This Week on the Breadboard: The Celestial Boost

[Chuck D. Bones]

Transistors can be used as diodes, so that's one way to obtain low leakage GE diodes. All of the D2x and D9x diodes I've measured are quite leaky. They can be used in a Klon or Distortion+ and that's about it.

 

[steviejr92]

Funny the first time i ever used those D9x diodes were in the Klon. They sounded incredible and that gave me the impression that GE diodes in general were the way to go in every cicuit. Boy was i wrong there are so many factors involved.

Im going to give that a try in the future. Thats a great tip!

 

[Chuck D. Bones]

There are a few ways to connect transistors as diodes. Assuming the transistors are NPN:
1. B = anode, C = cathode, E is not connected.
2. B = anode, E = cathode, C is not connected.
3. B = anode, C = cathode, E is shorted to B.
4. B = anode, E = cathode, C is shorted to B.

1 & 3 will have slightly higher Vf.

 

[steviejr92]

Question: Why do scenarios 1 & 3 have a slightly higher Vf versus the other two situations?

Im assuming if they were PNPs the same can be said about the Vf but in scenarios 2 & 4?

 

[Chuck D. Bones]

Question: Why do scenarios 1 & 3 have a slightly higher Vf versus the other two situations?

It has to do with the geometry of the transistor. The C-B junction must be able to sustain a much larger Vr than the E-B junction. The C-B junction must also have lower capacitance than the E-B junction in order to maximize switching speed and bandwidth. These requirements cause the C-B junction to be thicker than the E-B junction and that leads to a larger Vf.

Im assuming if they were PNPs the same can be said about the Vf but in scenarios 2 & 4?

No. 1 & 3 still have the larger Vf with PNP transistors. The anode & cathode nomenclature is swapped, that's all. Ge PNPs are generally (but not always) lower leakage than NPNs, so PNPs are usually the better choice for making low-leakage diodes. That being said, you still have to test every one of them to be sure the leakage is low enough.

 

[steviejr92]

Ahhhh ok i get it now. Thats just how they're built and function. Makes sense seeing as (most times) you see BJTs with their Collector connected to the VCC.

 

[bhcarpenter]

That mode selection is brilliant! I might try swapping D1 and D3 so that the “Fuzz” mode’s clipping is asymmetrical like the Golden Fleece.

 

[Chuck D. Bones]

It's already asymmetrical.

 

[bhcarpenter]

It's already asymmetrical.

Do you mind explaining how? Via Q2 clipping?

 

[Chuck D. Bones]

Q2 and R10 do not drive the diodes symmetrically. At lower GAIN settings, the drive is nearly symmetric and we see only odd-order harmonics in the spectrum. At higher GAIN settings, Q2 is able to sink a lot more current than R10 can source. The result is the peak current in the even numbered diodes will be higher than the peak current in the odd numbered diodes. This asymmetric clipping causes even-order harmonics to appear. At even higher GAIN settings, Q2 can be driven into cutoff (Ic goes to zero) and we see large even and odd-order harmonics.

In FUZZ mode, Q1's gain is high, causing it to contribute 2nd & 3rd harmonics.

 

[bhcarpenter]

Whoa that’s really neat. Thanks for the explanation!

 

[Chuck D. Bones]

As the note decays, the various harmonics fade in and out. Q2's bias shifts also.

 

[spi]

Very nice. I was dreaming up what overdrive pedal I would design and came up with a 3-stage:
COT50 -> ODR1-based soft clipping op-amp stage -> Baxandall tone. Still haven't gotten around to breadboarding this, but your circuit has a lot of similarities.

 

[Chuck D. Bones]

Great minds think alike, ha ha ha ha ha.

 

[Chuck D. Bones]

OK, looks like I'm done. I did some things to increase the brightness. Increased R2, decreased C4 & R20. I added C19 & R13 to add some brightness when it's in Boost mode. I also bumped VR2 back up to A250K.

 

[steviejr92]

C19 and R13 are creating a high pass filter correct? Just challenging myself!

 

[Chuck D. Bones]

Yes. Any impedance between Q1-S and GND makes negative feedback which reduces Q1's gain. Before this latest mod, in Boost mode the only thing between Q1-S & GND was R4. With C19 & R13 in place, there is a now path around R4 made up of C2 + R6 + C19 + R13. This causes Q1's gain to start rising above 600Hz and level out around 3.5KHz. The total gain increase from 250Hz to 10KHz is 7dB. The easiest way to see this is with LTSpice. When we talk about gain, it makes more sense to look at Q1's drain current since Q2 is current-driven by Q1. The plot below shows Q1's drain current as a function of freq. The roll-off below 80Hz is due to C1 & R2.

 

[steviejr92]

I need to work with LT spice more this is pretty fascinating.

It’s pretty interesting that by you increasing the gain you also increase the brightness. Makes sense whenever I turn my treble knob up on my drives it also sounds like the gain is being increased a bit. Especially when maxed out.

 

[fig]

The little bit of exposure I've had to it was very educational. At some point I'd like to dig deeper into it. I side-railed it for another shiny bit, UV printing...which I didn't seem to have much of an aptitude.....appetite galore, just from admiring all the work I've seen here. No matter, as my curiosity remains a driving force.

Whoops, kinda rabbit-holed there a bit. It's best just to read the first sentence and move on

 

[Chuck D. Bones]

"Brightness" means more treble. If we increase the gain in the treble frequencies, then the sound is brighter.

In some pedal circuits, turning up the TONE control can also increase the overall volume. Turning up the GAIN in a distortion pedal makes more harmonics. Turning up the TONE in a distortion pedal emphasizes the harmonics coming pout of the distortion stages.