Hi all,

Our local band (vox, 3guitar, drums) usually (and very satisfactory) runs on an EV1200 amp with EV tops and subs, sorry can’t remember the model numbers but not really relevant to my question, but gives an idea of the output for our small gigs!). We use Soundcraft mixer that also works well.

Sometimes (not too often) we could do with a bit more power for larger/outdoor venues and hence my question:- we have a spare Samson TXM20 (don’t knock it, it too does very well when we use it) powered mixer that the manufacturer specs at 2x500w into 4Ohm or 1000w into 8Ohm bridged. We are thinking of using the Samson to boost up things when required and buying some extra speakers to match it. I always think I know the answer to this, but then confuse myself when thinking/reading about it… I’m happy in that 250-300w would be fine for this amp into 4Ohm speakers (each side). Or if we wanted to go big on an extra sub at 500-600w into 8Ohms (bridged mode)… hoping I’m right so far.

Here’s where I get lost…

1, can I assume that the manufacturers spec will automatically translate to 2x250w into 8Ohms or should I ask Samson (it isn’t in the spec sheet), which will mean 125-150w rated speakers each side. This isn’t a question about wiring multiple speakers in series or parallel (for some reason I’ve grasped that and can work it out!), just assume one speaker per side.

2, What are the pro’s/con’s between using 4Ohm or 8Ohm (given the choice).

Thanks for any info.

PS: the Samson spec sheet is here, if it helps:-
http://www.samsontech.com/products/relatedDocs/TXM20.pdf

a rule of thumb ive always used is that 8 ohms is roughly 70% of the 4 ohm wattage

pros/cons? 4 Ohms will give you more from the amplifier

It's not a bad rule of thumb to be fair but it all depends on the amplifier.

An amp thats 100% efficient would give double the power at 4ohms as it would at 8, and again double the power at 2ohms as it will at 4. In reality no amps are 100% efficient. That all said, there are a few amps on the market (normally switch mode and various digital type amps) which will claim over 90% efficiency but the majority lay in the 50 to 70% range.

Thank you very much Sound In Gloucestershire & Rob, all understood.

I suppose the safe bet would be to ask Samson what the spec of the amp is at 8Ohm.

Samson’s spec’s have confused me a little because they use 2x500w into 4Ohm and then use 1000w into 8Ohm (bridged). Does this mean that amps (depending on design etc) are less efficient in bridged mode seeing as Samson are quoting less load for the same (combined) output… or am I missing something/confused, or is this perhaps indicative of a poor design by Samson!?

When running bridged, the two sides of the amp are essentially splitting the load between them - it's easiest to think of each side supplying half the power to half the load. When driving 1000W bridged in to eight ohms, each side of the amp is contributing 500W into (essentially) four ohms which tallies with their quoted spec for running stereo in to four ohms.

It also depends on the power supply in the amp - what its voltage and current capabilities are. Generally you are voltage limited with higher impedance loads and current limited with lower impedance loads.