Fairfield Barbershop: Tone switch

[Plexi]

I'm starting this one.. sounds great on YT demos.


Which could be the trick over the new "tone" switch?
Maybe a smaller/bigger C6? C1?

I'll try there.

[duck_arse]

why not abuse C4? it has a moderate and constant/know load on it, it should be easy calcs.

[Plexi]

Before all, Fairfield guys... where did you get those J201s?
Man...the 8k2/9k1 resistors are far away from bias the jfets.

why not abuse C4? it has a moderate and constant/know load on it, it should be easy calcs.

I've considered that one too.. but I don't really know what is doing there, some kind of coupling cap?

Discarded C6: it doesn't change anything, at least in a good way.
Tried from 1nf to the noted 100nf (full "bass"): less than 10nf only start to make the sound poor and without presence.

[samhay]

"Discarded C6: it doesn't change anything, at least in a good way"

Bad idea [if you mean remove C6] . Unless the volume is turned all the way down, you will have DC on the output.

Edit - maybe that was list in translation?

C4 is what I would be playing with too.

[nocentelli]

Before all, Fairfield guys... where did you get those J201s?
Man...the 8k2/9k1 resistors are far away from bias the jfets.

I seem to recall the chap from fairfield stating that they should be but 6v at the drain (presumably at minimum sag) if that makes any more sense.

[Plexi]

Thanks both!

Sure nocentelli, that's what I thought when I use the 10k pot and start to oscillate like crazy (way too slow V at Drains)
The guys on Fairfield let them overly-feeded Jfets in a middle term before start playing with the "SAG" knob.
I biased them at 4.5v and only get oscillation almost all the sweep.

So, 9k1 and 8k2 are fine there.

I though that when the Sag pot was full (no resistence) you get a "normal" setting; but this setting is about 50% of the SAG pot.

[Plexi]

If everything is ok, it's more transparent than I thought.
TONS of volume: no joke.. I can't past the knob about 9 o'clock.
Used an KSP2222 (hfe 195) on Q3.

Questions:
- How modify the response 2M2 instead 2M in R3?
- How can I make this to get a bit more gain? Reduce R2 and R6?

I'm missing something.. Sag knob almost doesn't do anything and I can't achieve the saturation/compression that eard in demos.

What I'm talking about is here, starting at 2:05

I'm in doub again about Jfets...
(http://www.diystompboxes.com/smfforum/index.php?topic=118914.new;topicseen#new)

[snk]

Hi,
I just built it yesterday, and it works fine, except that the SAG knob doesn't seem to do anything  on my build either
In the end, did you manage to find what was making yours not working as expected ?

Also, i think it rolls off the bass range a bit too much to my taste. Should i increase the 10nF cap right after the input, or maybe the 100nF cap at the output ?

[Mark Hammer]

When I was chatting with a fellow from Fairfield at the Festival Sonore, about 5 weeks back, he said something to the effect that they find they have to be really picky about selecting components for the "Sag" part of the circuit.  Of course, he didn't say which components, or what they looked for, just that they found it tricky so they had to be picky.  I know zilch about those MOSFETs used in that subcircuit.

They're actually not that far away from me, and he invited me to drop by some time, so I may just do that.  Note that I do like to protect the IP of friends, though, so don't expect any great revelations or industry secrets.

[Rob Strand]

Maybe you can only hear the effect of the Sag Pot when it's driven by a hot signal? If you measure the voltage on circuit side of the Sag pot I'm sure you will see some voltage drop as you increase the Sag pot resistance.

[snk]

Thank you for your answers.
I haven't yet measured the voltage at the SAG pot pins. I will do it by the end of the week-end.
By the way, the overdrive sounds quite good even without it, so it's more a matter of curiosity (trying it with the SAG working as expected) than a real need

One thing which annoys me more is that the pedal is cutting quite a lot of bass (while it is supposed to work fine with bass heavy instruments, at least from what i have read on various forums).
I think it should sound find on an electric piano, but on a synth there is an audible bass loss below 100hZ, and a 10dB loss around 50hZ.
Here, in pink, you can see the DRY sound, and in GREEN, you can see the processed sound.

And hear the clip here : https://vocaroo.com/i/s1hbEMYHXUHY

[Rob Strand]

Try white noise and see if you get the same gap   Maybe the lows are clipping more than the highs.
(I haven't analysed the ckt for bass drop.)

[snk]

Thank you for the advice, Rob.
I just tried with some noise, and it's clear that the pedal cuts the low range.

[Rob Strand]

Thank you for the advice, Rob.
I just tried with some noise, and it's clear that the pedal cuts the low range.

Something is going on there.   I bashed out a few number and there some cumulative roll-off due to C3, C4, C5
but it seems like that roll-off alone cannot explain what you are seeing.   Maybe C6 is causing a roll-off with the sound-card input resistance?  I guess the way to prove that would be to put a big cap across C6 then see if the roll-off changes.

[snk]

Maybe C6 is causing a roll-off with the sound-card input resistance?

Not with the soundcard : the pedal is plugged into the AUX of a mixing console (this is the way i often do with dozens of pedals, and until now it never behaved in a similar way). So, obviously, the impedances are not the same than with a guitar, but whatever the wiring scenario is, it shouldn't cut so much bass, isn't it?

I guess the way to prove that would be to put a big cap across C6 then see if the roll-off changes.

By "across", do you mean "instead", "in series" or "in parallel" (sorry, english is not my native language, therefore i am not sure to understand your suggestion  )

By the way, one little thing to know (and that i need to check) : when building the pedal, i wanted to use two 20K trimmers instead of the 8.2k and 9.1k resistors. But i didn't have any trimmers left in stock, so i used sockets, and put two 8.2K resistors (as i was lacking 9.1K resistors too). I have read a couple times that with the Barbershop, tweaking the values and biasing correctly the transistors was key to having it sound right, but could such a small resistor variation (8.2k instead of 9.1k) make such a big sonic change (cutting so much low frequencies) ?

[Rob Strand]

Not with the soundcard : the pedal is plugged into the AUX of a mixing console (this is the way i often do with dozens of pedals, and until now it never behaved in a similar way). So, obviously, the impedances are not the same than with a guitar, but whatever the wiring scenario is, it shouldn't cut so much bass, isn't it?

The mixing console could have an input impedance as low as 10k or 47k, pretty low compared to a guitar amp anyway.   For a 10k input impedance you might end up with a -3dB point above 150Hz.  Perhaps not as high as your plots show but getting close.    It could be the cause of the bass roll-off.

By "across", do you mean "instead", "in series" or "in parallel" (sorry, english is not my native language, therefore i am not sure to understand your suggestion  )

In Parallel.  Connecting in parallel makes the cap bigger and lowers the bass cut-off frequency.

By the way, one little thing to know (and that i need to check) : when building the pedal, i wanted to use two 20K trimmers instead of the 8.2k and 9.1k resistors. But i didn't have any trimmers left in stock, so i used sockets, and put two 8.2K resistors (as i was lacking 9.1K resistors too). I have read a couple times that with the Barbershop, tweaking the values and biasing correctly the transistors was key to having it sound right, but could such a small resistor variation (8.2k instead of 9.1k) make such a big sonic change (cutting so much low frequencies) ?

It's quite normal to have adjustable drain resistors on a JFET amplifier.   The true aim is to adjust the drain resistor so the drain voltage is the "correct voltage".  That is more important than setting the drain resistor to match the schematic.  Variations in the JFETs are will cause much more variation than small changes in the resistors.    For this circuit the "correct voltage" isn't marked on the schematic.    1V *change* on the drain voltage is noticeable.  Most circuits target drain voltages of around 4 to 6V but there are exceptions were you deliberately change the voltage to get a certain tone.

[snk]

Hi,
Further investigation report with the Barbershop :

- I tried the pedal with a buffer first, the sound is unchanged.
- I tried the pedal with a buffer after, the sound is changed : the bass is back ! So, Rob, you were right about the impedance mismatch issue.
- I wired a 150nF cap in parallel with C6 (100nF) : and the bass range is here again (without changing the overdrive "color" at all)
- I wired a 150nF cap in parallel with C4 (47nF) : i didn't hear any change at all.

- I measured the voltage across the SAG pot's 1st lug : it gave me 11,3V, and turning it didn't change anything.

- I measured the voltage at the FET lugs : with a 8.2K resistor, Q1 was at 5.4V, and Q2 was set at 6.1V. I will try with other resistor values until i hit the 6.6V sweet spot, but i already like the sound with the 150nF ("bass mod") !

[snk]

I think I will add the 150nF cap on a switch, as i like the tone as it is...
Should i add a buffer after the volume pot, just before the output jack socket, to prevent similar behaviour to happen again ?
http://tagboardeffects.blogspot.com/2014/08/buffers.html
like this one ?

[Ice-9]

  I know zilch about those MOSFETs used in that subcircuit.

Mark, The mosfet and zenner etc is just polarity protection.

[bluebunny]

^^ Yep, pretty much the circuit from R.G.'s "Advanced Power Switching and Polarity Protection for Effects" at Geofex (the MOSFET is P-channel).