[sdiy] Wah pedal modifications

[Michael Bacich]

On May 1, 2006, at 9:49 AM, Aaron Lanterman wrote:

> Speaking of which, I've got another student working on a voltage- 
> controlled crybaby circuit. I turned him onto using a gyrator to  
> replace the inductor. He grabbed an off-the-shelf crappy op amp  
> (LM301, maybe? I don't recall the number), and it gave him 5 times  
> the mH he was wanted to get. Then I gave him a TL08something, he  
> swapped that in, and it gave him what he wanted.

Maybe you already knew this, but just in case you didn't:

The LM301 is a non-frequency-compensated opamp, and in most  
applications, must have its high frequency response compensated with  
an external capacitor.  Without the cap, the amp with have a very  
different frequency response than any of the typical internally- 
compensated opamps that we are used to (pretty much all modern  
opamps, with just a few exceptions).  It could also have serious  
oscillation problems, depending on the gain factor and type of  
circuit configuration.  This means an LM301 will likely not work as  
you might expect a 741 or TL081 to work.  This could explain why the  
gyrator circuit responded so peculiarly..

To make the LM301 behave like a standard amp, usually it just means  
connecting a 30pF cap between pin 8 and pin 1.

See the LM301 data sheet for tips on external compensation:

http://www.utdallas.edu/~hellums/docs/JournalPapers/LM101A.pdf


BTW, ARP synths are FULL of LM301's.


MB

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