all 7 comments

[–]lithiumdeuteride 8 points9 points  (2 children)


The first block of your circuit is an op amp in a 'non-inverting amplifier' configuration. The gain (ratio of output to input) for a non-inverting amplifier is given by this equation:

A = 1 + (Z1 / Z2)

where Z1 and Z2 are generalize impedances. In this case, Z1 is a 10k resistor, 470p capacitor, and two diodes all in parallel. Z2 is a 1k resistor and 50n capacitor in series.

If the circuit didn't have capacitors or diodes, the gain would be:

A = 1 + (10k / 1k) = 11

which would produce the original signal multiplied by a factor of 11. The 470p capacitor acts increasingly like a short circuit at higher frequencies. At very high frequency, it has essentially no resistance, so it brings down the impedance Z1 to zero, which reduces the gain to: A = 1 + Z1/Z2 = 1 + 0 = 1

This shows that very high frequencies will still be present, but they will only be amplified at unity gain.

The 50n capacitor also behaves like a short at very high frequencies, but it's in series with a 1k resistor, so Z2 will never go below 1k. However, at low frequencies, capacitors act increasingly like open circuits (high impedance). At very low frequency the 50n capacitor's impedance will climb towards infinity, making Z2 very large. At low frequencies, therefore, the gain is: A = 1 + Z1/Z2 = 1 + Z1/inf = 1+0 = 1

So again the circuit limits the gain of low frequencies to unity. Middle frequencies will be amplified the most. That gives the circuit a 'mid boost' effect.

Now we consider the effect of the diodes. A diode is like a one-way resistor whose resistance starts off very large, but drops precipitously when the voltage across it is big enough. So, if we input a large enough signal, the diodes will conduct when the signal swings too high (or low), chopping off the top and bottom of the waveform, which we hear as distortion.

However, since diodes exist as part of Z1, they only distort the part of the signal that's proportional to Z1/Z2. The complete gain is 1 + Z1/Z2. That means the clean, undistorted signal still comes through, simultaneously with a distorted, mid-boosted signal from Z1/Z2. This kind of gain stage is described as 'soft' clipping, because the clean signal and distorted signal are both present in the output.

The second part is simply diodes shunting signal to ground. It chops the peaks off the signal just as before, but no 'clean' signal gets through. The full signal is chopped by these diodes. If it weren't for the capacitors in the soft-clipping gain stage, those diodes would distort in exactly the same way as the diodes shunted to ground. The only difference is that the soft-clipping stage lets some clean signal through as well.

Usually, you would put a small resistor (1k) in-between the op amp output and diodes, forming a voltage divider and protecting the op amp from putting out too much current through a forward-biased diode.

As for the op amp recommendation, I like the TL072 for its high-impedance inputs, high gain-bandwidth product, and low noise. The RC4558 is also a good choice.

[–]mpigsley[S] 0 points1 point  (1 child)


Thanks so much for going so far in depth. It means a lot. I will definitely look into both of those op amps as well!

[–]lithiumdeuteride 2 points3 points  (0 children)


Once you master op amp circuits, I recommend trying some of the JFET and MOSFET circuits at Their distortion character is entirely different from your typical op amp + diode device. Good luck.

[–]FireThestral 4 points5 points  (1 child)


Okay, this is a dual-distortion amplifier network. There is gain and a filter, but we won't worry about those.

The hard clipping usually is associated with distortion pedals. This is because it is connected to ground. When the threshold is passed current will either drain to or from ground depending on whether the op-amp output is positive or negative. Because it snaps to ground when the threshold is passed it is called hard clipping.

Soft clipping is usually more associated with overdrive. These diodes are connected within the feedback loop. These are essentially resistor-bypass. After the threshold has been met current will flow through the diode, skipping the resistor (sort of, but for our purposes assume this to be true). This allows more current around the op-amp, which has a similar effect to the hard clipping, but because it is in the feedback network instead of the output it gives a more rounded signal. This creates a softer sound than the hard-clipped signal.

This circuit has both, which is not uncommon. They can be tuned depending on the makeup of the diodes and a few other factors. Most good quality pedals on the market have some of both to give a unique sound.

Hope this helps!

EDIT: Op-Amp selection. I don't really have any specific suggestions, but depending on where the output is going, you are going to want one that has sufficient rail voltages for the application. If this is going to your computer input it should be around 1V amplitude, so there is no reason to operate with 15V rails. Just be mindful of other components in the system.

[–]mpigsley[S] 1 point2 points  (0 children)


Completely answered my question! Thanks a lot for this. I wasn't really paying attention to the diode turn on voltage.

[–]eunzie 0 points1 point  (1 child)


Someone correct me if I'm wrong, I'm not super sure. But I'm pretty sure the diodes in the feedback path (soft clipping) are there to 'round off' the top of the waveform, which sounds nicer than the abrupt 'cut' of the waveform peaks caused by amplifying the input signal to an amplitude larger than the supply rails of the opamp. It's stuff to do with the 0.7v drop required by the diode, and the feedback scheme. I'm saying I think it's a power limiting type operation, maybe the hard clipping is to add some distortion?

No idea on opamp selection, but the chip I just replaced in my blues pedal was a 4588. But according to this website, they aren't the best thing out.;prev_next=next

[–]mpigsley[S] 0 points1 point  (0 children)


Hey thanks for the reply first of all! I think you're right about the signal being boosted larger than the supply rails. I might just have to bring it in to an oscilloscope and play around with it.

As for the opamp, the LM741 I'm using is just super noisy. It could just be the unregulated source I am using for the rails. Again... something I need to look into at a lab. I'll definitely be looking into the 4588 though! Thanks again friend.