Distortion + limited range on distortion pot

[jeff g]

Just finished the GGG Distortion plus build, with some debugging.

It works, but it sounds like s**t - really thin and harsh and the distortion pot has really limited range - it's only on for the last bit of the rotation (maybe 10%)

I still haven't got IC voltages to match the specs, even with many rechecks of continuity and solder joints bridges etc.

IC voltages are

Pin 1 = 0.18
Pin 2 = 4.11
Pin 3 = 1.74
Pin 4 = 0
Pin 5 = 0.18
Pin 6 = 4.45
Pin 7 = 9.00
Pin 8 = 0


Any suggestions ? Maybe substituting some part values from the DOD250?

thanks

[billings]

My Distortion+ has the same thing going on.  Probably worse;  I couldn't get an rlog pot easy, so I tried to fake it with a parallel resistor trick (as described in the geofex pots article)...  well, that trick only works if you're rlog tapering up to max resistance, not rlog tapering down to min resistance (which is what the Distortion+/250 require). 

So the right thing to do is apparently to get an rlog pot and wire the ground to middle and the op amp loop cap to the dime lug.  Alternatively, you can take an audio taper pot and wire ground to middle and the the cap to zero lug;  that will put the max volume on the left, though.  Or you could also wire a smaller value linear pot like the rlog pot, sacrificing some of the clean range.

Of course you may already have that taken care of, but I'm certainly not going to delete all that text since it's there already.

If somebody else posts something on this and has a specific idea of what might be wrong here, listen to them first!  They probably have more experience than I do.  Given that, though...

The non-inverting pin (3) is really low; according to the circuit it should be hooked up to the bias network formed by C1 and R2-3, which should be keeping it reasonably close to the reference voltage.  Maybe something like this would work:

1) Double check that the op amp is in there the right way.

2) Check and see if the bias network is correct.  Since the two one meg resistors evenly divide +9V and ground, the voltage at the point between them should be right around +4.5V.

3) If it is, double check the resistance over the 1M resistor between the network and the positive 741 input.  Since there is nothing of note in parallel with that resistor, it should be 1M.

4) If that's right, too, check and see if there is a voltage drop across R5.  Since it's hooked up to a capacitor, there should be no current across it, and thus 0V.

If all of the above are correct, then either something else is wired to pin 3 or something is wrong with the op amp.

[burnt fingers]

I'm not sure about the thin sounding thing.  You could probably change the input cap to fix that.  Try .022uf,.047uf or even .1uf.

What kind of pot are you using for the gain?  On this and the 250 they used rev log to keep the gain from bunching up at the end.  If you're just using an audo taper you will get a very limited range.  A linear will help a little more.  

Double check your componant values too.  I've put a 22k where a 2k2 should go and a 1k where a 10k should go.  it's easy to miss when your soldering it up.

There are some mods for these things floating around.  If you search the forum you'll find them Im sure.  I hope you get this working to your liking.  I have a 250 and I dig it and I have a D+ laid out on perf just waiting for some time to solder it up.  

Scott

[jeff g]

Thanks for the replies

I guess I should have been clearer - I installed what I thought was a Rev Log pot for gain (It's what I had ordered), but I'm not sure that it actually is. 

It's marked A 1M, but no 'R'  - how can I verify that it's reverse?


Also, I swapped a .22 u input cap - seems to sound better, but still trebly.  I'll search the archives for more mods.


Is there a schematic / clear (read: step by step) instruction to install a diode clipping switch?

[DryRoasted]

It's marked A 1M, but no 'R'  - how can I verify that it's reverse?

Simply put, if the drive knob has little effect up until the little 10th of a turn or so, then its not reverse, but if it behaves "normally" then its reverse.

(Like I said, simply put )

[petemoore]

  Eventually, I just bypassed that gain pot. I use a pre-boost to adjust distortion...'above 10'...
  I used a 100k pot there, I find little use for the lower gain settings on the DIST+...not that they can't be cool, but I just use a different something for those type tones, and leave the DIST+ settings set up as hard distortion.
  Using a 100kL pot eliminates some of the lower gain settings but allows a 'finer tuning' to the remaining 'higher gain' settings..

[Mark Hammer]

pete's correct.  The first 900k of resistance on that pot do VERY little, which is why the recommendation is to use a reverse-log taper - so you can get through it quickly and get to the 100k portion that does more.  Which begs the question:  If what you REALLY want and need is the steak, then why go to the trouble of making an undesirable soup more palatable?  Why not just go directly to the steak?

Keep in mind that the Dist+ came out when MXR did not have the Micro-Amp.  Consequently, one of its functions was to partly serve as a semi-clean boost for part of its range by providing modest amounts of boost that would not result in diode clipping.  Do you REALLY need it to do that?  I think not.  Does it do anything interesting with gains below x10?  No.  Skip the soup and go to the steak.  Get a 100k pot and make the whole rotation useful.

The thinness is a result of a deliberate strategy that I seem to have to explain every 4 months.  With noninverting op-amps, the gain is set by the ratio of the feedback resistance and the resistance from the inverting pin (pin 2 on a single op-amp like a 741) to ground.  You can either make the resistance to ground smaller to increase gain or you can increase the feedback resistance to increase gain.  When the feedback resistance is used (see TS-9 and Rat), placing a small value cap in the feedback path in parallel with the feedback resistance results in more treble being shaved off the top as the resistance is increased and gain goes up.  This has the side-effect of "smoothing out" the raspiness of distortion as distortion amount is increased.  When the ground resistance is used, a cap in series with that resistance has the effect of rolling off bass as the resistance gets smaller and gain is increased.

In the Dist+ and DOD250, the .047uf cap to ground results in little audible bass rolloff at lowest gain settings, but as gain is increased, bass gets chopped more and more until at max gain the rolloff is around 720hz or so. 

Why the heck did MXR do this? I see two rationales.  One is that fuzz simply sounds buzzier and nastier when you cut the bass out.  Not everyone's cup of tea, but it does sound a little more evil.  As an adjunct to this,n you get a sort of combined tone-plus-gain control so that tone is automatically adjusted when gain is increased.  The other rationale is that hum from single coil pickups and poorly shielded instruments (highly typical of the 70's) can be very objectionable when you amplify it 200 times (max gain is x213).  By rolling the low end off any hum at 60hz or 120hz does not get amplified as much as higher frequency content does, thus protecting the listener from loud hum.

If you have a reasonably hum-rejecting instrument and rig, and would like more bass at higher gain settings, simply replace that .047uf cap with a .22uf unit.  That will still lop off a little bit of bottom, but not in a terribly objectionable way (rolloff is at 154hz at max gain).

[jeff g]

Thanks everyone.

Out goes the "reverse" log pot and I'll try the 100KL


Again, would anyone have a link / schematic for installing a diode switch?

[Fret Wire]

Keep in mind that the Dist+ came out when MXR did not have the Micro-Amp. Consequently, one of its functions was to partly serve as a semi-clean boost for part of its range by providing modest amounts of boost that would not result in diode clipping.

I kinda doubt that, not to mention the fact that it even distorts without diodes. Was this supposed boost function stated by anyone in MXR? I kinda think it was intended as nothing more than a distortion. Plus, nothing in the MXR interviews in last year's two stompbox books even hint at that.

I'm not offering this up as fact, but the "thinness" may have been to cut through the mix in a band situation, much like a tubescreamer. When the D+ first came out, woofy-assed fuzzes were still used alot.

All this steak and soup talk is making me hungry. But first we need a fork, or we can't eat the steak. Here's the fork Jeff: the reason these endless pot value and taper questions keep coming up eveytime someone builds a D+ is because just about every schematic posted has the wrong pot values and tapers. The original MXR spec is for 500k rev log (gain) and 50k log (volume).The rev log taper for gain is simply the most natural progressing to the ear.

It's probably the most mislabled ckt on the net. I've brought this up before, and you'll usually get the "EH response", meaning that MXR probably just used what was available and cheap at the time, like Electro Harmonix supposedly did. Or you get, " I had one a long time ago, and if I remember(??), it had 10k and 1m". Well, I have one in the here and now (non-led block), and it's 500k rev log and 50k volume.

The "sub theory" just doesn't wash. Check every schematic on the D+ and you'll find all these different supposed values for gain: 1m log, 1M lin, 500k lin, 500k log, 500k rev log, 100 lin. Same for volume: 10k, 50k, & 100k - all in either log or lin tapers. Plus, you'll find different schematics with different combo's of all those different gain and volume values and tapers. Do you really believe they subbed that much?

Another problem with the "sub theory" is that quite a few of the other 1590b sized script and logo ckts used the 500k rev log. So we are supposed to believe that MXR "just used what's available" on the D+, while at the same time, the phase 45, phase 90, and other MXR ckts that were spec'd for a 500KC always did come with the proper 500k rev log.

Most D+ builders/users already know the third problem with the "sub theory". The gain progression is so natural to the ear with rev log, and so bunched up with other tapers, that this would have been a big thing to mess with, or sub for. On the volume side, the original 50k doesn't even have a lot of output. That's why a 100k log volume is a popular mod. Subbing a 10k in would really kill the ckts chance of pushing an amp. Unity gain is hard enough with a 10k. Would they really have done that to a pedal so popular at the time? Also again, would they sub a 10k for the volume when other MXR ckts of the same time used (and came with) a 50k log? Doubt it.

Now we have our fork, lets get to steak and eggs. Jeff, how will you be using the D+?

Bedroom style or larger amp clean channel: meaning either a small clean amp or a large clean channel amp relying on a most of the pedal's distortion, instead of the amps?

Or with an already overdriven amp: needing only a little more gain and volume boost? Here, it's most of the amp's overdriven character coming through, with some xtra gain and unique voicing coming from the D+.

If you need a lot of gain from the pedal, than Pete's right, all you need is 100k. But, remember that whether you use 1m, 500k, or 100k, it will still be bunched up with a non-rev log taper. So, if you need only the heavy gain end the ckt, use 100k rev log. That way you'll still get a more fine and natural adjustment to your ear. With 100k lin, it's still going to be bunched and harder to dial in the exact sweet spot.

Another thing to consider, by using the original 500k rev log, you will have all the gain adjustment range in a very natural progression to the ear, and you can use it with different amps or situations that may need alot of gain from the pedal or only just a little gain. And of course, no matter how much gain you use/don't use, you will always get the pedal's voicing (bass/treble rolloff) coming through.

To save future builders from the "my gain is all bunched up" dilemma, I think soon that I may attempt to email Keith Barr, and link him to a few of these D+ threads. Hopefully, we'll get the final answer on if there were any pot value/taper subs. It just doesn't add up that there was.

[Mark Hammer]

I think Fret makes some strong and well-reasoned points.  Although the point about the Dist+ never cleaning up, given the way the 741 was used, is quite accurate, the fact remains that even with a 500k pot and germanium diodes, there was a sizeable chunk of the pot rotation where diode clipping simply doesn't kick in unless one is using VERY overwound pickups and slamming out power chords like there is no tomorrow.  Whatever pot value/taper was used, the 1M feedback resistor remained constant, meaning that the last 100k or so of whatever pot was in there was the only bit of resistance that stood a remote chance of generating diode clipping.

Notwithstanding, MXR did make unannounced parts subs occasionally, which sometimes came with changes to the labelling on the box, and sometimes didn't.  I'm inclined to agree with him that whatever subs they made tended NOT to be of the sort that we were accustomed to from the earliest days of E-H ("Let's see.  What do we have in the parts drawer here that's close enough?").

Just as an aside, I have a drawing I made from a buddy's Dist+ in 1978, plus a photocopy of the board.  His had a 10k volume pot and no .001uf cap in parallel with the diodes.   I had to add the cap to mine after I grew so fed up with the ugly fizzyness.  Now, before that spins out of control, I need to provide some context.  My "buddy" was a tech in the biomedical engineering unit of the teaching hospital I was working at, and he made HIS copy by photocopying the board of someone else's and cloning it.  Possible that HE made some subs, and that my drawing simply parroted what he had done?  Yeah, I suppose.  And I suppose that's how some of these stories get started.

[jeff g]

I appreciate all the replies, but having just gotten into pedal building a few months ago, alot of the technical info is beyond me at this point.

I really just enjoy building things and getting different sounds with effects - hopefully, understanding theory will come later.

For the D+, the input cap swap to .22 uf sounds better to me and I can live with the limited range - I've switched to the 100KL and I may put in another rev log pot later

Fret Wire - regarding your 'steak and eggs' question - I'll probably use the D+ in both the clean and distorted channels, but not having heard the lower portion of the distortion range, I really don't know   

Either way, I'll probably build a crap load of additional distortions and other effects (given that I've built 8 pedals since I started this hobby in March - with very limited free time) and I may only use the D+ for max distortion - who knows?

Anyways, thanks again for all the replies and info everyone - this is a fantastic forum. 

Hopefully, I'll be able to contribute something back as my knowledge and experience increases

[Mark Hammer]

The "input cap" is not the one you want.  The change to .22uf from .047uf is the cap that is connected to the gain/distortion control.  Input (and output) caps play a role too, to be sure, but in this case the root cause of the thinness is that .047uf cap.

The circuit that comprises the Distortion+ is about as fundamental a building block as one could imagine, and can be modded and reconfigured 50 ways from Sunday.  That, and its simplicity and low cost, have made it the subject of many thousands of posts here.  If you are interested in learning how things tick, it's a great place to start.  Build it and tinker, and I promise you you'll be smarter by October.