Looking at building a wah pedal.
I have Rej's Kwawk PCB and it calls for a LF351..
Anyone know a direct replacement?
I'm guessing a tl071 would work but I'm not familiar with the circuit.
I guess I'll have to do the work.. Just being lazy if anyone knows off the top of thier heads...or any other part of your brain..hehe
You could get away with a 741 or any single opamp but the 351 claim to fame is the high slew rate. My somewhat limited understanding of the term means it doesn't sag under rapid gain/freq changes. Single op amp will at least get the thing out of the water till you find one.
Sounds right to me Gordo. I think it is the speed at which the amp responds to changes. I know the term from microphones. The faster, the better at catching attack transients. I guess that would be good in a wah pedal with sweeping frequencies goin on.
I hate it when you just need one part..
Right? I'll dig thru my stash in the morning and see if I have one.
Cool. Thanks.
I think I post it in the beg a part thread..
Maybe I'll just order one of Brian's Weeners!
Probably spend less money than trying to order one chip...
His wiener wah PCB doesn't have any chips on it...hehe
You could make one from the SMD version (LF351DT) for a little more than a dollar.
https://oshpark.com/shared_projects/z8hGhQoa - SOIC8 to DIP8 adapter, 85 cents for 3 (no shipping cost)
https://www.arrow.com/en/products/lf351dt/stmicroelectronics The chip, 37 cents (no shipping cost)
Use some resistor leads as legs on the adapter, or splurge and get some header pins and you're good to go (just be aware of what size header pins you're getting if you intend to socket). Might take a week or two with OSHPark's fabrication turnaround, but its an option.
I would try a TL071 as the nearest equivalent, because both it and the LF351 have JFET inputs. For some circuits this seems to matter, but often an opamp with BJT inputs (like 741) works also.
The slew rates of LF351 and TL071 are typically 16 and 13 V/microsec from the datasheets. This is how fast they can change the output to follow the input. These are both way faster than should ever matter in an audio circuit. Really old opamps like the LM308 have a slew rate less than 1 V/microsec, but even then I've been a little skeptical about whether it matters in a guitar pedal. A way to think about this is: the fastest I would expect an audio signal in a pedal to change is about 4 volts at 40 kHz (guitar amps really cut off at about 5 kHz, so this is a very generous margin). That is a slew of 4 volts / (1/40 kHz), or 4 volts in 0.025 millisec, or 0.16 V/microsec, much less than the slew that most op-amps are capable of.
Adding to the last post. Here is a calculator for slew rate. To even get close to the specs of those two you need 200khz at 9v.
http://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-slew-rate.php
Quote from: bsoncini on December 20, 2017, 09:44:33 AM
Adding to the last post. Here is a calculator for slew rate. To even get close to the specs of those two you need 200000hz at 9v.
http://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-slew-rate.php
OK! There it is. Much appreciated Bsoncini. I get it now. Not so important at 5k.. @25k might be more important as the AC wiggles a lot faster... I know technical terms..(wiggles)