4 ohm amp with 8 ohm speakers - AVS Forum
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post #1 of 29 Old 01-04-2011, 05:58 PM - Thread Starter
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I have a new set of RF 83 which are in need of better power, so im looking at a new amp but unsure of my approach.
I am looking at sole power amps specifically this one. Rotel RB1562.

I am planning to bridge this amp to create 200 watts @ 4 ohm load but i want to make sure of safety of the speakers and if it would be better to just find a 8ohm amp.

Obviously the speaker will try to pull out power it needs from any amp so wouldn't using a amp that's lower than the speakers nominal impedance be better than just a amp rated the same, because the amp is rated to run at 4 ohm respectively without fail and will provide the speaker the power it needs at any level ??.

thanks.
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post #2 of 29 Old 01-04-2011, 06:18 PM
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If an amp can handle 8 ohms, it can certainly handle 4 ohms (tube amps are potential exception, but you are not asking about tube amps)

If you bridge the amp, you need 4 amp channel for 2 speakers (I did not look up the amp to see if it has 4 channels.)

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post #3 of 29 Old 01-04-2011, 06:22 PM
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Quote:
Originally Posted by MichaelJHuman View Post

If an amp can handle 8 ohms, it can certainly handle 4 ohms (tube amps are potential exception, but you are not asking about tube amps)

If you bridge the amp, you need 4 amp channel for 2 speakers (I did not look up the amp to see if it has 4 channels.)

I looked at the manual. It can't be bridged.
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post #4 of 29 Old 01-04-2011, 06:22 PM
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why are you in "need of better power"? those speakers are ridiculously easy to drive...

and even if you did need more power (which you don't), why do it the hard way?

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post #5 of 29 Old 01-04-2011, 06:28 PM
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Quote:
Originally Posted by MichaelJHuman View Post

If an amp can handle 8 ohms, it can certainly handle 4 ohms (tube amps are potential exception, but you are not asking about tube amps)

If you bridge the amp, you need 4 amp channel for 2 speakers (I did not look up the amp to see if it has 4 channels.)

michael, i think you want to look at your first sentence again... if an amp can handle an 4 ohm load, it can certainly handle an 8 ohm load, but not the other way around...

or did i misunderstand what you said?

i think the op may be a bit confused by the fact that the amp will "double down" to 200 watts at 4 ohms...

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post #6 of 29 Old 01-04-2011, 06:31 PM - Thread Starter
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Im using a 80 watt Marantz atm and i have a budget sony amp as well, but i am not getting any good mid/bass sound from the speakers not to make me go wow thats awsome etc.

I believe that i need a better amp with more volt/power to hit a sharper and harder mid/low frequency sound.
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post #7 of 29 Old 01-04-2011, 06:32 PM
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I think the OP also doesn't understand that even if that amp could be bridged. He would then need two of those amplifiers. One amp for each speaker.
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post #8 of 29 Old 01-04-2011, 06:38 PM - Thread Starter
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It has 8 terminals if that helps otherwise yes im unaware and now aware it will only give me a mono output.
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post #9 of 29 Old 01-04-2011, 06:40 PM
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Quote:
Originally Posted by [DANGERDAN] View Post

Im using a 80 watt Marantz atm and i have a budget sony amp as well, but i am not getting any good mid/bass sound from the speakers not to make me go wow thats awsome etc.

I believe that i need a better amp with more volt/power to hit a sharper and harder mid/low frequency sound.

nope... that's not going to help... you can have all the power in reserve in the world, and it won't matter, since you won't be drawing on it...

eq, speaker positioning, room treatment, etc. WILL help... or switching to speakers that have more of a pronounced mid/low range...

but a "different" amplifier won't help...

edit: btw, i saw your other thread on this... the thing that someone posted about the "stiff woofer requiring more power" is, ummm, one of the more interesting theories i've seen here on avs...

sensitivity numbers DON'T lie...

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post #10 of 29 Old 01-04-2011, 06:47 PM
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Quote:
Originally Posted by [DANGERDAN] View Post

It has 8 terminals if that helps otherwise yes im unaware and now aware it will only give me a mono output.

Thats so it can be bi-wired. You don't need to do that.
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post #11 of 29 Old 01-04-2011, 06:51 PM
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Quote:
Originally Posted by ccotenj View Post

michael, i think you want to look at your first sentence again... if an amp can handle an 4 ohm load, it can certainly handle an 8 ohm load, but not the other way around...

or did i misunderstand what you said?

i think the op may be a bit confused by the fact that the amp will "double down" to 200 watts at 4 ohms...

Correct, I said it backwards. Was eating dinner and trying to think at the same time.

"But this one goes up to 11"
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post #12 of 29 Old 01-04-2011, 06:51 PM
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Quote:
Originally Posted by [DANGERDAN] View Post

It has 8 terminals if that helps otherwise yes im unaware and now aware it will only give me a mono output.

It is a stereo amp with no built-in bridging capability. The four pair of terminals are for bi-wiring or could be used to drive two pair of 8 ohm speakers at the same time.

With the RF-83, being an 8-ohm nominal speaker, the amp's rated power of 100WPC will apply. There is no inherent way to make this amp put the (summed) 200W power from both channels into one 8-ohm speaker.

Just because there is a knob doesn't mean you should turn it.
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post #13 of 29 Old 01-04-2011, 06:56 PM
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Quote:
Originally Posted by MichaelJHuman View Post

Correct, I said it backwards. Was eating dinner and trying to think at the same time.

lol... eating and posting at the same time does that to me too...

and forget about drinking and posting... a little bit of wine goes a long way to making my brain not work...

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post #14 of 29 Old 01-04-2011, 07:14 PM - Thread Starter
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But 8 ohm nominal rating is not accurate to the point where the speaker can drop its impedance down to 2 almost so no matter what type of watts a amp puts out its how it provides the power, how strong, how fast, how clean.
So in saying that finding a amp that can provide the power better would increase mid/low frequency sounds due to having better power when it needs it.

http://www.youtube.com/watch?v=fgVPH...eature=related

This guys 8 inch klipsch woofers move really well on low frequency sounds, but when i play that song even with bass on full mine do not come close to what his do.

Efficiency these speakers have yes and they can go loud but quality is something different that watts alone can not deliver.
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post #15 of 29 Old 01-04-2011, 07:31 PM
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^^^

wrong again... "quality of watts" is a myth... amplifiers amplify a signal... that's all they do (and all they should do)... watt 1 of amplifier "a" is the same as watt 1 of amplifier "b"...

as far as impedance dips... impedance dips (and yes, i know all about the famous klipsch impedance dip) will not necessarily cause a problem... they DO have a dip, but nothing that won't be handled... if it WASN'T being handled, your amplifier/avr would be going into protection, or you would be clipping... the 8 ohm nominal IS correct...

fwiw, if paul klipsch was still alive, he'd show his button to anyone who claimed that his speakers required a bunch of power to "open up"...

two other points:

- the person in the video has the volume cranked up to a ridiculous level in order to make the woofers move like that...

- note that he is driving the speakers with a budget avr... nothing "special"... no 200 wpc... no rotel (or insert name here) amp... just a plain jane run of the mill budget avr...

you have a choice... you can "believe", or you can "learn"... that's up to you which one you choose...

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post #16 of 29 Old 01-04-2011, 07:49 PM - Thread Starter
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I have two amps a sony which is 185 watts x 7 @ 6ohm and a marantz 80 watts x 2 @ 8 ohm, out of the two even though the sony has more watts the marantz has a better sound with a bit more bass.

Amps sound differently and also provide different frequency quality regardless of watts because its the quality of the amp that does it, my marantz is probably by far better quality build that of the sony.

The lower down the frequency range the more power is required but not so much more power its more better quality power faster harder cleaner power, if you get down in technical terms it works like this ( i would imagine).

You have a amp which has a nominal impedance of 8 and the amp has just enough volts to provide that resistance level, and the current being pushed into that resistance is enough for the speaker it would perhaps run.
But say that speaker was requiring more power due to the difficulty of the sound its producing and lets just say the amp has twice the watts the speaker would ever need so what would be the problem, well its not because the amp doesn't have enough watts its because it cant get the power it requires fast enough.

Its like with the drilling for oil, we have plenty of it but you can only pump so much of it at a time.

This is just my belief, i like to get down and technical and believe what physics tells me.
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post #17 of 29 Old 01-04-2011, 08:58 PM
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Quote:
Originally Posted by [DANGERDAN] View Post

The lower down the frequency range the more power is required but not so much more power its more better quality power faster harder cleaner power...

More power is needed at lower frequencies maybe because there's simply more harmonics there. There's very little above a few khz for example.

I am going to ignore faster and harder power as I can't quantify that.

Cleaner power just means you are not distorting, IMO. And many amps have very little distortion when operated within their limits (often below .01% THD.)

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You have a amp which has a nominal impedance of 8 and the amp has just enough volts to provide that resistance level, and the current being pushed into that resistance is enough for the speaker it would perhaps run.

You are correct that all amps, at some point, will clip. Either the amplifier will hit the nominal supply voltage and clip, or the supply voltage will sag under load...same problem...clipping. All amps will clip if you push them too hard. All amps won't clip if you don't push them too hard. All other distortion is either noise, or minimal harmonic distortion due to amp imperfection.

Quote:


But say that speaker was requiring more power due to the difficulty of the sound its producing and lets just say the amp has twice the watts the speaker would ever need so what would be the problem, well its not because the amp doesn't have enough watts its because it cant get the power it requires fast enough.

Not sure what you mean by difficulty of sound. Higher frequencies do require a higher slew rate, but in general, we would expect any competent receiver or amp to have sufficient slew rate to avoid distortion from that source.

Amp don't really have a bunch of watts sitting around. What they have is a power supply that provides a fairly stable voltage (there would usually be some ripple.)

If you overdrive your amp due to too high of a volume, a few scenarios could occur which which result in distortion from clipping. It's not that you ran out of watts. You either exceeded your nominal supply voltage, your supply voltage sagged under load, or your amp/receiver's limiter circuits starting limiting supply voltage to protect itself - all three result in clipping.

An impedance dip could result in a problem if the signal in conjunction with the speaker's response instantaneously presented an low impedance to the amp. It might clip for that short period of time, or it might simply shut down. Is this a realistic scenario which happens a lot? I have my doubts.

"But this one goes up to 11"
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post #18 of 29 Old 01-04-2011, 09:11 PM - Thread Starter
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So your saying if you can drive the speaker to the SPL that you like without distortion then the amp is supplying enough power and there is no need for a more powerfull amp ok i can understand your theory.
But with speakers from polk i always here that they need really powerfull amps to fully open up, even though they can currently play the speakers at a respectable SPL level.

So there has to be a thin line on low end amps and high end amps where the low end can probably drive the speakers to a good SPL but to get the full benifits from the speaker you would need to drive it with a fairly decent amp to fully open up.
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post #19 of 29 Old 01-04-2011, 09:16 PM
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Quote:
Originally Posted by [DANGERDAN] View Post

So your saying if you can drive the speaker to the SPL that you like without distortion then the amp is supplying enough power and there is no need for a more powerfull amp ok i can understand your theory.
But with speakers from polk i always here that they need really powerfull amps to fully open up, even though they can currently play the speakers at a respectable SPL level.

So there has to be a thin line on low end amps and high end amps where the low end can probably drive the speakers to a good SPL but to get the full benifits from the speaker you would need to drive it with a fairly decent amp to fully open up.

Well I have heard this whole "my speakers really opened up" with more power statement many times. Not sure what to say about it. If you only need 4 watts of power to drive your speakers to an average output of 90 dB SPL two meters from your speaker, I fail to see how having 200 watts will "open up" your speakers at this sort of SPL level. Certainly if you push your system hard, you will reach a point where sound quality goes down.

IMO, music tends to sound better as you increase volume, maybe that's a bit of what people are talking about. You need enough power to be able to increase to your desired SPL (and some people like it very loud.)

"But this one goes up to 11"
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post #20 of 29 Old 01-04-2011, 09:38 PM - Thread Starter
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That's why i am trying to work into the amp deeper, i too don't believe that the more watts is what opens up the speaker more. But it may be a factor but more importantly its how the power is delivered, what i meant when i said difficultly of sound was the frequency it was producing because the tweeter itself only uses probably at guess around 20 watts and this is the main reason why they are so efficient.
Its not until you start going down from below 1khz that the power is really needed, now the tweeter is probably getting the power it needs but the woofers may possibly not and distortion is not appearing because the SPL is already at a acceptable level.

But if you give it the power and again im not talking about watts, im just talking about the quality of power and perhaps it could give more woofer movement to fully open up, better quality at lower volumes because it can do more.

Just thinking aloud.
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post #21 of 29 Old 01-04-2011, 10:38 PM
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I own two books on amp design, and neither talks about any of that stuff.

The books talk about having a suitable power supply. They don't talk about having a suitable power supply for better bass. In fact, a few engineers reject the idea that some amps will have tighter bass, etc. The power supply needs a large enough transformer to handle the max average load. So it has to have a large enough VA rating. It needs to have a suitably high RMS voltage at the transformer's secondary to achieve the desired supply voltage for the amp's output stage. There's other considerations such as keeping signal lines away from the power supply as much as possible and transformer vibration, but I have read nothing about anything special to ensure better bass. In some ways, lower frequencies are easier to deal with on the amp end, because they are such low frequencies. Amp/speaker interaction could be a potential issue I guess, but only if there's a problematical impedance dip that coupled with some very specific signal causes the amp to be seriously overloaded (see Self's book, chapter 8.)

When talking about stuff besides the power supply, all they talk about is the various types of distortion which are created by the various amp stages.

Slone (RIP) talks a bit about bass response. Specifically he talks about limiting bass response to some practical frequency. In other words, he argues against designing the amp so it comes close to a flat response down to DC. Nothing about tight bass or anything like that. Just practical circuit design for audio frequencies.

If you can show me where I missed the discussion of the points you bring up in Self's book or Slone's book I would appreciate it.

"But this one goes up to 11"
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post #22 of 29 Old 01-04-2011, 10:55 PM - Thread Starter
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""Technically "power is power" and it would sound the same. However two amps with the same power ratings are NOT identical which is why you do hear a difference. One amp might be Class A, another might be Class AB, one might be Class H, and so on. That can easily change how the amp sounds. Also the capacitors can have an effect on sound. The transformer used can effect sound, although pretty much everything uses a toroidal transformer now so that isnt really an issue. If the amp is solid state or a tube amp, producing the same wattage, it would sound different. Using different circuit design would effect sound. The amp's damping factor can effect sound because if one amp has a low damping factor it cant control the speaker as well as the amp that has a good damping factor. Also one company might rate their power output differently than another company.""

From a mate of mine who was helping me, it describes quite a bit about what has effects on sound.
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post #23 of 29 Old 01-04-2011, 11:05 PM
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Quote:
Originally Posted by [DANGERDAN] View Post

""Technically "power is power" and it would sound the same. However two amps with the same power ratings are NOT identical which is why you do hear a difference. One amp might be Class A, another might be Class AB, one might be Class H, and so on. That can easily change how the amp sounds. Also the capacitors can have an effect on sound. The transformer used can effect sound, although pretty much everything uses a toroidal transformer now so that isnt really an issue. If the amp is solid state or a tube amp, producing the same wattage, it would sound different. Using different circuit design would effect sound. The amp's damping factor can effect sound because if one amp has a low damping factor it cant control the speaker as well as the amp that has a good damping factor. Also one company might rate their power output differently than another company.""

From a mate of mine who was helping me, it describes quite a bit about what has effects on sound.

First off, the amplifier class could affect how an amp sounds. Class A, for example, lacks the crossover distortion seen in class B (or AB.) But see Self's explanation in his book of why class A is not needed for high fidelity sound (accurate reproduction.) But most amps are class B or a variant (AB, G or H.)

Can transformers affect sound? Hopefully not power supply transformers, in a well designed amp. An output transformer for a tube amp? Maybe. Certainly, Bob Carver seems to think the output transformer is important in tube amps.

Tube amps CAN sound different. Especially when clipping. But I did not think we were talking tube amps here...so let's not...(and a tube amp could probably sound identical to a solid state amp if designed that way, and was operated in it's limits.)

Damping factor is perhaps overrated. See the many articles on this topic by engineers who have shared their knowledge with us (not that it has no relevance, just that in the real world with normal amps, it does not.)

As for power being rated differently, obviously so. But we were not talking about THAT either. I was talking about amps not being overdriven, which ignores whatever the rating says...either it's clipping, or it's not. And believe me, it takes very little power to play quite loudly without clipping. A few watts average with 15 dB of headroom allowed will be very loud in most people's setup. Certainly some people will want more than this. That's also irrelevant. As we all agree that clipping is to be avoided. In other words, if you lack sufficient power to avoid audible distortion, buy a bigger amp! That goes without saying (or simply reduce volume, a cheaper option.)

It's what's happening when an amp is not clipping that (I thought) we were discussing.

"But this one goes up to 11"
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post #24 of 29 Old 01-06-2011, 01:53 AM - Thread Starter
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Why is there impedance drops, if a amp could provide 200 watts @ ohm would the speakers still drop in impedance or has it got something else to do with providing the power rather than the ammount ?
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post #25 of 29 Old 01-06-2011, 03:45 AM
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Speakers are resistive loads and far from "perfect". The impedance curve is either a by product of some compromises made during the design of the speaker, or it is flat out intentional. Certain amps claims to deal with irregular impedance curves with more ease then others and you'll hear all kinds of "explanations" from amp designer as how this achieved. In any case as pointed out electronics will do very little to influence the overall sound compared to acoustics and speaker/ LP placing.

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post #26 of 29 Old 01-06-2011, 03:48 AM
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Quote:
Originally Posted by [DANGERDAN] View Post

Why is there impedance drops, if a amp could provide 200 watts @ ohm would the speakers still drop in impedance or has it got something else to do with providing the power rather than the ammount ?

The speaker has an impedance curve. The impedance curve is simply a property of the speaker and you can't change it, much like the height or weight.

Although speakers are spec'd to be "8 ohm" or "4 ohm", the impedance actually varies with frequency. So an "8 ohm" speaker may actually be 5.1 ohms (or whatever) at a given frequency. The impedance curve has "dips". Each speaker model is different, but some speakers have deep dips (like an "8 ohm" speaker that drops to ~3 ohms at some frequency) and others don't.

An amp will deliver power as needed, into the impedance dictated by the speaker, and subject to the demands of the music and the current position of the volume control. If those conditions call for more power than the amp is capable of delivering, it will clip or distort.

-Max
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post #27 of 29 Old 01-06-2011, 05:24 AM
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Originally Posted by [DANGERDAN] View Post

Im using a 80 watt Marantz atm and i have a budget sony amp as well, but i am not getting any good mid/bass sound from the speakers not to make me go wow thats awsome etc.

I believe that i need a better amp with more volt/power to hit a sharper and harder mid/low frequency sound.

While a different/more powerful amp might help, there's a lot that needs to be addressed/explored first.

My reading about RS 83's indicates they are space and placement sensitive. They seem to sound better in larger rooms. A lot of people feel they need to be EQ'd to sound good. How big is your space? Also, have you experimented with placement? Maybe there's some room suckouts in the mid bass range. If so, a bigger amp won't fix the problem.

Are you using any EQ, like Audyssey? Maybe some room treatment, different placement and EQ will fix the problem.

Finally, how did you decide to get those speakers? Did you audition them and if so where?
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post #28 of 29 Old 01-06-2011, 01:04 PM
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I believe impedance curves are heavily influenced by enclosure. You can see different impedance curves with different enclosures

A plot of one speaker type you will see shows impedance rising to a high peak (at resonance I thinK?,( then starts dropping down to some minimum, then starts increasing with increasing frequency.

I am sure speaker drivers contribute their own impedance to the situation, but are a smaller influence than enclosure. What I find interesting is how speakers manage fairly flat (anechoic) response in this environment. I believe it's partly because at resonance impedance is high, but then the speaker itself has higher energy at this point. But I am not 100% sure of this My knowledge of speaker design is low.

One thing is clear, the nominal impedance is heavily influenced by the impedance peak if any sort of normal math is used. It would not surprise me to learn than instantaneous impedance seen by the amp is frequently lower than 8 ohms, but once again that is a guess. And how would you know what the real worst case scenario is?

I am pretty sure that one factor that's no obvious is that a real music/movie signal is not going to look like a test sweep where pure sine tones are being generated. The speaker is vibrating in a complex way which is contstantly changing over time.

I think there's no way to know what is going on without knowing what the current draw graph looks like. As the amplified signal interacts with the speaker and the speaker interacts with the cabinet, there's an instantenous voltage, current and impedance. Which could only be seen via a good simulation or real time measurement to see what the worse case scenario actually was.

"But this one goes up to 11"
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post #29 of 29 Old 01-06-2011, 07:01 PM
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Quote:
Originally Posted by MichaelJHuman View Post

I think there's no way to know what is going on without knowing what the current draw graph looks like. As the amplified signal interacts with the speaker and the speaker interacts with the cabinet, there's an instantenous voltage, current and impedance. Which could only be seen via a good simulation or real time measurement to see what the worse case scenario actually was.


Most reputable mfg of speakers probably have the impedance graphs, but whether they're published or not is a different matter. If the mfg won't provide them, perhaps the speaker in question has been reviewed and this measurement provided.

Here is a graph measured by Stereophile for one of my pairs of speakers. Impedance is the solid line and scale to left, phase angle is the dashed line and scale to the right.

You can see they drop below 4 ohms for a decent spread between roughly 55Hz and 200Hz. They like a beefy amp with good low impedance output capability.
LL

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