# What difference do Tommy ICs make, and why?



## smsms (Feb 23, 2020)

Something I've noticed with my own Tommy (v2) build: I couldn't find a JRC4559, so I bought a TL072 and a JRC4558 to use instead. The 4558 sounds good but at lower gain settings, the clipping doesn't engage at all. By contrast, using the TL072, with the gain knob barely above 0 it will clip a little, which suits my purposes much better.

My question to the EE-minded amongst you: why is this the case? I've looked at the datasheets, but it's all hieroglyphics to me.

Thanks


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## Chuck D. Bones (Feb 26, 2020)

I would not expect the opamp to make a significant difference in the sound of a Timmy.  The diodes are responsible for the clipping. Neither opamp stage should saturate.  I suspect your build may have an error in it.  What are you using for power?


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## smsms (Feb 26, 2020)

I'm using a normal MXR DC brick - it's not the quietest power supply, but it's workable. I'm wondering if there's something about the circuit where, depending on the IC, if the gain pot is below some threshold the IC doesn't engage. I know very little about EE, but my debugger brain is fixated on this possibility.


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## Chuck D. Bones (Feb 26, 2020)

As long as the power supply make 9V or something close to it, this pedal should work properly.  The opamps are always "engaged." This is not like a Tone Bender where a transistor can be under-biased and gate the sound.  The fact that your pedal is sensitive to which opamp is installed makes me think you have a bad part or something else that isn't quite right.  Would you care to measure the opamp voltages?


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## smsms (Mar 1, 2020)

My (cheap, Aliexpress) multimeter probes are busted - i'll measure when i get a chance


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