distortion project and adding an active tonestack

[rocket8810]

so i've decided to embark on another build of my own design, but inspired by something that exists. i'm having a little trouble with understanding something that i know i should, but i'm missing something stupid. the basic idea is adding a tonestack to a circuit, and where to place the volume control.

now, i have used and added passive tonestacks, that had a gain recovery stage, and trimmer to control the final output, but i'm looking at adding a an active tone control which has me thrown for a loop.

so for example: if i take the ross distortion



and want to add this following active bax tonestack



in my head, i'm thinking that if i remove the output from the ross distortion i should replace it with a fixed resistor, say 50k to have max output from the ross, to control the output into the first gain stage of the tonestack opamp, set the trimmer to prevent clipping, and to have a master output i would have a simple voltage divider at the end of the tonestack. but i'm not 100% sure that is the best idea. i really like the idea of having an active tonestack, and have plans to use this for a few different projects, one being using it in the tiny giant amp, but i have to get a a little better understanding of how to properly add it to another effect first, rather then in a separate enclosure.

so to make a long story short, what is the most efficient and effective way to combine the two circuits, and have a master volume. according to the notes on the bax the trimmer acts as a gain/volume control for the first opamp stage. i also want to make sure that the first gain stage of the tonestack to clip.

and as usually, thanks in advanced for any help guys.

[aron]

Can't you leave the Ross as-is and take the output from the 50K gain and then go to the 1uF capacitor into the IC1A? Then put a resistor on the end of the output to ground?

[rocket8810]

so the ross's output, or really any effect that i would add this tonestack to, would still be a master volume control if i just hooked it up together like that? i didn't think that would work that way. i thought that would act more like a distortion control, similar to the sustain pot in a BMP, since it would be controlling the amount of the signal going into tonestack, and if the signal is too much it would cause the tonestack to distort. also, this may be a stupid question, but why would i need a resistor at the output of the tonestack?

[aron]

Anti pop resistor on the end to bleed the capacitor to ground. I think it would work ok on the end like I described. If the level is too high adjust the trim on the tone control circuit and if necessary put an series resistor going to the 50K level pot.

[rocket8810]

beautiful. thanks man, i'll give it a try. btw, any suggested readings on this subject, so that i don't ask such a silly question like this later. always looking for a ready for more info.

really this idea is kinda based on the idea of creating something similar to the quantum mystic. i know vic had worked on a schematic for quantum mystic,



so just making some comparisons to what i'm looking at doing and what was done here. hence, the reason for my question about the approach to this, and some other ideas i've got running around in my head.

[PBE6]

You can remove the 50k pot from the output of the Ross Distortion and the 1M pull down resistor from the input of the Baxandall EQ and connect them there. If you remove R3 from the feedback loop of the  inverting opamp it will let you go all the way down to zero volume so no replacement is needed for the 50k pot.

Looking up the 1N270, it seems like the forward voltage drop is about 1V. It's handy to know that the output of the Ross will never get above 1V so you can design the gain and EQ stages to prevent excessive clipping. I haven't simulated or calculated it, but I'm guessing the EQ will give you at least 15 dB of gain. Conservatively speaking, if the max voltage shouldn't exceed 3V to prevent clipping in the last stage then the gain on a 1V distortion signal shouldn't exceed 3V / 10^(15dB/20dB) = 0.5V. This is actually an attenuation of the distortion signal, but you're not always going to be boosting the EQ all the way up so it's probably reasonable to use a 100k (or maybe even a 250k) pot for the inverting gain volume control.

You'll probably want to add a pull down resistor to the output of the Baxandall too, after C6.

[rocket8810]

i'm a little confused PBEG. are you suggesting something like this?



i didn't put in values for obvious reasons, just trying to figure out the topography and methodology first.

i did this using the the schematics of the ross and the active bax, what you suggested, and the quantum mystic schematic. from what i've read about inverted op amps, R2 & R3 sets the gain, and would be equal to R2/R3 and should be chosen based on the input impedance. no?

[PBE6]

That's the right connection between the circuits. I was suggesting that you control the gain by using a 100k in the feedback loop of the inverting opamp, although it does run the risk of overdriving the EQ at extreme settings. If you do that, a pull down resistor on the output is a nicety.

If you keep the inverting opamp at a constant gain as you've shown in your diagram, you can remove the pull down resistor at the output as the output pot will allow the capacitor to discharge instead. This method may be slightly noisier at lower volume settings, but it's probably not that important with a distortion pedal.

[rocket8810]

hmm, if 100k as the resistor in the feedback loop could induce overdriving the EQ, could that not be prevented by using a higher value resistor? btw, i'm glad the rough schematic looks alright, it's what i was thinking it should look like. i'm taking that really i should just treat the active tonestack just as i wold with a passive tonestack. thanks for the help so far, i definitely appreciate it.

[PBE6]

A larger input resistor or a smaller feedback resistor would cut down the gain in the inverting opamp stage, thus preventing overloading in the EQ stage. I think a 100k pot would be fine, worst case you can increase the input resistor to allow full travel of the pot without clipping the last stage. Alternatively you can let it clip and see if you like it!

[rocket8810]

Very true. I think this could work very well, once everything is figured out. Since the Ross will have Ge clipping diodes the output won't be that great, so I think having the eq be active will really help with that, not like we really need anything above unity, but it's nice. 

This is one of those times that I got to say, thank god for having a breadboard. I mean I already figured out a layout, so I can just place sockets for certain components I'm unsure of what their value should be. But, high ho, high ho, it's off to breadboard I go. breadboard Unfortunately, I won't be able to do anything until next week so I'll keep messing with the values of some components in various calculators and work on the maths until then.