bi-amp vs dual monoblock

Hi Folks. Decided to run with a different setup.
Managed to get my hands on a Rotel RC995 pre amp and 2 Rotel RB971 power amps.
My question is this. How do i set it up, do i bi-amp, running the bass speakers from one amp and the tweeters from the other, or do i monoblock one speaker per amp.

Advice would be gratefully appreciated.
Regards
John

 

Good question that.

Given the amps are of equal power, if you bi-amp the tweeters will get more than they need, the bass cones less so.

If you bridge them for more power, then the bass drivers will appreciate it, but you won't get quite the same seperation.

Personally would probably bridge them - or flog them both and get a bigger fatter power amp.

 

I would use one amp for each channel and use them to power the bass and treble separately.

 

what he said...

try whats called Horizontal Biamping, one amp per speaker with one channel driving the bass and one the treble.

You get advantages in channel seperation and the traditional advantages of biamping, but because your logically splitting the power supplies between speakers, each amp is capable of better dynamic swings when something big comes along. Something you wouldnt get in a traditional biamp setup.

Ive been a firm advocate of this setup for years, as im not a fan of bridging stereo amps, doing so is a compromise from the outset, and is not the ideal way to get the best sound quality.

 

The two channels of each amp also have different signals reducing the effects on the PSU and channel separation.

 

?

 

Try the normal biamping.

My experience with a Rotel RA-971mkII and RB971mkII was it made not the slightest bit of difference. Those amps do not lack for current delivery and you still have the same amp driving things and unsurprisingly it sounds the same.

I would recommend monoblocking them with short lengths of speaker cable (ie sit the amp directly behind the speaker). This will give you the potential for an actual improvement to the sound.

An even bigger improvement will be gained by using the amplifiers fully differentially. The RC995 has XLR balanced outputs - you will need to make up cables that take the two signals out of the XLR and feed out to separate phono jacks, and feed those into the left and right inputs of the RB971. Wire the speaker up as if the amplifier is bridged (ie between the two red terminals), but leave the bridging switch set to off. What you've done is moved the bridging into the preamp, and keeping a larger part of the signal chain differential will have much more significant improvements over everything else.

 

Using the XLR Outputs sounds very interesting. So would i use each RB971 as a monoblock without the monoblock jumper being on, and use 1 XLR to 1 phono into the left chanel input of one amp and 1 XLR to 1 phono into the right chanel of the other amp. i think that is what you mean. If that is the case, i have some XLR connectors and will make up a couple.
Thanks
John

 

No, that's not what I mean...

In the XLR you have three pins - Ground (pin 1), +ve signal (pin 2) and -ve signal (pin 3). Split out of a single XLR to 2 phonos, so the first phono has ground (collar) and +ve signal (pin) and connect that to the left channel of the 971. The second phono from the same XLR with ground (collar) and -ve signal, and connect that to the right channel of the 971. Then connect speaker red terminal to left channel red terminal of the 971, and speaker black terminal to the right channel red terminal of 971.

Repeat for the other amplifier and speaker.

 

Very interesting. So, if i have this right, XLR to 2 phono, ground (form xlr) to casing (the standard phono -ve) on both phono, positive (from xlr) to middle pin on one and negative (from xlr) to middle pin on the other. make up 2 oif these, one for each amp.
Regards
John

 

You've got it.

Let me explain bridging, so you can see what is going on.

The simplest form of an AC signal is like a phono connector carries - an AC signal and a fixed level (ie ground). This is what you will find on the output of many (note - not all) amplifiers at the speaker terminals - the black output being ground and the red output being the signal.

This is "Single Ended".

If you now change it so that you no longer have a fixed level, but instead you have an inverted copy of the signal as the other line, you now have a differential signal. The absolute level of the signals doesn't matter - only the difference between them does.

In amplifiers this is known as "Bridging".

If your single-ended amplifier has a voltage swing to +10V or -10V, then you can see if you use two of them bridged together you can now have +10V on one and -10V on the other - equivalent to +20V, or you can reverse that and have -10V on the first and +10V on the second which is equivalent to -20V. Thus you've doubled the voltage swing and potentially quadrupled the power (subject to current availability out of the power supply).

Flicking the bridging switch in the amp simply feeds the one input into the other channel but inverted. Doing what I suggested with the cables achieves the same thing, but earlier in the chain, which gives a greater degree of noise immunity, plus the components used to do so in the 995 are almost certainly of higher quality than those in the 971.

 

using an amp bridged in that manner (e.g. feeding one channel negative and one positive) sounds like a very dangerous idea indeed. But then having never tried it i cant really say either way.

But then i hate bridging amps, as in a bridged configuration, each amplifier section only see's half the speaker impedance... e.g. 4 ohms for an 8 ohm speaker... if your 8 ohm speaker then dips to say 4 ohms (as many do... and below... e.g. B&W) then your left running 2 or less ohms out of your amps.

The rotel are not in my opinion sufficiently well endowed to handle dynamic swings into a 2 ohm load, and very few amps are. This is why in my own opinion, bridging is a waste of time. It cannot possible keep decent grip on anything approaching a difficult load, and any perceived sound benefit from the extra power will soon be lost when the amp encounters a slightly tricky situation.

Ive tried a few amps bridged, and the loss of delicacy, fine detailing and texture to the sound was enough to put me off for life.

bridging is by definition a compromise... an amp cannot be designed to perform optimally in both stereo and mono, so if its designed to work in stereo, this is who IMHO it will sound best. Nothing ive encountered in many years of playing with various kit has persuaded me any different.

if you want monoblocks... go buy monoblocks.

 

I hate to point this out, but the Bel Canto S300 is a bridged amplifier design...

 

bridged in what way?

its two seperate class D ice modules under one box. If the modules themselves are bridged then they have been designed from the ground up to work that way. Not an add on feature thought of later

However, i do keep meaning to update the signature. I got rid of the S300 as i found it had no guts into difficult speakers, and wasnt great at volume. Thoughts that may well echo my earlier comments about bridged amps. They struggled into 4ohm dynaudios horribly and just about did ok with my focals but lost out on the low end these speakers can do.

Went back to using audiolab 8000M's for a while, which overall have a much better grip on things.

 

The modules are themselves bridged. I should point out at this point that I design class-D amps for a living...

In design terms it's not an "add on feature thought of later". It's simply the way you design bigger amplifiers. There is no difference between using an amplifier that can be used as two single ended amplifiers as a bridged amp vs a designed from the ground up bridged amp. For example, note that the bigger rotel power amps can't be bridged (eg RB 991) - because they already are. The circuit used is very similar to the smaller amps.

As for grunt, the RB971 is immensely powerful, absurdly so for what they cost. Although rated at 60Wpc, on the test bench I had over 110W (8ohm) unclipped from it. I'd seriously suggest that if you like the 8000M over the S300 then give some rotel power amps a try, they might be right up your street.

 

Cables made, about to plug them in, wish me luck. I will report back soon.

John

 

SUCCESS.

The sound out is fantastic. I am a happy man. The system sounds like something costing many times what it actually did. I made up my own XLR solid silver cable to connect the pre and 2 power as suggested and WOW!!!!!!. I cannot believe how good it is. Thanks for your help and advice Isaac, i can heartily recommend this way of connecting amps. Just as a comparison, i tried them as a bi-amp (standard) and it was good but not in the same league as connecting them like this. I also tried a monoblock setup with one amp driving each speaker via phono, again great but in a much lower league.

Many thanks

John (now very happy)

 

Im actually in the market for a big meridian amp.. the 557 in particular. I find the rotel stuff a little forward sounding for my tastes, and a smoother amp will suit the 1007's well.

the class D thing didnt work for me, well not with the S300 anyway, but ive spent a LOT of time with the ref1000s and cant really say theres much wrong with them.

 

Johnandchris - I am glad that worked out well. The next step is to place each amp behind the speaker and use very short lengths of speaker cable. Balanced interconnects (which is what you've effectively made) don't suffer from being somewhat longer, but the benefits in reducing the speaker cable length are significant.

Shrink - fair enough. I found the rotels reasonable in the treble, although it depends on the speakers used - with metal tweetered B&Ws I would agree on the forward description, and I've heard the Be tweetered focals and think they probably need very careful matching. How about a NAD S300? I still miss mine at times...

 

Oh, and I should add...

What I recommended for Johnandchris to do works for the specific rotel amplifiers he has. Don't try it at home kids.