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Breadboarding Overdrive

Tube Screamer Like Pedals

Now, let’s talk about Tube Screamer like pedals. Maybe not in the sound but more in the circuit topology. In this one I’ll be covering Boss Super Overdrive – SD-1.

This post continues building on previous posts: breadboarding the pedal, how it shapes the tone and I even did an alternative version.

To be honest, this post can be extrapolated into nearly any pedal using Tube Screamer like circuit. I’ll cover quickly the schematic, and then do a quick analysis of it – in comparison to the TS808 circuit. I’ll compare the sound of the SD-1 pedal to my breadboarded version, and I’ll try to get them to sound the same. Who knows how will this turn up – you’ll have to watch the video to see for yourself.

Btw, I threw in SPICE circuit in there if you want to play with it yourself.

Schematic

Let’s have a look at the schematic:

Boss SD-1 Super Overdrive schematic
Boss Super Overdrive schematic (click for full size image)

I marked all components to match what I have on TS808 circuit in one of the earlier posts. Designators are offset by 400, so R401 on the schematic here corresponds to R1 on TS808 schematic, C411 corresponds to C11 and so on.

The circuits are nearly identical. I mean, lots of components have different values, but the circuits are nearly the same. I was flipping between the two images in the video and only if I had aligned them better there would be very few differences.

Analysis

OK, let’s get into details here. I’ve annotated the schematic as in some earlier posts:

Schematic of Boss SD-1 with elements that might be affecting the tone annotated.
Tone shaping elements of the Super Overdrive pedal (click for full image)

You’ll notice that there’s a lot of highlighted elements in red indicating that they might be affecting the sound. I also added a little warning sign onto the ones that are different from TS808.

For example, 720Hz high pass filter in clipping section affects the sound but has exactly the same cut-off frequency as TS808. On the other hand, SD-1 has much more gain than TS808 in the clipping section.

Let’s zoom in a bit.

Clipping Section

Let’s look into the clipping section.

Annotated schematic of clipping section of SD-1 pedal
Clipping section of SD-1

In the post about what does it mean to be a Tube Screamer, I covered this section in more detail. Looking at this on the first glance there are lots of differences.

At the input of the section, we have a filter attenuating anything under 88Hz for example. That ought to make some difference, but I wonder how big of a difference it makes with all the other filters in the circuit.

720Hz high pass filter is exactly the same as on TS808, but Super Overdrive has a greater range of gain. Minimum gain is 17dB for SD-1 vs 21dB for TS808 and it’s 47dB vs 41dB. 47dB vs 41dB is really double the gain!

Finally, in my opinion, the biggest difference between the two pedals is in clipping. SD-1 has asymmetric clipping compared to symmetric clipping of TS808. SD-1 will be louder for sure, but also, at higher gains, it must be more compressed I would think. Not to mention difference in harmonics added due to the clipping.

One thing to note though is the taper of the (over)drive pot. SD-1 has linear pot, TS808 has log pot. To compare them, SD-1 around middle will be about the same gain as TS808, but I might have to go down all the way to 8 o’clock on SD-1 to match 12 o’clock on TS808.

Tone Section

Tone section differences look even more pronounced:

Annotated tone section of Boss SD-1
Tone section of SD-1

The tone sections look nearly identical (if we ignore component values). The only difference is C413 cap that was added to SD-1. There’s more gain in the active tone section, and the filter values are all very different.

It would be very easy to fall into a trap here thinking that the frequency response has to be vastly different between the pedals. But keeping in mind how the tone section works – input low pass filter is compensated by the active section with high pass filter.

The very high cut-off frequency and high gain of the section just means that this compensation may go for longer, but there’s another low pass filter there to yet compensate for that as well.

All said and done, this really is just an adjustable low pass filter (I’m oversimplifying it I know, just roll with it). Way more controllable one though, compared to a passive adjustable filter in RAT for example (RAT filter controls 300Hz-30KHz range cut-off frequency, SD-1 roughly 400Hz-4KHz).

Tone Section Frequency Response

What I said earlier, let’s look into the frequency response of the tone section and compare SD-1 and TS808.

SD-1 Tone control on the left

OK, when you move the slider to the right you reveal SD-1 tone section response, when you move to the right you reveal TS808 tone section response. I forgot to mark them. If you look closely, huge part of the sweep overlaps.

If I move the SD-1 graph down a bit (imagine I roll down output level a bit), I can align the graphs better. Look at the result:

Aligned SD-1 and TS808 tone section frequency response

This all looks very much the same. Quite frankly, unless you can notice very, very small differences in sound (maybe if you have a trained ear) I doubt you’ll hear any difference. Now, this is all great in theory, but we’ll need to definitely confirm this in practice.

If this is all true, all I need to do is add a diode to make my breadboarded TS808 version sound like SD-1 for the first half of the overdrive knob rotation.

SPICE

Here’s the SPICE file if you wish to play with it yourself:

Trying it Out

Finally, let’s try this out:

That went pretty well I think. While I did get very close to getting the sound, there’s still some difference in sound, especially audible when I was playing chords. I’ll try at some stage to get sound to more closely match the two pedals, but that’s for another day.

Now, it is super silly to mod super expensive pedal to get way cheaper pedal. But the mod works both ways. There’s even a video out there by Wampler Pedals covering how to modify SD-1, part by part to get TS808 – a bit excessive I think, but totally doable.

What I’m going to do next is look into another SD-1 like pedal. But more and more I think about this, I’m looking into a possibility of designing a pedal that would have different clipping options, maybe slightly different tone section to actually get some different voicing etc. Stay tuned 🙂

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