What is a Tube Screamer?

I did a nice breadboarding exercise in one of the previous posts. I breadboarded TS808 – Tube Screamer. Pretty famous pedal, and this, sorts of continues my “Screamers” series of posts. I’ll go totally techno-philosophical with question – “What is a Tube Screamer anyway”?

What differentiates a TS from the other pedals? I’ll approach it from a technical point of view a bit, but you should totally hear from somebody who knows how to use it properly.

In the end, I’ll try to breadboard a simplified version of the pedal, just to see if we still get the same sound. In the adage of a “duck test” – if it sounds like a TS and it plays like a TS – it is a TS. We’ll see together how that goes.

If you don’t want to read this you can always just skip watch the video, but do check the page out as well – all relevant diagrams and links are here. Here’s the video:

Shaping the Sound

You should always use your ears to test a pedal. Let me get that out of the way first. No amount of theory or visual representations of frequency responses/sound waves/anything other than sound – should convince you that something sounds good or not. But, using some theory should help us get to a right ballpark of where we want to get to.

Let’s look at all possible parts of our TS schematic that might be shaping the sound:

Input and Output Buffers

From the previous post we know that there’s an input buffer and output buffer, and I’ve added filter cut-off frequencies there for the buffers:

I loosely added cut-off frequencies, for example – C8 and R12 form a high-pass filter with approximately 3Hz cut-off frequency. Cut-off frequencies, as shown on the diagram – are pretty much out of audible range. This means that the buffers should not affect the sound.

This makes sense, because TS808 does not have a true bypass. Bypassed sound goes through these buffers – and not affecting sound makes a lot of sense.

Hmmm … I’m taking notes – I should be able to eliminate these buffers if I’m using true bypass – right? Now, the buffers do provide an extra function though. For one – input buffer ensures high input impedance which we want. Output buffer provides low output impedance … which we want too. This just means that some thought has to be put in – I probably can’t just remove them right away.

Clipping Section

If I just look at the clipping section, there are few important bits shaping the sound here:

High pass filter with about 720Hz is ensuring that some of the low end is not as amplified as the rest of the frequency spectrum. It is a bit silly talking about attenuation when the minimum gain of this section is 20dB (or 10x).

Now – the gain and the filter – this is affecting clipping. Clipping is symmetric – there are two diodes – one diode clipping positive, the other clipping negative side of the signal. It is important to stress out that – this is clipping part – tone section that follows is affecting the harmonics and overtones introduced by the clipping section.

The last filter – low pass filter formed by C4, R6 and RV1 will most likely not affect the signal that much. Especially if you don’t crank up the gain to the maximum where the cut-off frequency is at it’s minimum: around 6KHz. It will soften the distortion a bit around maximum gain.

Note though that the speakers used in most amplifiers have a sharp drop off after 5KHz. Maybe if you plug it in directly into DI and try it out you can hear more difference – give it a try, so how it works both ways.

At any rate – the gain – 21-41dB, the 720Hz cut-off frequency of the filter and symmetric clipping – that’s what matters the most.

Tone Control Section

Finally we have tone control section and level control:

Finally – tone control section. Very important, the first filter with 723Hz cut-off frequency attenuates lots of harmonics introduced by the preceding section, and the op-amp compensates somewhat for this attenuation.

You can control the gain of that section from about 0dB to 13dB – but be careful inferring anything out of this. The cut-off frequency is about 3.3KHz – but that just means that the combination of the passive filter and this active filter forms a bandpass filter 723Hz-3.3KHz at maximum tone setting.

Essentially – no treble on minimum setting, cut-off at 3.3KHz at maximum setting. That’s why the tone control response (just that section) looks like this:

If we look at the passive + active filter responses independently – combined they give the graph above, but decomposed here how they look like:

The passive filter is slightly affected by how the active filter is implemented – in blue. In green you can see where that 0-13dB gain is coming from for the active filter.

Note on the filter at the output formed by C7, R11 and Level pot (RV3) – that should not affect the signal significantly, or at all. The filter should only affect audible spectrum at the very low output levels at which, I’m sure, most of us won’t even notice the difference.

Conclusion

If we circle back to the whole diagram – there’s several important bits. Probably the most important filters are the one for clipping at about 720Hz and the other at the input of the tone section – 723Hz. That gives the recognizable hump to the Tube Screamer’s frequency response.

Gain of the clipping section is important, but more important is that the clipping is symmetric. I would say that minimum gain is important because that makes the minimum gain setting very much usable.

Finally – the tone section is important – I’m not sure that 3.3KHz filter is the defining feature though. Anything above 3.3KHz would cover frequency response that TS covers, but if you never use it at the max tone setting you can get away with lots of fiddling there and still get the sound that you want.

Can we actually get the sound ourselves?

So does my breadboarded version sound as the actual commercial pedal? Well, they sound pretty close. If you want to torture yourself a bit you can try hearing my half an hour comparison video. There’s no talking – just playing the same thing over and over again – it is tedious.

Here’s the video – watch it at your own risk:

What I learned was that I hate editing these kinds of videos – oh boy, how tedious it is. But I also learned that while the pedals don’t sound exactly the same in the video – they are pretty close. In some cases I got the sound exactly the same, in some cases not as much.

If you saw my previous video or read the post – I did not use the same taper for tone pot as the actual pedal. My tone control is thus squashed, so a very small change in the knob position may affect the tone a lot. And that’s most likely what is the difference. The takeaway point should be – if I fiddled with the knobs a bit more – I could’ve probably get them to sound the same.

Minimal Version

Finally – let’s talk about the minimal version of the pedal. The whole post about what Tube Screamer is was really to get to this point. There are some parts of the pedal that we can tweak, or even remove, and the overall sound should not change.

So if I just remove buffers, things should work just fine? Well, I did elude to it – that won’t exactly work. We’d need to preserve high input impedance for example. But fear not – we can always call our good friend TL072 to help.

The other thing is – the way the input buffer is wired up – we’d be missing pull-down resistor if we were to just remove the buffer. The resistor helps with the pop when the pedal is engaged so it’s a good idea to put something in it’s place.

Without further ado – here’s the minimal version of TS808:

If you notice – I changed the op-amp, then did some resistor adjustment to increase input impedance. I removed quite a few components but everything bar the 2 resistors and replaced op-amp stayed the same.

Well, that should sound the same as the “full” version. If you haven’t seen the video by now – maybe it is good idea to check it out – it’s at the top of the page. Until next time!