What clips first: opamps or diodes

K

kweefthief

Active member
I just watched the most recent TPS video and them talking about diodes made me think: what do we hear clip first usually?

I bet it's varies from circuit to circuit but i'm curious nonetheless! I tend to keep the gain on my pedals below 3 o'clock so this is very interesting to me.
 
As far as I know It depends a lot on the set up where it happens and what happens, in some cases the op amp wont clip at all, but if you're running a rat/250 style hard clipping for example, the diodes are clipping to ground at whatever the diode forward voltage is, somewhere around 0.7v, which is going to be well before the op amp hits the rails on a 9v power supply.
 
Really not possible to answer this any way but "it depends".
What is the forward voltage on the diodes, soft or hard clipping, what kind of op amp, voltage at which the circuit runs, is it a stage that is already boosted by a previous stage, etc.?

If you ever built a Rat or TS with a clipping switch where you can also select "no diodes", you will find that you get clipping with the gain high even without diodes, but you'll have more clean headroom (usually). The way that the op amp clips does contribute to the tone as well especially on high gain settings, which is why LM308 Rats tend to sound more fuzzy and saggy than other op amps (and I know people will disagree with me, but I do in fact have a fancy Rat with an op amp rotary with which I have tested this extensively).

Transistors certainly do clip and you can also have soft and hard clipping in/after transistor stages. Usually transistor clipping sounds "nicer" than op amp clipping, which is why most op amp circuits have diodes to help out with the clipping character. Cascaded JFET/MOSFET circuits like the Plexi Drive or Box of Rock have no diodes, a Big Muff has "soft-clipping" diodes in 2 of its transistor stages and Electras like the EQD Speaker/Special Cranker are basically a transistor driving hard clipping diodes.
 
jesuscrisp said:
Really not possible to answer this any way but "it depends".
What is the forward voltage on the diodes, soft or hard clipping, what kind of op amp, voltage at which the circuit runs, is it a stage that is already boosted by a previous stage, etc.?

If you ever built a Rat or TS with a clipping switch where you can also select "no diodes", you will find that you get clipping with the gain high even without diodes, but you'll have more clean headroom (usually). The way that the op amp clips does contribute to the tone as well especially on high gain settings, which is why LM308 Rats tend to sound more fuzzy and saggy than other op amps (and I know people will disagree with me, but I do in fact have a fancy Rat with an op amp rotary with which I have tested this extensively).

Transistors certainly do clip and you can also have soft and hard clipping in/after transistor stages. Usually transistor clipping sounds "nicer" than op amp clipping, which is why most op amp circuits have diodes to help out with the clipping character. Cascaded JFET/MOSFET circuits like the Plexi Drive or Box of Rock have no diodes, a Big Muff has "soft-clipping" diodes in 2 of its transistor stages and Electras like the EQD Speaker/Special Cranker are basically a transistor driving hard clipping diodes.
Click to expand...
very interesting thanks for the detailed explanation! what would happen in a typical blues breaker?
 
You can see a lot of this happen visually using a free simulation software called LTspice. I have a bluesbreaker put into the software here. This is messy because I didn't want to make a ton of images to make it really easy to see what happens where.
green is a 0.3v input sine wave at 1khz, trying to approximate some guitar pickup output level here.
blue is after the first gain stage, sweeping the pot 0%, 25%, 50%, 75%, 100%
red is the inverting stage with clipping, you can really see the wave form distort here
teal is after the tone stage right before the volume, this makes the waveform look pretty wild and harder to tell whats going on, but I thought it was interesting so I left it.
1682808145461.png
 
kweefthief said:
very interesting thanks for the detailed explanation! what would happen in a typical blues breaker?
Click to expand...
Again depends on what you have. In general the BB has a "dual gain stage" with a gain control that affects both the first and second half simultaneously. The Made in England 1v1 has very low gain with most of the distortion happening at the very end of the sweep. With the 2 diodes in series per side and the series 6K8 resistor you have more headroom from the diodes themselves too. The made in Korea 1v2 has much higher gain in the first half of the gain stage with a much brighter overall tone as well. I'd say you get substantially more op amp AND diode clipping from the 1v2 as the first stage is pushing way more volume into the second (1v1 "boosts" up to 17 dB, 1v2 up to 34 dB). If you REALLY want to know what is doing the clipping though, breadboard or build it and try with different diodes or no diodes. Otherwise I'm in the camp of "I don't care if it sounds good".

DGWVI said:
I'd be interested in seeing more of this setup, if you've got time
Click to expand...
It's called the Drunk Beaver Bat series. I have a Heavy Bat that includes LM308, a soviet 308 copy, LM741 and TL071 on a rotary. Let's you mess around with different op amps in a Rat with the EXACT same parts tolerances and without physically removing the ICs to do so. As close as side-by-side op amp comparison as you can get.
 
jesuscrisp said:
It's called the Drunk Beaver Bat series. I have a Heavy Bat that includes LM308, a soviet 308 copy, LM741 and TL071 on a rotary. Let's you mess around with different op amps in a Rat with the EXACT same parts tolerances and without physically removing the ICs to do so. As close as side-by-side op amp comparison as you can get.
Click to expand...

Rad. Looks like theirs just uses a 2p4t. Might whip something similar up on the next Rat or Blue Clipper I build
 
I’ve built a RAT style circuit recently. It’s really all about its setup. If you have the opamps gain set too high it doesn’t matter what the diodes are doing you’ll hear only the opamp.
 
steviejr92 said:
I’ve built a RAT style circuit recently. It’s really all about its setup. If you have the opamps gain set too high it doesn’t matter what the diodes are doing you’ll hear only the opamp.
Click to expand...

After I first played around with ProCo's Clean Rat, I discovered how much I like the sound of the straight Op-Amp clipping, so that's why I'm not terribly interested in diode options. For my use case, they just limit the volume and the dynamics once the opamp starts struggling. But, the chip does matter quite a bit once you get up to around noon on the Distortion dial. The 308 and OP07 are my favorites because they start collapsing sooner, but 301, 5534, 709, and 3130s are all fun to play around with
 
The OP07 is what I ended up going with in mine. I wanted to try a 308 but I was out. I want to give an op amp based rat a try sounds interesting
 
You must log in or register to reply here.

Similar threads

Chuck D. Bones
Opamps for Dummies - part 1
Replies
16
Views
2K
Chuck D. Bones
Chuck D. Bones
Chuck D. Bones
Opamps for Dummies - part 4
Replies
2
Views
744
Chuck D. Bones
Chuck D. Bones
Chuck D. Bones
Opamps for Dummies - part 2
Replies
11
Views
1K
hamerfan
hamerfan
Chuck D. Bones
Opamps for Dummies - part 3
Replies
5
Views
948
Chuck D. Bones
Chuck D. Bones
MichaelW
DEMO Southern Belle Ge (Greer SouthLand)
Replies
9
Views
1K
Barry
Barry
Back
Top Bottom