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Discussion Starter #1
Hi everyone,

Recently I hooked up a 2 ch power amplifier that was waiting to be installed into my system. I went from the AVR's onboard 90wpc @8 ohms capable amp to a 150wpc @8 ohms capable outboard amp. The 2ch amplifier is used to power the main L\R speakers and they are set to small with a crossover point of 60hz to dual subwoofers.

I noticed that the sound had changed after setting the preamp output level on the AVR to match the same 75db reference level with the outboard amp. I'm not talking about a night and day change, but a noticable difference in bass clarity for the worse. I noticed a very slight increase in bass output - enough to warrant turning down my subwoofer level down one notch in the AVRs bass management (related in user menu as 1db). Once I turned the sub level down a hair everything fell right back into place. There is now no discernible difference to my ears between the 2 amps, but of course the power amp has much more headroom and doesn't sound slightly hard or brassy when pushed.

I'm a healthy skeptic and pragmatist when it comes to anything relating to audio, but I don't like to draw lines in the sand one way or another. Can anyone explain a technical reason for this perceived difference? By technical I am talking gain structure differences or electrical signal. I'm not into the whole amps are unique and special debate. To me, they are tools to get a job done and for some reason one seems to do a different job at low frequencies.
 

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AVR's 90WPC shared power and channels (5 channels?) compared to 150WPC (2 channel)dedicated?
That explains the extra "headroom" and perhaps the "perception" of now being less "slightly harsh or brassy when pushed".
 

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Hey Ratman. :)

The AVR was tested at 91.5wpc 2ch driven and 82.6wpc 5ch driven in sound and vision. Not crappy, but not a dedicated amp for sure.

Thoughts anyone?
 

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And?
What does S&V have to say about the new 2 channel amp you now incorporated into your system?
IMO, adding an additional dedicated 60WPC should be of some benefit regardless. ;)

How about make/model numbers of the AVR, new amp and a link to the S&V tests?

The AVR was tested at 91.5wpc 2ch driven and 82.6wpc 5ch driven in sound and vision.
What was the test criteria to generate those results/numbers?

Thoughts anyone? :)
 

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I'm sure their testing is credible. I was just curious about the 91.5 wpc.

"Continuously driven" generates the question: "At what frequency? One frequency or from 20Hz-20KHz?"

Anyway... you nor I can answer accurately that would be of benefit. So... if you're happy with the new setup, enjoy.

Just a thought to consider. It takes more power to reproduce low frequencies. So perhaps that extra 60wpc helped to offload that task to the new amp, thus the increase in bass output and the less hard and brassy sound since the AVR doesn't have to allocate power to the front L/R channels any longer.
 
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Hey Ratman. :)

The AVR was tested at 91.5wpc 2ch driven and 82.6wpc 5ch driven in sound and vision. Not crappy, but not a dedicated amp for sure.

Thoughts anyone?

Hi,

I might be a little skeptical that an amp rated at 91.5wpc in 2-channels gets 82.6wpc in 5-channels. That seems like unusually consistent performance, but some amps are capable of that, so let it pass. But what you are really comparing, I think, is the 82.6wpc to the 150wpc in the separate amp. That should translate to about 2.5db of additional headroom. (A doubling in power = 3db.) A lot of people can hear much smaller volume differences than that, so perhaps that explains it. I know that an older good AVR I used seemed to struggle a bit in 5.1. When I moved to 7.1, I re-purposed it to run just my two most demanding speakers, and let my new AVR run the other 5. All concerned seem much happier now. :p Having some extra headroom can definitely help sometimes.

Regards,
Mike
 

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But what you are really comparing, I think, is the 82.6wpc to the 150wpc in the separate amp. That should translate to about 2.5db of additional headroom. (A doubling in power = 3db.) A lot of people can hear much smaller volume differences than that, so perhaps that explains it.
Not likely unless the listener was cranking up the volume till the amp clips. In such case, the listener has other bigger problem to worry about. Can you guess what that is? Hint, it's biological.
 

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82 wpc @ 5 channels driven is unusual performance for an AVR (or, I should say, "AVR's power supply.") It indicates a stiffer (i.e., more regulated) power supply than usual.

I've seen numbers as low as 30 wpc/5 channels on AVRs.
 

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Discussion Starter #10
Yes, the Arcam does seem to have a sturdy and controlled output. I assume this is to reduce distirtion at peaks in program material. But the distortion is there (albeit very subtle.)

LFEer, I know what you are getting at. When enjoying the system at reference level, or even below (where its more comfortable) the peak to average dynamic range in good recordings/soundtracks is more than enough to cause distortion. Luckily it does it gracefully.

I am more interested in what theoretical aspects of a power amplifier cause the speakers to have a different perceived low end output. I assume it is signal gain related, but I cant be sure.

Many people claim huge differences in amplifiers, in my experience I am starting to think its because the system is not recalibrated wrt signal gain vs spl at the listening position.

I will use an analogy of sighting in a rifle scope. Once sighted in the range is set for a specific distance. The rifle is dead on at that specific distance. If you take that scope off and put it on another more powerful rifle, the accuracy is the same at that range, but remains close to accurate for a longer distance. But you still have to re-sight the scope between setups. Once calibrated, they perform the same within their respective ranges.
 

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Different types and brands of amplifiers have different sensitivities and gain outputs; which if not accounted for can cause problems when mixing brands/models.
If running all the same brand/model then the change would only be in the upgrade itself, which would be global SPL increase or decrease.

Some amplifiers allow you to control the sensitivity and gain, a good example is the Lab Gruppen FP+ 4000 to 14000; very good series of amps (especially for the size and weight).
I don't know of any HiFi brand amplifiers that do though.

The biggest difference I've heard was going fully-active with external XO's.
It does give the cleanest signal, with highest headroom.
That's how I run my LCR and subs. Every cone has it's own amplifier/channel.
Expensive though because it requires arrays of amplifiers + lots more wires and idle power consumption.
 

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When enjoying the system at reference level, or even below (where its more comfortable) the peak to average dynamic range in good recordings/soundtracks is more than enough to cause distortion.
Rated peak output of amp is higher than rated continuous output.
Luckily it does it gracefully.
The important question is, is it audible in level matched DBT?
Many people claim huge differences in amplifiers,
I've seen many of them over the years at audio meets. But they couldn't say the same after level matched DBT of those components they brought.
 

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Discussion Starter #13
LFEer, do you have anything to offer in terms of a more technical explanation as to why in room response could change between amplifiers? So far not being "matched" at the signal vs spl level is also my best guess.
 

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Discussion Starter #16
Maybe its just a Festivus miracle ?
Could be! It wont be over till someone pins Frank! :D

Ok, measurements.

A while ago I was taking some measurements in my room during the room tuning phases. These graphs were taken at the time using 2ch full bandwidth pink noise at 1/12 octave resolution. I was trying to test general frequency response in my room as I added treatments and changed channel levels. The graphs were carefully taken for this purpose, they illustrate a pretty accurate comparison between the two setups mentioned. The only changes I made between setups were changing the preout level to achieve a 75db level at the mlp. All else remained the same, including microphone position. I simply added rca cables between the units and swapped the speaker connections over, then reset the channel levels to a measured 75db for the seperate amplifier. I had to add about 1.5db to the R\L channel levels to reach the same reference level.

Before anyone mentions, I know the db scale is too large. I was jumping between many tests and forgot to lock the ranges and it reset. :p These graphs are old.

The tests were taken at the mlp in my theater with subs crossed over to the mains at 60hz. The sub level settings were not changed between amps. A 2ch pink noise test signal was used. It is from the stereophile test disc and was played with an Oppo 103. I do not use EQ in my system. There was no EQ applied to these tests.

Here was the AVR at the time. R/L with subs. Pink noise 1/12 octave measurement.



Here was the seperate amplifier at the time. R/L with subs. Pink noise 1/12 octave measurement.



This is what I am talking about. The graphs are virtually identical until we get to about 100hz. It supports what I perceived the difference to be. The subs required a new setting to bring the low frequencies back into balance. It sounded worse before the sub level was tweaked back a bit.
 

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Discussion Starter #17
Different types and brands of amplifiers have different sensitivities and gain outputs; which if not accounted for can cause problems when mixing brands/models.
Definitely noticed that too. :) It is easy to see how measuring becomes crucial if you want to get everything dialed in.
 

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The only changes I made between setups were changing the preout level to achieve a 75db level at the mlp.
That explains the change in subwoofer output. Two amps (AVR's and separate's) you compaired have different sensitivity to input signal. When you up the preout to match for one amp that's less sensitive, you end up increasing the preamp signal going to subwoofer thus the difference in spl at low end.

Try the measurements with subwoofer turned off.
 

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Discussion Starter #19
Aha! I knew there had to be a technical reason that made sense. Thanks! I will try again with the subwoofers removed via the AVR software to confirm. If this is the case, I think we have nailed why the perception of boosted bass is present in my setup with the external amplifier. :cool: It may in fact be signal related.

I will try full range pink noise without subwoofers.
 

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Discussion Starter #20
I was able to take new measurements with the subs out of the equation today. The response plots are virtually identical now. It was indeed signal related. 1/12 octave smoothed, pink noise response with 5db divisions for ease of spotting differences.

AVR



Seperate Amplifier



Note to others, if you have an outboard amplifier, and use a sub crossover setting, check you are not boosting your sub frequencies with the main loudspeaker preout settings! :)

Lesson learned. :cool:
 
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