How to build a device to reduce impedance of preamp?

[veenife]

hi, im a audio engineer student and im about to write essay about how load imedance affects the frequency response from microphones...

in the end i want to build two devices... one that raises the mpedance and the other that sinks the impedance of the preamp input....

to raise the impedance i guess its pretty easy... you can just build a pot in between mic and amp and there we go....

like seen here https://www.youtube.com/watch?v=r1yPMDHiRuc
....(correct me if im wrong)

but then, how can one build a deivce to reduce the impedance in a preamp???

 

[godfreyl]

The video you linked to shows how to reduce the input impedance of the preamp, not increase it.

 

[veenife]

Really? im a total ignorant sorry... anychance to get that explained? i mean i thought a poti in the middle of the way would cause more resistance and more resistance incrase the impedance... or??

but yeah, so how to make a device that increases the impedance?

 

[Audioguru]

A resistor IN PARALLEL with the input of the preamp REDUCES the input impedance.
A resistor IN SERIES with the preamp input increases the input impedance.

Note that the effect is barely heard and works only with a "dynamic mic" (coil, diaphragm and magnet).
I wonder why the effect was not demonstrated in the video?

 

[godfreyl]

anychance to get that explained?

See here or here. The explanation in the video wasn't very clear.

Anyway, adding a 10K resistor in series with the preamp input will significantly increase the input impedance, while adding a 100 Ohm (or even 10 Ohm) resistor in parallel with the preamp input will significantly reduce the input impedance.

Adding resistors in series or parallel is certainly the cheapest and easiest way to change the input impedance, so it's ideal for experimenting to see the effect on frequency response. However there is a bit of a penalty in terms of sound quality as the noise performance will be degraded.

 

[veenife]

Thats all i was looking for, that helps a bunch already, Thanks! :-D