Prince of Tone

All about modern commercial stompbox circuits from Electro Harmonix over MXR, Boss and Ibanez into the nineties.
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tommymariot
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Post by tommymariot »

I'm building a Prince of Tone for my Pedalboard but I found 2 different schematics: Aion's one and PedalPCB's one.
In addition of the different value of the input capacitor, the difference between the two schemes is in the clipping section.

Which one is correct? Or simply are different revision of the same pedal?

Someone can help me?
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mauman
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Post by mauman »

From the Aion website: "The Achilles is a direct clone of the Prince of Tone with no modifications." Kevin (owner of Aion) is very transparent about his traces, and if/when his PCBs vary from the originals, it's usually an improvement and he talks about why. I'd trust that.

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Post by george giblet »

As much as I respect the accuracy of the aion's tracings, the actual PCB shows the diodes connecting to two resistors and the switch (very likely the center pin) and one of those resistors goes to the tone pot, all of which follows the PCBPedal schematic. There's also a cap near the 1k, possibly the 100nF, but it's not possible to follow the connections from the top of the PCB. More than enough to put some doubt on the aion schematic.

While none of the schematics follow the original parts designators, the parts on the original PCB are: R31, R8, D2, D4, C31.

Another difference between the two schematics is aion's C7, which is not present on the PCBPedal schematic. Don't know what is on the original pedal.

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Post by idy »

The two schematics are identical regarding the diodes. Two ways of drawing the same thing, switching diodes between feedback loop and output of the opamp to ref/VB.

One schematic has a small cap, 47pf, in the feedback loop, always a good idea in these kind of things.
The other has a 100n DC blocking cap on its output, I think it is not necessary. In fact, since you are clipping to Vref, that cap is a bad idea because the signal is then referenced to 0v DC and so will only clip in one direction with that cap there. You would then want to clip to ground and not Vref.

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Post by george giblet »

The two schematics are identical regarding the diodes. Two ways of drawing the same thing, switching diodes between feedback loop and output of the opamp to ref/VB.
When the diodes are in the feedback loop there is a difference:
PedalPCB shorts C14, R15. Only 1k (R9) between opamp and tone control.
Aion: leaves R10 in circuit. Both 1k's (R10 and R11) in series between opamp and tone control.

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Post by tommymariotti »

george giblet wrote: 09 Jan 2023, 08:10
The two schematics are identical regarding the diodes. Two ways of drawing the same thing, switching diodes between feedback loop and output of the opamp to ref/VB.
When the diodes are in the feedback loop there is a difference:
PedalPCB shorts C14, R15. Only 1k (R9) between opamp and tone control.
Aion: leaves R10 in circuit. Both 1k's (R10 and R11) in series between opamp and tone control.
Exact. Looking at gutshot on the web it seems that the DPDT has every pin soldered. So I think that Aion’s schematics is more accurate on the diode clipping section.

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Post by george giblet »

Exact. Looking at gutshot on the web it seems that the DPDT has every pin soldered. So I think that Aion’s schematics is more accurate on the diode clipping section
If all contacts on that DPDT are used then it's the only aspect in favor of the Aion schematic ; all switch contacts are at least wired to the PCB. The PCB tracks don't match the Aion schematic, they look much like the PedalPCB schematic.

When I look at the PCB all the parts marked in Red are visible on the PCB. The Blue parts aren't so obvious. The points that don't make sense are in the text on the top right.
POT - identifiable tracks and parts.png

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Post by aion »

To clarify - I didn't trace a PoT myself... I think I sourced this one mainly from the PPCB schematic along with looking at some photos, but I've never taken one apart.

The 47pf feedback capacitor was just a "good practice" addition to the circuit and is not in the original.

The missing 100n capacitor was an oversight, likely from designing the KoT at the same time. The KoT does not have this capacitor and neither does any other Bluesbreaker variant, and it's not necessary since the diodes are already clipping to Vb rather than ground (as in the Drivemaster from which the hard clipping concept is adapted). So I wasn't in a rush to get it added, although I do have a revision ready for next time I reorder the boards just to be as accurate as possible.

The switch was a bit more of an editorial decision. On the PPCB schematic as drawn, by leaving the diodes connected to the junction of the two 1k resistors, it shorts the 100n and 1k when in soft-clipping mode and they are only part of the circuit in hard-clipping mode. A year ago when I designed the Achilles, I was convinced it was a tracing error, so I drew mine how I thought it should be. Looking at it now I'm not so sure—although I would still say it makes no audible difference. In the PPCB version, the soft-clipping mode is identical to a KoT/Bluesbreaker since there's just one 1k resistor and no capacitor. In hard clipping mode, it's different from the KoT, which just connects the hard-clipping diodes to lug 3 of the tone control, after the single 1k resistor rather than in-between two 1ks.

I will likely revert back to the PPCB switch wiring in the next revision since I can see the intention behind the design. Even if mine is sonically identical, it's not electrically identical and that's something I always aim for.
Last edited by aion on 11 Jan 2023, 16:38, edited 1 time in total.

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Post by aion »

idy wrote: 09 Jan 2023, 07:46 The other has a 100n DC blocking cap on its output, I think it is not necessary. In fact, since you are clipping to Vref, that cap is a bad idea because the signal is then referenced to 0v DC and so will only clip in one direction with that cap there. You would then want to clip to ground and not Vref.
The volume control does supply Vref to this section if it's blocked from the opamp output (think of it as a fixed 100k resistor). I would agree that it's better to put a cap there to isolate from the opamp's reference voltage, but it does work as shown... plus the Vref stuff is all straight from the original Bluesbreaker, and there is a great deal of religious fervor that makes it a fool's errand to try to improve it!

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Post by george giblet »

Thanks for filling in some of the blanks.

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Post by george giblet »

Referring to my mark-up with the original part designators:

I can confirm that R7 is the 6k8 marked on the schematic and R6 is the 220k marked on the schematic.
R6 is carbon film and R9 is carbon comp and both are 6k8 but it is that way.

The C31 problem (C31 on the real PCB/my markup). I'm starting to think this whole area needs checking.

If we look at the PCB there is a 100nF (C5 on PedalPCB schematic, C6 on Aion schematic).
On the actual PCB I can't see what the part designator is but it's the large poly cap near the switch and the IC.
This cap is 100nF.

The problem: C31 is a poly cap but it is much smaller than this 100nF cap. So I doubt C31 is actually 100nF.

The size of C31 looks more like the input cap C1 (or the other poly cap C82 which doesn't appear on either of the above schematics). That would make it anything from 4n7 to 22n, but not 10n as it doesn't seem to look like the other 10n's on the PCB.

A possible explanation: If C31 is a smaller value, then maybe it isn't connected like C14 on the Pedal PCB schematic.

What else could it be? Maybe C31 is a cap connected across the diodes (like an MXR distortion+).

That would also explain the unconnected switch contact on my markup. In distortion mode C31 is connected across the diodes. Something like this,
POT - speculative corrections 2023_01_14.png
A minor variation on this is C31 connects to VREF and not D1 and D3.

The whole idea hinges on the fact C31 isn't 100nF, if C31 is 100nF this fix cannot be correct either.
Despite this fact this fix ticks all the boxes, it is speculative and unverified.
Someone needs to check what is going on on a real unit.

FYI:
100nF part shown is confirmed
C31 is a smaller part
POT C31 small.jpg

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Post by aion »

george giblet wrote: 13 Jan 2023, 20:49 The whole idea hinges on the fact C31 isn't 100nF, if C31 is 100nF this fix cannot be correct either.
Yeah, that's super frustrating. Panasonic ECQ-V 100n caps are in that smaller size, so I wouldn't say it's impossible, although in photos I've seen of the KoT, e.g.:



...it pretty consistently uses the larger size for the two 100n's. They look like a mix of Panasonic ECQ-V and ECQ-E.

But, to tip the scales a bit - If you haven't seen the trace thread, check this out:
https://forum.pedalpcb.com/threads/prin ... stion.3278

It was traced by a forum member and then confirmed by PPCB tracing a second unit (a Duke of Tone), so it seems unlikely that both would have made the same mistakes on cap values and switch connection.

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Post by george giblet »

Yeah, that's super frustrating.
Indeed.
Panasonic ECQ-V 100n caps are in that smaller size, so I wouldn't say it's impossible, although in photos I've seen of the KoT, e.g.:
...it pretty consistently uses the larger size for the two 100n's. They look like a mix of Panasonic ECQ-V and ECQ-E.
Agreed. The fact there's a 6k8 carbon comp and a 6k8 carbon film is a little odd to me but it shows using different packages isn't off the radar.
But, to tip the scales a bit - If you haven't seen the trace thread, check this out:
https://forum.pedalpcb.com/threads/prin ... stion.3278

It was traced by a forum member and then confirmed by PPCB tracing a second unit (a Duke of Tone), so it seems unlikely that both would have made the same mistakes on cap values and switch connection.
I have seen those threads but I don't have accounts on those forums so I couldn't get the images or schematics. I couldn't extract enough details from the text to piece together the history. The second tracing adds a lot of weight for the correctness (providing it was traced from scratch and didn't use the first tracing as a starting point.)

Even if we accept the 100nF cap is a different packages. The hanging issue is what to do with the unused switch contact on the PedalPCB Schematic? If we stretch things a little we could even say the switch selects one pair of clip diodes in distortion mode, like the KOT. However that means the middle of the four diodes would need to connect together in order to use a single switch contact.

I'm happy to accept the schematics are the best knowledge so far but when things don't add-up 100% you start to suspect a subtle problem. I certainly don't believe any of my suggestions. They are only possibles to look at. The units are what they are.

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Post by aion »

It's a switch configuration I've never seen before - but I believe I can see what Mike was going for, so maybe we can confirm it psychologically if not physically.

Goal 1: Move the inner DIP switch function to the face of the enclosure so it's more easily accessible. (New feature, makes the POT more commercially attractive)
Goal 2: Use only four diodes to get two distinct clipping modes. (This cuts down production cost & makes supply last longer since the exact diodes are out of production.)
Solution: Use a DPDT switch to move the two pairs diodes between two positions, rather than engaging or disengaging two sets of diodes as in the KoT.

Technical consideration #1: Hard-clipping diodes need to come after a resistor, but he didn't want to connect them in parallel with the tone control as in the KoT. (Not sure why, but I know I also look at my old designs very differently when adapting them today, and with more years of knowledge I can better see how past design decisions could be improved upon.) So, isolate them from the tone control with the existing 1k resistor, but then add another 1k resistor in front.

Technical consideration #2: It's probably not good practice for the diode clipping to be DC-coupled to the opamp, so put a capacitor in between, large enough to pass full frequency.

Technical consideration #3: Neither of the previous considerations are relevant in soft-clipping mode. The signal path of the KoT is sacrosanct, and the tiny details are what pay the bills, so if there's a way to bypass the capacitor and resistor in soft-clipping mode then you might as well.

So, the end result is a pretty creative configuration that 1) moves the diodes in-between the feedback and hard-clipping positions, and 2) shorts the newly-added capacitor and resistor when in feedback-clipping mode so they're only active in hard-clipping.

All speculation as to what Mike was thinking about when he made it, but to me this seems like a plausible sequence for the design process.

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Post by george giblet »

You can't go wrong with the toggle switch.

FYI, I found a pic of the DOT and I can clearly see two 100nF boxed poly caps, oddly enough with different color packages - nuts. One of those caps 100nF has designator C31 - so that gives a lot of credibility to the 2x100nF's line of thinking.

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Post by dmc777 »

Anyone want me to open up my POT and verify c31 value or anything else?

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Post by karul »

dmc777 wrote: 20 Jan 2023, 14:12 Anyone want me to open up my POT and verify c31 value or anything else?
sure

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Post by george giblet »

Anyone want me to open up my POT and verify c31 value or anything else?
There's plenty of things to check.

The main doubt surrounds how the toggle switch is connected, what contacts go where.

If you look at the pic I posted on 11 Jan 2023, 02:42 it has the part designators which should match the PCB.
- Low doubt about the left contacts SW1.1 going to R7, Vref, D1, D3.
- The doubts are around SW1.2:
Low doubt: SW1.2 center terminal/Pole to D3,D4,R31, R8
Medium/Low doubt: SW1.2 contact going to R31 and pin 7 of opamp
*** Largest doubt: Where does the other contact of SW1.2 go?
It doesn't go any where on the schematic but all contacts of the switch have wires!

It's up to you if you want check the low doubt stuff but the last item simply cannot be guessed or resolved without a real unit. IMHO the final piece of the puzzle :)

One detail, if you get a connection from the "outer" switch contacts to a part you need to flip the switch to see if it *stays* connected. Obviously the "outer" switch contacts connect through to the center terminals (and parts connecting to it) in one of the switch positions.

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Post by dmc777 »

I’ll try and make some time tomorrow to open it and take some close ups. It’s from 2020 or 2021 though I doubt it matters much.

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Post by aion »

george giblet wrote: 20 Jan 2023, 21:56 *** Largest doubt: Where does the other contact of SW1.2 go?
It doesn't go any where on the schematic but all contacts of the switch have wires!
I see that all six pins are soldered, but only the center-left pin has a visible top-side trace coming off it, with the rest on the bottom. I always include pads for unused pins because it provides better mechanical connection. I don't see anything in the photo that casts doubt on the schematic, although I could certainly be missing something basic!

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