Yet more noodling around with a Distortion+

[Mark Hammer]

I was looking for something in the parts bin and realized I had some old metal can LM741CE chips.  So I figured, what the heck, I don't have a Distortion+ in the stable at the moment; may as well put one together.  Having a piece of perfboard just the right size helped too.

The problem with the stock Dist+ is that turning up the gain removes the bass.  My own unsubstantiated belief is that, when the original units came out, hum from single coils was still very much an issue.  So by rolling off the bass as the gain was increased, you didn't amplify the hum inherent in single coils quite as much.  You ended up with a piercing distortion, but without quite as much of the boosted hum.

Obviously, hum would not be an issue with either humbuckers or any of the other low-hum or hum-rejecting designs of the present day.  The contemporary solution to the reduced bass at higher gain would be to exchange the .047uf cap adjacent to the gain pot for a higher value, like 100nf or even 220nf.

For this evening's build, I borrowed a trick from the Rat.  The Rat uses two parallel ground legs in a non-inverting op-amp: one that provides a sort of baseline boost across the spectrum, and another that adds much more boost above 1500hz.  As you increase gain, you add more boost for the mids and highs than for the bass.

What I did instead was to use one ground leg for a fixed full-spectrum gain, and another to provide the variable gain.

So, where you see the R3 / R2 / C3 path below, there are two paths.  One has a 100k/22nf network to ground, providing a constant gain of 11x, applied across the entire spectrum (well, a rolloff below 72hz).  The other parallel path is a 3k9 in series with a 100k pot and a 68nf cap, for a max gain of 257x with a bass rolloff around 600hz at max gain, compared to 720hz at max 213x gain in the stock unit.

Yes, I know the stock unit uses a reverse-log 1meg taper gain pot, but I had no intention of ever seeking something verging on clean from this, so a 100k pot was fine.  I also wanted juuussst a bit more sizzle fro the unit than a stock D+.  Diodes were some unspecified germanium type and volume pot was 100k for more output.

The dual-path arrangementallows for more "bite" at higher gains, without sacrificing the bass bedrock.  In other words, it sounds full rather than thin.  In retrospect, I think I might prefer the 100k/22nf path to be 150k/27nf for a slightly mellower tone at minimum gain.

I should also note that the 1meg feedback resistor (R4 below) has 82pf in parallel, and the clipping diodes have 3300pf in parallel (C5 below), to take out the fizz and hiss.

Pretty much turned out as I hoped for.  Feeding it into the dirty channel of my little Champion 110  sounded great.

[EBK]

Diodes were some unspecified germanium type . . . .

I have some of those exact same diodes laying around! 
And, you already know a Distortion+ is in my queue (http://www.diystompboxes.com/smfforum/index.php?topic=116757.msg1082459#msg1082459), so I'm totally going to follow your lead here. Thanks, Mark! 

[davepedals]

Thanks, Mark!  Me thinks I'll perf one up this weekend since I have the parts.

[EBK]

Any chance someone could quickly whip up a new schematic with all Mark's changes?

Also, Mark, did you stick with the reverse-log taper?

[Mark Hammer]

Also, Mark, did you stick with the reverse-log taper?

The reverse-log taper is a function of the original circuit that attempts to go from near-clean (veterans will confirm it never gets clean) to fairly dirty.  Keep in mind this was a design from a time when no one had a pedalboard and single pedals were expected to accomplish a lot with one or two knobs.  Really, all the action of note with a Dist+ occurs in the last 100k or so of pot resistance.  The need for a C taper is simply to get through that first 900k quickly.  Using a smaller-value gain pot foregos all the higher-resistance/low-gain settings and starts off dirty, with no pointless banal attempt to get semi-clean.  If I want clean boost, I have lots of other pedals for that.

The logic of the dual ground-leg is that when the ground leg is used for adjusting gain in a non-inverting configuration, the bass is rolled off at 6db/oct.  So, if max gain in a stock circuit rolls off bass at 6db/oct starting at 720hz, then 360hz is 6db lower, 180hz 6db below that, 90hz is 6db below that, and so on, down through to DC (obviously you won't hear that continued rolloff with guitar).  The dual ground-leg strategy creates a "floor", whereby the rolloff created by the adjustable ground leg stops when it reaches that floor.  So, while the adjustable ground-leg still loses bass as you increase gain (by reducing the resistance to ground for that leg), it doesn't lose quite as much, because of that "floor".  The fact that I used a slightly-higher-than-stock value for the cap (.068 rather than .047) also means that I lose some bass when increasing gain, just not so much that it sounds too thin.

[Cozybuilder]

Any chance someone could quickly whip up a new schematic with all Mark's changes?

How about with a volume boost & power protection & filtering?

[EBK]

Thanks, Russ!

[Gus]

Mark

I posted this in 2008 IIRC not the same as yours. Buffered input to a adjustable high pass to a dist + like circuit(you could use a big cap in the - to gnd leg for more full range) etc. to a gain of two this is to increase the level after the diode limiting. The adjustable high pass is what is a little different

[Mark Hammer]

That looks about right, Russ.  What's missing is the 1meg/1meg divider to provide the 4.5Vref.  But apart from that, spot on.  Thank you!

And, just a little note.  Your diode-selection arrangement is more complicated than it needs to be.  If you have two pairs of back-to-back diodes (of whatever type) in series, all that selecting between them needs to involve is bridging whichever pair you don't want.

So, let's say you have a Ge pair in series with an Si pair to ground.  Get yourself a 3-position on-off-on SPDT toggle and tie the common to the junction of the two pairs.  IN one direction you bridge the Ge pair, making the Si the functioning pair.  In the other direction, you bridge the Si pair, making the Ge the functioning pair.  And in the middle position, neither pair are bridged, making the clipping threshold set by the sum of the two pairs.

RE: Gus' circuit.  That's a smart idea, Gus - carve away from the bottom what you don't want/need.  I guess the only thing that needs mentioning is that the proximity to the clipping threshold will depend somewhat on how much bass is included in the signal, since more of the overall amplitude is found at the low end.  That's neither good or bad.  Just bear in mind that increasing the bass content will have an effect similar to increasing the gain.  Not exactly the same, since the harmonic generated will be different, but it will push the diodes harder.

[blackieNYC]

I really must restore my dist+ to its glory, after having done all possible mods. This looks great.
In Gus's buffer/filter, could the high pass be incorporated into the buffers fb circuit, and use the buffer as a booster to make up gain as bass is reduced?


 

[Mark Hammer]

I won't take anything away from Gus' ideas.  I'll just say that my own suggestions were based on helping out a circuit relying on a single 741.

[Gus]

I like Marks one 741 IC idea and build.

I posted the schematic as another idea.
The adjustable highpass filter does not move like Marks does when the gain is adjusted.
I also added a x2 gain stage after the diode clipping because people sometimes want more output level when using Ge diodes.

[Mark Hammer]

I like Marks one 741 IC idea and build.

I posted the schematic as another idea.
The adjustable highpass filter does not move like Marks does when the gain is adjusted.
I also added a x2 gain stage after the diode clipping because people sometimes want more output level when using Ge diodes.

Thanks, Gus.  I was actually thinking of adding another post-clip gain stage for precisely the reasons you mention.  But, I built the thing because I had a piece of perf that was just the right size for a single op-amp build, so.....   

[mac]

Mark,
Mine has a switch to choose caps instead of two paths. The stock 47n and 150n iirc.

mac

[Mark Hammer]

I've done that too, although you have to admit, it turns it into a somewhat different pedal.