwidth, or change the cap and have a different relationship that precludes the classic thin shriek the unit can deliver.  With gain being multiplicative, and a pre-boost added, one has the option to hit the diodes every bit as hard as before (even harder, actually), but do so with a wider bandwidth than a stock Dist+ has.

Case in point:

Let's stick a simple x5 boost ahead of the main clipping stage.  To get something akin to the standard x213 gain (at max) normally found in a Dist+, one would only need a gain of x53 in the second stage, or a ground leg resistance of around 19k.  With a ground leg resistance of 19k and a stock .047uf cap, the low end rolloff begins around 178hz.  Big difference for a guitar.  Not only do you get the low end girth, but the diodes are actually pushed a little more at that gain largely BECAUSE the low end is retained.  So, by incorporating a variable front-end boost with the standard yoking between gain and rolloff in the stick Dist+ clipping stage, you can achieve varying kinds of "thickness". 

These days, I tend to like the 1N4148 diodes in mine, but that is largely because of the volume boost they give.  With a post-clip recovery stage, one has the freedom to use germanium diodes and still get a decent output level so you can squeeze more crunch from the amp itself.

How could one produce such a 3-stage version easily?  One way might be to use a 741 for the stick circuit, and a dual op-amp for the before and after.  Another way might be to use a simple FET booster for the front end (the wiper-to-ground dual gain pot could be used with the AMZ MosFet booster and an op-amp stage), and a dual op-amp for the 2nd and 3rd stages.  Alternatively, one could usethe same discrete booster for the front and rear ends and a single op-amp for the Dist+ part.

[WGTP]

When it starts to squeal/howl uncontrollably.  I wondered when you guys were going to get to that "Never Enough" stuff.

If the Mosfet boost was too bassy, reduce the size of the input cap.

Check out the AMZ Overdrive Pro.  The first stage is from a BMP.  Use the shunt to ground R/C network from the Rat (may need to reduce 1M resistor though).

I don't see this mentioned much, but using an op amp with rail to rail capabilities seem to make things louder and provide more signal for the diodes to clip.    

[RaceDriver205]

...um, when the stellar primary of the planetary system undergoes gravitational collapse and blows 85% of its mass off into space in the form of gamma rays, X rays and other radiation and a shell of relativistic plasma?

You call THAT gain?