Impedence- A few more details. With Speaker matching

[newtoaudio90]

Hey guys

As many of you know I am new and have been posting and reading ALOT. Trying to learn as much as I can.

I wanted to confirm my current understanding of Impedance

I understand impedance is resistance. A smaller impedance means less resistance and thus requires more power ( in watts) to drive it. and allows this power in due to the lower resistance

using the hose analogy (more water).

Now here is where the questions start. I read if you hook up two speakers to an amp that has inputs for two speakers then they will be set up in 'parallel' and thus the impedance half's?

In the example of the speakers I have been looking at the Minimum impedance is stated at 4ohms and the Nominal impedance is 8 ohms.

Now from the various responses on here I gather that Nominal is the impedance listed when a speaker only lists one value and is essentially the average impedance however minimum impedance is very important as it requires more flow of current at that level and if they amp cannot provide this then ...your screwed.

Now if we consider amp and speaker matching. I have listed some specs of the amp I purchased the other day below. Essentially considering you dont want a distortion of 10%.

Now if you take two speakers with a Nominal Impedance of 8O then the impedance will be 4 Ohm per channel????

Therefore when looking at the amp data you look at the 4ohm specs. i.e. as below ( 2x48W) into 4 ohms at 0.1% THD.

Then finally consider the min impedance. 4 ohms per speaker = 2 ohms per channel when running in parallel fashion. Now although this amp isn't rated for 2ohms, it wont have to run this all the time and should be powerful enough to handle it if not overdriven.

(I understand all the impedence varies with the frequency curves and thus all speakers are going to make the amp provide power at lower impedence levels at times that they may not be rated for.

Ultimate questions:
1. Is this calculation and statement correct in terms of speaker and amp matching?
2. Am I right to look at the 4ohm rating of the amp seeing there will be 2 channels plugged in and thus 4 ohms per channel (with 8 ohm speakers)
3. Is the way I have considered Nominal and minimum impedance correct.

These are the concepts I have struggled with as a newbie in the last week so your help is much appreciated.


Specs for the amp below


Power output: 2 x 80W (into 4Ω @ 10% THD), 2 x 48W (into 4Ω @ 0.1% THD), 2 x 50W (into 8Ω @ 10% THD), 2 x 27W (into 8Ω @ 0.1% THD) • Dynamic range: 102 dB • Signal-to-noise ratio: 98 dB • Power efficiency: 81% @ 80W into 4Ω, 88% @ 50W into 8Ω • Input sensitivity: 200 mV • Weight: 11 lbs. • Dimensions: 11.375" (288.9 mm) L x 6.625" (168.28 mm) W x 3.375" (85.73 mm) H.

 

[PENG]

I understand impedance is resistance. A smaller impedance means less resistance and thus requires more power ( in watts) to drive it. and allows this power in due to the lower resistance

using the hose analogy (more water).

I think you've got the idea. In the stricter sense impedance generally implies it is a complex number consisting of components more than just resistance. It may contain a combination of resistive, inductive and capacitive reactance. Yes the lower value it is, the more current it would allow to pass through the circuit but it does not mean more power will be developed. For example, you could have a very low impedance that is made up of almost pure inductive reactance. In that case you get high reactive power (unit in VA) but almost zero real power (unit in watts). Again, if you ignore the technical terms I think you got the concept.

Now here is where the questions start. I read if you hook up two speakers to an amp that has inputs for two speakers then they will be set up in 'parallel' and thus the impedance half's?

That is only true if you hook up two speakers to the same amp channel. If you hook up one speaker to each of the two channel than each amp channel will still see the full impedance of the speaker, NOT half.

Now from the various responses on here I gather that Nominal is the impedance listed when a speaker only lists one value and is essentially the average impedance however minimum impedance is very important as it requires more flow of current at that level and if they amp cannot provide this then ...your screwed.

I would not say nominal necessarily mean average. You need to ask the individual manufactuer. I am not aware of an univerally accepted/adopted definition of "nominal impedance". As to whether you are screwed it depends on other factors. If the minimum impedance occurs at a higher and narrow frequency range and where the phase angles between the current and voltage phasors (or commonly referred to as vectors, a term that a high school science student would be familiar with) are small, say less than 30 degrees, then it most likely won't be an issue for most amps. That is unless it is really low, like 3 ohms or less. Conversely, even 5 to 6 ohms could be tough for many amps if such values occur over a wider and lower frequency range; and coincides with where the phase angles are large, say >45 degrees.

Now if we consider amp and speaker matching. I have listed some specs of the amp I purchased the other day below. Essentially considering you dont want a distortion of 10%.

Agree, to me anything more than 0.1% is not good, except for tube amps.

Therefore when looking at the amp data you look at the 4ohm specs. i.e. as below ( 2x48W) into 4 ohms at 0.1% THD.

In many cases yes, but the amp you listed outputs only 27W into 8 ohms, at 0.1% THD. So in this case I would consider it a 27W per channel amp, at the very maximum.

Then finally consider the min impedance. 4 ohms per speaker = 2 ohms per channel when running in parallel fashion. Now although this amp isn't rated for 2ohms, it wont have to run this all the time and should be powerful enough to handle it if not overdriven.

Again, if you connect one speaker to each of the two amp channels, the amp will not see a halved impedance.

(I understand all the impedence varies with the frequency curves and thus all speakers are going to make the amp provide power at lower impedence levels at times that they may not be rated for.

It does not seem to make much sense in a strict sense but again I think you understand the principle so I am not going to be critical on its lack of accuracy in terms of proper engineering language.

Save you Googling, use the following formula to calculate equivalent resistance:

1/Req=1/R1+1/R2+.........+1/Rn

Example: if you have two 8 ohm resistors in parallel,

1/Req=1/8+1/8=1/4 therefore Req = 4 ohms

or one 8 ohm and one 2 ohm in parallel,

1/Req=1/8+1/2 = 5/8, therefore Req = 1.6 ohms

Now people just don't parallel two disimilar speakers so the second example is just for illustrating how to calculate the equivalent resistance. For impedance it gets more complicated but if you are interested I am sure you know how to google or where the libraries are.

 

[newtoaudio90]

Thanks very much for that. I am very glad you clarified speakers are not running in parallel when they are plugged into their own inputs.

Very comprehensive response I appreciate your time.

 

[MR.MAGOO]

Would it be safe to assume low impedance 4 ohm + high power (watts per channel amp) = big room; high impedance 8 ohm + mid-power = normal size room?

Trying to understand ohms vs watts thing. And how it relates to AVR's, speakers, room size, etc.

 

[TLS Guy]

Would it be safe to assume low impedance 4 ohm + high power (watts per channel amp) = big room; high impedance 8 ohm + mid-power = normal size room?

Trying to understand ohms vs watts thing. And how it relates to AVR's, speakers, room size, etc.

You have to be careful. For most speakers you have really no clue what the impedance is unless you measure it with a dynamic measuring program, the manufacturer or a third party has published the impedance and phase angle curves.

A quoted minimal impedance is helpful, as the minimal impedance + 10% is a reasonable rule to arrive at what is the likely loading of the amp. That rule is by no means infallible by the way.

Manufacturers tend to average out the curve which is useless. The fact is that most speakers drop their impedance in the range where speakers are hungry because of baffle step compensation.

These days unless there is a published curve the wisest course is to assume speakers are 4 ohm.

If there is more than one woofer, or mid/woofer you should definitely assume it is 4 ohm unless there is cast iron proof to the contrary. Evidence to the contrary means a reliable impedance and phase angle curves. Absent that ignore what a speaker manufacturer claims.

And yes, as impedance goes down the speaker will attempt to draw more current form the amp, doubling the power draw for every halving of the impedance. If the current is not available, either the amp will clip, go into protection or blow up, and any combination of the above.