Extension cab advice

[Deep Thought]

OK, I've got a Trace 7215SMC 1x15 300 watt combo, which by Saturday night's performance I reckon could do with a bit more top end-not so much volume, just ability to cut through-I play a P-bass pretty much exclusively. I'd like to get a 2x10 cab for it-it says minimum impedance 4 ohms-and I understand that at present I'm only getting 200watts of the 300 without the 4 ohm cab. I have my eye on a Peavey 210 TVX which is 4ohm but rated at 175 watts RMS-is this enough wattage or do I need minimum 300? I'm assuming that the 200 the internal speaker's running at at present, plus the 175 should be enough, but am I right in this? It seems 4 ohm 2x10's are a bit hard to come by in my price range-second hand isn't really an option because I have a bass I want to part ex. Alternatively, how much difference will it make if I use an 8ohm cab, which seem a bit more plentiful? After much studying of sticky's and previous threads I've not been able to come up with a definitive answer to this one, so I must throw myself on the mercy of you splendid fellows. Ta.

Edited March 23, 2008 by Deep Thought

[Musky]

You'll need an 8 ohm cab to match your trace. When you add an extension speaker the total impedance is halved, giving you your amps minimum impedance of 4 ohms - i.e. 8 ohm internal speakers + 8 ohm extension = 4 ohms. If you add a 4ohm cab to your combo you'll get a total impedance of about 2.6 ohms, which will blow your amp!

The wattage will be shared equally between two 8 ohm cabs, so you'll need a cab capable of handling at least 150W. There's a sticky on this at the top of this forum, which probably explains all this a bit better than I can.

[clauster]

Omni 10.5 with the basslite and piezos, more hi-mids and highs than your combo

[bass_ferret]

Trace Bright Box?

[Deep Thought]

Thanks all, especially Musky (you were great in Deputy Dawg, BTW ). Now I understand-I naively assumed that because it said 'minimum 4 ohms' on the back, that I should use a 4 ohm cab, but the bit about the 8 ohm cab makes sense now. Also that's good news because there several 8 ohm cabs around in my price range. The BFM's are a great idea, but I have just bought a bass I wish I hadn't, and need to go back to the same shop to swop it for a new cab (and perhaps a decent mike as well). Much encouraged, thanks Folks.

[gary mac]

I've got the same amp. On the last gig I played I added my Hartke 2 x 10", which is rated at 150 watts @ 8 ohms.

It worked very well and certainly gave me a cut through sound, which was much needed to keep up with keyboards and five guitars.

I would prefer a trace 2 x 10", but the Hartke has the added advantage of being very light, which is a relief after having carried the Trace combo out of the car and into the venue.

All the best Gary.

[Harrythebassdawg]

If you relace an 4 ohm cab, with two 8 ohm cabs, would the two cabs together be louder, even though the impedance is the same?

I'm just wondering as im deciding what cab(s) to buy

[Musky]

[quote name='Harrythebassdawg' post='162589' date='Mar 24 2008, 12:21 PM']If you relace an 4 ohm cab, with two 8 ohm cabs, would the two cabs together be louder, even though the impedance is the same?

I'm just wondering as im deciding what cab(s) to buy[/quote]

I simple terms, if your replacement cabs were comparable, then yes.

In general, doubling the number of woofers adds 3db of volume, but any volume increase would be dependent on both the efficiency of the drivers and the cab. So it is quite possible to use a pair of new cabs that are less efficient than your existing one and actually get a volume drop.

Since joining up to these forums I've been discovering just how complicated speaker design is, so maybe someone else would like to jump in here.

[bass_ferret]

It depends. If you replace a 4ohm 4x10 with 2 8ohms 2x10 with the same drivers and the same porting you will get the same volume.

[Happy Jack]

Doesn't the power rating come into this somewhere?

If you're using a 250W RMS amp (turned up high, of course!) into, say, a 350W 210 at 4 ohms (so there's plenty of headroom), and you plug instead into a pair of 350W 210s at 8 ohms, then my [i][b]extremely limited[/i][/b] understanding suggests:

1. Impedance is the same as before, at 4 ohms.
2. There are twice as many speakers to be driven.
3. There's no further power to be extracted from the amp.
4. Volume will be - crudely - the same.
5. The sound will be a little "fuller", perhaps.
6. The same would still be true if your pair of 210s were rated for only 200W each, since 400W is still way higher than the amp can output.

I have no doubt that this analysis is wrong, but could someone explain where it goes wrong, and why?

Thanks.

[Musky]

My understanding (largely gained from Alex's posts) was that the amount of air shifted contributed to the overall volume. The 3db figure when doubling the number of woofers was something Alex mentioned, though there may well be some kind of caveat to that.

Though of course, like bass ferret points out, you're not necessarily adding any greater displacement by doubling up on the number of cabs. Didn't think of that one!

[Lfalex v1.1]

Lots of variables here.
Too many, in fact, to be able to reach a valid conclusion without a bit of "field-testing"!
The numbers alone are not enough.
Particularly when we're relying on manufacturers' (notoriously inaccurate) performance figures!

Two other BIG contributory factors are Efficiency and Frequency Response.
The former can contribute greatly to the perceived volume of any given combination of cabinets, whilst the latter governs the tonal presentation of the ensemble.

[lowhand_mike]

adding an 8ohm 2x10 will certainly add the higher end you are looking for wether you choose one with or without a horn, i am currently in the reverse position to you with a 2x10 combo and NEED to get a 1x15 to add more low end. one thing i have metioned before and has been mentioned by alex and BFM is to put the 2x10 extension cab on its side so that the speakers are vertically stacked, this will add to projection quite alot, there are technical reasons why but the simple explantion is it works, it might look a bit odd as the cabs might not match in width in this configuration but its worth the weirdness for the sound.