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Thread: Like Duncan's Tone Stack but for Presence, Resonance and Vox Cut controls...?

  1. #1
    Supporting Member Steve A.'s Avatar
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    Like Duncan's Tone Stack but for Presence, Resonance and Vox Cut controls...?

    Like Duncan's Tone Stack Calculator but for Presence, Resonance and Vox Cut controls...?

    I guess you could do this with SPICE but I think it would be really cool if someone wrote a standalone program like TSC, but it would handle some of the common auxiliary tone control circuits in guitar amps and even your basic RC networks between tube stages or in a guitar tone control. (Like TSC a graph showing the frequency response curve would be generated.)

    Or if all of this can be done using SPICE maybe what would be needed is a tutorial for n00bies unfamiliar with the software. (I'd always thought of using SPICE to model complete electronic circuits and never just the building blocks like tone stacks.)

    Another idea... how about a USB audio interface to hook up to your computer so that you could actually hear the results of signal processing with TSC or a circuit modeled in SPICE?

    I know absolutely nothing about the computer circuitry and electronic hardware to actually accomplish this but I think it would be really cool...

    Steve A.

    P.S. Duncan just came out with an update to TSC in March but I haven't checked it out yet:

    TSC

  2. #2
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    It's actually quite problematic to derive response of feedback loop -based tone controls in an amp that has high output impedance (e.g. tube power amp). Yes, basically "Presence" and "Resonance" controls are nothing but shelving low and high frequency controls where the range is difference of open loop gain and closed loop gain.

    BUT

    Open loop gain of a high output Z amp is affected by load impedance. When you hook in a reactive load, like a loudspeaker, it will introduce a frequency response for the power amp because amp's voltage gain is higher to higher impedances than to lower ones. Since different speakers are different each speaker will naturally introduce its unique impedance characteristics, and therefore unique response for the amp. It can not be predicted.

    Basing calculations on "fixed" load impedance will not provide correct results because at the range where those tone controls work the load impedance is much higher than the nominal load impedance. In practice open loop gain at those frequencies will be higher than generally expected, which in turn increases range of those controls and therefore changes their response.
    Not to mention, the impedance is not "fixed" value. At speaker's resonant frequency ("Resonance") the impedance peaks and at upper high frequencies ("Presence") it gradually slopes upwards. Such characteristics will change the response into something different than simple "shelf" since overall response reflects load impedance.

    Things get even more complex if we add combinations of different speakers and crossover networks to the mix. They will all affect load impedance and amp's response.

    If you know impedance characteristics of the speaker (or load) you can calculate the response of those controls WITH THAT SPECIFIC SPEAKER (or load). The response will be different when you plug in another speaker (or load) though so I'm not overly convinced about usefulness of such simulations in general.

    Even TSC provides incorrect results if the load resistance is not set correctly. And that's an easy one... Assume a scenario where load is not a fixed resistance but an impedance, which is also moderately random and practically unknown.

    Yes, there's a learning curve to SPICE but it's one of those things you have to live with if you want more advanced simulation features than simulating a bunch of "premade" circuits. When you learn SPICE you are not limited by limitations of software, such as TSC. You can simulate practically anything. In my opinion, that's worth the effort of learning how to use that soft.

    Despite its simplicity I could never see myself going back to using TSC. It's just way too limited. With SPICE I can simulate any circuit from a simple resistive divider or RC filter to a complete amplifier.

    Another idea... how about a USB audio interface to hook up to your computer so that you could actually hear the results of signal processing with TSC or a circuit modeled in SPICE?
    In LTspice you can assign a .WAV file for any current or voltage source, and also "record" a .WAV file from any node current or voltage. I'm pretty sure that even today the computation process is too slow for "real time" operation, though.
    Last edited by teemuk; 06-14-2017 at 08:08 PM.

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