Vintage Tone Circuit - Improvements?

[Box_Stuffer]

I found this really cool circuit from Elementary Electronics magazine from the early 70s. I set it up on the breadboard.







I have been tinkering with it and substituting some caps to try to get different sounds, but I have not been able to make it sound much better than the original design. Although I do like a 47nf for C3 instead of the 50nf because the bass is a little tighter and you get less flubby distortion when it gets overloaded. It is supposed to stay pretty clean, but if you have hot pickups and hit it pretty hard, there is some distortion, which is why R6 is there to adjust the gain. The effect is most pronounced when R6 is wide open, but you do get that unwanted overdrive. As you can see the article says to use the highest gain BJT available and I am using MPSA13. The only others I have are 2N3904 and 2n2222.

As you turn the R8 control, you get a varying sweep of EQ boost from low to mid to high. You only get the treble boost at about the last 5% of pot travel. It would be nice to have a wider variance of treble tones. That is really the main complaint I have.

Does anyone see some possible areas for improvements? Thanks!

[GibsonGM]

I wonder if a reverse log 10k pot would give you more play with the treble, since you say you only have about 5% and are using a linear pot (you are, right?).    Maybe you can taper it to see what effect it has:

http://www.geofex.com/article_folders/potsecrets/potscret.htm

[Box_Stuffer]

I'll have to try that. I am using linear pots for both. I will have to order up some of the reverse log pots.

[GibsonGM]

If you read R.G.'s site I linked, there is a way to use 'tapering resistors' on your pot. Check the 2nd set of graphs halfway down; it deals with reverse tapers.    You could fudge something together and see if it works!   You may still get the reverse log pot, but this might help to see if it'll work...

[Rob Strand]

To stop the circuit barfing with the pot on minimum all you need to do is add a small valued resistor in series with the pot.   Make it just large enough to keep out of the barf zone.

These circuits are very fickle.  You can often make "improvements" by making R9 larger than R7 and C4 smaller than C3.   It might shift the frequency a bit but you can compensate for that by scaling the caps.  However, the transistor can load the RC network down and stuff it up.   See if you can find the inductorless wah thread perhaps from around 2018.   It's all a delicate balancing act.

[R.G.]

That circuit screams "Twin Tee wah circuit" at me. The relevant filtering is a mildly mis-tuned Twin Tee filter in a feedback loop to give a boost at its notch frequency. There was at least one build-it article in Popular Electronics back in the late 1960s or early 1970s on making a wah this way. Bongiorno as an author comes to mind, although it's foggy from this distance in time.
Yeah, it ought to make a variable boost that's mildly sweepable.

Duuuh. The reference is in the Technology of the Wah at geofex. See page 45
https://www.worldradiohistory.com/Archive-Poptronics/70s/1970/Poptronics-1970-01.pdf

[Box_Stuffer]

If you read R.G.'s site I linked, there is a way to use 'tapering resistors' on your pot. Check the 2nd set of graphs halfway down; it deals with reverse tapers.    You could fudge something together and see if it works!  You may still get the reverse log pot, but this might help to see if it'll work...

I'm not very good with these equations. So, if I am using a 10k pot, what size resistor would I need for R3?

[Rob Strand]

I'm not very good with these equations. So, if I am using a 10k pot, what size resistor would I need for R3?

That formulas are for a voltage divider but you have a variable resistance.   The parallel resistance trick is much more resticted in what you can do with a variable resistance.

The short answer is you can't use the parallel resistor to approximate a Reverse log pot with a linear pot.

For a reverse log pot in that circuit you would use pot terminals 2 and 3.   With the pot at 100% rotation the resistance is zero, with the put centered the resistance is about 20% of the pot value, with the pin pot at 0% rotation the resistance the pot value.  It turns out there's no parallel resistor that gives you that result.

For log pot using terminals 2 and 3.     With the pot at 100% rotation the resistance is zero, with the put centered the resistance is about 80% of the pot value, with the pin pot at 0% rotation the resistance the pot value.    If you have a 100k linear pot and you add a 33k resistor across pins 2 and 3, when the pot is at 0% rotation the 33k is in parallel with the 100k pot so the "new" pot looks like 25k ie. the "new" pot value has changed from 100k linear to 25k log.   When the linear pot is a 50% rotation terminals 2 and 3 are at 50k ohm.  50k ohm in parallel with 33k is 20k ohm.   Notice now that 20k is 80% of new pot resistance 25k so we have kind of created a log pot used as a resistor between terminals 2 and 3.

Unfortunately using a log pot between terminals 2 and 3 isn't very useful.

What's useful is a reverse log pot between terminals 2 and 3 but it can't be done.  The parallel trick works the wrong way for us.

When you have a voltage divider the goal is different.  Also you have the option of connect the added resistor across pins 1 and 2 or pins 2 and 3.   For a variable resistor it only makes sense to connect resistors in parallel with the *two* terminals you are using - the other two terminals have no effect.

[ElectricDruid]

Having a look at that circuit in the simulator shows up a few things:

1) The frequency peak moves from about 200Hz to about 770Hz.

2) The 2K pot attached to the transistor emitter doesn't do a hell of a lot. Mostly it narrows the range of the other control. The widest range is obtained with the emitter shorted to ground.

3) As well as the mid boost, there's a heavy roll-off of treble. It's more of a resonant lowpass response than a simple mid-peak.

Overall, it's definitely in "static wah" territory.

[Rob Strand]

As well as the mid boost, there's a heavy roll-off of treble. It's more of a resonant lowpass response than a simple mid-peak

That's why the colorsound inductorless wah (and my inductorless wah) add a treble boost.   It's kind of a fudge/fix to correct the treble.   

I've got RC newtorks at the input and output to correct the treble,
https://www.diystompboxes.com/smfforum/index.php?topic=121430.0


IIRC, you can reduce loading with buffers but the circuit starts to grow.

[Box_Stuffer]

Having a look at that circuit in the simulator shows up a few things:

1) The frequency peak moves from about 200Hz to about 770Hz.

2) The 2K pot attached to the transistor emitter doesn't do a hell of a lot. Mostly it narrows the range of the other control. The widest range is obtained with the emitter shorted to ground.

3) As well as the mid boost, there's a heavy roll-off of treble. It's more of a resonant lowpass response than a simple mid-peak.

Overall, it's definitely in "static wah" territory.

The 2k Pot is useful to turn it down so you don't get the unpleasant distortion. It is really like an intensity control. It does sound best with it wide open ( no resistance) and it is possible in most positions but there are a couple spots where you need to dial it back to take away the buzz - mostly in the mid boost zone. As you turn it down you lose the phase effect as well as gain.

You can dial in some sweet "%^&*ed wah" tones for a rippin' Thin Lizzy style solo, but there are more subtle tones that you might just leave on all the time. Turned all the way down and on a neck pickup, you can do some bass lines if you were jammin' with buds.

[antonis]

<The 2K pot attached to the transistor emitter doesn't do a hell of a lot. Mostly it narrows the range of the other control. The widest range is obtained with the emitter shorted to ground.>

<The 2k Pot is useful to turn it down so you don't get the unpleasant distortion. It is really like an intensity control. It does sound best with it wide open ( no resistance) and it is possible in most positions but there are a couple spots where you need to dial it back to take away the buzz - mostly in the mid boost zone. As you turn it down you lose the phase effect as well as gain.>

It's just a poor BJT single stage CE amp, trying to compromise high open loop gain(*) with low distortion ..

(*)for more effective implementation of Twin-T configuration inside the NFB loop..

[Box_Stuffer]

I tried a 10kB pot in front of it as a pre-gain and grounded the emitter. It works pretty good this way. I get 100% of the effect plus I can reduce the input to prevent the distortion. Having both controls would give even more options. Maybe I can figure out a different method of intensity control instead of messing with the emitter.

[sergiomr706]

This seems to be the same circuit with another name, maybe It helps you http://www.diystompboxes.com/smfforum/index.php?topic=56112.0
https://tagboardeffects.blogspot.com/2013/01/70-funk-box.html?m=1

[Box_Stuffer]

I put it all together with the pre-gain rather than the emitter gain and it sounds pretty good. It really sounds great through bigger speakers - like 12" and it seems to respond more to humbuckers than single coils.