Amp match

Sabby

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I have the opportunity of obtaining a pair of old Rotel RB960BX power amps. I would like to run them in bridged mode, but the manual states that the speaker impedance should never be less that 8 ohms when run in this mode. My Monitor Audio Silver S6's have a nominal impedance of 6 ohms. Would I cause any damage to either the speakers or the amps in running them in this configuration?
 

Dougal1331

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They would be absolutely fine. They may throw out a bit more heat as a result, and as long as you don't max the volume the amps should never run out of steam. The current trend of 6ohm speakers is relatively recent, and quite common with 2.5-way designs- not so long ago they were nearly all either 8ohm or 4ohm.

I imagine that when the amps were made, the designers simply recommended against 4ohm speakers, which ended up as 'not less than 8ohms' in the manual. There will be a good margin for error!

I'd buy them if I were you!
 
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Anonymous

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when you are running the amps in bridged mode, you are increasing the maximum power output of the amplification circuitry, but you are effectively halving its current handling capability - per watt.

When you try and draw more current out of an amplifier than it is capable of providing the voltage output to the speakers will drop. Being as peak power is consumed when your signal is at a high peak or a low trough this will manifest itself as horrible distortion in an excessive case but a more reasonable case you will just start to loose control of your bass cones as they require the most power to move. Your amp might throw the cone forward, then not have enough power to stop it moving forward so much and 'overthrow' the cone. I don't mean overthrow to damage, just to distort your sound. You'd end up with horribly muddy mid/bass if driving your amplifier to its current handling limits.

That said, a speakers impedance is actually a speakers 'nominal impedance at 1000hz' which is precisely it's electrical resistance to a 1Khz signal. However, impedance itself is not a static figure and is highly frequency dependant, particularly in speakers. A few nice graphs of various types of speakers here: http://www.churchsoundcheck.com/imp1.html so you see what I mean. I really wish all manufacturers would provide graphs like this for impedance, response and phase shift btw.

I would be prepared to wager that your manual states not to ever connect smaller than 8 ohms impedance speakers because the manufacturer was wary that they would not sound as good due to the extra current requirements. Thing is this will probably only manifest itself at volumes you're not going to be using anyway.

Bottom line is though, you are NOT going to break your amplifier or speakers by rigging the two together. The very worst that could happen is the amplifier go into thermal shutdown (let it cool off and try again) or blow an easily replaceable fuse... but I give that a 1 in 1000 chance of happening.

Plug it in and have a go, feed something you could call reference music through and try it at different volumes. If the amplifier starts to run out of juice before your ears run out of hearing ability or your speakers start to overdrive then it should be pretty apparant. Sure you could continue to run your amp at those volumes if your ears can take it and if you're happy with listening to a struggling amplifier....

My best guess is that they'll hook up fine and be totally ok for your listening though.
 

Sabby

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Thank you to Dougal1331 and Mr_Poletski for your response to my query. The detailed and analytical response from Mr_Poletski was very enlightening. I will certainly never turn up the volume on the amps to ear splitting levels and can therefore assume that this setup will work well. The amp is rated at 60 watts per cahnnel at 8 ohms and 180 watts into 8 ohms in bridged mode. Am I correct in assuming then that the amps would drive the speakers much better in bridged mode as opposed to one amp driving both speakers in normal mode.
 

Dougal1331

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Within the limits mentioned by Mr Poletski, the extra power would come across as more thump and control at moderate to high levels, but in general, at lower volumes, 60W will be perfectly adequate, and you'd probably not notice the difference!
 
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Anonymous

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HI Dougal, Talking of the 960bx power amp, i have one myself with the matching pre-amp. For some time now i have been trying to acquire another 960 power amp to bridge my set up. My speakers, Mission 753 freedoms, great!, but i want more punch & control. I know of a 970bx power i could get my hands on, but even though the tech spec is the same, would it work with the 960bx.

As for bi-amping what gains would i achive by going down that road? Would this set up give me more punch / control?

I am seriously condidering the new Roksan Kandy partnered with a cyrus cd6se. I must admit i am unsure of what to do for the best to taylor my sound to be punchier & warmer that at present. I would greatly appreciate your or anyones views on this.

Thanks.

Krezz.
 
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Anonymous

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Hi Mr Poletski, Only recently joined the forum, just reading your reply on the Amp match thread with regard to Rotel power amps etc. Your knowledge has already answered a couple of my questions. I have posted a message this morning 15/9/09, i do hope your goodself & Dougal may be able to help me with my queries.

Many thanks.

Krezz.
 
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Anonymous

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Hi Sabby, Have you acquired the two 960bx power amps yet? I have just missed out on one for sale on E bay due to my account being locked up. Well naffed off! as i have been trying to get another myself to do the same as you. I have posted a question to others on the site with regard to Rotels 970bx power amp, but so far no replies. If your are now sorted with yours i really would like to know what your outcome is. I have had my Rotel pre & power for a long time and i still think they can give more modern amps a run for their money, Happy listening,hope to hear from you.

Cheers.

Krezz.
 

Sabby

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Hi Krezz, unforftunately not able to obtain a second Rotel 960bx, extremely difficult, especially here in South Africa. I sold the 960bx and managed to get a 980bx which is rated at twice the power of the 960bx. Very good amp for what I paid and it was in mint condition. Sound is now much better, especially in the bass which now has greater authority and slam. I hope you get your second 960bx as it is also a very good amp and as you mentioned stands up well to newer amps.
 
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Anonymous

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Hello again Sabby, The 980bx, well there's something i could also consider. Pleased you are now sorted & happier with your sound. More slam & authority in the bass, that's also what i am after. Just as a matter of interest are you in vinyl / c.d or both?

Happy listening.

Thanks for the reply.

Krezz.
 

Sabby

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Hi Krezz. Yes the 980BX if one of Rotel's classic power amps from the 90's. I am not yet into vinyl but am eager to give it a try as soon as I find a suitable deck. I would then also have to get a phone preamp as my amp doesn't have one.
 

steve390

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Just a quick question as I've never used amp as a monoblock
I have a Rotel 960 and am having problems finding another one would it be OK to use along side a 970?
Thanks Steve
 

Sirolway

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I had one of these Rotel RB power amps new in the early 1990's and loved it - it produced a far better sound than it should for the price. I remember I auditioned the 960BX and one of the higher ones - either the 970 or 980. I had been intending to buy the bottom of the range (960) but the higher level ones were simply much better, so I bought one of the better ones ... but I can't for the life of me remember which of the better ones I bought - the RB-970BX, I would guess ..

So you might want to try to get hold of a 970, rather than a 960, if you possibly can
 

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