About the VOX tone stack

[JohnnyMcFly]

Hello all...

I have a question about the VOX Tonestack...


When calculating the frequency of the above outlined RC Filter. I get a value of
33,862.8 HZ

That can't be right? What am I missing?
If its not right please let me know how you calculated the right Frequency for it....

Also does R1 in some way act on this value too?


Thanks your help is appreciated...

[Dave H]

I think you need to move the decimal point one place to the left

[JohnnyMcFly]

But then the math isn't right?

I'm using
2*pi*Resistor*Capacitor/1
Where Resistor = 4.7e-012 and Capacitor = 1e+006

Is there something I'm doing wrong in the math? Or can someone verify that circuit is correct? Its right out of the TSC 1.3

Thanks for your help and suggestion though..

[JohnnyMcFly]

haha man I must be a super noob.. I figured you guys at this forum would have this figured in seconds...Thats why I say I'm a super noob because I'm probably doing something so stupid its not apparent..
haha

[Bruce / Mission Amps]

Well, just using 1Meg and 47pF isn't going to really get what you want but a simple RC circuit of 1M and 47pF has a 3dB point of:
3K386Hz, or 3,386Hz or 3.386KHz.

[Jag]

Dave's right!


fc=1/(2piRC)
fc=1/(2*3.14159*1000000*0.000000000047)
fc=1/0.0029530946
fc=3,386.29 (rounded to 2 decimal places)

[Jag]

Cross posts! Sorry Bruce!

[JohnnyMcFly]

Jag...your calculation uses
0.000000000047
Is that not 4.7e-11?
I thought 47pF is equal to 4.7e-012 in scientific notation...which would be
0.0000000000047. Thus altering the decimal point and the overall value goes from
3,386.29 Hz to 33,862.9 Hz..Which we all know cannot be correct...

Can anyone confirm that these are the real the real VOX tonestack values?

So, I am very confused and feeling like a complete moron.. Is there some rule of thumb you guys use when doing these calculations that I don't know about or something. I don't want to seem ungrateful for all your help it is most certainly appreciated, but I am still missing something? I use the exact same formula..Dave, how did you get that number?
Thanks guys I am sorry to keep on with this, but I really need to understand why this tonestack works if these numbers are correct...As far as I can tell using the numbers I get it should not work...

[Robert M. Martinelli]

Jag...your calculation uses
0.000000000047
Is that not 4.7e-11?
I thought 47pF is equal to 4.7e-012 in scientific notation...which would be
0.0000000000047.

Johnny,
1 pF is 1*10e-12 F in scientific notation, so 47 pF is 47*10e-12 F or 4.7*10e-11 F if you like it better.
This is the reason why you obtained a frequency which is greater than the actual value by a factor of ten.

The answers you got from our most knowledgeable friends are right.

Hope this helps

Best regards

Bob

[d95err]

If the purpose of your investigation is to learn filter math, then go ahead and calculate.

If the purpose is to figure out how the Vox tonestack affects tone, I suggest you download Duncan's tone stack calculator (Google for it).

[pdf64]

As I understand it, there's an error on the Vox model in the TSC, in that the Vox tone stack is driven by a cathode follower, but the TSC assumes a source impedance of 56k. It would be more accurate to modify it to 1.3k, as used in the TSC Marshall model.
Fortunately it's simple to edit it to other values.
Re the original query, I much prefer engineering notation, in which exponent only moves in steps of 3, and so matches up with the milli (e-3), micro (e-6), nano (e-9) and pico (e-12) - using that helps me avoid getting into the above type of muddle, which is all too easy otherwise.

[d95err]

If the purpose of your investigation is to learn filter math, then go ahead and calculate.

If the purpose is to figure out how the Vox tonestack affects tone, I suggest you download Duncan's tone stack calculator (Google for it).

Oups - looking at the screenshot of the original post, it seems that it is indeed taken from Duncan's tone stack calculator. So, obviously you already know about it. Sorry!

[JohnnyMcFly]

Yep you are correct Jag...Thank you for that...see I was a Super noob after all... How did I miss something so obvious!

Just so you know...What this is for is so that you can take the TSC files from the Duncan Tonestack Calculator and place them directly into our VST Amp Sim. The amp sim then uses that tonestack. We have just started this edit so I don't know how accurate it will be given the nature of it. We have to sort of MacGyver it into position as we had never planned on changing tonestacks in this way. So far it seems to be working, we are getting the correct response and shape, but what is in question is its interaction with the rest of the components. Our models are not meant to be 100% accurate representations anyways, they are meant to give you a good tone, and thats it. A a matter of fact, we don't attempt to model to 100% accuracy on any of our sims. We'll leave that for those that believe a tube amp can be digitally modeled(let them bat their heads against a wall for while), we as a company don't believe it can, there is just too many variables to try and control. Our point of view is simple..there is only one place to get a Marshall sound, and thats by using a Marshall.

Anyways guys thanks for your help and if you like I'll make sure that the software gets released here too as it will be freeware. No this is not an attempt to sway you to the Dark side..haha...We just want to give our users more options..

[Jag]

Scientific notation nails me once in a while too, if I'm tired or have had too much coffee. :-)

[JohnnyMcFly]

Well I tell you what...I FINALLY got these numbers right and I'm thinking Great! Lets plunk it in the software..expecting something that would behave a little like a VOX tone stack...NOT EVEN CLOSE! The Marshall and Fender tonestacks worked out very well, but the vox is not even close..I have the shape when in full off and full on positions, but in between is nothing even similar. I believe it stems from the way the MID is functioning..The bass and mid in this configuration work in tandem..if I'm correct. So, when the bass knob is turned down, the mid is full?