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Preamp volume knob controlls bass too?

Xeperiset

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I've had the schiit stack (modi 2 + magni 2) for a long time. Recently I got the SYS preamp for the purposes of switching outputs. The actual intent of it is switching inputs but I was told this will work by the Schiit customer service.
I'm using it to switch between my headphone amp and speaker amp, and it goes into the modi, and then to my computer.
If I lower the volume on the SYS itself instead of the amps the bass lowers along with it, having it at 1/3 volume has very low bass and 1/4 is basically nonexistent. It makes the volume knob on it pretty much useless.

Is this because I am using it 'in reverse' or is it unrelated?
 

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NTK

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The simple passive attenuator circuit looks like this. I think this is how volume control is implemented in the SYS passive pre-amp. This circuit doesn't work in reverse, which means you can't use it to control volume properly if you reverse input and output.

pot.png

(source: https://commons.wikimedia.org/w/index.php?curid=20737313)
 
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Xeperiset

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So what is it even doing? It does lower the volume to 0 at position 0%.
 

NTK

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When used in reverse and the volume is set to 0, it basically shorts the input, and that's why you are not getting any output.

Used in reverse, the volpot becomes a variable resistive load (goes from its rated resistance to 0) to the input.
 
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Xeperiset

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I'm wondering why on the way to 0 it decreases the bass as well. Should I basically have it at 100% all the time?
 

NTK

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My guess is this is how your source react to the varying load impedance. Yes, you should leave it at 100% and your SYS should then function as a pure switcher.
 

Blumlein 88

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You might get an effect like this if some of the gear in the chain is capacitor coupled.
 

Zek

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You can increase the value of the capacitor.
 

Blumlein 88

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Yes increasing the value of the capacitor if you know where it is and can get to it.

You really were given bad advice to use it in reverse. When you lower the volume the impedance being seen by the Modi goes down. The Modi is supposed to be direct coupled no cap. Still with a low enough impedance maybe it can't keep the low end flat in response.

I could puzzle thru this further, but really you shouldn't use the SYS backwards like that. Or if you do, control volume elsewhere and keep this unit turned all the way up. When you turn it down you are asking the Modi to look into a very low impedance. It may not hurt it over time or it may. Turned all the way up, the Modi is seeing a 10 k ohm impedance which should be fine.
 
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