Distortion + IC change out problem!

[WorkBench]

I changed the IC in my MXR distortion + to a JRC 4558.  It will not work now.  It works in the bypassed position, but when I switch on the effect....NOTHING!! I tried putting it in both ways, checked all the solder joints, changed out surrounding components, still nothing!  I even tried to put the original IC back in, and NOTHING!  Does anybody have an idea of what may be causing this?  Thank You all very much!
Chris

[cd]

You put a dual opamp in a single opamp socket.  Wrong pinout.

[mikeb]

You put a dual opamp in a single opamp socket.  Wrong pinout.

... but it still should work when the original opamp is put in, I would think (hope?).

Do you have a multi-meter? If not get one, then measure the voltages on the IC pins. Errr ... changing out the surrounding components might have been a little premature, you may well have complicated the repair (although I admit some software I've worked on has got more and more broken before becoming 100% correct and functional a moment later!)

Mike

[Lonestarjohnny]

did you install the old IC correctly, if you look at the socket you'll notice 1 end has a u'd slot in 1 end, and if you look at your IC it will have the U shape in 1 end, they should match when installed properly.
Johnny

[petemoore]

I tried putting it in both ways, [ no no]
 I don't think I caought the part about, is the circuit wired for a single or a dual opamp?
 Does it work when you put the 'original chip back in?

[WorkBench]

I was not aware of the circuit being wired for a single op amp.  i really did not know there was 2 different op amps like that.  Can you get a JRC 4558 for a single op amp circuit?  thanks!
Chris

[petemoore]

Yes singles and duals have different pinouts. Just look them up for their data sheets...
 There are 'better' single OA's [for noise etc.] than a 741...NE5534 for instance, that's very clean OA, you might want something 'thicker' sounding...I can't recall OA types and #'s just now.

[WorkBench]

Yes, thicker sounding, with more bottom end for the MXR distortion +, any body know a good ic for that, a single op amp ic that is!
Chris

[Mark Hammer]

The "problem" with bottom end you are having is that as the gain of the op-amp is increased (and especially because of the WAY it gets increased), the bass is rolled off more at higher gain settings.

I posted a very long note about this here recently, but I forget which thread (maybe someone will be good enough to mention it).

Here's the short version:

There should be a .047uf (47nf) cap in series with a 4k7 fixed resistor, which is in series with a 1meg gain pot.  The resistor/pot combination, when combined with that cap going to ground, sets the low-frequency rolloff at a frequency determined by the formula: F = 1/(2*pi*R*C).  With the gain turned down all the way (max resistance), that frequency is: 1/(2*3.14*.047uf*1.0047M) = 3.4hz.  With the gain at max (minimum pot resistance), that frequency becomes 1/(2*3.14*.047uf*.0047M) = 720hz.

THAT is where your bottom went.

The solution is simple.  Solder a second parallel cap to the back of the board between the pads where the .047uf cap goes.  A value of 0.1uf (100nf) would give you a combined capacitance of 0.147uf, and a rolloff around 230hz at maximum gain, which will restore much of the lost bass and "meat".  Alternatively, replace the .047 cap with maybe a 0.15 or even 0.22uf cap.  If you wanted a "deluxe" version, you could have the extra capacitance toggle-switch selectable.

Incidentally, the changing bass rolloff with increased gain could be considered a "feature" by many people because as you change the gain you shift the tonal emphasis not only at the top end (by increasing/decreasing harmonic content) but also at the low end.

[WorkBench]

Thank You Mark!  I put the original IC back in and then put a .1uf in parrallel with another .1uf.  Now the bass is there!  Again, thanks!
Chris

[Mark Hammer]

You're welcome.  I like it when simple things change everything, like a cherry on top of an ice cream sundae.

All lurkers should please note that this is a common feature to all op-amp based distortion units that vary gain by changing the resistance of the path between the '-' pin and ground.

When you see one of these, do the math first, using the formula, figure out what the change in rolloff will be with the minimum and maximum pot settings (plus any fixed resistor there), and decide if it is something you can live with.

As mentioned in the previous post, sometimes people like that tonal change which accompanies gain, but if you don't now you know what to do to fix it.

I expect a credit on the back of the CD, Chris!   :wink: