Rat mods - prevent IC clipping

[add4]

Hello,
I'm currently playing with my Rat, i am trying different diodes configurations: on a DPDT on off on, i put :
IN feedback loop    --   TO diodes      --  IN Hard clipping
OUT feedback loop --   FROM Diodes --  OUT Hard clipping (ground)

So when the switch is left, i'm in soft saturation/overdrive mode, when the switch is centered, i use no diodes, so from what i understand, this is supposed to be a clean boost, and when the switch is in right position, i'm in stock rat mode: hard clipping the signal from the IC to ground.

Then i'm breadboarding the diodes, trying LEDs, germaniums, and diodes/FET combinations.

I have a few questions about this :
- Even when no diodes are engaged, i hear a high pitched distortion when the drive is high. i guess it comes from the IC. What can i do to prevent that? lower the gain of the IC? This distortion is not musical at all it's very harsh and i hear it also when the diodes are engaged. i would love to get rid of it. Any suggestion?

- I hear no BIG difference between soft clipping and hard clipping modes. Is that normal? is there a way to emphasize the difference? i was thinking of putting it on a switch, but it doesn't seem to be worth doing it, except with the germanium diodes, where the dynamics and sound is really different. the LED and FET clipping modes seem much closer in hard and soft clipping modes.

- i have put the ruetz mod on a pot (R8 -> 1k pot), and i when the pot is "open' no resistance between the feedback loop and the capacitor to ground. the guitar starts to feedback like crazy. i'm thinking of putting a resistor in eerie with the pot to avoid that, but i really like the sound i get when it's almost fully open. Any explanations about this phenomenon, how could i keep the audio characteristics and get rid of the feedback?

Thanks

[Jaicen_solo]

I could be wrong, but from what I understand, the RAT uses slew rate limiting as part of its sound, which is why it wants a specific IC. That's probably what you're hearing.

[tubelectron]

Hi,

If you have a LM308N in your RAT, then it's the best. This OP amp has excellent overdrive/distortion sound, and in the RAT, it distorts nicely, along with the clipper : that's the peculiar sound of the RAT. If your distortion sound is bad, it's because you use another IC, OR your circuit may be faulty. IMHO, Discard the OP07D, accept the 741, and choose the LM308N... It's what I have done on mine, a RAT Big Box RI returned to the 1981 Rev.B specs :

[stringsthings]

I have a few questions about this :
- Even when no diodes are engaged, i hear a high pitched distortion when the drive is high. i guess it comes from the IC. What can i do to prevent that? lower the gain of the IC? This distortion is not musical at all it's very harsh and i hear it also when the diodes are engaged. i would love to get rid of it. Any suggestion?

what you're probably hearing is oscillation ... it's a very, very high pitched sound that's not really musical ... op amps configured as they are in the RAT circuit can go into oscillation when they're asked to provide a lot of gain ... using a breadboard also lends itself to the possibility of oscillation ...

when the circuit is put into an enclosure ( and the feedback loop connections are kept on the PCB or as short as possible ) the oscillation will go away ...

for breadboard purposes, try putting a capacitor ( perhaps 500 pF or higher ) in the feedback loop

[add4]

Thank you all for your replies.
I had the LM308N plugged in and i desoldered the 30pf cap between pins 1 and 8 following Jack Orman's fat rat book advice and tried other opamps.
I also find the LM308N to be the best sounding and musical op amp into the rat.
I guess i should put the 30pf cap BACK into the circuit if i use the LM308N opamp? Maybe the small not musical distortion comes from the fact that this cap is not there at the moment. Would it be worth trying other cap values in your opinion?

The circuit is the stock rat 3 circuit, with all mods provided by jack roman in the rat book, and R6 replaced by a 1M pot (this is the pot that produces the feedback sound when opened all the way. I have to check whether the value of the cap behind the pot is still the same.

Overall with all these mods, the rat sour MUCH better to me than it sounded stock.

[tubelectron]

I guess i should put the 30pf cap BACK into the circuit if i use the LM308N opamp?

Yes, indeed !

Overall with all these mods, the rat sour MUCH better to me than it sounded stock.

If it's a return to the 1981 Rev. B circuit (original big box RAT), I should say again : Yes, indeed !

A+!

[Electron Tornado]

The datasheet says that cap will influence bandwidth and slew rate.  Does connecting that cap from pin 8 to ground affect the tone? Also, how would changing its value (and thereby bandwidth and slew rate) affect the tone?

[Electron Tornado]

The datasheet says that cap will influence bandwidth and slew rate.  Does connecting that cap from pin 8 to ground affect the tone? Also, how would changing its value (and thereby bandwidth and slew rate) affect the tone?

Self, there's some helpful info in this thread: http://www.diystompboxes.com/smfforum/index.php?topic=4290.0

You're welcome, 

Me


PS - Quit being lazy. Use the search function next time. 

[stringsthings]

I guess i should put the 30pf cap BACK into the circuit if i use the LM308N opamp?

Another vote for "Yes!"  

depending on the type of op amp, the 30pF cap ( frequency compensation capacitor ) may not be needed ... one of the first super popular op amps, the 741, didn't require an external compensation cap ... later on, chip manufacturers gave designers/users more options by providing the option of choosing the value of the compensation cap ...

in our case, when we're dealing with distortion that we purposefully desire, some of the specs aren't nearly as important as the end result ( a pleasing distortion to one's ears )