Amps and 4-ohm loads

[discomo]

Any generalizations or directions on what to do to drive a 4 ohm load with an amp rated for 8 ohms??
I am under the impression that SS amp(s) can driver higher than stated impedances w/o too many problems. If I find a tube (power) amp with only an 8 ohm tap, what can I do (if anything) to use this with a four ohm load?
Thanks-
Mike

[TheGimp]

That in part depends on what power level you are talking about.

I routinely drive 4 Ohm loads with amps designed for 8 ohms at moderate load levels (ie, I'm not using a guitar amp driven at full output).

Driving a miss-matched load with a tube amp with the load below the optimum load results in slightly more distortion, and depending on the operating point may result in more power out or less power out. Unless the amp is poorly designed it should not result in red plating or other damage to the amp.

[Vinylsavor]

Hi!

There is no general answer to this. It depends both on your speaker and on your amp. Those impedance numbers are quite soft parameters and leave a lot of room for interpretation. You should consider two more aspects:

The variation of the impedance of the speaker over the freqeuncy band. In general higher speaker impedances are easier for tube amps but what counts more is less variation of the impedance. A 4 Ohm speaker can drop as low as 2 Ohms or lower in extreme cases.

Also important is the output impedance of your amp. This leaves a lot of room for variation. Especially no feedback SET amps tend to have a high output impedance.

Mismatching is generally a compromise. It would work but the amp should be able to cope with the speaker impedance. An amp with a highish output impedance (low damping factor) will cause colorations when it drives a speaker with a impadance curve with large variation. In extreme cases a 8Ohm amp can drive a 4 Ohm speaker better than a 8 Ohm amp, if the speaker has less impedance variation.

If the amp has several taps it makes sense to also try the 4 Ohm tap with 8 Ohm speakers. This often sounds better since it copes better with impedance curve variations.

Try it out and use what sounds best

Thomas