I’m trying to understand the cut off frequency of this tone control… but need help!

[Flying]

I’m getting myself in a mess because I don’t know how to do the calculations! This circuit is Spaceman Effects Charon, traced by Deadend FX. I’ve bread boarded it and think the fuzz is quite appealing, but I feel the tone control does nothing, it is supposed to cut bass as you turn it clockwise. It’s quite likely I’ve gone wrong on the breadboard and just can’t see my error but I was hoping someone might take pity on me and help me understand what is going on from C8 onwards.



This is my best guess, but it is based on very little knowledge:

A high pass filter is a capacitor followed by a resistor to ground, so when the tone pot wiper set at lug 1, there is 470nf capacitor with a resistance of 150k* to ground giving a cut off frequency of 2.2Hz (I used an online calculator).

(*I’m guessing the volume pot also has an influence, but I confess I don’t know how to calculate that.)

And when the wiper is at lug 3 the 470nF and 15nF are in series making the total capacitance 14.5nF and a resistance of 100k to ground giving a cut off frequency of 109Hz? Basically the same frequency as the open A string on a guitar.

I suspect the above shows my complete naïvety and none of it is even remotely right! But if you happen to know the secrets to some of this magic please let me know.

Best wishes, Chris.

 

[Flying]

User Error!

I started trying different value resistors for R6, 10k had a really noticeable effect, it got quite thin as I turned the pot, so I tried a 51k, still working, not quite as thin, so I tried a 68k, and was getting into the territory I was expecting! So I just thought I’d put that 100K back in, perfect! I wish I could say where I went wrong before, I thought I’d checked everything.

It’s great that I resolved the issue, however it also shows my maths above is completely wrong as the cut off frequency I’m sure has to be much higher than 109Hz. Time to go to electronics school

 

[JTEX]

You can download Tina-Ti and open the txt file I attached (change its extension from .txt to .tsc). Then press CTRL-A and you'll get a frequency response plot for different pot settings. Something like this:

 

[Flying]

Hi @JTEX,

Wow that’s brilliant, I wish Tina-Ti would run on my Mac. Thank you so much for running the simulation and taking a screen grab, it’s so much easier to see, and thankfully now hear what the tone control is supposed to do.

Thank you again,
Chris.

 

[JTEX]

I haven't tried this myelf, but apparently there's a way to run Tina-TI on a Mac using Wine:

How to run TINA-TI on your Mac - for free!

I'm pretty stoked. I have found a way of running Windows applications, including TINA-TI, on my MacBook Pro with the Apple M2 silicon. That seems to be a pretty common desire, so I thought I'd put a guide together on how to make that work. I have used vmWare Fusion in the past and really liked...

www.diyaudio.com

 

[giovanni]

I’m trying to build a bit of intuition on this. I find that ignoring the coupling cap and the volume pot is a good way to do that. The best I got so far is that when the tone pot is all the way to the “top”, the tone circuit is essentially bypassed, that is, the signal retains all frequencies. When the tone pot is all the way to the bottom, you get a voltage divider between the 100K resistor and the parallel of the tone pot and the cap. That gives you a high pass filter (not of the first order). From my napkin math, the filter cuts some lows but not by a lot (I’m getting less than 6dB which doesn’t match the simulation but it’s probably because I ignored the coupling cap and volume pot), so I’m not surprised you don’t really hear a big difference when turning the knob. I think you could get a more pronounced effect by using a larger value for the tone pot, but you would have to change the cap accordingly to maintain the same corner frequency. You could also use a lower value for the 100K resistor theoretically but I wouldn’t recommend that because it would absorb a lot more current.

Edit: the filter does start kicking in at 200 Hz according to my napkin math.

 

[JTEX]

What I don't get is how the two transistors work, seeing that they don't have a DC path from their emitters to ground. Also, the PNP seems to go between... GND and GND at both ends??? Makes no sense. It's not powered in any way.

 

[giovanni]

What I don't get is how the two transistors work, seeing that they don't have a DC path from their emitters to ground. Also, the PNP seems to go between... GND and GND at both ends??? Makes no sense. It's not powered in any way.

WTF are we sure the schematic is correct?!?

 

[giovanni]

OK I thought of a more intuitive way to think about the filter. When the pot is at the lowest spot (meaning filtering the most), you get the voltage divider as I mentioned above. However I should have considered the volume pot in parallel with the 100K resistor, that is a 50K resistor at max volume. For low frequencies the capacitor is an open circuit so you can ignore it and you get the voltage divider formed by said 50K resistor and the tone pot (also 50K total), giving a 6dB attenuation (1/2 the voltage) per the simulation. This is also useful to understand the effect of the volume pot: if you are at half volume, you get a parallel of the 100K and a 50K resistor, about 33K, so the bass frequencies are attenuated a bit more than 6dB (33/(33+50)~0.4). At high frequencies the capacitor acts like a short circuit, so you get no attenuation at all.

 

[Flying]

What I don't get is how the two transistors work, seeing that they don't have a DC path from their emitters to ground. Also, the PNP seems to go between... GND and GND at both ends??? Makes no sense. It's not powered in any way.

Hopefully with the voltages on the schematic it will help it make sence, I sort of think I know it works becuse the Ge tranistors emitter is at a more positive voltage than it's collector, but as I have already proven I don't know much! But that's partly why I'm breadboarding and trying to understand things.



Thanks everyone for their help, I’m really liking this fuzz!

 

[Sturdag Lagernathy]

Hopefully with the voltages on the schematic it will help it make sence

No, that schematic would not work as drawn. Maybe a couple of lower value resistors missing, about 1k, maybe less, parallel to C4 and C5? Got a pic of the build? Now I'm curious..

Edit: online schematics are the same, Dirtbox Layouts used it and has a "verified" vero.(1 user reports) I'll admit I'm at a loss. Investigation continues..

 

[giovanni]

Hopefully with the voltages on the schematic it will help it make sence, I sort of think I know it works becuse the Ge tranistors emitter is at a more positive voltage than it's collector, but as I have already proven I don't know much! But that's partly why I'm breadboarding and trying to understand things.

View attachment 88143

Thanks everyone for their help, I’m really liking this fuzz!

Transistors need to be DC biased to function, that is, they won’t even turn on if not biased. Both transistors are connected to ground via capacitors which blocks DC, so there is no path for biasing them. Even assuming that the second one is connected to Vcc instead of ground next to the trim pot, they would both be completely off in this schematic since either the emitter (for the NPN) or collector (for the PNP) are floating. If there is a working vero layout there must be a path to ground, maybe a missing resistor as @Sturdag Lagernathy mentioned.

 

[giovanni]

@Fuzzonaut built one on vero, maybe they can shed some light!

 

[Fuzzonaut]

Well, I got it back home now and just hooked it up again and ... it is a cool fuzz, but the TONE control is surely not you're "usual dark to bright" kinda thing like on a Muff. I would rather call it something like "Thight", as it does seem to tighten up the lows ... on a bass.

But yeah, you might think it doesn't really do anything, if you're not closely paying attention. It's very subtle, at least on my build.
I used the dirtbox vero layout and just went with it, no changes.

As for circuit/schematic analysis, y'all are way ahead of me,

 

[giovanni]

Well, I got it back home now and just hooked it up again and ... it is a cool fuzz, but the TONE control is surely not you're "usual dark to bright" kinda thing like on a Muff. I would rather call it something like "Thight", as it does seem to tighten up the lows ... on a bass.

But yeah, you might think it doesn't really do anything, if you're not closely paying attention. It's very subtle, at least on my build.
I used the dirtbox vero layout and just went with it, no changes.

As for circuit/schematic analysis, y'all are way ahead of me,

From the vero diagram, I can't see a path from E1 or C2 to ground, but maybe I am looking at it wrong? Can you tell?

 

[giovanni]

I think the only explanation is that those two 47u electrolytic caps are providing a path to ground via their leakage currents. On the Vero layout there is also a 100uF electrolytic cap from the trim pot to Vcc, which I think is how Q2 gets biased. Very strange way to design a circuit.

 

[lowpitch]

From the vero diagram, I can't see a path from E1 or C2 to ground, but maybe I am looking at it wrong? Can you tell?

Marked the ground tracks in green. Looks faithful to the schematic to me. Q1 E goes to ground through the two 47uF caps. Q2 C goes to ground through the 22K resistor and 20K trimpot. Both emitters are tied together.

 

[giovanni]

You’re right. Still unclear how Q2 is biased since both C and E have a path to ground but not to Vcc.

 

[giovanni]

Ok, I think the emitter of Q2 is at a positive voltage because it’s connected to the emitter of Q1. So that’s how it’s being biased maybe? Not sure about the base of Q2. I suppose it’s biased via R5 but I’m not super familiar with such a configuration.

Edit: it doesn’t help that Q2 is drawn upside down.

 

[Flying]

Edit: it doesn’t help that Q2 is drawn upside down.

This is the way I've always seen it: