This Week on the Breadboard: The Celestial Boost

[Chuck D. Bones]

This circuit was inspired by the Lovepedal COT and Xotic BB Preamp. The COT is essentially a hopped-up Electra Distortion. I replaced the COT's BJT with a JFET, added a clipping option and active tone controls similar to the BB Preamp.

It's still early days, but I'm pleased with the results so far. Q1's Vgs,off is -2.1V. Any JFET in that neighborhood should work. Like the COT, the GAIN control varies both the negative feedback at Q1's source as well as the bias point. This helps keep the clipping fairly symmetric as the GAIN is turned up. With the CLIP switch in the OFF position, the first stage runs clean at low GAIN settings. The LEDs are always in the circuit and cause Q1 to limit gracefully instead of saturating. When the CLIP switch is in the ON position, the Si diodes are engaged and we get some smooth, mostly symmetric clipping. I placed the VOLUME control between the 1st and 2nd stages to maximize the available headroom. The 2nd stage is a variable Mid Boost centered at 750Hz. I "borrowed" that part of the circuit from the DSM OmniCabSim. The 3rd stage is a James-style active tone stack, derived from the BB Preamp. The maximum clean output is 5Vp-p. This is not a high-gain pedal. It provides boost, tone shaping and some distortion when the CLIP switch is on. Any decent dual opamp will work; I went with the NE5532A this time around.



Left to right: VOLUME - BASS - MID - TREBLE - CLIP - GAIN

 

[Feral Feline]

I just popped this in the Wish List...

https://forum.pedalpcb.com/threads/celestial-boost.16845/

I know it's early stages, and it might get tweaked some more etc, but I love boosts.

 

[Chuck D. Bones]

A bit of tuning would be req'd for use with bass.

 

[cooder]

Another one on the list then, yay!

 

[jjjimi84]

I like the way this is shaping up! With the amount of eq capabilities this would end up being a really great always on tone shaper

 

[music6000]

Something Like This:

 

[Chuck D. Bones]

I thought I'd try adding Fuzz & Distortion options. This is where I ended up. If Boost is all you want, then the previous circuit does that nicely.



I added anothe gain stage (Q2). The toggle switch has three positions Boost, Fuzz and Distortion. It's kind of a BFD One half of the MODE switch selects the clipping diodes. I stacked Germanium, Silicon & LEDs in series. In Boost mode, all three pairs are in circuit and the clipping threshold is high. In Distortion mode, the LEDs are shorted out and the clipping comes from the Ge & Si diodes. In Fuzz mode, the LEDs & Si diodes are shorted out and the Ge diodes do all of the clipping. The other half of the MODE switch controls the gain and bass response of the 1st stage. Low gain for Boost, medium gain for Distortion, and high gain with some bass cut for Fuzz.

 

[steviejr92]

How did i miss this?! I know what im building soon!

 

[jjjimi84]

I want this!!!!

 

[Chuck D. Bones]

How did i miss this?! I know what im building soon!

I suggest that you try the first circuit and see if that does what you want. If you think the GAIN knob doesn't go high enough, then try the 2nd circuit.

 

[steviejr92]

Will do! That clipping section looks very interesting! I like the movement of the Volume knob, definitely going to try to implement that somehow in the future.

 

[music6000]

I thought I'd try adding Fuzz & Distortion options. This is where I ended up. If Boost is all you want, then the previous circuit does that nicely.

View attachment 46842

I added anothe gain stage (Q2). The toggle switch has three positions Boost, Fuzz and Distortion. It's kind of a BFD One half of the MODE switch selects the clipping diodes. I stacked Germanium, Silicon & LEDs in series. In Boost mode, all three pairs are in circuit and the clipping threshold is high. In Distortion mode, the LEDs are shorted out and the clipping comes from the Ge & Si diodes. In Fuzz mode, the LEDs & Si diodes are shorted out and the Ge diodes do all of the clipping. The other half of the MODE switch controls the gain and bass response of the 1st stage. Low gain for Boost, medium gain for Distortion, and high gain with some bass cut for Fuzz.
View attachment 46843

 

[Chuck D. Bones]

I like the movement of the Volume knob, definitely going to try to implement that somehow in the future.

The Mid boost and Bax tone section have gain when the tone controls are turned up. Putting the VOLUME control in front of the tone sections allows us to dial the signal down so we can avoid saturating the two opamps.

 

[steviejr92]

Ahhhh ok im familiar with this!

May i ask what R7 is doing? Is that achieving a higher series resistance for the Diodes?

 

[Chuck D. Bones]

That's right. It softens the knee a little bit, to kind of mimic the V/I curve of Germanium diodes.

 

[steviejr92]

That 2nd one is a real BFD

Is Q2 germanium? This is insane! If so is that adding a bit of character amongst those diodes clipping?

 

[Chuck D. Bones]

You ask some good questions.
I think the circuit would work fine if Q2 was low-HFE Silicon, but it was easy to go with Germanium so I did it. I tried a few different Ge transistors: AC127, 2N1308, 104NU71 and MP38A. The last two are my favorites, but they can all be made to work by adjusting the bias with R8 and R9.

D1 & D2 should be low-leakage. You can also try Schottky.

If you look at at the first two stages, it pretty closely resembles the FET-Ge boost. When I got the idea to add a 2nd gain stage, I looked at four circuits: COT50, Emerald Green Distortion Machine, Pink Purple Fuzz and FET-Ge boost. I tried a few different configurations in the Celestial Overdrive, and ended up with the circuit shown above. I have a couple more component value tweaks to try, but it's 90% there. It's not a high-gain pedal, but there's plenty of compression and sustain when it's in Fuzz mode. between the MODE switch and the tone controls, the are a lot of voicing options. This is is no one-trick pony.

 

[steviejr92]

This is the ultimate drive IMO. What more do you want?

For D1 and D2 I’m imaging if you wanted to go Schottky you can run the 100 ohm resistor back in series and get similar response. I would also imagine if you didn’t have the ge diodes or didn’t have low leakage ones this could be a good option?

 

[Chuck D. Bones]

Yup, good idea. They don't have to be super low leakage.

 

[steviejr92]

It's weird I've had more luck finding low leakage GE transistors than I have low leakage GE diodes.

I have a crap ton of GE diodes and the ones ive found to be low leakage (almost running out) are the 1N34As i have. All the DXX series seem to be high leakage. I put a pair in a Big muff and it just didn't sound good. Lost all of its "mojo". I do have those D18s though ive never used them, never even tested them...Need to get on that...

EDIT: Those D18s are are good to go measuring over 2M in some cases.