BillFitzmaurice.info

Yeah all cabs will be single loaded, without buying a large van there's little chance of me transporting anything bigger. The GX5 states it'll do 4ohm but no less, QSC 'removed' that functionality to keep the cost of the amp down as not many people used it apparently.

Assuming the amp was hooked up and the resistance was too low, would the amp just continually shut down or would it try to push the load and then die in a hail of sparks and smoke?

If the GX5 couldn't handle the jandal it would get hot and presumably shut itself down or die an early death from running hot all the time. Giving up in a hail of sparks and smoke, not likely.

I think it was Bill saying the reason they made the Labs 6 ohms was to come out at 8 ohm in a horn application. Most of us are running amps that are ok down to less than 4 ohm. Afaik you're the first BFM builder to run into the issue. It should be fine but I'm no audio engineer.

I guess the best way to know for sure is to take all the kit out to somewhere it can be given the boot for a few hours straight.

Appreciate your input, cheers.

Running amps @ 2 Ohms increases the distortion, no? Not to mention it only buys a measly couple hundred Watts of extra output, if that. Best to run at 4 Ohms, play it safe.

I remember Sydney's words: There's no such thing as a free lunch.

Running at 2 ohms gets you additional output from the amp but it's at the top of the crest factor where distortion and clipping are most likely due to amplifier and driver non-linearity.

Also, read this thread where he discusses another Philosophy of Implementation where amps can't supply full voltage swing to the drivers (anything that gets a +100 from Bill should be mandatory reading for all of us):

http://billfitzmaurice.info/forum/viewt ... ementation

There's also a report (probably in the education section) on how running amps at 2 ohms shortens their working life considerably. The figures were significant enough for me to decide never to do it.

Generally the advice with audio and especially pro PA audio is not run anything (or everything!) at the bleeding edge.

Build more cabs (if you want louder) or buy more amps (if you want them to run cooler and last longer). And the system will sound better too.

We already saved a bunch of cash through Bill's genius anyway.

The GX5 I have is rated for either 4 or 8ohm loads, with both channels driven at 4ohm it claims 700w (at 1khz).

I guess the question is, would the output of a single pair of T39s running full whack be comparable to a cluster of 4 T39s being driven like Miss Daisy? I'm not for killing my equipment without a good few outings under their belt anyway.

I took a look at the spec sheet for the GX5 and looking at the heat it outputs it's almost scary. Running full whack with an 8ohm load on each channel it outputs 1891 BTUs per hour. 1/3 power on a 4ohm load per channel it's doing 2162 BTUs so it's getting warm. At full power on a 2x 4ohm load it's hitting 3754 BTUs per hour. I wonder if the amp lasted the full hour during this test...

Talking myself out of running a 4ohm load on each side until I either get a bigger amp or just a matching one to share the duty. More expense but my birthday is coming up soon which is kinda why I was asking about the 4 cabs, I was considering blowing my own horn and providing my services as DJ (along with a couple of friends) and sound guy at my own party.

*edit: small print found.

1/8 power represents typical operating conditions
1/3 power represents peak program levels
Full power is breaker limited to short periods

You don't need to drive the amp into oblivion to achieve a good result. The 700W peak is what you can hit with your loudest dynamic passage of music before the amp runs into a higher level of distortion and starts to clip. Most of the time you're well below.

It'll still be pretty damn loud if you set your limiter to whatever voltage would be 700w for an assumed 3 ohm, no box impedance, calculation.

I really thought Bill would have chimed in by now. I guess there's something in the new plans about two Labs per channel being fine for a 4 ohm load?

Bill Fitzmaurice wrote:

Mikey wrote:I REALLY wish there was a forum sticky regarding impedance calculations (IITP)...

Why? The plans state exactly what the load is for each driver configuration.

from another current thread

OK so instead of creating yet another thread I'm just going to pop this on the back end here. The GX5 I've got is a good solid amp and seems perfectly fine playing at volume for extended lengths of time, just what I ask of it.

At some point I'll be building some more T39s, probably slims and be looking at hooking them up (4 in total) to an amp parallel meaning a 4ohm load per channel. The GX series is capable of that but as it's kinda pushing the limits of the design I'm looking for something which is very happy with a 4ohm load all day every day. Are there any brands out there which will do such loads at high volume without complaining? I don't require any DSP functions as I've got a DEQ and DCX to look after those duties right now.

Peavey IPR is stable to 2 ohms, sounds great (I have 12 of them between 2 systems now) and they really are stupid lightweight.

Not to mention the price.... call Cal Mazzara at Full Compass (tell him i referred you) and you'll be suprised.

They rate it for 4 ohm operation, all day every day. Put it where it gets a good air supply and it will cool itself.

Your programme material is nowhere near full power most of the time unless you have no dynamics left from over compression / running into the limiter all the time.

I'll look into getting the next model up and using the GX 5 I've got for just the tops instead of the piddly amp that's doing them at the moment. Greg if my shit sets on fire I'll be very upset with you...