Warmanized passive tone filter circuit. Your thoughts?

[ssstonelover]

Someone elsewhere brought this passive Warman circuit to my attention as an alternative to the usual tone controls or even PTB. I can't find a single review online though Warman published this 3 years ago, and though I suspect someone or more has  implemented it at some point.
The Warman link is: passive tone filter circuit.
It looks very intriguing due to the continuous tone sweep across the frequency spectrum.
Here is an image with part of the story.



At first blush it appears to be a powerful and useful solution to the usual tone controls, but having never done anything like this, or seen other alternatives of the same basic architecture, I'd like input and suggestions on plus and minus points and advice before I'd proceed down such a road.

[sumgai]

Bethany,


I must start out by saying that I'm going to be quite negative in my analysis.

First, no matter the pot's rotation, the signal is always going through a 0.1µf cap, and that always raises a flag for me. Is that value large enough to pass the entire frequency spectrum without loss of signal level? If so, then why have it in the first place?  It's not like we're playing with a mix of high voltages and/or AC and DC here.

Next, let's look at the input side. Here we see two paths for the incoming signal to get to the pot, one going through a cap, and other through a resistor. But wait.... each of those paths also go to ground through yet another cap or resistor, depending. So right there we see some signal strength being siphoned off to ground, before even getting into the pot. And that's the intention of the circuit - to cut signal strength at certain frequencies designated for each path, and then to "choose" from one path or the other by means of a pot... sort of a blend between the two paths.  But that 'blend' won't put the whole picture back together again, at some point someone is going to say "I'm not hearing that full tone that I expect from this pickup".

And that sums it up.  In short, if you were Dick Clark, I'd tell you that I can't dance to that, it's got no beat.
HGTH




sumgai

[ssstonelover]

Sumgai,

'No free lunch' is what I'm hearing, or maybe I'm hearing 'the cure is worse than the disease', too bad.
I've always heard that passive tone controls were subtractive, meaning this one would be no different but likely worse than a PTB in that the control is always 'engaged' to a degree here.
How would is stack up to the usual TBX circuit in your opinion?

[JohnH]

Maybe it might have some tones that could be liked, but it looks that it will always be taking some tone away and so that the natural sparkle of a simple standard wiring is diminished. So if you want to try it, maybe do a reversable lash-up version to test it. Then if you like what it does, I'd suggest that the pot could have a push-pull switch so it's completely bypassed unless you pull the knob.

[JohnH]

PTB circuits are great because both the bass and the treble side can be set to max and have no extra effect. In fact the treble on a PTB can be no-load, if you want the option of a bit more instead of less.

TBX's have their own quirky-ness (see our tone control discussions) but Phostenix' re-imagining of it as a combined treble/bass control is very clever.

[ssstonelover]

JohnH,

Yeah, a 100K push-pull pot could be used to selectively bypass the 'Warmanizer' control for no-load function, or that if I wanted to use a push-push I could buy a 250K and then throw a resistor across lug one and 3 to get the effective resistance down to ~100K. In any case I can't say I'm dying to try this that badly when there are other acceptable solutions available.

The PhosteniX mod of the TBX is more appealing I'd say overall, in that if I do that much work (the mod), it could work on a lot of other projects well, so stand me in good stead, plus is one less hole on the body than a PTB, though that is just a minor consideration, compared to the slicker way of rotation between treble and bass cuts with a single knob. I should review his video to see how much of a PITA is might be LOL, but likely I am up to the challenge.

[JohnH]

Sure, I think the TBX mod by Phostenix is just a clever way of wiring the pot, but doesn't need a mod to pot itself, if you already have a TBX pot.

[reTrEaD]

This is clever idea in an amp (such the Supro Thunderbolt) or a pedal (such as the Electro-Harmonix Big Muff) but in a passive guitar it's not clever at all.

We can ignore the useless .1µF capacitor and break the rest of it into sections.

The 4nF cap and 100kΩ resistor form a high-pass filter with a (approximately) 400Hz -3dB rolloff frequency.
The output of that filter feeds the CW end of the pot.

The 39k resistor and 10nF cap form a low-pass filter with a (approximately) 400Hz -3dB rolloff frequency.
The output of that filter feeds the CCW end of the pot.

Under ideal conditions, with the pot at its midpoint, we would expect about 5dB of insertion loss and a modest mid-scoop of about -3dB. Rotating the pot clockwise "boosts" the treble above the initial -5dB loss and cuts the bass slightly. Rotating the pot counter-clockwise "boosts" the bass above the initial -5dB loss (to about -2dB) and cuts the treble at a rate of -6B per octave.

However, driving this circuit with passive guitar pickups is far from ideal. The loading caused by the high pass filter is almost tolerable by itself. Roughly equivalent to a 200kΩ fixed resistor at 400Hz. But it does get worse as the frequency increases. The loading caused by the low pass filter is worse. Roughly equivalent to a 80kΩ fixed resistor at 400Hz and it too gets worse as the frequency increases.

I don't have a way of modeling the effect the inherent loading will have, but I suspect it won't be pretty. Putting this in a test mule with a DPDT to bypass it (and have a traditional tone control) in the bypass loop would be a good way to evaluate it firsthand.

[reTrEaD]

btw, when I first glanced at the thread title, I thought it said Womanized passive tone filter circuit.

[ssstonelover]

Retread,

A test mule would be interesting, generally speaking, but given your comments, I think I will just stay clear of this circuit period! (At least for a guitar anyway)
I will have enough loading (HHH rails on this build) so don't want to drag down the signal any more in the higher frequencies.

As far as the name, yes, kind of a play on words. I wonder if that name was Warman's version of British humor LOL.

[Yogi B]

Not really adding much on what other have already said...

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Sept 18, 2023 14:09:11 GMT -5 sumgai said:

First, no matter the pot's rotation, the signal is always going through a 0.1µf cap, and that always raises a flag for me. Is that value large enough to pass the entire frequency spectrum without loss of signal level? If so, then why have it in the first place?

There's no other reason other than because it's present in the Big Muff Pi (C3 in their schematic), from where the circuit's been lifted.

Warman's only difference being the larger 100k resistor in place of the typical 22k. In the original circuit, the effect of a larger resistor would be to raise the cutoff frequency of the low pass filter half of the control, presumably with the aim of eliminating the mid-scoop present in the response (see the green 'flat' response in §4.1 of the previous link). Also, as others have mentioned, note that at the mid setting (and assuming the notch has been filled) we still have around 6dB insertion loss (a halving of the signal volume).

However a further thing to note is the usual analysis is in the context of the rest of the BMP circuit: the tone control is being fed from a fairly low, and constant, impedance (around 2k at low frequencies, depending on transistor gain, and falling to even less at higher frequencies). In these circumstances the around 30k (at 1kHz) of loading the the tone stack places upon the previous stage can largely be ignored, as a larger loss comes from the tone stack itself.

Whereas, the output impedance of a typical guitar pickup is more — much more particularly around its resonant peak (usually in excess of one hundred times that 2k value). Therefore, 30k of loading represents a much bigger issue and will cause significant loss of high end even at the 'full treble' end of the tone knob. (This is also where the Warman's change to a 100k resistor in place of the original 22k has a secondary effect, the original 22k would further worsen the loading.)

The following frequency response plot hopefully illustrate what one can expect from the combined insertion loss & loading effects. In red we have a typical Strat pickup loaded with two 250k pots, 470pF of cable capacitance & 1 Meg amp input impedance. The orange trace removes both 250k pots and adds the 'Warman' tone control set at full bass, yellow has the control set half way, green is set at full treble.

[ssstonelover]

Yogi,

Ah, based on the Big Muff Pi....
The graph is very illuminating high end losses across all the settings in this circuit in addition to the volume loss.
Quite an impact you're quantified due to the 'misuse' of what might work well elsewhere but not with the naturally high pickup impedance.

[ashcatlt]

Sept 18, 2023 19:08:32 GMT -5 reTrEaD said:

…thought it said Womanized...

I assumed we were talking about that Clapton thing at first.

[ssstonelover]

Yes, that would be better than Warman's bad word play.

[reTrEaD]

Sept 21, 2023 21:40:12 GMT -5 ashcatlt said:

Sept 18, 2023 19:08:32 GMT -5 reTrEaD said:

…thought it said Womanized...

I assumed we were talking about that Clapton thing at first.

That was my first thought.

My second thought:

[ssstonelover]

Ah yes, Britney.... but that is more on the aggressive side, not the passive one LOL