Solenoid Drive (modified)

H

HamishR

Well-known member
I worked out how to add the bass pot rather than relying on the switches inside - not too difficult thanks to the resources online these days! So now it's built and working. I had to reorganise the pots to add the bass pot so I though why not rearrange everything so I don't get confused??

On almost all of my overdrives and distortion pedals I like to have gain top right, volume top left, bass lower left, treble lower right. If there is a midrange control I place it between the bass and treble which means removing the Tayda pot condom - some masking tape (a couple of layers) does the trick of insulation. So I rearranged the knobs which meant short wire traces around the pedal but it actually wasn't as busy as I feared.

But now I'm wondering if all the trouble was worth it! It's a bit of a strange one, this pedal. I think I can see the kind of tone they were chasing - it's very Hendrix Strat-into-Marshall sounding - well, with a Strat. Plenty of thump but the trebles sound a bit thin to me. It's a cool sound but light years away from the sound I normally head towards. As usual the board is excellent - even with all my carry on it worked flawlessly first time and didn't mind my messing it up a bit

I think I see why they made the bass adjustable with switches rather than a pot, too. Maybe it was for space, but it seems to me that the mid boost pot acts suspiciously like a bass pot as well, so you're almost doubling up. It's not really a traditional midrange control. There is no TS-style mid-hump here which is probably why the trebles seem thin. Anyway you guys with all the technical knowledge will know what is going on better than me. All I know is that the bass and mids controls are a little confusing to use. Soulext.jpg Soulint.jpg
 
I think it's a fair point that the labeling of those controls is confusing.

The midboost control is wired like the Voice control in a Zendrive. It adjusts both the gain and the bass cutoff frequency (almost all dirt pedals cut bass before clipping, and this is a big contributor to the voicing (example, TS cuts 720Hz and below).

The gain adjustment can be seen by this formula: Gain = 1 + R2/R1 (see attached pic). R1 is the midboost pot + the 1k8 resistor. As the midboost pot is turned up, the gain increases. Of course, the gain is also adjusted by the gain knob, because R2 = the gain pot + the 18k resistor.

The bass cutoff frequency can be seen by this formula: F = 1 / (2*pi*R*C) where R is the midboost pot + the 1k8 resistor, and C is the 100n cap. So at midboost pot max the cutoff frequency is 884Hz, and at min its 135Hz. Given those frequencies, it makes sense that it would sound sort of like a bass pot.

The bass pot adjusts how much post-gain bass boost, centered around 120 Hz, so it's technically a different range than the mid pot, but in practice there's probably some overlap in the sound.

As to the lack of tube screamer mid hump... the TS has the 720Hz bass cut before clipping, and also a 720Hz treble cut right after clipping, but before the tone pot. The tone pot then either further cuts treble, or actively boosts it back up. I think that combo of bass cut at 720 and treble cut at 720 is what gives the TS it's mid hump. The Soul Driven doesn't have that 720 treble cut before the tone pot. It just has a single passive treble cut (again, cutoff frequency is F = 1 / (2*pi*R*C)). This tone pot configuration is like the Timmy, Rockett Animal, or Zendrive, all of which are regarded as less mid-humpy, more transparent than the tube screamer. Also, the diode configuration (2 each direction instead of 1 each direction) sounds less compressed, and can contribute to a pedal feeling more "transparent" (again, see Timmy and Rockett Animal).

Hopefully this is helpful. One of my favorite things about getting into DIY and learning about the circuits is that I can get an idea of how a pedal might sound (or at least what it compares to) by looking at the schem. This is especially true for op-amp with diode clipping circuits. Of course there's no substitute for hearing it for yourself, and small component changes can drastically change the sound.

FIY the pic is from the electrosmash tube screamer article, which is how I learned most of this.
 

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Thanks, I really appreciate your response even if some of it (well rather a lot of it) is hard for me to grasp. I absorb technical info very slowly. I am getting to grips with the way diodes affect clipping - gradually! For example I find that I usually prefer pairs of clipping diodes over one each way, for exactly the reasons you mention.

As you say, the "midboost" knob affects gain in a very noticeable way. And yes, there is a lot of crossover. I don't really mind building a pedal only to find it's not to my taste. I like to build and realise that not many things will really click. There are only so many circuits which will suit any player.
 
HamishR said:
Thanks, I really appreciate your response even if some of it (well rather a lot of it) is hard for me to grasp. I absorb technical info very slowly. I am getting to grips with the way diodes affect clipping - gradually! For example I find that I usually prefer pairs of clipping diodes over one each way, for exactly the reasons you mention.

As you say, the "midboost" knob affects gain in a very noticeable way. And yes, there is a lot of crossover. I don't really mind building a pedal only to find it's not to my taste. I like to build and realise that not many things will really click. There are only so many circuits which will suit any player.
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