Ohm Matching - AVS Forum
Forum Jump: 
 
Thread Tools
Old 06-15-2014, 11:28 AM - Thread Starter
Member
 
kawie01's Avatar
 
Join Date: Jun 2010
Posts: 160
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 6 Post(s)
Liked: 13
Ohm Matching

I ran into a guy that was saying how critical it is to match the exact ohms from AVR to speaker. He kept saying that I would notice a big difference if i made sure that my Speakers were receiving the proper ohms from AVR. I did some digging around and he appears to be incorrect. Any feedback?

All my speakers are 8 ohms paired with Onkyo 818
kawie01 is offline  
Sponsored Links
Advertisement
 
Old 06-15-2014, 12:55 PM
AVS Special Member
 
bass addict's Avatar
 
Join Date: Dec 2003
Location: A padded room
Posts: 4,157
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 224 Post(s)
Liked: 327
I'm not quite sure what you're getting at. Most receivers are designed for 8 ohm speakers, with the newer ones being able to allow lower ohm speakers to be used. Lower ohms allow more current, which allows for more power, albeit at extra heat from the amplifier. Older receivers would shut down when hooking up lower ohm loads than they were designed for. Newer receivers allow you to select 4/6/8 ohm speakers, but it doesn't work the way an external amp does. An external amp that is designed to drive 4 ohm loads will effectively double power (not always the case but a good rule of thumb) from an 8ohm to 4 ohm load. Receivers essentially have a built in current limiter that will allow you to hook up lower than 8 ohm loads, but you are not really getting any additional power from the on board amps.

So really, it doesn't matter whether your current speakers are 4/6/8 ohm, the receiver is going to produce about the same amount of power regardless. If you want to maximize your listening experience, I'd be more concerned with sensitivity than ohm ratings.

Achievement Unlocked

Psychotic Episode Averted

bass addict is offline  
Old 06-15-2014, 01:59 PM - Thread Starter
Member
 
kawie01's Avatar
 
Join Date: Jun 2010
Posts: 160
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 6 Post(s)
Liked: 13
Quote:
Originally Posted by bass addict View Post
I'm not quite sure what you're getting at. Most receivers are designed for 8 ohm speakers, with the newer ones being able to allow lower ohm speakers to be used. Lower ohms allow more current, which allows for more power, albeit at extra heat from the amplifier. Older receivers would shut down when hooking up lower ohm loads than they were designed for. Newer receivers allow you to select 4/6/8 ohm speakers, but it doesn't work the way an external amp does. An external amp that is designed to drive 4 ohm loads will effectively double power (not always the case but a good rule of thumb) from an 8ohm to 4 ohm load. Receivers essentially have a built in current limiter that will allow you to hook up lower than 8 ohm loads, but you are not really getting any additional power from the on board amps.

So really, it doesn't matter whether your current speakers are 4/6/8 ohm, the receiver is going to produce about the same amount of power regardless. If you want to maximize your listening experience, I'd be more concerned with sensitivity than ohm ratings.
I understand the sensitivity of speakers. This guy was going off on how, for example, if your speakers are rated at (enter number here) ohms, you should be sure the speaker end is at the said ohms. With distance (speaker end) there would be a diminished signal, ohms, or whatever. So if your speakers are rated at 8 ohms, one should make sure that 8 ohms is the readout at the speaker for the best possible sound.

Sounds like hogwash but I dunno.
kawie01 is offline  
Old 06-15-2014, 05:46 PM
AVS Addicted Member
 
MichaelJHuman's Avatar
 
Join Date: Nov 2006
Location: Colorado Springs, CO
Posts: 18,955
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 238 Post(s)
Liked: 118
Tube amps require some impedence matching. Some tube amps even have a switch. This seems due to the fact they have an output transformer.

Solid state amplifiers actually have a very low output impedance. You can't match it.

What you do need to be careful with, is the minimum impedance. If the speakers have too low of an impedance, the amp/receiver could overheat, shutdown, or in the worst case scenario fail.

4 ohm speakers are the minimum you can usually use with audio-video receivers. They are usually designed for 8 ohm speakers, not because of impedance matching, but in terms of the power supply and the heat sinking and such.

Short answer, you don't need to impedance match with solid state amps, but you should heed the manufacturer's direction on minimum impedance.

"But this one goes up to 11"
MichaelJHuman is online now  
Old 06-15-2014, 06:04 PM
AVS Addicted Member
 
MichaelJHuman's Avatar
 
Join Date: Nov 2006
Location: Colorado Springs, CO
Posts: 18,955
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 238 Post(s)
Liked: 118
Quote:
Originally Posted by bass addict View Post
I'm not quite sure what you're getting at. Most receivers are designed for 8 ohm speakers, with the newer ones being able to allow lower ohm speakers to be used. Lower ohms allow more current, which allows for more power, albeit at extra heat from the amplifier. Older receivers would shut down when hooking up lower ohm loads than they were designed for. Newer receivers allow you to select 4/6/8 ohm speakers, but it doesn't work the way an external amp does. An external amp that is designed to drive 4 ohm loads will effectively double power (not always the case but a good rule of thumb) from an 8ohm to 4 ohm load. Receivers essentially have a built in current limiter that will allow you to hook up lower than 8 ohm loads, but you are not really getting any additional power from the on board amps.

So really, it doesn't matter whether your current speakers are 4/6/8 ohm, the receiver is going to produce about the same amount of power regardless. If you want to maximize your listening experience, I'd be more concerned with sensitivity than ohm ratings.
There's a few points here that seem incorrect. Not trying to be a pain, but I wanted to address them.

I don't think it's a general rule that newer receivers have better capability to do with 4 ohm loads than older receivers. The cheaper receivers likely have less ability to deal with 4 ohm speakers than the more expensive receivers, and that has nothing to do with how old they are. More expensive receivers, will have bigger heat sinks and bigger power supplies which seems likely to help. You would expect their protection circuitry to be less...aggressive...than cheaper receivers and therefore more likely to not have issues with 4 ohm speakers.

As for current limiting, the usual way receivers allow for lower impedance speakers is via a switch. But the switch does not limit current as such - it reduces voltage, reducing maximum power output (which does limit maximum current output, but I would not call it a current limiter.) The protection circuits in a receiver may limit current, but in my experience it's more like short protection - they shut down to protect the receiver. Perhaps some receivers due have an active current limiter while still operating, but I have not heard of such a thing.

"But this one goes up to 11"
MichaelJHuman is online now  
Old 06-15-2014, 07:15 PM
Senior Member
 
russ_777's Avatar
 
Join Date: Jan 2005
Posts: 270
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Liked: 20
Below is a plot of the impedance (magnitude and phase) versus frequency of a popular loudspeaker. Can you explain to me how one would possibly select an amplifier that "exactly matches the ohms" of this speaker? Bear in mind, this plot is very typical.
Attached Thumbnails
Click image for larger version

Name:	1113PSBT2fig1.jpg
Views:	10
Size:	51.5 KB
ID:	119698  
GIEGAR likes this.
russ_777 is offline  
Old 06-15-2014, 08:43 PM
AVS Addicted Member
 
MichaelJHuman's Avatar
 
Join Date: Nov 2006
Location: Colorado Springs, CO
Posts: 18,955
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 238 Post(s)
Liked: 118
As you say, the impedance varies. Even so, my understanding is that you should use speakers with the proper nominal impedance with tube amps. From what I have read, there's no major issue with a small enough mismatch. But too much or too little impedance seems like a bad idea with tube amps.

With solid state amps too low is the main issue.

Personally, I don't buy 4 ohm speakers for use with AVRs. IMO, there's more disadvantages than advantages. But if your receiver/amp can handle them, and all else being the same, you can push more power into the 4 ohm speakers and play them louder. I can appreciate the allure in that case.

"But this one goes up to 11"
MichaelJHuman is online now  
Old 06-15-2014, 08:51 PM
AVS Special Member
 
bass addict's Avatar
 
Join Date: Dec 2003
Location: A padded room
Posts: 4,157
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 224 Post(s)
Liked: 327
[quote=MichaelJHuman;24993506]
Quote:
Originally Posted by bass addict View Post
I don't think it's a general rule that newer receivers have better capability to do with 4 ohm loads than older receivers. The cheaper receivers likely have less ability to deal with 4 ohm speakers than the more expensive receivers, and that has nothing to do with how old they are. More expensive receivers, will have bigger heat sinks and bigger power supplies which seems likely to help. You would expect their protection circuitry to be less...aggressive...than cheaper receivers and therefore more likely to not have issues with 4 ohm speakers.
OK, just for you I'll rephrase. Most new receivers are designed to handle low impedance loads, compared to receivers of yesteryear. I'm willing to bet there is a much greater percentage of receivers today (regardless of cost) that can handle 6 and even 4 ohm loads. A few years ago, only few would withstand reference level hooked up to a 4 ohm speaker. I'll let you argue over the semantics.

Quote:
Originally Posted by MichaelJHuman View Post
As for current limiting, the usual way receivers allow for lower impedance speakers is via a switch. But the switch does not limit current as such - it reduces voltage, reducing maximum power output (which does limit maximum current output, but I would not call it a current limiter.) The protection circuits in a receiver may limit current, but in my experience it's more like short protection - they shut down to protect the receiver. Perhaps some receivers due have an active current limiter while still operating, but I have not heard of such a thing.
I'm sorry for not giving the perfect "audio" definition. In the end they limit current, you can pontificate about how that happens.

Achievement Unlocked

Psychotic Episode Averted

bass addict is offline  
Old 06-15-2014, 08:59 PM
AVS Special Member
 
GIEGAR's Avatar
 
Join Date: Jan 2013
Location: Australia
Posts: 1,214
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 50 Post(s)
Liked: 332
Quote:
Originally Posted by kawie01 View Post
I ran into a guy that was saying how critical it is to match the exact ohms from AVR to speaker. He kept saying that I would notice a big difference if i made sure that my Speakers were receiving the proper ohms from AVR. I did some digging around and he appears to be incorrect. Any feedback?

All my speakers are 8 ohms paired with Onkyo 818
It sounds like this guy has no idea what he's talking about!

Speakers don't "receive ohms from the AVR". An AVR is a voltage source. Ohm's Law is basically: "Voltage pushes current (amperage) through a resistance". In a reactive circuit, or complex load such as a speaker, impedance is substituted for resistance. Have a read up on it in this article: Understanding Ohm's Law, Impedance And Electrical Phase 101, and tell this guy he's full of hogwash.

Your Onkyo TX-NR818 carries THX Select2 Plus certification. This means it's certified to drive three 4Ω (nom.) impedance front speakers and four 8Ω (nom.) impedance surround speakers simultaneously. If all your speakers are 8Ω (nom.) impedance, you've got nothing to worry about. In fact, if your speakers are at least 89dB/1W/1m sensitivity, the 818 will play any commercially available multichannel program material to cinema (THX/Dolby etc.)reference level in a "Select2" sized room and viewing distance (~56m^3; ~3 to 4m).

Last edited by GIEGAR; 06-15-2014 at 09:06 PM.
GIEGAR is online now  
Old 06-15-2014, 09:02 PM
Senior Member
 
russ_777's Avatar
 
Join Date: Jan 2005
Posts: 270
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Liked: 20
The point of my posting that was to illustrate how preposterous the comment was that was made to the OP that it is critical to match the EXACT "ohms". There is no such thing as exact ohms. The impedance is variable and complex, period, end of story (unless you're talking about a few unique speakers whose drivers are mostly resistive).

A nominal impedance is not much more than a wild ass guess about how well or poorly behaved a given speaker will be with most PAs. There is no standard way of measuring or estimating it. How do you know a given speaker has "proper nominal impedance" if you don't know how it was measured or determined? Blind faith? It is infinitely more instructive to get access to a plot of the measured impedance and assess it across the frequency band, both magnitude and phase.
russ_777 is offline  
Old 06-15-2014, 09:53 PM - Thread Starter
Member
 
kawie01's Avatar
 
Join Date: Jun 2010
Posts: 160
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 6 Post(s)
Liked: 13
Thank you all for your input. I politely listened to the guy and thought about what he was saying later that day.
kawie01 is offline  
Old 06-15-2014, 10:06 PM
AVS Special Member
 
Worf's Avatar
 
Join Date: Sep 2000
Posts: 1,892
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 25 Post(s)
Liked: 22
First, let's consider a system that consists of an ideal voltage source (0 output impedance, infinite amperage, etc) an impedance device we'll call "source impedance", and a load impedance, wired in series. In a real system, the amplifier can be modeled as the voltage source and the source impedance, and the load is your speaker - so your speaker system sees the voltage source of the amp, and the source impedance.

Matching impedance (having the load and source impedance be the same) results in the maximum power being transferred in the system. However, efficiency is low - 50%! Half your power is wasted in the amp! Higher load impedances mean the efficiency goes up, but the total power transferred is lower.

Let's take some numbers - let's say the voltage source can output a max of 10V, while the output impedance is 0.5ohms. The most power that can be produced by this is 10 watts (into a 0.5 ohm load). If we increase the load impedance to 4 ohms, the amp produces just over 2 watts. However, the efficiency goes up from 50% to 89% so your amp isn't heating up as much.

Practically speaking no amp will be able to handle the theoretical maximum - the power supply limits the output voltage and current.

In fact, because speaker impedance varies with frequency, the load presented to the amp varies, which means the amount of power transferred can vary which results in the volume varying by frequency. A rule of thumb is that the output impedance of the amp should be 1/8th that of the load - which results in the output varying under 0.5db from 8 times source to infinity. Now, a 4 ohm amp you'd think would mean 0.5 ohms, but no, a 4 ohm speaker can have significant frequencies where it drops to 2 ohms or so, meaning you'd want an amp with 0.25ohms or lower. In a poorly matched system, this means the amp and speakers do some EQ on the output signal - at some frequencies, the amp is louder, and other frequencies, the amp is softer, instead of being the same volume throughout the entire frequency range (the ideal amp response - the amp should be flat, not doing EQ!)

In the tube days, this required work because tubes have a higher output impedance, so you'd need matching transformers to bring it down. But modern solid state amps can have arbitrarily low output impedance.

For all practical purposes, a modern amp doesn't need impedance matching because the designers know this and design it to have a very low output impedance.

The only reason you have "4 ohm" settings on amps is that it drives the amp more conservatively which limits the output power that can be produced so the finals don't overheat because the amp can produce more power into a lower impedance load. And because the power supply can be power limited, so a high powered signal can cause clipping which can destroy speakers as well.
GIEGAR likes this.
Worf is offline  
Old 06-16-2014, 09:02 AM
Advanced Member
 
SiGGy's Avatar
 
Join Date: Jul 2004
Location: Lenexa, Kansas
Posts: 741
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 89 Post(s)
Liked: 90
Quote:
Originally Posted by russ_777 View Post
Below is a plot of the impedance (magnitude and phase) versus frequency of a popular loudspeaker. Can you explain to me how one would possibly select an amplifier that "exactly matches the ohms" of this speaker? Bear in mind, this plot is very typical.
I would ignore/throw away any impedance measurement above 500hz. All of the power/headroom of your amp will be driving the frequencies lower then 500hz. In fact really 200hz and below would be the grunt of it. I do see the OHMs drop to almost 2 at 600hz, but it's really a limited to 600-650hz. I wouldn't expect too much info to be around there, as 700hz ish is right near the ear fatigue range if it's to high in the mix.

The load on the amp to drive the mids/highs isn't much compared to the low frequencies. I would get something capable of 4ohm load.

There is no "one size fits all". These measurements were probably done using a frequency sweep. Might be interesting to see how it performs with pink noise as well.

p.s

Finding an AMP to drive a 2 OHM load without breaking a sweat is hard. I have some old carver TFM-55's that will do it. But most receivers today would probably start on fire

-SiGGy

Last edited by SiGGy; 06-16-2014 at 09:16 AM.
SiGGy is offline  
 
Thread Tools


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off