Impedance and speakers

[QC]

I am buying my son some newer (for him) bass guitar equipment. He has outgrown his existing rig. The amp that I bought is capable of putting out 300W at 4 Ohms. I already have a cabinet capable of handling 400W at 8 Ohms which should work well, but I want to add another cabinet too. I'm looking at a cabinet rated for 150W at 4 Ohms. What happens when I drive those two cabinets from parallel outputs from the amp above?

 

[ratracer]

Re: Impedance and speakers

Since the 2 speakers will be in parallel the impedance as seen by the amplifier will be (8*4)/(8+4) = 32/12 = 2 2/3 Ohms. That may or may not be a problem for the amp over the long term, if the amp happens to also have a 2 Ohm rating from the manufacturer it certainly wouldn't be a problem.

 

[ratracer]

Re: Impedance and speakers

The other point to mention is the possibility of blowing the speaker(s) in the new cabinet because its maximum power rating is less than that of the amplifier. As a practical note since we're talking about a 300W (I assume this is an RMS rating, not peak) amp and a 150W-rated cabinet this is generally not a problem unless the amp itself is being driven to the point where the output signal is clipped which distorts the signal.

 

[aspeck]

Re: Impedance and speakers

Ratracer sounds like he does some work with sound amplification. His advice seems solid to me and I will ditto what he said.

 

[QC]

Re: Impedance and speakers

Yes, it is 300W RMS and it sounds like I would probably be OK as long as he doesn't run the second cab alone.<br /><br />Thanks guys!

 

[BoatBuoy]

Re: Impedance and speakers

Typically, an amp that delivers 300W RMS into a 4 ohm impedance load, will only deliver 150W RMS into an 8 ohm load, and 75W RMS into a 16 ohm load. The converse is somewhat true. Below 4 ohm, you may be experiencing greater than 300W, but to the detriment of the amp.<br /><br />I would run them in series. That would give you 12 ohm impedance with plenty of headroom.

 

[QC]

Re: Impedance and speakers

If I run them in series, does the load get split evenly between all of the speakers? i.e. I will end up with 4 10" speakers in the one cab and one 12" speaker along with a horn in the second cab.

 

[BoatBuoy]

Re: Impedance and speakers

The load will be split somewhat according to the impedence. Sound produced will depend on efficiency, and since you may not know what that is, just try it and see what works. Some speakers are so efficient that 10w will rip your head off on the next block. Others, you can't even hear below 100w.

 

[QC]

Re: Impedance and speakers

Roger. What I am also considering is that I have a Fender Bassman 60 Combo (what he grew out of) that I am thinking of converting to a cabinet only (amp is dead, but the speakers work), the 12" is rated for 75W at 8 Ohms and I think I'll be pushing it. I would end up with a 4x10" cab rated at 400 watts 8 Ohms AND a Quitecat modified Fender 1x12" plus a little horn. The 12" speaker is rated at 75W 8 Ohms.<br /><br />Thoughts?

 

[BoatBuoy]

Re: Impedance and speakers

I wouldn't worry about the power per speaker. You don't run speakers like outboards, WOT. Nearly all will clip and cause damage. Get the impedance in the range (4-16 ohms) and try it. <br /><br />Little background:<br />Impedance formula for speakers in series is r1+r2+r3+....rn, where r is the rated impedance of speaker.<br />Ex. one 4ohm + one 8ohm and one 16ohm, all in series is 4+8+16=28ohm, which wouldn't work.<br /> <br />Impedance formulas for speakers in parallel is 1/(1/r1 +1/r2+1/r3+....1/rn)<br />Ex. one 4ohm + one 8ohm and one 16ohm, all in parallel would be:1/(1/4 +1/8+1/16)=0.4ohm impedance. Obviously this wouldn't work either. Solution would be a combination of the two.

 

[QC]

Re: Impedance and speakers

Yeah, I am getting there, but need to study. Impedance and I have a history of not understanding each other . . .<br /><br />Thanks for the help!!!

 

[chuckz]

Re: Impedance and speakers

Quietcat,<br /><br />Don't do it! Hooking up two mismatched cabinets in either series or parallel may produce sound but it will be severely distorted.<br /><br />In parallel 2/3 of the power will be consumed in the lower rated cabinet. In series 1/3 of the power will be consumed in the lower rated cabinet. Either way you will have a problem with a mismatched system. Impedance mismatch equal distortion and a loss of efficiency.<br /><br />You need some sort of power divider/impedance matching network between the amplifier and the speaker cabinets. Go to a PROFESSIONAL sound company and get what you need to do it right.<br /><br />OR<br /><br />Get him a 300w 4 ohm cabinet and throw out the 8 ohm cabinet. The 4 ohm cabinet will allow twice the current flow as the eight ohm. The limiting factor in the amplifier is voltage. This means at the same voltage the 4 ohm cabinet will have twice the power of the eight ohm. This does not necessarily mean it will be twice as loud, but it will be louder.

 

[QC]

Re: Impedance and speakers

OK. Not going with the mismatch stuff at all. <br /><br />I bought the 8 Ohm cabinet yesterday, so I am not going to LI Chuck it . It is a 1999 Gallien-Krueger 410RBS 400W @ 8 Ohms, paid $240. I agree and kinda understand on the 4 Ohms to match the Amp rating, but wouldn't a decent 8 Ohm 115 or so match up well when used together for bigger gigs? Wouldn't that get me the 4 Ohm match when they are connected in parallel?

 

[Xcusme]

Re: Impedance and speakers

Hmmm, my math finds that 3 speakers (4,8,16)connected in parallel equals 2.667 ohms...<br /><br />You could connect them in a series /parallel arrangement too.<br /><br />Connect 4ohm and 8ohm speakers in series= 12ohms.<br />Add the 16ohm in parallel and you end up with 6.857 ohms.

 

[QC]

Re: Impedance and speakers

Ya know, I'm starting to feel like these guys that wonder why their 3.0 Merc can't swing a 24 inch pitch prop . . .<br /><br />I am gonna assume that Xcusme didn't see the last info. Again, I would be OK, if not really good, if I ran two 8 Ohm cabinets with similar ratings in parallel, driven by my 300W RMS 4 Ohm amp?

 

[BoatBuoy]

Re: Impedance and speakers

I believe you'd have a winner there.<br /><br />

Originally posted by Xcusme:<br /> Hmmm, my math finds that 3 speakers (4,8,16)connected in parallel equals 2.667 ohms...<br />

Ha! I wasn't even close. That's what I get for using Windows calculator instead of writing it down. Actually I get 2.29 but even when calculated correctly, it won't work. Good thing I'm not working for NASA. No telling where the shuttle would end up.

 

[chuckz]

Re: Impedance and speakers

I'm a bit too much on the engineering terminology and not enough on the end user terminology. I believe in "audio speak" you need what they call either a bridge or a crossover box to match the two speaker cabinets to the amplifier. It can be done but not just by wiring them in parallel.<br /><br />The problem with puting two eight ohm speaker boxes in parallel is that they have an impedance of 8 ohms, not a resistance of 8 ohms. So without a lot of trigonometry I'll cut to the chase, two eight ohm speakers in parallel do not necessarily result in 4 ohms. This is why you need a power splitter/impedance matching type device in line with them.<br /><br />I'm assuming if your son wants to do bigger gigs he wants to please an audience. Doing it right will result in a more pleasant experience for all involved.

 

[QC]

Re: Impedance and speakers

Thanks all. One thing my son and I both share is a very good ear. No problem evaluating the sound. I will definitely not settle for any combo that sounds like carp!! Even if his band employs two "screamers" etc., the bass sound will be right . . .<br /><br />Now, if I add the bridge or crossover can the 3.0 swing the 24P? <br /><br />Thanks again.

 

[BoatBuoy]

Re: Impedance and speakers

Only if the power output meets or exceeds the power consumption of the prop.

 

[ratracer]

Re: Impedance and speakers

Impedance ratings of speakers are meant to be representative, NOT an absolute measurement. There will be variations from speaker to speaker; for any given speaker there are fairly wide variations as well based on the frequency of the input signal. In real life, problems only occur when the impedance is so low that a) the amplifier can't properly handle the higher current and/or b) there isn't a wide enough gap betweeen the effective impedance and the output impedance of the amplifier, which among other things affects the dampening characteristics of the amp/speaker combination. <br /><br />A crossover is used to split a signal based on frequency, which makes it useful for a multi-driver loudspeaker system (lower frequencies to a woofer, middle to a mid-range, higher frequencies to a tweeter etc. etc.) but not necessarily for this application. That's not a completely techincally accurate description, but I'm trying to write this so you don't have to be an engineer to understand it. Empericaly, crossovers are one of the biggest causes of distortion in audio applications. <br /><br />The simplest solution is for you to just buy a 2nd speaker system identical to the first.