### SOS impedance article Q

Posted:

**Thu Apr 02, 2020 4:01 pm**I've just been reading Hugh's excellent SOS article "Understanding Impedance" (http://www.soundonsound.com/techniques/ ... -impedance) and understand it apart from one concept which I am struggling to figure out!

When talking about matched impedance there's the example given of a 0dBm signal from a mixer with 600R output impedance connected to a tape recorder input with 600R input impedance which registers 0dBm also. I can't seem to work out why this is the case! For calculating dB loss with input / output impedances I've been visualising them like a resistor divider and using the formula Av (dB) = 20*log10(Zin / Zin + Zout). Following that through for the voltage matching examples given later in the article gives me the same results as Hugh calculates (0.04dB etc..) but this first example would seem to give a -6dB loss with matched impedances, and with 2 x 600R in parallel (300R equivalent) I calculate a -9.5dB loss. I understand how having two 600R resistors in parallel causes half the total voltage to develop across each of them but can't make that sit with the equation above. I'm obviously doing something wrong but would really appreciate any pointers to show me where!

Thanks in advance.

When talking about matched impedance there's the example given of a 0dBm signal from a mixer with 600R output impedance connected to a tape recorder input with 600R input impedance which registers 0dBm also. I can't seem to work out why this is the case! For calculating dB loss with input / output impedances I've been visualising them like a resistor divider and using the formula Av (dB) = 20*log10(Zin / Zin + Zout). Following that through for the voltage matching examples given later in the article gives me the same results as Hugh calculates (0.04dB etc..) but this first example would seem to give a -6dB loss with matched impedances, and with 2 x 600R in parallel (300R equivalent) I calculate a -9.5dB loss. I understand how having two 600R resistors in parallel causes half the total voltage to develop across each of them but can't make that sit with the equation above. I'm obviously doing something wrong but would really appreciate any pointers to show me where!

Thanks in advance.