SD1 - try getting a real transparent overdrive - I'm not able

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nooneknows
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Post by nooneknows »

Hi,

since we are again in lockdown and I had to occupy my time, I thought What about taking out one of my old SD1 out the drawer and mod it to get the most transparent overdrive I can get ,like my Timmy for example? I love the Timmy transparency, that pleasant grit on the highs, the crunchy round bass.
My idea was to get rid of all the component that trim the bass and cut the high, flatting the mid hump, It turned out it's not that easy as I thought.

so (referring to Boss schematics):

- I think the SD1 buffer is pretty transparent, the stock 18nF C2 value after an emitter follower seems to fit right without big tone shaping, anyway I enlarged it to 100nF;

- I raised R6, starting with 10K, to tame down the dist and lower the bass rolloff freq to 338 Hz - it farts, I tried lowering the cap too but it ended up that with a roll off below 600 Hz the bass starts being unpleasant;

- played with R7-C4 and R8-C5 trying to enlarge the passing band at the output of the dist section, no luck treble starts being squealing and , however the mid hump roundness is always present;

- got rid of C6 - trebles like nails on a blackboard, so I modified R8-C5 to lower the cut off but the mids then came out again;

- in a final attempt I removed the tone control, reshaping the tone stage as a plain buffer, in a OD1 style, no cap around : again, mids still there, scratching highs. I had to put a 10nF on C6 again to control them;

But if I take a look at the Timmy topology, apart the variable bass and treble cut, well it's not that far from a SD1 without the tone control and with non inverting stage instead.
Where am I wrong? is there anything I am understimating? why I'm not able to get the same kind of transparency ?

Any hint is welcome,
thank you

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mauman
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Post by mauman »

Maybe this will provide some ideas? https://modyourownpedal.com/blogs/news/ ... drive-mods

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nooneknows
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Post by nooneknows »

mauman wrote: 07 Nov 2020, 05:02 Maybe this will provide some ideas? https://modyourownpedal.com/blogs/news/ ... drive-mods
thank you (btw, the schematics in the link contains errors for example it lacks the 1st opamp feed to ground that shapes the bass rolloff corner) the boutique mod reported there enlarges coupling caps, remove C6 and implements the TS tone stack, I've done it several type of this back when I bought from Brian one of his first books on modding.

What I noticed is that, however, the mid hump and the characteristic of the SD/TS style pedal always came out, what I don't understand is why I'm not able to obtain the transparency of a Timmy that shares the same topology even if I remove what I think are the components which provide the hump (I always refer to it because it's the overdrive I like the most, always in my pedalboard since day 1, thank you PaulC forever, one of my best purchase ever).

Actually my SD1 has become this:
boss-sd1-con mie modifiche OD1.gif
as you see, no particular filter in the signal path apart C6 as mentioned above, to cure the squealing highs, and the cap across the feedback diodes to round the corner of the clipped signal

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Klay1975
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Post by Klay1975 »

So by changing the opamp configuration it becomes more transparent? (Inverting)

Is your mod r6 and c3 still creating a mid hump of 723Hz ?

C2 hpf lowered to around 30hz?

Thanks

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