Op-amp Muff Fuzz only works with JRC4558, why?

[commathe]

Hey everyone, I was breadboarding a muff fuzz to learn a bit more about how it works and also because it is the only stompbox I've seen with a "textbook" inverting amplifier set up where the audio is fed into the feedback loop rather than the tube-screamer style one where the audio is fed into the non-inverting input along with the "bias" vref voltage.

So, why does the muff fuzz not work with my TL072s? Pulling up their respective datasheets the most obvious difference is that the TL072 has FET inputs while the 4558 has BJT transistors. Why does this make such a difference? Does this mean that no FET input op-amps would work? Why not?

Also, how does the "tubescreamer" style op-amp set up work? I'm not talking about the diodes, I'm talking about the cap to ground/vref in series with a resistor.

[commathe]

Annotation: I think I may have fried my TL072. That could explain the problem. How sensitive are they excatly?

[bluebunny]

How sensitive are they excatly?

To static or to heat?  Unless you get zorched by lightning, I think they're pretty robust when it comes to static electricity.  And I think you can get away with a couple of seconds (but no more) with a hot soldering iron.  Or else use sockets.   

On the other hand, if you meant, "sensitive to insults", then I can't help you with that.  I'm always loving and cuddly towards my TL072s.   

[commathe]

Bummer. Shorted/fried would have made more sense. Now I'm just confused why the circuit works with one kind of op-amp but not the other...

[Seljer]

I've fried a bunch of TL072s by accidentally connecting the supply rail or ground where it shouldn't go, or swapping the two  

They're rugged but not that rugged  


For a basic test to see if an opamp is working, set up up as a non-inverting buffer (output connected to - input), and put half the supply voltage (you're vref) on the + input, use a voltmeter to measure the output. If it matches the + input voltage the part is passing DC voltages ok.



In general you can quickly deduce is something is awry in opamp circuits by just measuring all the pin voltages and see if everything correctly standing at 'virtual ground'

[bluebunny]

Sorry, I thought you meant you had indeed fried your 072.  It could be that the different nature of the two chips make a difference, but you'll need to wait for someone more learned to stop by with a cool explanation. 

[lungdart]

Hey everyone, I was breadboarding a muff fuzz to learn a bit more about how it works and also because it is the only stompbox I've seen with a "textbook" inverting amplifier set up where the audio is fed into the feedback loop rather than the tube-screamer style one where the audio is fed into the non-inverting input along with the "bias" vref voltage.

So, why does the muff fuzz not work with my TL072s? Pulling up their respective datasheets the most obvious difference is that the TL072 has FET inputs while the 4558 has BJT transistors. Why does this make such a difference? Does this mean that no FET input op-amps would work? Why not?

Also, how does the "tubescreamer" style op-amp set up work? I'm not talking about the diodes, I'm talking about the cap to ground/vref in series with a resistor.

It would help to see your schematic. But there is no reason a "text book" inverting amp would not work on a TL072. The most obvious issue I would say is to check for any pinout differences. If you reversed the power rails, the IC could be fried.

As for your other question, the cap and resistor going to VGND on the inverting input of a tubescreamer style gain stage is an active high pass filter.