Power supply VA rating and how much is really needed

I have a general question. Two actually.

How much capacity of a power supply is needed to allow for each channel in a multi channel amplifier feeding demanding speakers preferred? 

Is 300VA-400VA per channel enough?  Will less be detrimental to performance and SQ? Is more always better?

What are ideal specs for a multichannel ( or stereo amplifier ) that will provide a pleasant experience.  Not looking for the 'best' of the best... but for a decent amp... what are specs that I should be focused upon?

I realize SQ is important, but specs play a role in narrowing down the list I have put together.

Second question:

How much output current capacity is need/required/preferred to drive speakers adequately?  And how to calculate the output if the spec isn't given.  I tried google, sadly I didn't find the formula I was looking for.

Is for example 40A enough? Is 30A?  

What factors need I consider in figuring out what spec matters most?


My speakers consist of B&W CDM 9NTs with matching center channel.  


Emotiva XPS 5ch
Monoprice 5-7ch 
Rotel RMB1585
Odyssey Stratos 2ch and adding emotiva basX 5-7ch or something like it ... possibly used market parasound, rotel, acurus, etc for surround duties.

Not sure which of these actually have the best specs that translates in to best bang for the buck purchase to provide sweet midrange, tight punchy controlled bass, and nice sweet highs but not to bright.

I know the Rotel is highly recommended based on blending well with my speakers - but when do specs trump sound? Or does it?

I am removing power cords, fuses etc from the equation for a moment.

Is 300VA-400VA per channel enough? Will less be detrimental to performance and SQ? Is more always better?

That's about 300-400 Watts per channel. Simultaneously. That's quite a bit.

What are ideal specs for a multichannel ( or stereo amplifier ) that will provide a pleasant experience. Not looking for the 'best' of the best... but for a decent amp... what are specs that I should be focused upon?

This is really dependent on the size of the room, the efficiency of your speakers and how close you sit to them. The efficiency of the speakers can vary by huge amounts. Get theater grade horns and 10-20 watts will be too much.

I think it's also important to note that 300-400 watts / channel is 1,500 - 1,600 simultaneously.

Specs never trump personal preference, but they do ensure that manufacturers don't try to cheat you too badly.


Just to clarify. And maybe I worded it unclear.  

I was asking about capacity of power supply. Ex: A 2 ch amp with a Tor PS of 1100VA ... you are saying it translates into 550w per ch? That doesn't makes sense if amp is rated at 8Ohms at 200wpc... 

If it was unclear - I want to know how much power a PS should have to drive difficult speakers... and how much output in current is suggested... 45A etc?

Sorry if there was confusion.

VA = AC watts.

This is really the upper limit of what the power supply can deliver.

However, the actual wattage spent is the power consumed by the speaker divided by some efficiency factor. So
200 Watts at the speaker / .6 = 266 VA continuous required. Probably more. LOTS more if class A. Class D, use about 0.8

Also, watts at the speaker are impedance dependent. Half the impedance = 2x the power required. 
A stiff 200 Watt amp at 8 Ohms may do 400 Watts at 4. If very stiff, 800 Watts at 2 Ohms.

Still, the power supply limits all, so if you have 550 VA available per channel,  you'll be lucky to see 330 max at the speakers.
Further, this is all compounded by the fact that music is dynamic. Short term power delivery may be more important for music than steady state.

Fun stuff, right? :)
Got it.

Thx. I am not trying to drive in circles - but just for kicks --

So - question -- all things being equal in the audio world -- I know they are not, but given the following -- is one better than the other in dynamics, , sq simply because one has more power from the supply available? And will I sonically experience a difference in SQ? I know MP amps are not high end - but I am trying to wrap my head around specs in general.

1. monoprice ati clone 2ch -- Toroidal Transformers 1 (800VA)
2. monoprice ati clone 3ch -- Toroidal Transformers 1 (1,025VA)

I am only using the above amp as an example because MP happens to have a full spec sheet, unlike the others I am looking at.

Although I don’t see the spec that Odyssey, Rotel, or Parasound lists on their spec sheets which is Current Delivery.

So where does the current in ’amperage’ output delivery come into play here? As I understand it - more current output -- better dynamics, better sq... and from what I can gather - better amps -- have higher current output, which is one of many reasons they sound better.

Odyssey Stratos ( since it is one I am considering) lists the spec :
Not trying to be a dumbass - just trying to ’get it’.

In theory at least, the sound quality doesn’t degrade until you reach the limits of the amp.

Are you going to run into those limits? :)

I mean, the argument can be made that overkill is the best kind of kill when it comes to engineering, but there is more to the sound quality than merely capacity. Noise filtering, storage capacity, decoupling and signal routing of the power supply matter a great deal as well.
It’s not really a spec that a lot of people should use. Fortunately the one thing that’s pretty well defined in terms of measurements are how you rate an amplifier, which is in steady state after a warmup at 1/3rd power. So assuming an amp meets its specs it will deliver as promised and leave the details of the power supply to the engineers.
It is important to note the power, impedance and number of channels driven at that specification.

Again, not sure overkill is something you can hear. A well designed small amp that is never used outside of it’s design envelope could sound fantastic.

But... let’s say you have some troublesome speakers like an ESL, or one that dips to 2-3 ohms in the bass. That will require more current. An amplifier needs to have 3 things to not wilt on a difficult speaker:

1 - Adequate power supply (VA) for the output desired
2 - Appropriate driver and output stages
3 - Low output impedance.
So, imagine a perfect amplifier with no output impedance and a limitless power supply. No matter the speaker, the Voltage will not change.

On the other end are high output impedance amps. Typically tubes. Their output is a little dependent on the speaker’s impedance itself.
You can see this in Stereophile review of tube amps with the simulated speaker load.

Is that bad? Well, it’s not precise, but you might like how it sounds.

Kind of on this theme, Nelson Pass has written a great deal about how we test, measure and evaluate amps. He makes the convincing argument that the 1st watt is the most important one.

What the distortion or noise is at 250W is irrelevant because no one uses it. You want an amp that is low noise, low distortion and performs beautifully in the first few watts, and has power to not compress.

And one thing I find VERY true is that a lot of amp makers rate the S/N at full output. It tells you nothing about the relative noise. The most careful makers rate Signal to noise at 1 watt. That tells you whether a 50 W amp is noisier than a 500 W amp much more reliably.

Thanks I will read up on what Nelson Pass has written.

Based on what you have stated. And your knowledge base - 

given a budget of approx max $2k-$3k... 

all things being equal ... which do you think is a better amp based on specs? And which do you think would be a good fit for B&W speakers?

monoprice 1800/2007 ati clone?
odyssey stratos 2 or 3 ch or khartago version
rotel rmb-1585
emotiva xpa5 
outlaw 7700

I realize that how things sound is the end game -- and these are not top tier products - as they are all midfi at best... but I don't have the budget for the likes of Pass, Levinson, etc... 

Am I missing any contenders in the amplifier space that I should consider?


Used, ATI and NAD's multichannel amps with Hypex class D stages.

Otherwise Rotel.

I have not heard Outlaw and Emotiva amps, but I have tried the top of the line, mail order HT processors and they were crap. I mean, worked well, looked fine, sounded like crap. Really really thin, gutless. No noise, but also no depth or credibility so I have never gone back that route. 
Sorry, I've been on somewhat of a hiatas from some frustrating audio testing and then thanksgiving and then getting sick. 

Generally speaking, I would not pay too much attention to how many amps output.  The specifications that say "40A output" are pretty meaningless.  The "output watts" is a specification that will indicate the overall size of the power supply.  A 500 watt monoblock amp is going to have a rather large transformer and a large power supply capacitor bank.  Obviously, you are not going to use all 500 watts for a single speaker, but the benefit is going to be much smoother sound, fuller midrange, and much stronger and fuller bass.  It also depends on how hard your speakers are to drive.  A smaller 150 watt amp would probably be fine on a more efficient high impedance speaker - such as a flat 8 ohm speaker that is 91db efficient.  Once you start going down in impedance and efficiency, you really want a beefier power supply - which translates to a larger power amp.

Another part is the sonic signature - do you want a very fast responding "Class B" type of amp, or do you want a very warm sounding "Class A" type - or something "neutral" in the middle.  It all comes down to personal taste.  I just finished my own evaluation of different amps and have heard a whole lot of different electronics when I went to RMAF.  In my testing, I would say that the Parasound was the best in fullness and natural sound, but it was too warm for my tastes.  Bryston cubed amps were very nice and very high resolution, but were too thin/fast for me (they are just about the closest to a Class B amp you can get).  The Classe Audio monoblocks I tested sounded very clinical and sterile in my system, but they did have good resolution. 

So, in my frustration, I just went back to Emotiva and purchased a pair of their 2-channel fully differential amps (the XPA-DR2 models).  I have one amp powering the front left / surround left.  And the second powering the front right / surround right.  Obviously, I'm balancing the power supplies in the amps so that they are used most efficiently, but at a rating of 550 watts per channel, it is totally fine.  I will say that after hearing gobs of ultra expensive electronics and a variety of different sonic signature, the new Emotiva differential amps are PRETTY DAMN GOOD!  I originally had Emotiva XPR-1 amps (1000 watt monoblocks) for my left/right which I sold earlier this year and regret that.  The new amps are only 4 days old and still burning in so I won't have a final judgment for another week or so.  It's very difficult to tell if these are going to be better than my original XPR-1 amps or not.  I will say my system sounds the best with Emotiva monoblocks (whether they are the old XPR-1 or the new XPA-DR2).  The Emotiva are very neutral amps, but they have excellent resolution.  The sounds/vocals are very realistic and they are extremely engaging with my B&W D3 speakers (which none of the other amps really gave me).  At this point, the amps do not quite have the amount of bass/midbass that I would like, but they are still burning in.  They are a slightly different amp board and the power supply is the switching power supply instead of a massive transformer and capacitor bank.  That may translate into how much bass strength I have.  I will know more in about 4-5 days.
I will agree with Eric's statements that Emotiva preamps / processors / DACs are not that great.  They are rather dull sounding and "closed in" sounding with lack of resolution, separation of instruments, and depth.  Their amps have always been very excellent, however.
Thanks for the info, Aux! :)

Glad some one else heard what I did.
Thanks for the input.  Have learned much already.