4 ohm load on 8 ohm amp?

[jmansion]

Are your drivers branded?, and do they have model numbers?.

Well, I have some to play with, hence my preference to use a DSP crossover and multiple amps.
I confess I don't understand quite why you are asking me that.

But I have (4-sih Ohm):
Peerless N95W-004
SBAcoustic SB23NACS45-4
SBAcoustic SB29RDAC-C000-4
Scanpeak 23W/4557T00
Wavecor TW030WA11-001
Vifa DX25SC-05

None of these are 'cheap little audio units'. The 23Ws (being discontinued) are about four hundred quid each.

I have other drivers too, but they are indeed billed as nominally 8Ohm.. I snap things up on eBay, and I'm a hoarder. ;-)

 

[kapitiaudio]

The Arcam's use different seconday tranformer windings for different speaker impedances. On some model's they have removed the 4/8 ohm switch and just specified 8 ohms. This is so they can save money with a cheaper transformer and omit the relays doing the switching.

 

[b_force]

As long as you stay at the same maximum current as the 8 ohm load, it will be fine.

It's the current that destroys stuff.

So for a 100W amplifier at 8ohm,this will give:

Sqrt(100/8)^2 * 4 = 50W

This value is lower because we are limited by current at this point.

Assuming that the amplifier doesn't have stability issues.

Since P=I^2 *R, be careful not to go over that current limit, because things will heat up very quickly.
Because of the square of the current, heat dissipation will go up fast.

 

[Marcel27]

I have a somewhat old multichannel receiver that I'd like to use for prototyping - it has DVD-A/SACD inputs that bypass the filtering etc which is handy for me, and claims 100W/channel. Its Arcam and very heavy so that might even be believable.
It says '8 Ohm speakers' on the back.
I have quite a few 4Ohm nominal drivers (Re is probably 3.5Ohm) that I'd like to play with, active DSP upstream of the receiver.
What's the worst that can happen?

I'm running here an very old Sansui B-55 on two 4 Ohm ASW Genius 300 speakers. The Amp is specified for 8-16 Ohm. If you don't push it too hard it will work.

 

[Bone]

Why dont you just put a 4 ohm resistor in series with each speaker. You will not notice the difference in output volume and you will protect the speakers.

 

[b_force]

Plus completely ruin the dampening factor (Q) of your loudspeaker.

 

[JMFahey]

I confess I don't understand quite why you are asking me that.

YOU are asking for help, it is your duty to supply needed data.

Your mystery speakers might be anything from high quality Hi Fi type to Music Instrument or PA types to Car type to cheesy pawnshop specials pulled from God knows where, so the question is very relevant.

Your "4 ohm" speakers might very well drop below 2 ohm at some frequencies.

 

[JMFahey]

It's the current that destroys stuff.

In power transistors, it´s SOA.

Which depends way more on voltage across transistor than current through it, just check any power transistor datasheet.

 

[b_force]

In power transistors, it´s SOA.

Which depends way more on voltage across transistor than current through it, just check any power transistor datasheet.

I wanted to keep it simple for everyone, but yes.

Or maybe a better answer is even: and/or.