Umm why is there an inductor in my passive I/V?

[eVITAERC]

Hi all,

I am in posession of a TDA1545 based Monica 2. Recently when I was toying with the notion of adding a buffer to the output so I looked up the schemetic, and something left me very puzzled.

The Monica 2 sports a very simple resistor I/V. Basically the Iout from TDA1545 first sees a 2.2k I/V resistor to ground and the voltage is tapped through a 4.7uF coupling cap. At least that is what I thought. Upon further inspection it is clear to me now that between the output cap and the I/V resistor there is actually a series 1mH inductor! What does that do? Does it form some better filter, or what? I am very puzzled. Thanks to anyone who can help shed some light on this.

[KT]

Are you refering to the little tiny green inductors, L1 and L2, that sit to either side of the I/V resistors?





I talked to Yeo briefly about this when I was changing out the I/V resistors (and accidentally clipped out the tiny inductors).

He said that with most sources these inductors aren't needed. But he did say a very few sources would send some sort of noise through, and this is what the inductors are meant to suppress.

I didn't get any more details than that.

I soldered some little jumpers into the spots where the inductors used to be and have had no problems driving the Monica II with a Rega Planet or Roku Soundbridge as the digital source.

Can't tell whether taking out the inductors changed the sound in any way, though, as I changed out some of the other components at the same time.

Best,
KT

[wa2ise]

Those inductors are intended to attenuate ultrasonic clock crud. Some audio amps may not like ultrasonic signal energy, might cause intermod or such. If you didn't hear anything different then your amp must not have a problem with ultrasonics. I'd leave the inductors in however, you may in the future use some other audio amp that might not like the ultrasonic crud.