Mixing speakers of different impedance

[Hewitson]

I have here a stereo system which is designed for 4 ohm speakers but my Dad insists on using his big 50W Kenwood 8 ohm speakers with it. What I'd like to do is connect one of the original 4 ohm speakers to one channel, and one of the Kenwood's to the other in order to show him how much better and louder it would be with speakers of the correct impedance.

Is there any reason why this isn't a good idea?

Thanks

 

[KJ6EAD]

No, an amplifier designed to drive a 4Ω load will just deliver a lot less power into an 8Ω load, but it's not really a fair comparison. Now if you were to connect his 8Ω speaker in parallel with a non-inductive 8Ω power resistor on one channel and your 4Ω speaker on the other, the qualitative comparison would be more valid.

 

[Hewitson]

In terms of sound quality, you are certainly right, but the main thing I want to demonstrate is how much louder the 4 ohm speaker would be (with the 8 ohms the system doesn't go very loud).

Is there any chance of damaging the amp with a 4 ohm on one channel and 8 ohm on the other?

 

[BradtheRad]

The hookup you plan sounds all right.

8 ohms is an easier load on an amplifier than 4 ohms. (Although there are some amps with a warning to avoid operating with no load attached.)

Even if you succeed in getting your father to recognize that his 8 ohm speakers have less volume, he may claim 'but they deliver higher highs and lower lows', or less distortion, or better imaging, etc.

To drive those 50W 8 ohm speakers to loud volume, it takes an amplifier with a higher internal voltage in the output circuit.

See if they have a switch or knob in the back, allowing you to choose volume or frequency response.

 

[KJ6EAD]

Is there any chance of damaging the amp with a 4 ohm on one channel and 8 ohm on the other?

No. Think about it. If you had a speaker on each channel and one got disconnected, one channel would have 4Ω and the other would have ≈∞Ω and no damage occurs.

 

[Kral]

For a given volume setting, your amplifier will deliver to the 8 Ohm load 1/2 the power it would deliver to a 4 Ohm load. This is a 3 dB reduction, which would produce a noticeable, but not dramatic reduction in volume, assuming the efficencies of the speakers are identical. However, you must take into consideration any differences in efficiency between the two speakers to determine the actual difference in sound pressure level. If Dad's speakers are twice as efficient as the 4 Ohm speakers, then the volume would be the same. Studies have shown that to produce an apparent (perceived) doubling in volume, a difference of 10 dB is required. This corresponds to a power ratio of 10:1! A 3 dB increase in power results in approximately 23% increase in apparent volume.