Watts @ Impedence

[jugleeson]

I'm looking at a bass head that has an output of 500 watts @ 4 ohms. I'm wondering if there's a calculation that will tell me what the output would be at 8ohms??

[es345]

Assuming that it is a transistor amp/ amp with very low internal impedance and a fixed maximum output voltage:
then it is 250W.

here is the calculation

Basic formula: U*U= P*R
Assumption above: Umax (4 Ohm) = Umax ( 8Ohm)

>> 500W (at 4 Ohm) *4 = P ( at 8 Ohm) *8

P (at 8Ohm) = (500W*4)/8= 250W

[g1]

The calculation above is true in theory. However in practice the power supply voltage does not hold up at full power into minimum load impedance. This is what I think es345 was hinting at with the statement "fixed maximum output voltage", which is rare in practical applications.
Typically, I think the number would be closer to 2/3 power rather than 1/2.
There is no exact calculation as different models of amplifier will respond differently to changes in load impedance.

[R.G.]

This is yet another consequence of a power amp being a small lump added onto the power supply. If the power supply can't supply it, the power amp can't use it.

Thomas Organ Vox amps used this to advantage. They had amps with 30, 60, and 120Wrms. All the amps used the same power supply voltages, but they changed the speaker impedance from 8 to 4 to 2 ohms respectively while beefing up the power supply to supply it on more powerful models.

[J M Fahey]

I can vouch for that.
In my early days, the Industry standard workhorse transistor was the 2N3055 (and its "cousins" such as 2N6254), which means safe +/- 35V rails, and selected ones for around +/-40V ones.
You'll find *Millions* of amplifiers designed along those limits: Standel, Kustom, Peavey, Gibson, Fender, Ampeg, you name it.
And what does it mean?
That it's reasonably easy to get around 20/22V RMS at the output.
Which means
50/60W RMS into 8 ohms
100 W into 4
200W into 2 (many classic Acoustics did so, such as the 360)
400W into 1 ohm , my own B400 from the mid 70's.
I paired them with two 4x12" cabinets, wired for 2 ohms each.
A real Stadium shaker, specially in those early days, where the standard was a 100W 4x6L6/EL34 Tube amp.
One famous incident was a certain Show at our Buenos Aires Luna Park (our "Madison Square Garden") when a Bass player used *two* B400 linked.
The show had to be stopped on people's complaints because it became a "bass solo".
Now the unagreed standard is rails about +/-70V, which means 300/350W into 4 ohms or 700 into 2.
Your amp probably has around +/-85V rails.
Which one is it?

[jugleeson]

Some good info, thanks guys.

It's a Hartke 5500C I was looking at for my buddy. He has two peavey cabs that he wants to run and was looking for a new amp. The peavey cabs are 350 watts @ 8 ohms per cab. He says he wouldn't run them both all the time, but wants the option of running just one or both.

[J M Fahey]

It will probably survive.
He must do his homework: add some noticeable compression which in Hartkes is in the preamp, not the power amp; avoid clipping like the plague and not go trigger happy with the graphic EQ lowest sliders.
Most times I see problems is when people is used to the sound 2 cabs provide, and unconsciously try to match that when using only one.
Even worse: I have seen (and had to repair) *a lot* of SWR Goliath 210 cabinets, which had a killer sound, but users wanted them to do the work of 4x10" ... which they were not.
The Argentine importer brought a container of them, sold them quickly ... and a lot died before the 1 year warranty date.
Most killers were Hartke 3500, with an odd Ampeg SVT something (the MOS hybrid ones) plus an odd Eden head.
In a nutshell: use the single cabinet but do not try to make it sound the same as two.

[Enzo]

Kinda what JM said. 350 watt cab with a 500 watt amp is fine - just don;t crank it to 10 and play as loud as you can all night.

Just because your car can go 120, doesn;t mean you should drive it that fast.

[kg]

Just because your car can go 120, doesn;t mean you should drive it that fast.

blasphemy!

[Enzo]

Well, of course I like to drink beer, kick ass, and drive real fast, but I can;t say that to the KIDS, now can I?

[olddawg]

In my humble experience the old rule of overrate the worst case scenario by 20% is still the best. I run my stuff on 11 boosted. Someone else is bound to do that also. It's like asking can I run my bass through my guitar amp and cab. Yeah sure you can. But you have to accept the consequences.