Can I make this montage:


Considering that Z is the speaker impedance and R is the resistor in series, this way (with the resistor in series with the speaker) the factor which multiplies the power in the speaker is [ Z/(Z + R) ]², and for R = Z we get [ Z/(Z + Z) ]² = 1/4.

I'm ultra noob in audio world, but I was just curious if I can or cannot make that.

Hi Furuyá,

welcome to HA

Of course you can do this, but you should know that the impedance of almost every speaker is not at all constant. This will result in a huge change in frequency response.

Why do you want to add a resistor?

Hi Sunhillow, thanks for the welcome


Wow, so it isn't recommeded at all doing that! Thanks for the information. I thought that the impedance was constant.

I was curious about that because someone in other forum wanted to reduce the power of his speaker and I had that ideia. But as I said I'm a noob and I didn't know if I could do that.


Regards and thanks for the quick response

Another reason for it being a bad idea is that it dramatically reduces the damping factor that the very low output impedance of the amplifier would normally give to the loudspeaker. Any well designed power amplifier should have an output impedance measured in milliOhms. Adding a resistor with a value of Ohms in series with the load could reduce the damping by a factor of 1000 in extreme cases.

If it's too loud, attenuate the input to the amplifier, not the output.

Cheers, Slipstreem.

Hi Slipstreem, I don't know about this damping factor but I quickly read about it and I understood that it's (load impedance)/(amp. output impedance) but didn't understand why it's bad to have not so low output impedance, since what matters to the speaker is a clear voltage signal.

But don't worry, it's ok, I'm in a learning process

Just a very quick explanation - the speaker is actually a two-way transducer. Applied voltage results in speaker cone motion, but speaker cone motion also results in voltage being generated. If the cone has a resonance (and all cones do) then if it starts vibrating at the resonant frequency then it tends to continue vibrating at that frequency. This then tries to generate a voltage, but the low impedance of the amplifier resists that, thus damping the vibration. If you add series resistance then the resonance is not damped nearly as well and the speaker has a maximum in its frequency response at its resonant frequency.