Adjusting input impedance according to first plugin


Wouldn’t it make sense if plugins like a fuzz could set the input impedance so that it matches the behavior of the simulated analog pedal with the guitar that is plugged in?
How is the input impedance adjusted in the mod duo?
Is it frequency dependent for instruments?


I was actually wondering about the input impedance recently also. The tech specs says there’s a “digital input impedance selector” for line/mic/instrument which I presume corresponds to the left-knob menu for input gain staging low/medium/high. But playing a passive pickup electric guitars through it, I always need tho “low” setting with the fine adjust turned down a few dB to avoid clipping the input. I would have expected electric guitars to require a high gain setting if the input is meant to work with line level signals as well.

Perhaps the input automatically detects the type of signal coming in and adjust line/mic/instrument and it’s independent of the low/medium/high gain setting? And if so, I would be curious if plugins are able to detect and change this setting as well.


This would be far above what LV2 plugins been able to do, means, it’s out of spec.
Also, I wouldn’t like a feature like that, and, if possible, disable it. No plugin should handle my hardware settings.

Surely not. You set what you need, no magic involved here.


Maybe I have to elaborate a bit more. The fuzz face for example has a relatively low input impedance and will load the guitar pickup:
So with this pedal the volume knob of the guitar would behave very differently compared to an high impedance input. There’s no way to simulate this behavior in any plugin afterwards.
This motivated me to think about setting the input impedance of the mod duo to the input impedance of the first pedal to be simulated. Of course, the setting itself is the job of the host not of the plugin. But the plugin could provide the information of its input impedance to the host.


It seems that the Line6 Helix is actually doing something like that:


No, it can’t. There is simply no feature in the LV2 spec which will allow that. And even if it could (the MOD could add a feature for that), it isn’t the host which do this setting on the MOD, it’s the user. And that should be so, I ain’t would see plugins changing my input impedance, that, is only allowed to me.
Given the guitarix fuzzface plugin, this one is developed with the high gain setting in mind, which, I understand now, wasn’t the best choice. Newer plugs from mine will handle that much better. The old ones will receive a update this year, to cover that issue.


Without adjusting the input impedance of the device itself you can’t cover that issue. I’m not suggesting to take away the user’s freedom to choose what input impedance is set. I suggest that the user has the option to set the input impedance of the device to the input impedance of the first active plugin (or whatever they like). I don’t see any option for that yet.


Wouldn’t it be enough when you put a tinygain plug on the input and adjust the “level” yourself to your needs while keep your input impedance?


Let’s assume source impedance is much higher than load resistance Ra >> Ri:
Ua = Ra·I = U0 - Ri·I
U0 = Ua
If that’s the case, you are right and it’s just about volume.

But if it’s not, we have to look at complex impedance. There are inductive and capacitive parts on the load side that have a combined resonance frequency. If the source impedance is low (guitar volume pot at 10), the resonance gets damped. If the source impedance is high (guitar volume pot low), the resonance gets louder because there’s less load at the source side where the resonance frequency is. So while I turn down the volume at my guitar the sound gets brighter.
But even if I don’t use the volume knob, the resonance behavior depends on what guitar, pickups and cuircitry I use. (And there are inductive and capacitive parts on the source side too. But let’s forget about that…)