The 32 OD Pure Germanium Overdrive pedal - design and build in words and pictures

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sclitheroe
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Re: The "32 OD" pedal - design and build in words and pictures

Post by sclitheroe » Sun Apr 04, 2021 12:34 am

Are Q31/32 always close enough in spec/tolerance that this would be considered a symmetric clipper?

Would insertion of a small resistor on the base/collector shunt on one of them be sufficient to alter the behaviour to make it asymmetric?

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laurie
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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Sun Apr 04, 2021 1:14 am

Yes, would always be close enough to symmetric. For asymmetric, put two in series on one side. I wouldn't bother. Winding the gain up to full (OD + distortion) will push it asymmetric.

More stable thermally. Q11 bias point isn't an issue. Bias for the voltage amplifier block is set by the RV21/R22 voltage divider on the base which won't really vary much. In a fuzzface there is only one base resistor so bias relies more on the gain of the transistor which varies with temperature.

With the RV11/R22 voltage divider the bias point will still wander around as the gain changes but the effect will be much less.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by Pepe » Sun Apr 04, 2021 6:44 am

laurie wrote:
Sun Apr 04, 2021 1:14 am
Yes, would always be close enough to symmetric. For asymmetric, put two in series on one side. I wouldn't bother. Winding the gain up to full (OD + distortion) will push it asymmetric.
Would asymmetric clipping affect the tone in a drastic and useful way in the low gain area? Could/should it be implemented from the start with a small switch then?
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laurie
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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Sun Apr 04, 2021 1:26 pm

Pepe wrote:
Sun Apr 04, 2021 6:44 am
laurie wrote:
Sun Apr 04, 2021 1:14 am
Yes, would always be close enough to symmetric. For asymmetric, put two in series on one side. I wouldn't bother. Winding the gain up to full (OD + distortion) will push it asymmetric.
Would asymmetric clipping it affect the tone in a drastic and useful way in the low gain area? Could/should it be implemented from the start with a small switch then?
It can be done, and here is what it looks like with two Ge transistors in series on "one side". Both pictures below use the same scale.

Asymmetric clipping will remove a lot of the overdrive effect. I'm not a fan and would definitely never do this for myself.
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symmetric_clip.jpg
symmetric_clip.jpg (57.07 KiB) Viewed 4452 times
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asymmetric clip.jpg
asymmetric clip.jpg (59.81 KiB) Viewed 4452 times

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Re: The "32 OD" pedal - design and build in words and pictures

Post by sclitheroe » Sun Apr 04, 2021 3:00 pm

That's pretty neat - the asymmetry is less visually than I would have expected. If the two pictures are the same scale on the scope, why is the asymmetric one higher amplitude?

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Sun Apr 04, 2021 3:13 pm

sclitheroe wrote:
Sun Apr 04, 2021 3:00 pm
That's pretty neat - the asymmetry is less visually than I would have expected. If the two pictures are the same scale on the scope, why is the asymmetric one higher amplitude?
Symmetric clips +250mV / -250mV (about 600mV peak-to-peak with the waveform rounding)
Asymmetric clips +500mV / -250mV (about 900mV peak-to-peak with the waveform rounding), plus it unbalances my carefully designed OD section ( :ugeek: ) a bit leading to a bit of overshoot
Last edited by laurie on Sun Apr 04, 2021 3:27 pm, edited 3 times in total.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Sun Apr 04, 2021 3:21 pm

I think it's important to recognise that the "asymmetric clipping" we all know for overdrive (e.g. the OD-1) is done in a completely different way...

https://d6a2e7ghqts3o.cloudfront.net/Ac ... uit700.jpg

It doesn't really apply to the 32 OD.

It is also possible to use asymmetric clipping in the circuit configuration in the 32 OD (similarly to swapping one of the clippers in a DS-1 for a LED), but again, the 32 OD doesn't really work that way - the DS-1 uses silicon "hard" clippers to get distortion, it isn't trying to be an overdrive.

The 32 OD is producing "saturation", rather than "distortion" (although, technically, saturation is distortion, just a specific kind).
Last edited by laurie on Sun Apr 04, 2021 3:41 pm, edited 1 time in total.

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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Sun Apr 04, 2021 3:38 pm

For reference, this is the waveform with symmetric "clippers" and the drive cranked to 100%

The asymmetry comes from the output of the voltage amplifier hitting the supply rails asymmetrically.
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OD+distortion.jpg
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Re: The "32 OD" pedal - design and build in words and pictures

Post by sclitheroe » Sun Apr 04, 2021 4:26 pm

laurie wrote:
Sun Apr 04, 2021 3:38 pm
The asymmetry comes from the output of the voltage amplifier hitting the supply rails asymmetrically.
Right, I was thinking about Q31/Q32 as diodes, hence my confusion.

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laurie
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Re: The "32 OD" pedal - design and build in words and pictures

Post by laurie » Sun Apr 04, 2021 4:32 pm

sclitheroe wrote:
Sun Apr 04, 2021 4:26 pm
laurie wrote:
Sun Apr 04, 2021 3:38 pm
The asymmetry comes from the output of the voltage amplifier hitting the supply rails asymmetrically.
Right, I was thinking about Q31/Q32 as diodes, hence my confusion.
Q31/Q32 are diodes... but they don't behave like "normal silicon clipping diodes".

If you had asymmetric silicon diodes the effect would be visually clearer because the "rounding of the corners" (saturation effect) would be largely absent.

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