Author Topic: LTSpice and MXR Dist+ Impedance question  (Read 193 times)

bushidov

LTSpice and MXR Dist+ Impedance question
« on: February 15, 2021, 01:25:41 PM »
Hi All,

Sorry to keep bothering everyone, but I keep getting curious about things and have questions. So, this time, its the MXR Distortion+. Using this as my schematic:


Per ElectroSmash, the input impedance is calculated as:
Zin = R1 + ( R2 // ZinOp-Amp ) + ZVirtual Ground
Zin = 10KΩ + ( 1MΩ // 2MΩ ) + 500KΩ = 1176KΩ or around 1M

I know ElectroSmash is something to take with a grain of salt, but this also matches with the official Jim Dunlop site for the officially declared Input Impedance of the pedal. However, because of C1 and it's reactance, shouldn't it be much much lower, like under 200K at 1kHz?

What is weirder is that LTSpice seems to agree.



At 1kHz, it is giving me around 150K of impedance.

Am I measuring it wrong?
Is the "declared impedance" called out for a different frequency (and I was wrong to assume 1kHz)?
Is this just something LTSpice doesn't do out of the box?
« Last Edit: February 15, 2021, 02:01:41 PM by bushidov »
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away."
 
- Antoine de Saint-Exupéry

antonis

Re: LTSpice and MXR Dist+ Impedance question
« Reply #1 on: February 15, 2021, 01:44:26 PM »
At 1 kHz, C6 capacitive reactance is 159 Ohms so you can safely ignore Zin virtual ground (500k)

C2 capacitive reactance is 15k900 added in series to R1..

So, you have 26k + (1M//2M) -> 692k at 1kHz..

No comes into game V1 output impedance toghether with C1 reactance at 1kHz (159k).. :icon_wink:
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

bushidov

Re: LTSpice and MXR Dist+ Impedance question
« Reply #2 on: February 15, 2021, 02:05:30 PM »
Doesn't the reactance of C1 play a role?

Also, when a pedal company is quoting "Input Impedance", at what frequency are they quoting it at, as by definition, impedance is frequency dependent?
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away."
 
- Antoine de Saint-Exupéry

antonis

Re: LTSpice and MXR Dist+ Impedance question
« Reply #3 on: February 15, 2021, 02:16:05 PM »
Sorry..!!! (typo)

"Now" instead of "No".. :icon_wink:

"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

PRR

Re: LTSpice and MXR Dist+ Impedance question
« Reply #4 on: February 15, 2021, 03:23:41 PM »
It may be clearer if you limit the frequency range and plot on log paper. But my Idiot agrees with yours.

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