Philosopher's Tone (AION version)

[Onion Ring Modulator]

Has anyone built this? I'm a bit disappointed in the volume drop. Did the original drop that much volume? I wonder if there's any mod i could do to get some back?

[antonis]

Volume drop could easily settled by raising R5 value..

What makes me wonder about "serious design" is C13 placement..

[aion]

Yep, the original is like this as well and it's been a common complaint since the first version. It's mentioned in the Polyphemus build docs under the controls section:

Volume is the overall output level. Be aware that even at maximum volume, it doesn't go much higher than unity gain, so it's not suitable for use as a booster.

What makes me wonder about "serious design" is C13 placement..

I wondered about this as well as I've seen many broad precautions that op-amps don't like this sort of thing - but figured I would trust Howard Davis on this one especially since Pigtronix hasn't changed the arrangement in any of the 5+ different versions of the circuit. There's a lot of interaction between the output of IC4 and the vactrol, to the point where even changing the R7 resistor value has a big effect on the resistance range of the optocoupler, so I'm wary of messing with it under the guise of fixing the mistakes of someone far more experienced than I am! But I did wonder if the 741 may have been an intentional choice for this position since they'll let you get away with a lot more than you can with a 'better' op-amp.

[Onion Ring Modulator]

Yep, the original is like this as well and it's been a common complaint since the first version. It's mentioned in the Polyphemus build docs under the controls section:

Volume is the overall output level. Be aware that even at maximum volume, it doesn't go much higher than unity gain, so it's not suitable for use as a booster.

Yeah, I read that in your doc. Having never played a real one, I didn't expect it to be this low. I'll crack it open and see what I can break 

[ElectricDruid]

That IC4 741 is just buffering the Vc reference voltage, which should *already* be smoothed and filtered by C14, so I don't see what the point of C13 is supposed to be anyway. And if the circuit *depends* on having an ancient old op-amp fitted and is very fussy about the R7 value, perhaps it's not that well designed?

I don't care how experienced they are, Howard Davis can have bad days like the rest of us. There are blatant errors in released products by major manufacturers all over the place, so deferring to authority is rarely a good policy. Sometimes the faults become so associated with a thing that they become features, but that's a case of "Phew! I think we got away with that one!" rather than it not actually being a screw-up in the first place!

The volume thing looks like an easy fix and doesn't affect anything else, so definitely fix that one at least.

[bluelagoon]

That C13 large cap at the output of the voltage follower VREF buffer, may well be an intentional part of the circuit design to give it instability at the only one place in the circuit it was applied to at R7 and the vactrol circuit section, otherwise why else would someone do it? Its not like he has used it as voltage bias vref on the opamps, just that one place at R7.

[antonis]

IMHO, C13 adds almost nothing to R7/C7//C8 LPF..
(dunno 741's output impedance value to consider it as a blocking oscillator safe margin in conjunction with C13 value ..)

[bluelagoon]

possibly wanted some oscillation into R7 and the vactrol circuit to enhance or promote sustain, I dunno? but figure there was something to leaning outside the rulebook?

[duck_arse]

opamps, what do I know? what's with the values of R16 and R15, are they the right way around?

What makes me wonder about "serious design" is C13 placement..

I wondered about this as well as I've seen many broad precautions that op-amps don't like this sort of thing - but figured I would trust Howard Davis on this one ....... so I'm wary of messing with it under the guise of fixing the mistakes of someone far more experienced than I am!

while I can see your point of view, this makes no sense. the blokes that engineered the IC wrote the datasheets which all state don't hang caps off the output of opamps. and then there is plenty of proper engineers herearounds who have said time and again don't hang caps, and then explained why not to hang caps, yet still people say .... that's how it was, so .....

[Onion Ring Modulator]

Okay, in the interest of closure for the original question:

I pulled out R5 and put a socket in. I started working my way up from 10k and I eventually settled on 100k. This provides unity, possibly even a little boost. I get some light distortion if I turn the Sustain up all the way, but I don't really plan on playing with the Sustain cranked max, so that didn't bother me too much. i suppose if one wanted to bring it all up completely clean it could be pulled back a couple values.

Edit - that was also with Blend all the way to the effect side, and looking at the schematic again I see that Blend is between Sustain and Output, so a more balanced blend would probably keep it clean as well.

[antonis]

I tend to believe that R7 should be placed between IC4 pin 6 and C13..

[ElectricDruid]

I tend to believe that R7 should be placed between IC4 pin 6 and C13..

That would at least make sense, which would be a big step forward in this circuit!