All you audio guys, impedance question

[PugnacAutMortem]

I'm just getting ready to build a couple of 2x12 cabinets that I want to run in stereo out of my power amp that is powering my Eleven Rack. I am having a bit of trouble wrapping my head around the impedance of the cabinet compared to the power amp.

I understand that if I put in 8 ohm speakers in the cabinet and wire them in series I have a 16 ohm cabinet, and if I run them in parallel I would have a 4 ohm cabinet. But when it comes to the power amp, if I run two 16 ohm cabinets in stereo, should I be looking at the wattage specs for the 16 ohm per channel or the 8 ohm per channel? From what I was reading somewhere if you run two cabinets in stereo it halves the ohms or something. It just didn't make sense, so I thought I'd try and see if I could get a more concrete answer.

And for example's sake, lets say the power amp is a Rocktron Velocity 300 (which from the specs: (unabridged/per channel) 150W @ 4 Ohms, 100W @ 8 Ohms, 60W @ 16 Ohms; (abridged/mono) 300W @ 8 Ohms, 200W @ 16 Ohms) and the cabinets are loaded with 2 Celestion V30's @ 60W apiece.

Thanks in advance for the clarification. Hopefully I phrased my question in a way that is understandable.

[Singlestack]

For stereo connections, you don't halve power amp impedance - "it is what it is". So if the specs say each output channel is intended to drive 16 ohm loads, then thats what you should connect to it. (As a brief aside some amps have rear panel switches that can be set to useful impedances to cover various cabinet specs). Keep in mind that a 2x12 cabinet isn't stereo since it has 2 speakers - it is a mono 2 speaker cabinet that (as you correctly say) can either be wired up as a 16 ohm of 4 ohm cabinet using two 8 ohm speakers. So if your amp is most efficient driving 16 ohm loads, then wire up the speakers in series and have a good 16 ohm match.

[PugnacAutMortem]

For stereo connections, you don't halve power amp impedance - "it is what it is". So if the specs say each output channel is intended to drive 16 ohm loads, then thats what you should connect to it. (As a brief aside some amps have rear panel switches that can be set to useful impedances to cover various cabinet specs). Keep in mind that a 2x12 cabinet isn't stereo since it has 2 speakers - it is a mono 2 speaker cabinet that (as you correctly say) can either be wired up as a 16 ohm of 4 ohm cabinet using two 8 ohm speakers. So if your amp is most efficient driving 16 ohm loads, then wire up the speakers in series and have a good 16 ohm match.

But I can run one 2x12 for the left and one 2x12 for the right and have a stereo rig that way correct?

[sellersm]

But I can run one 2x12 for the left and one 2x12 for the right and have a stereo rig that way correct?

Yes.

[james_bond_007]

But I can run one 2x12 for the left and one 2x12 for the right and have a stereo rig that way correct?

Disclaimer: I'm trying to not get into the gory details of AC signals, and how they actually change with frequency in this discussion.
So rather than having a "techy" debate in this thread, I'd ask that you start a new one, so we don't confuse the OP.

For a stereo sound system each channel (Left or Right) is considered independent of the other. Each channel is using it's own amplifier.
Although both amps may be located in one box, the "stereo amp" performs pretty much like 2 separate mono amps (one for L and one for R).
(Let's not get into "Bridged mono" configurations in this discussion...if you want to know how they work, ask about that as a separate question)

So your load is (2) 12"@8 ohm speakers per channel (left or right).
You have 2 identical "boxes'; one for L and the other for R

If wired in series, the load will be about 16 ohms.
If wired in parallel, the load will be about 4 ohms.
( I say about, because the impedance and reactance is frequency dependent, and the speaker responds differently at different frequencies. Thus an 8 ohm speaker is not exacty 8 ohms at all frequencies, but is close enough not to worry about it for this purpose).

Many amps are capable of driving 4, 8, or 16 ohm loads. (Check the specs)
Given the same voltage, a lower impedance load will offer more power delivered to the load.

Amps typically refer to Volts or Watts in terms of "RMS (Root Mean Square)" .
This is "sort of" a DC approximation of an AC signal (i.e. an audio signal) and useful for quick calculations.

For DC (or AC rms) and for general audio calcs, the DC equations are "close enough"
( P=IV or P= I^2R or P= V^2/R where P=power(Watts), I=current(Amps), V=Voltage (Volts) and R=resistance (Ohms))

EX: Say Voltage is 10V rms

Power P=V^2/R
for 16 ohm load : (10V rms)^2 / 16 = 6.25 W rms
for 8 ohm load : (10V rms)^2 / 8 = 12.5 W rms
for 4 ohm load: (10V rms)^2 / 4 = 25 W rms

So if your amp can handle a lower impedance load, wire them as 4 ohms, and you can get more power across the load.

Finally, YES, you should have
A "left" box : 2x 12" @8ohm (parallel) = 4ohms, as seen by the amp's LEFT channel
A "right" box: 2x 12" @8ohm (parallel) = 4ohms, as seen by the amp's RIGHT channel

Also, if this is for a not for guitar or bass, you would benefit by filtering (i.e a crossover network or low pass filter or eq. ) out the high end at or before the signal gets to the speaker. 12" speakers are not typically capable of producings "highs". Guitars and basses are not capable of producing highs, such as those produced by cymbals etc. and sort of "self filter" their sound.

There is still considerable energy in the high end that gets to the speaker, even if the speaker cannot reporduce it. Since the speaker is not optimized to reproduce highs, the high freq energy is often converted to heat and can fry the voice coil. (lots of other reasons this can happen as well- that's yet another discussion).

Great place for parts : www.parts-express.com

I hope this helps...please ask about things that I may have caused confusion with.

PS If you need any amps, I'm getting rid of my QSC 1450 and 850 amps, and other audio stuff (mixers, speakers, cables, etc.)
Let me know.

[asmo]

Wait.. No has brought up yet that your resistance changes at various power levels and load.. Lets go crazy!!!

[PugnacAutMortem]

***Much knowledge condesned to save unnecessary scrolling***

Thank you VERY much for your response Mr. Bond. There's a ton of knowledge all condensed into one place that I can now use instead of trying to piece it together from all corners of the interweb.

First off, I actually will be using this for guitar (and bass, if I can figure out a way to incorporate a 4x10 cabinet into this rig) so the speakers I'll be getting should be optimized for guitar frequencies. But would there be any disadvantages of wiring the same 8 ohm speakers in series and running both cabinets as 16 ohms? It seems like the particular power amp I'm looking at runs 150W per channel @ 4 ohms, but 60W per channel @ 16 ohms. If my speaker cabinets are going to be 100W cabinets (two 50W speakers), then wouldn't I need to run it at 16 ohms to prevent blowing the system?

[jerrymrc]

Wait.. No has brought up yet that your resistance changes at various power levels and load.. Lets go crazy!!!

I am waiting for the "I have two amps and one set of speakers" thread. Tim K builds Spikkers, I have built some (But love my Magneplainer's) Brutal likes JBL's but works on vintage Yamaha stuff.

[james_bond_007]

I am waiting for the "I have two amps and one set of speakers" thread. Tim K builds Spikkers, I have built some (But love my Magneplainer's) Brutal likes JBL's but works on vintage Yamaha stuff.

Magneplaner's are at the top of my list of Great Speakers.
You are are very fortunate to be able to enjoy those.

[james_bond_007]

Thank you VERY much for your response Mr. Bond. There's a ton of knowledge all condensed into one place that I can now use instead of trying to piece it together from all corners of the interweb.

First off, I actually will be using this for guitar (and bass, if I can figure out a way to incorporate a 4x10 cabinet into this rig) so the speakers I'll be getting should be optimized for guitar frequencies. But would there be any disadvantages of wiring the same 8 ohm speakers in series and running both cabinets as 16 ohms? It seems like the particular power amp I'm looking at runs 150W per channel @ 4 ohms, but 60W per channel @ 16 ohms. If my speaker cabinets are going to be 100W cabinets (two 50W speakers), then wouldn't I need to run it at 16 ohms to prevent blowing the system?

Glad my post helped...

The devil is in the details.

What is the exact amp and speaker models you will be getting ? (Point us to the specs/datasheet for them)

We need to make sure you are not confusing Wrms with Wp-p (peak-to-peak) or Wprog (program etc.)
There are also ways to protect your speakers from seeing too much power.

Also, the shape/size, and material of the cabinet plays a significant roll in the quality of sound reproduction.
Just making a plywood box with 2-12" holes and a 1/4" TR input might not cut it.
Speaker cabinet construction is outside of my scope.
JERRYMRC suggests you tap into TIM K's brain for info on that.
(Jerry's a "good guy"...so listen to his advice)