Passive subs and impedance question.

uncledick

I was connecting some speakers at a gig last weekend which consisted of an 8 Ohm passive sub and then a cable from there to a 4 Ohm 'wide-range' speaker which sits on top (same both sides).  

Can somebody confirm that the in/out connections on the speakers are just in parallel and that I've actually given the power amp a load of somewhere under 3 Ohms?  I've assumed that there is not usually a cross-over and that this function is just a result of the relative speaker characteristics.  Is this right?  

The regular guys in the band are very much 'if the plug fits it's fine' so I didn't like to ask!

ESBlonde

I take it one power amp was used for both cabinets which would require a passive crossover between the boxes. The full range would have a crossover between the cone driver and horn (unless it's a pizzo type).
A sub only reproduces the lowest frequencies and some kind of filter is required. A passive crossover at low frequencies requires a lot of copper windings and will be heavy if it's mounted inside the cabinet. If not you fed full range to both boxes and would have got mush out of it in a very inefficient manner.
If I was running this rig I would use an active crossover (or DSP) before the amp and have one half of the stereo power amp driving both top cabs in parallel and the other half driving one sub on it's own giving a 4 Ohm load or both of them in series to give an 8 Ohm load to the amp.
That way the amp would be way more efficient and the cabs would also not be producing wasted effort trying to reproduce frequencies they cant.

It would be interesting to know the make/model of the gear you are using.

ICBM

It depends. Theoretically the sub should have a crossover.

You should be able to test it without taking it apart - if you plug the amp into the 'output' jack, if there is a crossover you will get no (or very little) sound from the speaker, because it would have to be going first through the high-pass filter (backwards, but it makes no difference) and then through the low-pass filter to reach the speaker.

If you get a good strong sound then the input and output are in parallel, even if there's still a low-pass filter before the driver.

Danny1969

Need to know the make and model of the speaker as some have the crossover in the speaker and the out socket is wired from that .... other systems use one four core cable and four pole speakon with typically the highs running off pin 1 + & - and the bins running off pins 2 + & -  ....... this kind of setup used on some HK looks like the bin and top is in parallel but it's actually completely separate ... just saves the hassle of running 4 cables  

Could be in parallel though and could be less than 3 Ohms or so .... as low as that is some amps are happy down to 2.5 Ohms. I've seen an old H&H 500D amp drive four bins and 2 tops without complaint for 200 gigs or so

ICBM

Danny1969 said:

Could be in parallel though and could be less than 3 Ohms or so .... as low as that is some amps are happy down to 2.5 Ohms. I've seen an old H&H 500D amp drive four bins and 2 tops without complaint for 200 gigs or so

Those are very, very hard to kill. I serviced a V800 recently which came to me with blown fuses, that looked like it had been through WWII and maybe a couple of other smaller wars as well - totally rusted and battered. I thought it might be a write-off but knowing how tough they are I thought I would give it a chance. The fault was that the mesh grille at the end without the fan had got pushed in and had shorted on one of the power transistors. I hoovered out god knows what amount of crud and filth from inside, replaced the grille and the fuses, powered it up and it ran perfectly!

uncledick

Thanks guys.  AFAIK the amp is an old Crown XLS of some sort.  I'll check out the speaker brands but it'll be a while.

ICBM

uncledick said:

AFAIK the amp is an old Crown XLS of some sort.

From memory those are rated for 2 ohms, so it should have been fine. Crowns are generally very hard to kill as well.

maltingsaudio

ICBM said:

uncledick said:

AFAIK the amp is an old Crown XLS of some sort.

From memory those are rated for 2 ohms, so it should have been fine. Crowns are generally very hard to kill as well.

Xls have odd non standard transformers so when they do go it's a pain.  Xls will work at 2 ohms though I personally don't like running anything at 2 because things tend to get too hot

PolarityMan

I think the clientele is definitely more impudent in massive pubs. Smaller places attract a more select crowd I feel.