Variable input impedance

[mike_mccue]

I am familiar with multi tapped transformers allowing for switchable input impedance on preamplifiers.

I am looking at a ART MPA preamp with a variable impedance that is adjusted via a rotary knob and a transistorized input component.

Can someone help me understand how this works?

thanks,
mike

[Steve Conner]

It's more than likely just a variable resistor connected across the inputs of an ordinary solid-state mic pre. It won't actually optimize the matching like a transformer would: just load the mic more or less.

[mike_mccue]

Thank you Steve,
I received a schematic of this circuit tonight and you are correct.

It appears they are using a dual ganged 5K audio taper pot in parralell and then the signal heads towards some 2N4403 chips.

I now understand that they are using resistance loading as a substitute for "variable impedance" but I'm not sure if I understand the implications.
I'm familar with the extra resistors on something like a Fender guitar amp inputs 1 & 2... They just seem to cause a general change in level (at least that's what I have always thought)


Is it fair to say that they are somehow scaling the impedance? I understand that impedance is an AC measurement. But I don't understand how resistance relates... in other words I'm wondering if resistance equates to flat frequency response across the AC spectrum (eg 20Hz-20kHz) or is that a complete misunderstanding. Is there any way to understand how they might relate?

I understand that transformers have other characteristics than just impedance and they can effect the sound... so I'm just trying to get a feel for what to expect from this combination design.


thanks again for taking the time,
mike

[mike_mccue]

FWIW,

Here's a link to the schematic:


ART-MPA-Gold-schematic.pdf

best regards,
mike

[Steve Conner]

Thank you Steve,
I understand that impedance is an AC measurement. But I don't understand how resistance relates... in other words I'm wondering if resistance equates to flat frequency response across the AC spectrum (eg 20Hz-20kHz) or is that a complete misunderstanding.

No, that is quite right. Impedance is a more general term that covers the resistance of resistors, but also the ways that capacitors and inductors react against the flow of current through them, which are more complex and subtle, in that their reactance changes with frequency, whereas the resistance of a resistor doesn't depend on frequency at all.

So if you're dealing with a 5K pot, well, that is just a resistor, so if you set it all the way up, it'll have a "resistance" of 5K Ohms. Or an "impedance" of 5K Ohms across the whole audio band, like you said.

If you want an executive summary, well, I'd expect the knob to have a less interesting sonic effect than the equivalent knob on a transformer-coupled tube preamp would. The impedance (not just resistance...) of its input transformer would colour the sound in different ways as you messed with the tappings. But with this circuit under discussion, you'll just hear whatever the change in load resistance does to the mic.

I have a Focusrite TwinTrak Pro with those variable input impedance knobs, but to my ear they never really did much. It seems to be a good sounding pre, but I just ended up leaving the impedance knobs at max.

[mike_mccue]

Thank you Steve. It almost seems like it's functioning as a volume knob of sorts.

Is that a reasonable comparison?

Also is there a reason it's dual ganged and apparently wired in parallel?

best regards,
mike