gtfields13
Member
I am setting up to build my next iteration of my Rat clone (Terrier Review) and decided to breadboard the basic Rat circuit from ElectroSmash, so I could evaluate alternatives to the LM308 OpAmp and the filter behavior. I was working from the ElectroSmash ProCo Rat analysis, but with my own modifications to the power supply - I've been building out my own voltage regulator circuit to setup for 12V supply and using a TLE2426 Rail Splitter as an alternative to provide the Vref for the op amp.
This has been a great learning experience for me, tracing the signal through the circuit, seeing how the different op amps behave with square wave inputs, adjusting the distortion potentiometer feedback to better control the pre-clipping amplification sensitivity, and looking at how different diodes behave. I settled for the asymmetric Rat diode setup, with 1N914 / 2x 1N914 giving me ~2.1Vpp for my output signal.
One thing that was puzzling me for a bit, though, was that there was asymmetric clipping at the output JFET, even when the distortion was turned down and the signal coming through the diodes was getting clipped asymmetrically (-0.63V first than 0.74V on positive side.
What I think I have worked out is that in the standard circuit, the gate was just above zero, the Drain was at my V+ (12.15V) and Source was at 0.63V, suspiciously exactly where I was clipping.
Now, biasing transistors is a new concept to me - I am still in learn as you go mode - so I did the new thing and went through this with ChatGPT (5, if you care) to try to understand this. So everything after this might be hallucination (mine or ChatGPT).
Where I ended up was changing from the 1M resistor to ground from the gate to a divider to life the Gate up to +3.3V (mostly because that was the divider I got from the resistor I grabbed, and with +12V supply so plenty of room). With this setup, after the output coupling capacitor I had the clipping I expected with the 2.1Vpp.
Now I am wondering if I have unintentionally killed part of the Rat sound, or if I just totally fooled myself into thinking there was an issue when there really wasn't. The clipping was not something I was hearing, but it was pretty obvious on an oscilloscope. At the end of the day, I also can't hear any difference between this breadboard and the "by-the-book" build I made using the PedalPCB board.
I also am puzzled when I think about the diode clipping stage at the output. While I see that I can change the range over which clipping occurs, I really don't see asymmetric clipping with the 1 / 2 diode arrangement. It's like the signal floats to the middle of the available range and still clips evenly on both sides.
I welcome any thoughts on where I have gone astray, or what I might measure to trouble shoot.
This has been a great learning experience for me, tracing the signal through the circuit, seeing how the different op amps behave with square wave inputs, adjusting the distortion potentiometer feedback to better control the pre-clipping amplification sensitivity, and looking at how different diodes behave. I settled for the asymmetric Rat diode setup, with 1N914 / 2x 1N914 giving me ~2.1Vpp for my output signal.
One thing that was puzzling me for a bit, though, was that there was asymmetric clipping at the output JFET, even when the distortion was turned down and the signal coming through the diodes was getting clipped asymmetrically (-0.63V first than 0.74V on positive side.
What I think I have worked out is that in the standard circuit, the gate was just above zero, the Drain was at my V+ (12.15V) and Source was at 0.63V, suspiciously exactly where I was clipping.
Now, biasing transistors is a new concept to me - I am still in learn as you go mode - so I did the new thing and went through this with ChatGPT (5, if you care) to try to understand this. So everything after this might be hallucination (mine or ChatGPT).
Where I ended up was changing from the 1M resistor to ground from the gate to a divider to life the Gate up to +3.3V (mostly because that was the divider I got from the resistor I grabbed, and with +12V supply so plenty of room). With this setup, after the output coupling capacitor I had the clipping I expected with the 2.1Vpp.
Now I am wondering if I have unintentionally killed part of the Rat sound, or if I just totally fooled myself into thinking there was an issue when there really wasn't. The clipping was not something I was hearing, but it was pretty obvious on an oscilloscope. At the end of the day, I also can't hear any difference between this breadboard and the "by-the-book" build I made using the PedalPCB board.
I also am puzzled when I think about the diode clipping stage at the output. While I see that I can change the range over which clipping occurs, I really don't see asymmetric clipping with the 1 / 2 diode arrangement. It's like the signal floats to the middle of the available range and still clips evenly on both sides.
I welcome any thoughts on where I have gone astray, or what I might measure to trouble shoot.