PEAQ: A colorful JFET-based booster

[niektb]

Hi all,

I've made a number of DSP boards and modified clones of existing pedals so now I thought I'd get my hands dirty on creating a pedal more from the ground up.
Meet PEAQ! First I wanted to do delay pedal based on a PT2399 (but tap tempo and all kinds of fun stuff) but inspired by Brian Wampler's video about boost types, I came up with the idea of creating a flexible boost pedal. (and seeing that this is also a pedal type i haven't on my board yet )



So essentially, it starts with two passive filters (LP and HP) to tighten up or remove top-end followed by an active baxandall tone stack (which is the same as in the Xotic BB) followed by the AMZ mini-booster but with 2N5457's instead. It also as dip switches for creating a presence boost effect

I've breadboarded and simulated this and it all seems to work pretty nice but what do you guys think? Do you have any suggestions? Let me know!

[iainpunk]

hey man, nice pedal.

i think its a bit redundant to have both bass and treble twice. i think it would be more useful to have a bass, treble, lo mid and hi mid control, so you can shape the tone to your particular amplifier.

i recommend looking in to 'Bridged-T' filters to sculpt the mids, or look into other "midrange" controls, or even the "mids" baxandall filter that floats around the web somewhere,

cheesr,
Iain

[Elijah-Baley]

Yes, it seems a nice schematic!
Me neither like too much Presence and Tight + Bass and Treble, but if you want all those control you can, or at least you can try. Presence and Tight will be finer controls, probably not so drastic like the Baxandall. I don't know if you thought about the aspect of your pedal, but you can use smaller knobs for Tight and Presence controls.

I see just a little effect of the dip switch with those caps value. Maybe you can just try some value and chose a fixed cap.

The mid control is always the more hard one. I tried the 3-band Baxandall version, it works fine, but that's it. An external part to add to the Baxandall could be a better idea. There are some solution to do it.

A little advice: I guess you could push a bit the gain in the first stage to avoid the little of loss signal due to the filters. You can try a small resistor in the feedback of the first opamp stage, 10k or more. Do someone else think that is a good idea?

[niektb]

Hey all! Thanks for your kind words! 
The reason I've opted for tight + bass is cause I tried emulating a resonant high pass filter (or at least, if you decide to use it that way  ) which is a trick often used in mixing & mastering for having a huge but tight low-end response. It's a trick also used in the Friedman BE-OD which is renowned for its huge bass (i have a clone here on my board and I can confirm it sounds absolutely massive ) I thought a resonant low pass could give a similarly nice effect but now that you point it out, I'm not too sure 

I know the bridged-T (i've used them in my TS clone) I'm not all too sure about the added value of a mid control. Turning back the bass and treble (or the tight and presence, the current schematic has multiple ways of turning into a midrange booster, each with different sounds) and boosting the volume a bit would give the same result, wouldn't it? Maybe if I could easily shift the frequency...?

Basically I see two use cases for this pedal:
1. Use it as a neutral/treble booster in front of an amp (for solos etc). In this case the dip switchs caps come into play. I myself like it with the 220pf but I only have a midrangey prs-style guitar to try it out with, I thought different values would come in handy with other guitar types top-end boost prolly sounds nice into the jfets (which is a possible soft-clipping stage) but maybe the dip switch is a bit redundant when there is already a treble band in the EQ Unfortunately I dont have a proper tube amp to try this out with (I need to take care not to clip the preamps of my audio interface ) but I ám planning to buy one in the future
2. Use it as a midrange boost in front of an overdrive/distortion pedal. I'm currently using a TS for boosting my Friedman but I would like to use this for other light/medium gain stuff, and im planning to use the peaq instead

I'm aware that I'm dropping a bit of volume in the filters but I'm already dropping a bit of volume just before the JFETs (so it doesnt saturate to much with high-gain pickups) so I'm not sure if boosting the 1st opamp stage would give me much benefits

[FiveseveN]

It's a trick also used in the Friedman BE-OD

Notice how the low-cut ("tight") control is before distortion and the bass control at the very end.

[iainpunk]

the BE-OD had a bass cut control before the distortion and a bass boost/cut after the distortion stage, so you can compensate. i personally think a resonant HPF is very useful before any type distortion, but i'd recommend a sallen-key resonant filter, there is a very nice calculator online for such circuits

http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm

i recommend C1 and C2 to be 47nF, R2 to be 100k and R1 to be a 1k resistor and a 100K log potentiometer, so more bass cut produces more and higher midrange. at max resistance there is only a cut in the very low bass, but at min resistance, there is 16dB of boost @ 350 Hz, which is really nice before a distortion.
(that's literally what my old favourite padal was, just that filter with those values)

cheers,
Iain

[niektb]

Quote from: FiveseveN on Yesterday at 11:37:32 AM

Quote from: niektb on Yesterday at 05:13:08 AM

It's a trick also used in the Friedman BE-OD

Notice how the low-cut ("tight") control is before distortion and the bass control at the very end.

You're correct! However, the jfet in my schem isn't meant to clip a whole lot (i think in that case the presence control would also be more appropriate at the end, to tame the distortion a bit) so it doesnt make a huge difference i think


Quote from: iainpunk on Yesterday at 03:13:30 PM

the BE-OD had a bass cut control before the distortion and a bass boost/cut after the distortion stage, so you can compensate. i personally think a resonant HPF is very useful before any type distortion, but i'd recommend a sallen-key resonant filter, there is a very nice calculator online for such circuits

http://sim.okawa-denshi.jp/en/OPseikiHikeisan.htm

i recommend C1 and C2 to be 47nF, R2 to be 100k and R1 to be a 1k resistor and a 100K log potentiometer, so more bass cut produces more and higher midrange. at max resistance there is only a cut in the very low bass, but at min resistance, there is 16dB of boost @ 350 Hz, which is really nice before a distortion.
(that's literally what my old favourite padal was, just that filter with those values)

cheers,
Iain

I already ordered the pcbs but your suggestion seems to be fun to explore as well! I'm currently fiddling around in simulation with the jfet as clipping stage with the S-K filter in front, and a presence and volume control afterwards. Expect some results soonish

btw, I simulated the version in the Topic Start in Tina-Ti but I don't seem to be able to upload the simulation files here?

[niektb]

Well, here it is lol! 

I swapped the 2Ns for J201, replaced the EQ with a Resonant HPF and added a hard clipping section based on AMZ's saturation control. (so hopefully the sound vaguely resembles a tube preamp followed by power amp distortion hahah  ). Finally I moved the presence control to the end in order to be able to round some of that distortion I have yet to breadbord it though, I have no idea what it sounds like 

[niektb]

Well, here it is lol! 

I swapped the 2Ns for J201, replaced the EQ with a Resonant HPF and added a hard clipping section based on AMZ's saturation control. (so hopefully the sound vaguely resembles a tube preamp followed by power amp distortion hahah  ). Finally I moved the presence control to the end in order to be able to round some of that distortion I have yet to breadbord it though, I have no idea what it sounds like 

Well I tried breadboarding this one and it surely farts! But other than that it isn't great hahah... But the HPF doesn't seem a lot, if I can find the problem, that could fix the first issue. The saturation control turns it into a warm, creamy overdrive (didn't have LEDs lying around so I used 5 1N34A's to introduce some assym hard clipping) but at the expense of a big volume drop
Prolly needs more gain also (but part of that is because I used 2N5457 instead of J201, just cause they were already on the breadboard )

[iainpunk]

hmmm, odd, i have had great success with the exact same HPF, even the same values

i must say that the bias point isn't ideal, far from it, i'd replace the 2nd op amp with a transistor buffer and use the opamp for a buffered Vbias, this should clean up some of the fartyness, as it has with experiments i did in the past.

cheers,
Iain

EDIT: it could also fart out if you have a voltage high enough to run current through the Gate of the Jfet. i have experience with that... not nice

[niektb]

Haven't had the time yet to look into the HPF but I did record a quick demo using my smartphone (and sorry for the recording quality XD), maybe you recognize what's going wrong

hmmm, odd, i have had great success with the exact same HPF, even the same values

i must say that the bias point isn't ideal, far from it, i'd replace the 2nd op amp with a transistor buffer and use the opamp for a buffered Vbias, this should clean up some of the fartyness, as it has with experiments i did in the past.

cheers,
Iain

EDIT: it could also fart out if you have a voltage high enough to run current through the Gate of the Jfet. i have experience with that... not nice

So you're actually saying that the Vbias can't supply enough current? and that the second opamp buffer hasn't enough headroom therefore?

[iainpunk]

Haven't had the time yet to look into the HPF but I did record a quick demo using my smartphone (and sorry for the recording quality XD), maybe you recognize what's going wrong

hmmm, odd, i have had great success with the exact same HPF, even the same values

i must say that the bias point isn't ideal, far from it, i'd replace the 2nd op amp with a transistor buffer and use the opamp for a buffered Vbias, this should clean up some of the fartyness, as it has with experiments i did in the past.

cheers,
Iain

EDIT: it could also fart out if you have a voltage high enough to run current through the Gate of the Jfet. i have experience with that... not nice

So you're actually saying that the Vbias can't supply enough current? and that the second opamp buffer hasn't enough headroom therefore?

no, the Vbias could have some signal bleed, which can have all sorts of weird behaviour. i don't think its about headroom. i suggest you use the 2nd op amp for a buffered Vbias, and use a transistor buffer instead of the original opamp's function
is that a Dutch accent i hear in the video?

de groeten
Iain

[swamphorn]

Alternatively, you could directly couple the output of the FET stage to the buffer and capacitively couple the clipping diodes to ground. This eliminates the biasing network altogether. As a bonus, it increases the load resistance and, thus, the gain of the mu amp.

[niektb]

For some reason, swapping out the 2N5457s with J201s doesnt work (cause the sound is completely gone). What could be a logical reason for that? The J201s were completely new...

Alternatively, you could directly couple the output of the FET stage to the buffer and capacitively couple the clipping diodes to ground. This eliminates the biasing network altogether. As a bonus, it increases the load resistance and, thus, the gain of the mu amp.

That's an interesting thing you propose there! But could you maybe explain how the clipping works? Cause intuitively I would this that the cap between the diodes and ground get charged to the signal dc offset minus the forward voltage of the diode. But that would mean one diode will never clip and the other always fully? Also the threshold would vary depending on the signal?

@Iain: uhmm yes indeed I'm a Dutchie XD
But I will first try to fix the low pass filter to make sure the issue isn't introduced in that 'broken' section before inserting a transistor buffer

[swamphorn]

 At AC, the capacitor is virtually a short, allowing the clipping diodes to shunt part of the signal to ground like a standard clipping circuit. At DC, the capacitor is charged to the bias voltage through the leakages of the diodes.

[niektb]

huh but I thought the diodes needed to clip around the signal bias right?

[niektb]

Alright I found the mistake in the HPF! I had the opamp input wired to the wrong cap... 

I also removed the 470k bias resistor at the input (cause it seemed redundant to me) and that does seem to improve the issue with the hardclipping thingy so I'm positive that a transistor buffer might eliminate the issue altogether

Furthermore, I've recorded a little something to demonstrate the sound of the pedal (with much better audio as I recorded it straight from the audio interface, rather than a smartphone camera ):
https://soundcloud.com/locrius-1/diary-of-a-pedal-prototype-overdrive-part-1/s-jHiRhAgXjTZ

My own remarks so far:
- The output volume seems a little on the low side (though I didn't dime in the demo above)
- The presence knob isn't superduper useful right now (although it does help to tame a little of the bridge pickup at high gain/saturation), so i'm thinking of adding a switchable presence cap to the jfet source.
- Also more clipping options would be nice... In the demo I used an assymetrical clipping stage with 5 1N34A's. (3 in one direction, 2 in the other) but LED's or Silicon diodes might add a little more grit (in which the presence knob will also be more useful). I'm not all to happy with the saturation pot, in the middle positions it seems to mix hard-clipped sizzle with the clean sound, so the resulting sound isn't very coherent (with 2 distinctly different sounds mixed together)
- Finally I'm thinking about replacing the gain knob with a 1Meg pot, but I will first try to insert the transistor buffer.

[niektb]

Alright, I implemented the suggestion by swamphorn and it all works like a breeze! Thanks a lot The aforementioned issue with regards to the nasty clipping seems to be gone! (there is still some hard-clipping with no diodes and most of the gain, but that is just probably opamp clipping 

I also increased the number of diodes for a bit more volume (and the ratio is altered a little for a bit more crunchier sound). I also increased the gain pot to 1Meg.
The only thing I don't understand is the behaviour of the tight pot (i swapped it with another 100K pot, same result...) @Iain: what opamps did you use for this filter ?

Here is the new schematic:


You know, I wasn't actually planning this pedal but I'm getting more and more happy with the sound (check the soundcloud link in the previous post) so I might even turn in into an actual pedal hahah 

[Elijah-Baley]

All those diodes? There's no other solution to encrease the volume?

[niektb]

All those diodes? There's no other solution to encrease the volume?

Well I could turn the opamp buffer stage into a little gain stage, but that also affects the clean signal (with the saturation pot fully CCW I mean ) which might/will introduce opamp clipping when driving the pedal hard. And this doesn't affect the volume difference between the saturation pot CW or CCW. Unless I do some fancy stuff with dual pots that is .

I could also use leds for clipping but I'm think to include that as alternate voicing