MXR Distortion + sounds crap.... help?

[mattpocket]

Ok,

So I got my MXR "working" and it sounds crap! There is no sustain and it sounds like its bubbling through water! haha It's distorting, but its crap, I know the circuit is right so its not that.

I tried swapping the diodes round and I got it sounding better, but it still sounds shite, it's not cutting through, I cant really describe the sound well...

Will swapping out any caps or resistors help?

Matt

[GibsonGM]

Wow, that's pretty unusual for that circuit, Matt.  The symptoms sort of suggest a bias problem...have you checked for shorts and stuff, around the 1M biasing network resistors and so on?  How does it sound with NO diodes - audio probing the output of the 741 will tell you more.  Should be a little dirty there, but not horrible sounding.  Read "What to do when it doesn't work" from the top of the forum, post some voltages, and try audio probing!  You'll get answers pretty quickly after that, I'm sure.

Other causes for this: Shorts around the IC socket. 741 in backwards. + and - input accidentally reversed.  blown or backwards electrolytic cap at input or output.  Dead 741.  Wrong values of resistance in feedback loop.  Can't say without more info 
It's a good sounding circuit...keep at it, you'll get it!   

[mattpocket]

I'll look into those things, and I will post all the information about the voltages etc... probably wont be until tomorrow now though...

Ive built it on my breadboard by the way and I used Dragonfly's layout (and I put the missing cut in too), I used the tantalium cap on output, and I have both 1n914 and 1n270 Ge diodes, but I am not sure which...

I will be in touch soon, if anyone can offer any help in the meantime, give us a shout!

Matt

[petemoore]

  I know the circuit is right so its not that.
  On to the next one then?
  You might want to take voltage measurements before concluding the circuit is right but the sound is wrong.
  Will swapping out any caps or resistors help?
  Absolutely ! If the values are incorrect, or a component has failed [but I very rarely encounter or read that].

[MartyMart]

Matt, sounds like you "Dont" have your Dist+ working  !!
It's a proven circuit that's been built hundreds of times, so YOU have a mistake
There's nothing wrong with that, I STILL make 'em after hundreds of builds 
Check usual stuff, soldering/component values etc etc and post back voltage readings.
It will then be obvious where the " mistake " is .....

MM.

[mattpocket]

Yeah its pretty obvious now somethings screwed, I just thought i'd already built it a few times and was sure it was right! haha

I measured the voltage on the IC earlier, but I didnt have time to post, but they were all above 8v? I had my black lead on the output sleeve, and my red one on an ic pin. Does that give you any indication whats wrong? IC screwed maybe?

I built it on breadboard, so its unlikely to be my soldering! hehe

Oh, also it seems the volume knob doesnt do anything whilst the effect is on either! :S

Matt

[petemoore]

I measured the voltage on the IC earlier, but I didnt have time to post, but they were all above 8v?
  See below...but I'd suspect there's a ground not made, test all marked ground points from a 'distant ground' point [like a sleeve, all connects between, then also get tested].
I had my black lead on the output sleeve, and my red one on an ic pin. Does that give you any indication whats wrong? IC screwed maybe?
  Not really, test the sleeve to pin 4 connection first, this establishes that you are measuring voltage from the ground reference voltage, in this case, 0.0Vdc.
  Reading the debugging thread and following those instructions generally causes the friction that causes the spark that lights the fuze, which ignites the powder that illuminates the area where the problem is.
  Make sure the PS Rails aren't shorted before applying power.

[mattpocket]

I will do the whole shabang from the debug thread tomorrow, and get back to you,

I'm not using the PS rails, I am using a battery clip, as though in the real circuit... its probably not best practice, but it works for me...

Thanks for the help so far, I had some problems with grounding already, see this thread, the edited layouts inparticular, the last one I posted is how the breadboard currently stands, apart from the breadboard obviously has a problem! haha http://www.diystompboxes.com/smfforum/index.php?topic=54026.msg412168#msg412168

Thanks

Matt

[petemoore]

  Whatever the schematic shows should be visible and measureable, except of course the caps, as long as they're in there...and connected only to what's shown as connected...
  I am using a battery clip, as though in the real circuit... its probably not best practice, but it works for me...   
  Battery is the supply of choice for when pre-testing a circuit, I just pre-check for a shorted V+/V-...this prevents the battery from possible short. Battery is also excellent source of 'pure' DC.
  Clip a DMM black lead to a ground point, test all grounds in beep mode, touch the V+ of battery clip, if it beeps, the battery will short out.
  Then switch to Voltage test, connect battery, take pin voltages/post them/ the inputs and output should be something close to 1/2v of supply from ground which should always be 0.0v on Dist+ ground points]

[mattpocket]

I dont think I have a beep mode (I'm guessing you mean continuity) on my DMM, is there any other method I can use?

I will rebuild the circuit later, and if I get the same results, I will post up the voltages from the board.

Thanks

Matt

[GibsonGM]

You can use your resistance setting instead of continuity....if you measure 0.0 ohms or very close, there is continuity.  Of course, this will change if there is a component in between the 2 points you're measuring 

Your symptoms really strike me as a mis-bias, or DC getting thru somewhere it shouldn't.  Gonna need those voltages, tho!  It's probably 1 resistor or wire going where it shouldn't.   

[mattpocket]

Well, I was taking the reading for the voltages, and I thought, balls to this, I'm just gonna rebuild it...

Took me ten minutes of logical thinking once more, and hey presto, it works this time!

Job's a good 'un...

I'm off to put it on stripboard now, and box it up... have fun...

It's snowing again here (UK) which is all nice, I'm going to snuggle down with my warm soldering iron and a cup of tea... what a wondeful afternoon! 

[mattpocket]

So, I finished it, and it sounds OK, but its not as good as the amp distortion (marshall mg100dfx)... it sounds kind of solid and regimented, is there some other pedal I could put before or after it to contour it somehow... like an EQ, booster, another distortion pedal, something else?

Matt

[Mark Hammer]

Something like this: http://aronnelson.com/gallery/album102/Freq_booster   can sometimes help to add some additional colour to a pedal like the Dist+.

I'm getting a bit tired of repeating this, but note that the .047uf cap in series with the gain pot means that you will lose bass as you turn the gain up.  At maximum gain, that cap will start to roll off the lows around 720hz.  In some respects that makes the clipping more apparent, but many complain that they lose "mass" in the sound because of it. 

Making the cap larger will keep more bass, and will also produce greater clipping because of that extra signal amplitude (or at least the same amount of clipping for a longer duration of the note's lifespan).  because of how op-amps work, making the cap bigger will still result in bass loss as the gain is increased, but it may be repositioned to a range where you won't notice it.  For example, with a stock .047uf cap, bass cut is at 3.4hz at min gain and 720hz at max gain.  With a cap of .1uf, that moves to 1.6hz at min gain and 339hz at max gain.  With .22uf, that moves down again, to .72hz and 154hz at min and max. 

Personally, I don't think anything interesting happens in that pedal until the last 100k of gain pot resistance (i.e., 900k is essentially useless).  If the 1meg pot was replaced with 100k, you'd have the gain go from x10.5 to x214, which is still a fairly wide range.  Using a .22uf cap, your rolloff would go from 7hz at min gain to 154hz at max.  Replace the 4k7 resistor in series with the gain pot with a 3k9 value, and max gain can be goosed up a bit to x257.  That's not exactly Rat-type gain levels, but with the extra bass in the signal, you should notice a meatier sound.

Should you actually find something to like in the stock cap value and gain-linked bass rolloff, consider sticking a .22uf and .1uf cap in series in place of the .047uf unit.  You can then use a toggle to shunt the .1uf cap, giving the full .22uf, or leave them in series giving and effective value of .068uf, which is close enough to the original value.  That and the Frequency booster should be able to yield a reasonably broad palette of thin and reedy through to woolly.