Rotel RB991 vs Acoustic Energy 309 - fail - diyAudio
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Old 19th July 2010, 09:08 PM   #1
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Default Rotel RB991 vs Acoustic Energy 309 - fail

Hi,

I am amazed at the stupidity of my own question, but missing knowledge, even if basic is still not there!

I am very partial to my current hifi setup (when in a large room).

The Acoustic Energy 309s are large sensitive, precise speakers that faithfully reproduce whatever you throw at them.

The Rotel RB-991 has a wonderful warm sound and has oodles of power.


The problem is that the RB-991 only sounds good when the RC-995 is at least 9 o'clock.

The problem is with the 991 not the 995, when running the 991 at low power levels it is known to be a bit weak.

but at 9'oclock the volume is significant due to 93db/w speakers and a small room.


My question is:

Is there any way of making the speakers or amp less efficient, so that the amp is operating in it's comfortable range?

The car analogy is I want to make the amp run in it's power band, currently the revs are almost at a stall.


So is this a stupid question, and is it possible?
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Old 19th July 2010, 10:04 PM   #2
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I presume that when you say 9 o’clock you mean that the volume is at 75% or of its travel. If that is correct then what you’re really saying is that the RB-911 only sounds at its best when played loud. Your problem appears to be combining efficient speakers with a powerful amp resulting in a sound that you’re not completely happy with. There is no easy way to make a powerful amp less efficient without resorting to a resistor to absorb most of the output from your amp, which is analogous to driving your car with your foot on the brake. If for instance you put an 8 Ohm resistor in series then it would absorb about half the power of your amp, so it would need to be rated at over 100 watts. I’ve never heard of this being done and am extremely doubtful of the outcome; however in theory it’s possible.
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Last edited by dave w p; 19th July 2010 at 10:05 PM. Reason: mispelling
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Old 20th July 2010, 12:15 AM   #3
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I did a computer program for power output into the speaker, output transistors and the heatsink.

It turned out the hardest the amp is being driven is at around 2/3 volume.
This is the time when most power is being dissipated in the output transistors.
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Old 20th July 2010, 02:33 AM   #4
jaycee is offline jaycee  United Kingdom
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maybe try increasing the bias? I think it's only 30mA in this unit. Judging by the number of devices and the heatsinking, you could probably double that.
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Old 20th July 2010, 06:39 AM   #5
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Hi,

Sorry I should have been more clear, by 9'Oclock, it is only about 20%.

Sticking a resistor in series was about the extent on my knowledge, but I assumed that would damage the sound more than it assists.


Excuse my further ignorance, how would adjusting the bias help?

Thanks,


P
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Old 20th July 2010, 07:03 AM   #6
Mooly is offline Mooly  United Kingdom
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It's not a daft question at all. I suspect your problem is more to do with the way we hear and interpret sound... what you need is an amp that performs the magic at low levels as well as high
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Old 20th July 2010, 07:17 AM   #7
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So any ideas on how to make that magic happen?
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Old 20th July 2010, 07:17 AM   #8
Mooly is offline Mooly  United Kingdom
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Just adding a bit more... adding a resistor is no good. That wastes power, meaning the resistor would get hot, but more than that, it drastically alters the response of the speaker as the load a spaeker presented to the amp is not constant but varies wildly with frequency. For example, a nominal 8 ohm speaker may have an actual impedance of anywhere as low as 3 ohms to as high as 20 or 30 ohms depending on the frequency.

Your problem sounds familiar... I suspect you find you want to sit and enjoy some music, you sit down to listen and find it hard to really get into the performance... you probably have certain recordings that sound good... but not maybe not many, you perhaps blame the recordings... after all if one or two are good the equipment must be good... it's also very critical as to what volume level does sound good. In the end you get bored and do something else instead.

Is that way off the mark or not ?

Bias with all due respect to Jaycee, I suspect that increasing the bias will be more a case of wishful thinking, in that it has changed the sound significantly.
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Old 20th July 2010, 07:28 AM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by inteificio View Post
So any ideas on how to make that magic happen?
Not I suspect without changing the amp unfortunately.
It might be worth you looking at some of Hugh Deans offerings on here, look under Aspen amplifiers and his AKSA designs.

Or would you go down the DIY route perhaps.
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Old 20th July 2010, 12:57 PM   #10
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beefing up the bias will help but this will be marginal and also will produce some extra heat for sure less than this """100W """ resistor

then gain a passive input and probably a small change in the gain of the amplifier will also might help ...

i wonder what type of drivers exist inside the amp ( will look for a schematic ) often if the amp is designed properly but gets dirty very easy changing to faster drivers will also have effect on your problem

doing all three of the above will alter the amplifier totally i think
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