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So this question is ultimately about the design of the Maxon OD-820 circuit, of which the green Pony is a clone.

The descriptions of the Maxon typically say something to the effect of.. the Maxon OD-820 uses the tubescreamer circuit, but adds a dual ganged potentiometer and a charge pump.

One difference I've noticed is that in the clipping stage, the tubescreamer (and byoc's "classic overdrive") has a high pass filter. A capacitor before a resistor, on the way to ground. This attenuates low frequencies.

In the maxon OD-820, and the green Pony, the resistor comes before the capacitor, which I believe is a high pass filter.

I haven't found any info about the Maxon that talks about that.

Is anyone familiar with these circuits enough to either explain how I'm misunderstanding this, or why there is indeee this difference.. at at least, validate that I am seeing what i think I'm seeing and that it is a notable difference?

Thanks!

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There are no passive HPF in either circuit. While I suppose there technically are, if you do the math, they aren't really filtering out anything in a useful range. The resistors that you are thinking are part of a HPF are bias resistors. The capacitors in question are decoupling input capacitors before the input of a transistor or op amp, so that signal needs the input voltage to be set to half operating voltage, via a resistor to vref in the case of the TS or ground in the case of the OD-820.

There are, however, passive LPF in both circuits that are present for the express purpose of tone shaping.

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Ok, thank you for that clarification.

I mixed my words up a little in original post, and I meant to say that the green Pony and OD-820 each have a low pass filter (I said high pass). Only the TS did I think have an HPF.

In the green Pony, there is the switch which adds more/less capacitance in the rc filter I was referring to, and that, I believe is the low pass filter.

But as far as the TS goes, I was looking at the schematic from electrosmash, and the clipping section(attached here) has the capacitor(C3) and resistor(R4) in the same spot as the green pony. They are comparable values too. The difference as I see it the resistor and capacitor are swapped.

I am no circuit wizard of though, I feel like I am still missing or misunderstanding this in some way, so I appreciate any correction, that's the only way I'll figure this stuff out.

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http://www.geofex.com/article_folders/t ... sxtech.htm

This will explain it better than I can.

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These two RC filters are both acting as low pass filters, despite the difference in component order. I believe this is because there is no signal take-off (no "OUT") between the components; both configurations just offer a path to ground, so the order doesn't matter. You can see what I mean by looking at the following diagram from THIS WEBPAGE.



You can calculate the roll-off frequency of the filter (i.e. where signal attenuation begins) using THIS ONLINE CALCULATOR. Plugging in the values for the TS (4700 ohms and .047uf) gives a roll-off "knee" at ~720Hz. For the GP, the "Thin" setting (middle/off switch position; 2200 ohms and .047uf) has the knee at 1539 Hz, the "Normal" setting (2200 ohms and .047 + .047 = .094uf) at ~770 Hz (very similar to the TS) and the "Fat" setting (2200 ohms and .047 + .33 = .337uf) at ~192 Hz.

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Ah, very help answer @duhvoodooman. I think I understand now!

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Happy to be of some assistance!

That low-pass filter in the TS op amp gain & clipping loop was one of the first things I learned about when I joined this forum in....2007 ( gulp!). I think this was the first time I posted about it, in the context of the original BYOC Overdrive (TS clone) circuit: viewtopic.php?p=29378#p29378

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2

Oh cool. You've been talking about this for a little while!

Ok, sorry, one other question re:
`the 0.047uf cap in conjunction with the adjacent 4.7K resistor determine the bass rolloff frequency of the pedal. Frequencies above that value will get the full gain of the clipping stage, while those below the rolloff point get progressively less gain/distortion`

That sounds like it's describing a high pass filter, rather than a low pass filter. Am I still confused about what those terms mean?

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No, it's a low pass filter--it's just that the lower frequencies are passed to ground, rather than cycling through that op amp gain stage and on to the pedal's output. That's what keeps the circuit from getting too muddy/boomy.

_________________
“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2