Inverting Opamp to Cut Oscillations

[LP Hovercraft]

I just made a Sansamp GT-2 clone.  Due to the fact that it such a high gain monster, I oscillates when hooked up to a switch.  To combat this, I have added a 741 op amp at the front end and put the signal through a 100k resistor, a .01mF cap, and a 10K resistor in that order to the 741's inverting input.  The inverting input is tied to the 741's output via a 1K resistor.  The non-inverting input is tied to ground.  It cuts the oscillations very nicely, but is rather noisy still.  Have any of you tried this mod for this purpose, and is there a schematic around for this kind of mod as not to stab in the dark at this kind of thing?  Thanks!

[Transmogrifox]

Yes, it works well.  A good start to reduce noise is to use lower noise opamps than 741.  I had a professor tell me something to the effect of,

"741's were great in their time, but if I ever catch any of you using one in a design, I'll come and strangle you.  You can get much better opamps cheaper than a 741".

I have been happy with the TL08x, TL07x series.  Try a TL081 in place of a 741.

[Jay Doyle]

Go for a NE5532, that ought to give you the best choice.

[petemoore]

It got to be true or one heck of a set of coincidences.
 741's are noisy.
 I agree with the Prof's Quote, because there are better OA's than 741.
 I built another Tube Reamer and it works much differently with TL082 or 062's in it than 741's, subtract noise and add 'clenliness' bout sums up the 741 compared to most other OA's.
 If I want noise I would go with a 386...they're so easy to wire up, and push hard.

[zener]

From using the search function, I've found this thread.

I just built a sansamp gt2 from tonepad and it oscillates like crazy. The squeal comes from moving the high and gain control. I have another post http://www.diystompboxes.com/sboxforum/viewtopic.php?t=21418 about my problem and Peter suggested something else. I tried it and it cut the squeal significantly but it was replaced by a louder-than-usual hiss.

Then I found this thread and i tried what was suggested here. I hooked up a TL081. Does the circuit supposed to REDUCE or totally REMOVE the oscillation? The squeal was still there but there at high settings but there is a slight to moderate improvement. With level at max, the squeal won't come if you increase only either gain or high control. I can't max both or else, this monster squeal will start filling my room. I just set the gain at max and the high somewhere at 30-50% of the pot. Also, it is still noisy although I've used a TL081 instead of the LM741.

Any further elaboration or addition on this or perhaps another solution to this problem?

I'm thinking of adding a treble booster http://bryant02.home.att.net/schematics/screamingbirdschem.gif in front of the output. Can it replace the treble control of the sansamp gt2 while I'll just fixed the setting original trebel control?

Thanks for any help :wink:

[zener]

:wink:      

[RDV]

I still like the 741 in a Dist+.



RDV

[D Wagner]

I still like the 741 in a Dist+.



RDV

Me too!  I think that the additional "noise" of the 741 is part of the overall distortion flavor that I like.

Derek

[LP Hovercraft]

Ultimately, I scrapped the 741 inverter idea as I felt that it sucked a lot of the crunchiness out of the tone.  It is also a good idea to use TL2262's as IC 1,2,3,  as suggested in previous threads-it is more stable and less noisy.  After using those, I was able to get more out of it, even setting the Gain, Treble, and Bass controls at 10.  The volume setting still remains an oscillation problem child, and anything slightly over unity gain still causes it to squeal like a pig.  I am thinking about using a trimmer in parallel with the volume pot that would set that volume at about halfway (unity gain on my GT2) no matter how far clockwise it's turned.  

The treble booster idea sounds pretty cool, let me know if you're on to anything with that.

[Manolo Dudes]

...It is also a good idea to use TL2262's as IC 1,2,3,  as suggested in previous threads-it is more stable and less noisy.

Yes, but the suggestion was for IC2, 3 & 4. Tonepad's layout, that is.