Sabro Drive One

I’m a real novice when it comes to how these effects work, but I’ve always wanted to learn so I figured I’d try to “design” one myself. You got to start somewhere, right?

There’s nothing new here whatsoever. There’s an op amp gain stage of relatively low gain with clipping LEDs in the feedback loop. After having read R.G.Keen’s article about the tube screamer over at www.geofex.com I thought it would be interesting to place the gain pot so it would simultaneously increase the gain and shift the frequency at which gain is rolled off towards the bass. If I’ve understood this about right the frequency gain roll off starts at approx. 500Hz at minimum gain and goes down to about 200Hz at full gain.

I used a “James” tone stack from Duncan’s tone stack calculator as a base and, helped by component values from the Wampler Triple Wreck and a few minor tweaks, ended up with what you see in the schematic above.

To give the final signal a little amplitude I added a make up gain stage of about 3 at the end. The thought was to also give this thing some boosting properties.

I built this in a 1590B enclosure and realized I didn’t really have enough room for both the “Bass” and the “Clip” switch, so I decided to drop the “Bass” one.

I’m very happy with how the build came together. For some reason I really like those stars.

But, to be honest, I’m not really that happy with the circuit result. I like the tone stack, but the gain stage ended up sounding a bit gnarly and unpleasant. Not sure why that is, but it indicates I have much more to learn yet ;)

Not really a recommended build. More of a curiosity.

Update (29.11.2011): Added C13 to remove some of the high frequency harshness I didn’t really care for. Sounds a bit better now.

Updated: 21 December, 2012 — 22:06

4 Comments

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  1. Yes, the last stage is an inverting summing amp. But who cares? It doesn’t make sense to put this pedal in a parallel looper, because it can add clean signal itself…
    I tweaked some parts values today and uploaded a new version.
    There’s a soundfile in this post: http://forum.musikding.de/vb/showthread.php?28362-Soundfiles-eurer-Treter&p=331988&viewfull=1#post331988
    That’s an older version of the circuit, with different input cap and silicon diodes for the hardclipping stage.

  2. Looks very cool, roseblood! If read this right the last opamp is inverting which would put the output out of phase with the input. Other than that it looks great. How does it sound?

  3. Have a look at this one, sounds pretty cool and has a lot of options:
    http://forum.musikding.de/cpg/albums/userpics/23524/rose_screamer_sym.gif

    -symmetrical supply (+/- 9V)
    -less couplin caps (JC Maillets “SRV special Mod”)
    -bass switch
    -wider gain range (from clean to lead)
    -clean signal can be added
    -lower noise because of better opamps
    -soft-/hardclipping switch

    Maybe C3 should be bigger, as in your layout for the Maxon OD-808 (which was starting point when I began to think about a “better” tube screamer.

  4. Nice one Harald – good to see some experimentation

    do you think that changing the clipping opamp would change the smoothness of the clipping ?

    cheers Hookey

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