Overdrive pedals are fundamentally "wrong" ways of processing an audio signal; as opposed to "right" ways that aim for making the output as identical to the input as possible - just louder. There is no single "right" way, but as with many things in life, there are always more "wrong" ways to do something than there are "right" ways. So the many hundreds and hundreds of designs and circuits that corrupt the signal and add varying degrees of harmonic content no present in the input should not surprise us.

But all of that is separate from whether a given circuit produces a noticeably different, and audible, consequence than another circuit. Sometimes, very different sorts of circuits can yield the same effect on the signal, while circuits that are near identical, but for one or two seemingly insignificant components, can yield very different effects.

In the particular case of the Tube Screamer, the chip was chosen because of availability and cost, with the sound being a negligible consideration. What makes the 4558 "special" in the case of the TS is the way it interacts with the components around it; in particular the feedback diodes. As Jack Orman demonstrated, however, one can use other dual op-amps and replicate much of the behaviour of the 4558 by simply sticking a 1k resistor in series with the diodes.

Are all overdrives merely variants on a TS? No. That said, many of them do use the basic gist of the TS, which is to place clipping elements in the feedback loop of an op-amp. You will know from your NAIT course (my wife is an '83 NAIT grad) that op-amps are designed to "want" to run full tilt. One sets the gain of the op-amp by providing varying degrees of negative feedback from the output back to one or the other inputs. Think of that feedback as putting on the brakes. The more efficient the path from the output to input, the harder one steps on the brakes. The capacitors in the feedback path provide an "express lane" for feedback of higher-frequency content, to severely limit gain in that part of the spectrum. We hear that as treble rolloff.

The overall efficiency of the negative feedback can be varied in several ways. Circuits like the TS provide an adjustable feedback path from out to in, but also provide a parallel path via the diodes. The diodes do not conduct until the output signal reaches around 500-600 millivolts. When the signal reaches that level, the diodes behave like an almost perfect feedback path, severely cutting the gain. BUT, once the gain is cut, the output of the stage is also cut and drops well below the signal level needed to make the diodes conduct. Conceptually, this is nearly identical to what takes place in a compressor: reduce the gain when the signal rises above some designated level. Small wonder that the TS has a "compressed" sound. And similarly, an equally small wonder that many other pedals that vary the threshold at which that negative feedback occurs - whether by number of diodes (like the Timmy, SD-1 et al.), or type (Hermida Zendrive, Menatone Red Snapper, et al.) - can sound somewhat similar but less compressed.

All of those are to be distinguished, in feel, from overdrives that use diodes going to ground, instead of in the feedback loop, to achieve clipping. In that instance, the gain of the stage is unaffected by the diodes. Whatever comes out of the op-amp is clipped by the diode complement on the output. Pedals in that category include the venerable MXR Distortion+ and Proco Rat, among many many others. The Analogman King of Tone goes so far as to allow the user to reconfigure the mode of clipping so that it can be either diodes in the feedback path OR diodes going to ground. Players like the KoT because those two variations provide different "feels".

However, and whatever, clipping elements are used, the sonic output is always a function of what one feeds in. Pre-clip tone-shaping matters. One of the goals of the TS was to arrive at a "smoother"-sounding clip. Since the amplitude of the lower wound strings is always hotter than the thinner unwound strings, achieving an equivalent intensity of clipping across all 6 strings was aimed for (and achieved for the most part) by reducing the bass content of the input to the clipping stage. That's what results in the legendary "mid-hump". Subsequent designs have aimed to re-introduce varying amounts of bass to achieve a fuller sound; albeit at the risk of disproportionate clipping.

I will recommend my good buddy R.G. Keen's analysis of the TS-9 circuit, that he posted on his long-running geofex site nearly 20 years ago: The Technology of the Tube Screamer

 

Should I even attempt to understand any of this?

Is my Box of Rock OD pedal based off of a Tube Screamer?

 

No. The BoR is essentially 3 cascaded near-identical Super Hard-On boosters. The Gain control is purely for the first one of the three, and the gain is fixed and slightly altered for the 2nd and 3rd. A 2-pole lowpass filter on the output rolls off the fizzies so that the Tone control can be used to alter what matters. Earlier versions omit the additional boost stage, which is also, not coincidentally, a 4th Super Hard-On stage.

Does that make sense?

 

I am very glad we have Mark as a resource on here.

 

While I am desperately trying to understand this I really don't.

Knowledge is power but should I take the time to investigate this? Is it worth my while?

 

What part do you want to understand most? I'm happy to cut your food up for you so it's easier to chew. My initial response to dorian2 was based on an understanding of what he had studied, but that doesn't need to leave you out. I'm always happy to get folks closer to their desired tone, and to be more self-sufficient in nailing it.

 

No. The BoR is essentially 3 cascaded near-identical Super Hard-On boosters. The Gain control is purely for the first one of the three, and the gain is fixed and slightly altered for the 2nd and 3rd. A 2-pole lowpass filter on the output rolls off the fizzies so that the Tone control can be used to alter what matters. Earlier versions omit the additional boost stage, which is also, not coincidentally, a 4th Super Hard-On stage.

Does that make sense?

This whole paragraph is in a foreign language to me. Would you mind dumbing it down for me into layman' terms?

A two pole bypass filter is what?

What makes the gain a fixed gain?

So what your saying is, as this pedal evolved it became more efficient and of better quality?

What is a super hardon stage?

Thx!

 

This whole paragraph is in a foreign language to me. Would you mind dumbing it down for me into layman' terms?

No problem

A two pole bypass filter is what?

A lowpass filter lets everything below a given frequency pass unaffected, and reduces content above that frequency. How much it reduces increases the higher up you go. This is usually described in decibels per octave or db/oct, for short. The more db/oct the steeper the rolloff, and more abrupt the cut. Each "pole" of such a filter produces an additionalk 6db/oct rolloff. So "two poles" produces 12db/oct. If you look at the parts labelled R17-C9-C10 on the far right of this drawing (which is the schematic for a ZVex Box of Rock), they form a single pole of lowpass filtering that begins to roll off treble starting just under 8khz. Because it is a single pole, that means content around 16khz would be 6db lower; 16khz being one octave above.

However, you'll see that R18-C11-C12 simply repeat what R17-C9-C10 do. So, we have not one, but TWO poles of lowpass filtering, which still begins just below 8khz, but reduces higher-frequency content by 12db when one gets an octave higher. So, more poles = steeper cut.

Look below at the Tone control. R15 and C8 also form a single-pole lowpass filter, providing a 6db rolloff, starting around 154hz. The other half of the TOne circuit is formed by C7-R16. That part forms a single-pole highpass filter, which has the opposite impact of a lowpass. That is, any frequency content above a certain frequency pass unaffected but frequency content lower than that is reduced. In this case the bass gets rolled off starting around 194hz. Rotate the Tone control in one direction and you get mostly bass with reduced treble, and in the other direction you get treble and mids, with reduced bass. So the Tone control of this type is also just a useful combination of high anbd lowpass filter sections; one pole of each. Folks in the pedal community will often refer to this configuration as a "BMP-type", since it is used in every single issue of the Big Muff Pi, in addition to a great many other distortion pedals...like this one. My buddy Jack Orman - who has been at this stuff longer than I have, if that's possible - has a number of useful tools on his site for thinking about filters. F'rinstance, here is a handy calculator: Guitar Pedals: R-C Filter Calculator And here is a nice little piece about modifying this type of tone control: AMZ - Guitar Effects & Tone Controls

What makes the gain a fixed gain?

Look at the left hand side of the schematic above. Better yet, look at the one below. This is the circuit for the ZVex Super Hard On booster. You can see - maybe not THAT easily, but with a bit of effort - that the components around the transistor labelled Q1 look an awful lot like th configuration in the SHO schematic below. The gain of that single-transistor circuit is set by the resistance to ground (GND) from one part of the transistor. Both schematics show that variable resistance is dictated by the setting of a 5 kilohm pot/control. Because the contact between the wiper and resistive strip inside that control is not absolutely perfect, it is subject to very brief discontinuities and re-contacts as you rotate it. These are not harmful, just irritating. But it results in what Zach warns users about when that control says "Crackle Okay".

Look at but further to the right in the BoR schematic and you'll see that both Q2 and Q3 have resistors going to ground from the same point on each of those respective transistors. Because R9 is set at 100R (ohms) and R13 is set at 330R, the gain in each of those stages is non-adjustable or "fixed". The 4-knob version of the BoR provides an additional boost stage, which is, once again, the same circuit as the one below. What is important is that it is preceded by the LEVEL control, so that a person can set the first 3 stages for max crunch at a reasonable level, and use the 4th stage to boost a bit for a solo.

So what your saying is, as this pedal evolved it became more efficient and of better quality?

Not exactly. Zach likes to re-use circuits as building blocks. In fact, the Super Duper 2-In-1 is a pair of SHO circuits, and the Box of Metal is FIVE, count 'em, FIVE cascaded stages that has an SHO at the start and 4 additional stages very similar to it, in a row.
What can I say? He likes the way these circuits clip when you pile them up, and lots of his customers do too. Of course, you won't know that unless you're Zach and do the I-wonder-what-would-happen-if experiments.

What is a super hardon stage?

Being as I am in mixed company, I will simply refer you to the schematic above.

Thx!

My pleasure.

There. Feel a little smarter?

 

BTW Mr. Hammer. I wish you were teaching the class OP Amps when I took this stuff. Especially considering our tendencies towards the guitar effects, which is a VERY small part (I know you know, others may not though) part of what these circuits are used for.

 

So when are you building yourself a pedal Lola? Can't wait for the outcome!!

 

Ya, as if! Lol

 

While that would be a delightful outcome, it is enough to simply understand how things work and why they sound the way they do.

When I was in grad school, 35 years ago, I took a course in my department that was all about machine-language programming and interfacing of micro-computers. It was a psychology course, taught by a psychologist. Huh? As he posed it to us, he was not expecting us to be able to do all the programming and hardware design ourselves. But he knew, and we knew, that somewhere along the line we would be using microcomputers for real-time control of experiments and data-acquisition, and he wanted us to understand enough about the inner workings of things that we could turn to the technicians in the departmental shop and tell them what we needed in enough detail that they could whip it up for us and it would do exactly what we needed it to do, right out of the gate.

The more you understand about your gear, the easier it is to get to a solution when things go wrong, isolate problems, ask for custom fixes in an informed manner and understand both the capabilities and limitations of things..

 

While that would be a delightful outcome, it is enough to simply understand how things work and why they sound the way they do.

The more you understand about your gear, the easier it is to get to a solution when things go wrong, isolate problems, ask for custom fixes in an informed manner and understand both the capabilities and limitations of things..

I would like to have a hands on approach under my belt!

While explaining things to me is very beneficial my mind has a much more dynamic understanding of things if I become involved in something physically as well as mentally.

Thank you for taking the time to explain things to me. I really appreciate this.

If left to my own devices I will take key words and find their meaning and apply that knowledge to the context of said subject.

This is a whole new frontier to me! I am not the kind of person to let this important information slip through my fingers hence the numerous questions I pose. I really want to and need to understand as much as possible!

Hope you don't mind.

 

I feel like I should enroll in an electronics course! I am going to investigate this.

 

I am having problems with my BoR pedal!

Maybe we can solve this situation together and I will have a better understanding of this specific pedal!

I will explain tomorrow! Right now it's dinner, a movie and then bed! I am not whining but I just got through the door from work! OT = extra $ for some extra quality gear.

 

The only amazing skill that I possess is my ability to solder!

I don't want to brag but I will anyhow! Lol

I did stained glass a number of years.

My soldering techniques evolved over the years! I really love soldering. It's so much fun. To be able to get a smooth bead of solder is very satisfying.

Now I need a basic electronics course to compliment my soldering skills.

 

I've been on the DIY stompbox forum, in its various incarnations, for over 20 years. In that time, through the myriad of resources of diferent kinds, I've seen many a person go from square one to expert helping others in avery short period of time. One of the advantages of the web is that if one explanation doesn't click, there will be another one out there that does. What facilitates individual progress is the firm belief that it is ALL eventually understandable, and that the limitations are not in the learner but in the compatibility between the learner's particular needs and the approach adopted by the explainer. If they don't connect, move on and you'll find something that does connect.

It's like learning a foreign language. It may seem hard at first, and a seemingly impossible task. But if even the dopiest people in that culture can learn it, then surely it does not require any special brilliance or aptitude. It's all a matter of being relentless and assured of the eventual outcome.

Since theremay be others with a BoR that would be interested, and this thread's header gives no clues to such persons, it might be best to start another thread. Or, if you are up to it, go to the diystompbox forum, and post your query there. People post repair or preliminary troubleshooting queries about commercial products there all the time. It is a phenomenally supportive and accepting group. We regularly see posts from people expressing what a blessed relief it is, compared to the snooty and unhelpful reception that novices receive in other places. I'm a regular there (coming up on 25,000 posts) so I'll be sure to see it and respond.