Author Topic: Duncan Tone Stack Calculator Zsrc  (Read 4726 times)

nero1985

Duncan Tone Stack Calculator Zsrc
« on: March 09, 2015, 08:41:26 AM »
Hi guys, just curious if somebody can help me with this, I was just wondering if there's a way to figure out proper value for the "Generator Source Resistance", i have a MuAmp circuit and i wanna tweak a tone stack, I was wondering how i can measure that value coming from the circuit into the tone stack to get a more accurate response on the calculator before breadboarding.

Thanks!

Digital Larry

Re: Duncan Tone Stack Calculator Zsrc
« Reply #1 on: March 09, 2015, 09:14:24 AM »
If you have a scope this will be more straightforward.  You put a signal through your circuit, measure the peak voltage, then put a load on it and see how it reacts.  The peak voltage will start to go down with some sufficiently low enough load.  It might be 100k, 10k, 1k, 100, 10, hard to know in advance (as I'm not familiar with your circuit).

Then you can get the source impedance using this formula, which I stole from myself.

Source impedance (ohm) = (Rload) * (Vopen - Vload)/(Vload).

Example:
Suppose Vopen = 3 volts.
Rload = 1k
Vload = 1 volt.

Rsource = (1k) * (3 - 1)/1 = 2k ohms.
Digital Larry
DSP tinkerer and former transistor twister

GibsonGM

Re: Duncan Tone Stack Calculator Zsrc
« Reply #2 on: March 09, 2015, 02:37:36 PM »
Nice quickie way to do the Z, Larry....Am I right in understanding that the absolute value of the load isn't important, just the amount it sags by?     Are we looking for the 'moment it begins to sag', like finding a resonance, or is the amount the peak reduces by unimportant, as long as it has dropped from the previous peak value?   Want to be sure I'm doing it right...thanks....
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duck_arse

Re: Duncan Tone Stack Calculator Zsrc
« Reply #3 on: March 10, 2015, 09:04:14 AM »
Rsource and Rload are two parts of a voltage divider. Vload is the divided voltage. Mr Ohm does the rest.
"So many things I would have done, but cows got in my way" - Both Sides Now

GibsonGM

Re: Duncan Tone Stack Calculator Zsrc
« Reply #4 on: March 10, 2015, 10:16:41 AM »
It works better if one takes their measurement BEFORE the coupling cap, LOL....I think the very odd results I was getting (triode gain stage simulation in LT) were due to partial bypassing...some results were like 800k Zout!   But when I do this before the cap, I am getting more trusty results.  Should've seen that coming, ha ha.    Rload working with a 22n cap, impedance changes w/frequency....

Cool  :)
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merlinb

Re: Duncan Tone Stack Calculator Zsrc
« Reply #5 on: March 10, 2015, 10:33:39 AM »
Hi guys, just curious if somebody can help me with this, I was just wondering if there's a way to figure out proper value for the "Generator Source Resistance", i have a MuAmp circuit and i wanna tweak a tone stack, I was wondering how i can measure that value coming from the circuit into the tone stack to get a more accurate response on the calculator before breadboarding.
If you show use the circuit we can probably tell you roughly what its output impedance is.
Or you can simulate it with your favourite, um, simulator.

GibsonGM

Re: Duncan Tone Stack Calculator Zsrc
« Reply #6 on: March 10, 2015, 01:43:41 PM »
Rather bone-headed question about this, but....if Rload = Zsrc, wouldn't the peak voltage observed between them be 1/2 the peak when open?
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...

slacker

Re: Duncan Tone Stack Calculator Zsrc
« Reply #7 on: March 10, 2015, 02:37:09 PM »
Yeah, that's the easy mathlessTM way to do it, adjust the load until you get half the voltage and then measure the load.

You can flip the idea upside and use it to measure input impedance as well, Get something will a low output impedance or that you know the output impedance of and hook it up through a series resistor or pot to the thing you want to measure.

GibsonGM

Re: Duncan Tone Stack Calculator Zsrc
« Reply #8 on: March 10, 2015, 03:32:06 PM »
I've done it for input impedance with a signal generator (fancy name for my AC source, lol) and a 1M pot, "Anderton Impedance Tester" method.

Figured it was actually the same thing!!
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nero1985

Re: Duncan Tone Stack Calculator Zsrc
« Reply #9 on: March 11, 2015, 09:11:52 AM »
Thank you for the advice! I was messing with the BSIAB without the last stage before the big muff tone stack

GibsonGM

Re: Duncan Tone Stack Calculator Zsrc
« Reply #10 on: March 11, 2015, 12:06:35 PM »
I did some digging, and this shed a lot of light on the subject for me:  https://www.youtube.com/watch?v=ieAhBejHe2M
MXR Dist +, TS9/808, Easyvibe, Big Muff Pi, Blues Breaker, Guv'nor.  MOSFace, MOS Boost,  BJT boosts - LPB-2, buffers, Phuncgnosis, FF, Orange Sunshine & others, Bazz Fuss, Tonemender, Little Gem, Orange Squeezer, Ruby Tuby, filters, octaves, trems...