Yo

I finished the Heliodor (OD-3) kit from Aion FX. I forgot to order some of the parts (: and ended up substituting in a 1n cap for a 1n2 and a 2SK208-GR JFET in place of a 2SK209-GR at the front of the circuit. It works! But I've never heard an OD3 in person to compare it to.

Their support email was prompt when I asked them about the JFET, saying it was probably fine in that spot. I didn't ask them about the cap because I saw they like public discussion for educational purposes, so I'm asking here.

So on the Heliodor, what is C10 doing? How much difference does 200n make? C10 is in front of Q6...my guess is that it's bleeding off some high frequencies to ground.

What is Q1? It's at the front. Buffer?

The Heliodor is my 3rd Aion build and my 5th pedal overall after two BYOC pedals. Building these has been fun. I have a Lumitron kit in progress and a few other effects I'm considering...

Hello and welcome!
Show us the schematic please so we know what circuit you're talking about and which ones are Q1 and C10.

my bad, here it is. this is the heliodor: https://aionfx.com/project/heliodor-amp-overdrive/

Welcome also..

Q1 indeed is a buffer, serving to buffer Q2's buffer input impedance..
(seriously now, it's just redundant - probably left there from the original circuit in which it served as bypass buffer..)

C10 is part of C9/R16/R17/C10 band-pass filter (actually is more complicated but you get the idea..)
200pF (that's 0.2nF and not 200nF) difference might or might not make audible difference..
(1nF instead of 1.2nF raises LPF's corner frequency about 265Hz - at 1590Hz from 1325Hz..)

Why are there two identical buffers in series, @aion?

Quote from: FiveseveN on November 01, 2023, 04:28:10 PMWhy are there two identical buffers in series, @aion?

Q1 is residual from the original circuit..
It often "happens" to maintain original circuit mojo tone..  
(classic instance is the two series caps bilaterally the ON-OFF switch in Guv'nor pedal,,)

P.S.
There aren't "identical", (very) strictly speaking..

Oh no, not my mojo! I need my TOAN.

I tacked on a 220pF ceramic cap in parallel with C10 and played a clean guitar clip through the pedal, getting the knobs as close as I could both times. I should've taped them to prevent movement. Here's the results.






The original 1n has slightly more highs past 1-2k, but the biggest difference is around -3dB at 10kHz and beyond. I don't think I could pick out which track was which in a blind comparison. The delta's highest volume level is -55dB.

I might leave it at 1n because one of my guitars is very very dark/warm.

Quote from: tootsMcgee on November 01, 2023, 06:08:45 PMThe original 1n has slightly more highs past 1-2k, but the biggest difference is around -3dB at 10kHz and beyond. I don't think I could pick out which track was which in a blind comparison. The delta's highest volume level is -55dB.

Generally you can only detect one E12 value difference (1n vs 1n2) with AB testing or listening for a *long* time (and know the original sound, sort of delayed AB testing).

In general the aion clones try to keep the original sound.

Quote from: FiveseveN on November 01, 2023, 04:28:10 PMWhy are there two identical buffers in series, @aion?

It's the full signal path of the original pedal in effect mode, converted to true bypass. I do edit out the in-line JFET switches, which would technically add <300R series resistance where they appear. But otherwise I keep it part-for-part to the original audio signal, unnecessary buffers and all.

Yeah you being aware that they are unnecessary only raises more questions.

Quote from: FiveseveN on November 03, 2023, 12:05:24 PMYeah you being aware that they are unnecessary only raises more questions.

It probably *avoids* a ton of other questions from "tone enthusiasts" about why Aion has dared to mess with the schematic of some sacred pedal or other. At least if you keep your clone totally accurate, no-one can argue with you about it, and you just *know* they would.

I agree with you, FiveseveN, I'd have dropped the extra buffers too because I don't think it would affect the sound and I like things as simple as possible, but I understand the logic of why Aion left them in.

Quote from: ElectricDruid on November 03, 2023, 12:48:24 PM

Quote from: FiveseveN on November 03, 2023, 12:05:24 PMYeah you being aware that they are unnecessary only raises more questions.

It probably *avoids* a ton of other questions from "tone enthusiasts" about why Aion has dared to mess with the schematic of some sacred pedal or other. At least if you keep your clone totally accurate, no-one can argue with you about it, and you just *know* they would.

I agree with you, FiveseveN, I'd have dropped the extra buffers too because I don't think it would affect the sound and I like things as simple as possible, but I understand the logic of why Aion left them in.

Thanks, yeah, this is exactly it. It's easier for those who know better to bypass the buffer than it is for those who don't know better to add it on if it wasn't there.

I've been at it long enough to know that if you expend energy trying to explain to people why certain things don't matter where the signal path is concerned, it won't be long before you don't have anything left to give. The mystique of originality goes far beyond rationality or science.

So, if there's space on the PCB—as there was in this case—I'm going to spend my energy designing the layout in a way that suppresses future questions, erring on the side of originality, rather than editorializing in a way that I may understand to be better but that invites legions of But Why questions from those who know just enough to read a schematic but not enough to understand it... since, in all fairness, that's my main audience base!

Thank you for all the discussion and answers! I'll just tack the 220pF on there better or replace it when I get the part and call it a day. The enclosure isn't here yet so I can't show you the completed build, but it will be very plain so you're not missing much.

Well. On the outside. The inside is embarrassing. I'm not sure what's more obnoxious; the 1uF cap pogo-sticked on top of two others because I bought box caps that are too fat, or the 3DPDT that I had to unsolder, giving the board a few good battle scars. The misaligned holes on the test enclosure and the pot with the jumpers. So much fun.

But it all works and as far as I'm concerned, it has a tone that I like, so I'm not touching it further!