PDA

View Full Version : Ok to connect 8 ohm speakers to 4 ohm stereo?


johnshum
May 25, 2008, 06:42 PM
I have a mini stereo that comes with speakers rated at 4 ohms. I want to hook up the stereo to the in-wall speakers in the bathroom, which are rated at 8 ohms. Is this doable as-is? If not, are there any (reasonable) means of making it so? Thanks for any advice!

KISS
May 25, 2008, 07:00 PM
No prpblem. Just won't be as loud.

Mr_am
May 25, 2008, 07:06 PM
Issue of impedance matching.. in your case would not allow for maximum power transfer.. as KeepIt said.. would not be as loud.

KISS
May 25, 2008, 08:16 PM
Actually audio amps are not transmission lines so impedance matching is irrelevant, In fact there is a term that is related to audio amplifier output impedance and that is "damping factor".

If an amp has a damping factor of 100 and a rated load impedance of 8 ohms. The actual output impedance of the amp is 8/100 or 0.08 ohms. This actually describes an amp I built from scratch.

An amp is constrained by V or I (current). An amp designed for a 4 ohm load places a higher I constraint and a lower V constraint than an amplifier designed for 8 ohm speakers. P is roughly (I^2)*R If R is taken as Z and a constant, and since the available I is lower then the maximum power to the speaker will be lower.

You learn a lot when you build one from scratch and you fixed them for a while.

Stratmando
May 25, 2008, 08:27 PM
If you wanted your amp to See 4 ohms, just hook 2 8 ohm speakers in parallel(+ to +) and (- to -).

johnshum
May 26, 2008, 08:09 AM
I'll test this by hooking the stereo up to a pair of old bookshelf speakers I have lying around. I believe those are 8 ohm units. I don't mind the bathroom speakers not being "loud enough" as long as they work. Don't plan to "sit" through an iTunes playlist, you know. I appreciate all the advice! :)

KISS
May 26, 2008, 12:17 PM
The formula for resistances in parallel is:

1/Rt = 1/R1 + 1/R2+... 1/Rn

Two 8's in parallel is 4.

Stratmando
May 26, 2008, 01:53 PM
Qoute "The formula for resistances in parallel is:
1/Rt = 1/R1 + 1/R2+ 1/Rn"
Or, the Reciprocal of the sum of Reciprocals, it is good for Resistors in Parallel, or Capacitors in Series.