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 (http://www.parts-express.com/?utm_source=google&utm_medium=cpc&utm_campaign=G-Brand-Main&utm_group=G-Brand-Main-Exact&utm_term=parts%20express)

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. [Flower] 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. [Flower] 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.[Beer]

[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[Coffee])

[jerrymrc]

Spikkers. About $150 to build these and I have to say they are still in my sitting room. http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup227Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup227Medium.jpg.html)http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup226Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup226Medium.jpg.html)http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup206Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup206Medium.jpg.html)http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup193Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup193Medium.jpg.html)http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup170Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup170Medium.jpg.html)http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup157Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup157Medium.jpg.html)http://i884.photobucket.com/albums/ac44/jerrymrc/audio/oldgroup156Medium.jpg (http://s884.photobucket.com/user/jerrymrc/media/audio/oldgroup156Medium.jpg.html)

[Singlestack]

Beautiful, simply beautiful. Well done...[Flower]

[PugnacAutMortem]

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.

Well I know these are the speakers I'm looking at getting (one of each in both cabs):

http://www.eminence.com/pdf/Cannabis_Rex.pdf

http://www.eminence.com/pdf/Screamin_Eagle.pdf

And I'm almost positive this is the power amp I'm getting: http://www.musiciansfriend.com/amplifiers-effects/rocktron-velocity-300-150w-rack-power-amp

Side note: What's with the dip between 1k and 2k on the EQ curve (or whatever the technical name for that chart is)? Is that all speakers or all guitar speakers or...?

[james_bond_007]

http://www.eminence.com/pdf/Cannabis_Rex.pdf ...

Side note: What's with the dip between 1k and 2k on the EQ curve (or whatever the technical name for that chart is)? Is that all speakers or all guitar speakers or...?

Speakers are essentially an electromechanical device. You put in an electrical signal, and the speaker mechanically "vibrates".

Ideally, one would like a flat response across all frequencies; that is, for say a single sine wave input of say 1V at any frequency, one would like the speaker to produce the same volume on its output. The graph (in your spec) shows the input frequency on the x-axis , and the output (in Sound Pressure Level) on the Y-Axis.
(Don't get too hung up on "dB" or "dBm or "SPL", just realize that it is a way to measure the output response of the speaker).

Like all mechanical systems, it has "sweet spots" and "dropouts" where it vibrates "better" or worse, depending on the frequency of the input.
So what you are seeing at 1.5KHz. is a frequency where it does not respond (aka a "bump" as well to frequencies (aka attenuates) in that range and produces a "softer" output than it does for signals in the range of 200-1KHz.

Likewise, it "likes" frequencies of just above 2KHz. and produces "louder" (aka amplifies) output for signals in that range (aka a "hump")

ALL speakers will have these, but better speakers are "flatter" (have fewer and not as dramatic "humps" and "bumps") than those of lesser quality.

Keep in mind that
1) the speaker response curve can be "corrected" by using a 31-band graphics equalizer (or "tone controls" ) to amplify/attenuate specific frequency ranges.
The sliders on the eq allows one to adjust the volume for only signals that fall into small frequency ranges, like 1K-to-1.3K etc.
31339

So you can "flatten" the response from most speakers with an eq. (tone controls are also "eqs" , but cover a much wider range with a few knobs...they cannot address a narrow range like the eqs)

2) Speakers are typically measured in a "perfect" room...one that absorbs all the sound and does not allow it to bounce around (aka an anechoic chamber...a room that does not have any "echos")

Each room also has its dropouts and sweet spots across the frequency range, so a perfectly flat speaker will still need to be adjusted for frequency (aka tone) in a room. And each room will be different. So paying a lot for flat speakers that are going to be moved from room to room as your gigs move from bar to bar is not such a great idea. Paying a lot for speakers that will sit in one place/room is worthwhile (aka this is why "studio monitors" are often very pricey)

Sometimes when turning up an amp, you will hear feedback. If you listen carefully, the feedback has a certain "sound/freq range ". The feedback is the result of "certain" frequencies being amplified by the room more than the others. They get loud enough that they are picked up by the mic and re-amplified, along with the source signal until they keep growing and make the undesired sound. One way to fix this is "turn down the master volume" for all frequencies. Another way is to use an eq to only attenuate only the frequency range causing the feedback. That way you can get more volume out of your system, while sacrificing only a narrow band of frequencies, rather than ALL frequencies.

3) The ear is not a "linear" device. That is, if you double the power on a signal, your ear does not perceive it as "twice as loud". The ear works more on a logarithmic scale, such that is takes "about" 10x the input to make your ear perceive the sound as being "doubled".

I hope this helps fill in some of your questions ...If not, just ask. There are a lot of knowledgeable and friendly people on the forum.

Someone on here will help you.[Beer]

[james_bond_007]

Well I know these are the speakers I'm looking at getting (one of each in both cabs): ...



So "I" have a few questions.

1) Why do you want to mix 2 different 12" speakers in the same cabinet?
From the specs, they don't seem to be too drastically different in response (Cannabis seems a bit better: humps/bumps not as pronounced, and flat part of output is at a higher SPL)

2) You have a "mono" guitar. Why do you want a stereo amp ?
Maybe...If you are using some effects, sometimes the effects can have a stereo output.
Maybe...If you need to separately control the volume of 2 cabinets, because they are located far apart.

But typically, the "guitar" speakers are stacked and located together.

3) Why not use bridged mono mode ?
Maybe...some amps invert the phase between the L half and the R half to their respective output jacks.

4) This amp has "2" output jacks for each channel (2xL and 2xR). Be careful.
The schematics show them wired in parallel. So if you plug "2" 4 ohm loads in the "2" L outputs, the amp sees a 2 ohm load...not good.

5) The amp is YOUR choice...I've not seen very good reviews on it so far. Seems the key selling point is that it tries to emulate a tube amp, but is reported to be VERY noisy.

6) Amp specs seem suspiciously lacking typical info.
Output power "type" is not listed (EX: RMS, peak, music, program etc.) but Input source IS spec'ed as RMS...hmmmmm.
No explanation for the 2 output jacks on each channel...I had to dig out the schematics to figure it out.
Input jacks on the rear panel...no issue, but typically they are on the front where one can plug in/out with ease.

7) What will you use for and how will you be using effects (as far as the signal chain goes) ? pedals ? another rack mounted unit ? etc.

8) Have you heard this amp ? Do the "tube simulations " really work well ? Do you need/want that ?
If not, consider using a solid/clean power amp, with a proven track record like a QSC.

[sellersm]

I used to have a set of Maggies! My all-time favorite speakers are the B&W 801 series! Could never afford them, but I used to love to listen to them, powered by Krell or Mark Levinson stuff. Just wow! Those are nice looking speakers there, Jerrymrc.

[james_bond_007]

Nice job, Jerry. [Coffee]

[jerrymrc]

Now that I think we have a direction I take it you are looking for pro-sound reinforcement as 007 is alluding to. I have only built a couple of pro-sound type stuff and that was long ago.