Another question on bi-amping

Hi,

Does anyone know about biamping.
I am currently biamping my speakers using a rotel RB-991 and a RB-993.
The amps are VERY similar but not identicle.

The only major difference between them is that the 993 has a lower dampening factor (280 as opposed to 500+ ) and a s/n ratio of 118 as opposed to 130 db.

They are 3 way speakers. one amp takes the tweeter and woofer and the other one the other woofer/sub.

I am assuming that the better amp (the 991 is better on paper) should be used on the high frequency and the other on the bass? but then again this is a poorly educated guess.

Does anyone have any suggestions?

p.s. for anyone who knows about rotel amps will know that the 993 is a 3 channel amp, that 3rd channel is used to power my centre channel for 5.1.

 

When you say sub are you talking about the largest bass driver on your speaker or a seperate subwoofer? If the latter is it a passive one or an active one connected via high level connection? Just trying to get my head around your connections.

The best answer to your query though is probably for you just to try the different alternatives and use the one you like best. If your current arrangement is to have the better spec amp driving your tweeter and mid driver, whilst a second drives the bass driver and possibly a seperate sub, then that would probably be my guess at the best way round but I would have a play to find out.

What are your speakers?

Edit - Oh, and you could try just using the better amp to drive everything rather than passive (I assume) biamping at all. Nothing lost having a listen to see.

 

Hi Peter,

Personally, I don't like biamping, because it tends to separate the sound at the speaker crossover point. This certainly opens up the music giving more detail and presence but at the expense of synergy, coherence and timing. This is a personal thing, and you may find that you prefer the gains to the losses.

If your amps are not exactly the same, it is likely to exagerate the effect. The one with the better (higher) damping factor is the one that should work best with the bass, but try it both ways round and see which you prefer.

Best Regards

Dave

 

My speakers are Acoustic Energy 309s

http://www.acoustic-energy.co.uk/products/aesprit/aespritloudspeakers.html

Unless anyone can provide a reason against I will setup the 991 to cover the mid/high. The dampening factor will come in handy for controlling the manically moving mid cone. the quality of bass i find to be less important.

On the advice of a friend i did buy a sub, unfortunately it added very little bass and disharmonised the sound. thankfully as i bought it online, to the seller discontent i used my 7-day cooling of and sent it back.

In the answer to just buying one better amp. Well, if you can find me a single amp that sounds better for less than £8k then I would be happy to give it a listen, but i have heard alot but nothing comes close at the price, even at 8x the price (2nd hand £700 for the both.

 

Peter,

Try the different combinations and let us know what you think!

Dave

 

ok done (turned one of the binding posts upside down without realising it, teaches me right for using a wrench to tighten them).

ok i have tried today:
1: 991 takes bass, 993 treble.
2: 991 takes treble, 993 bass.

in truth on a blindfold test sitting on my sofa i doubt i could tell the difference betwen them. Though the sound is a lot better than non bi-amped.
If i sit at my computer desk (positioned directly between the speakers) the sound now sounds really hollow.

Bazarly enough i think the best sound in total was when i had the amps crossed. That being having the bass of one speaker and the treble of another being wired through each amp. That is how i set it up first time when i wasnt looking to hard at where stuff got plugged in.

strange.

 

try 991 on left, and 993 on the right as well. I'd be interested to know your thoughts.

 

Linn people do this a lot and the conventional wisdom is to put the better quality on the top. Crossed could well work, or one amp per side too. The most important thing to get right is the gain of the amp, in dB, i.e. ratio of output to input. Linn's are about 28.5 dB so one can mix in the same family without gain problems at least. Some pple think all the amps should be the same, others go as far as for example a tube amp for the mid. Note that gain is not the same as watts, for the same reason as 10 to 1 is the same as 20:2

 

Good points....I've tried a fair bit of this with previous parts of my system,and used Krells(KMA and KSAs) for the bass and mid of my Keltik system,with tubed amps for the treble,with excellent results,but it's not something I'd suggest for all!

I do think that reserving the most powerful amp for the bass is worthwhile,especially if the bass and mid/treble amps are of similar quality,but as has been tried here,the simplest way is to just try a few combinations and see which works best.

 

Assuming Rotel's damping factor figures to be accurate (and from bench testing I have done on my own rotel amp, I can assure you they're not) then I personally would prefer the one with the higher damping factor on the bass. However, since in reality both will have around 1000 damping factor into 8 ohms at low frequency, I wouldn't worry too much about it.

Given that rotel's design methodology is very much evolutionary rather than revolutionary, the amp circuits in the two amps will be extremely similar indeed. The main difference will be power supply headroom, which will be greater in the 991 than the 993. For that reason I'd put the 991 on the more current hungry application (the low bass driver).

But really just try things and see what you think. If you can't hear any difference (and I wouldn't be surprised) then I wouldn't worry about it.

 

I can't find the info on whether the amps are more powerful than each other. I would expect the amps to be very similar; the 993 is not the evolution of the 991 just the 3 channel version. They were designed to be matched together in a surround sound setting, I assume they would be fine together as they were designed for it.
I am unwilling to try one amp on the left and one on the right as I cannot see that affecting the sound in a positive function (if the amps are similar it would sound the same, if different it would scew the stereo imaging). not to mention the wiring is a hastle.

I always assumed that the midrange cone would require the most powerful amp as that is the one that is being trhow about the most (movment visably is about 3 times as much).

I', exhausted here so kinda rambling, I will see if i can think of something more coherent to say later.

bye

 

Both the same @ 200W/channel into 8 Ohms 20-20kHz for 0.03% THD:

http://www.cinenow.com/pdf/rotel_rb993-991thx_bro1_us.pdf

Really? You can visually see the mid driver moving more? I'm a bit surprised but I guess it depends on the particular implementation of the speaker? Any idea what the lower cross-over is?

 

Peter - the point is that all rotel amp circuits are very closely related, from the bottom of the range to the top of the range. The circuits in the 991 and 993 will be identical schematically and in terms of components, but the 993 will have less capable psus per channel.

The impedance characteristic of a speaker is typically most unfriendly (ie lowest impedance) at lower frequencies... it's not a smooth decline, but one with peaks and troughs. They can be as high as 16-20 ohms and as low as 3 ohms for an 8 ohm nominal speaker. The mid/treble region will usually be relatively free from deep troughs, but these will occur in the bass region.

 

that's a pity that you're prepared to dismiss it out of hand without trying it. I do think that it makes a huge difference to the soundstage. basically it gets both channels amplification on separate power supplies. I don't see any benefit in bi-amping with one amp doing treble and one bass. the real benefit to my ears is to get the two channels electrically as far apart as possible.

I'm not suggesting that you should keep it like that, only as an experiment to see if you could hear a difference. if not fine, but as you were looking for ideas about bi-amping, my experience has shown me that this is the way to go. I'm talking a major improvement.

 

Kennyk does have a point.

Interestingly, the NAD S300 (which has separate transformers per channel) has a very wide soundstage, whereas my rotel (RA971) has a less wide soundstage, but a more solid centre image. The crosstalk in the rotel acts as a crossfeed centralising the sound more.

 

Ok I finally listened to your advice. crawled behind my hifi case unscrewed the binding posts and found a problem. I cut all my cables to the exact lengths. They wont reach the other side of the amp =-(
Two on top is OK but there isn't enough play to move to the other side of the amp.
This blind test will be done with one speaker!

 

I am afraid i do not know what the crossover on my speakers is.
I use Acoustic Energy 309s.
I have only seen one other speaker with the mid and bass cones being the same size. On those speakers the mid cone was the most mobile aswell. (they would be the infamous Willison Benesch ACTIIs that I melted the voice coil on the mid cone).

So taking into account the movment would the more dampened amp still be reserved for the bass?

 

Well I gave it a go. The cables are to short to be of any use. I have longer but cheaper speaker but this is the wire I bought for the job! not cheap either (black rhodium s300x2)

Expensive wire that i got for free by asking black rhodiums boss nicely (and tradign in some wire i didnt use)