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Amp for RF-7 stable down to 2 ohms


dbossa

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Hi guys,

With the 2.7 ohm dip of the RF-7, I figure I'd be best getting an amp that is stable down to 2 ohms. Can any of you point me in the right direction? I was figuring something with at least 200 watts per channel at 8 ohms. Probably a 2 channel amp but I might be willing to go as high as a 5 channel amp since I'm doing 5.1.. or maybe just add a 3 channel amp later on. Anyway, some makes and models would be really appreciated!

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You'll find that most people will recomend what they own (myself included). You shouldn't have trouble finding a 200 wpc amp that will drive the RF-7's. It's the lower end stuff that might struggle with their impedance dip.

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I use the Monster Power SS MPA 2250 (see signature) to drive my RF-7s. This is an amazing amp in my opinion. It is rated at 250wpc continuous into 8ohms but is capable of delivering significantly more when needed. These amps (as I understand it) are really two completely separate amplifiers (one for each channel). Where each amplifier has it's own huge tordial (sp) transformer and capacitors (to provide power for short spike requirements). This means that the power available to one side is not depleted when the other side is demanding a ton of current (which is what often happens in lessor amps). This also explains why these things weigh so much (>85 lbs). As you can see from my signature I also use the 3 channel version (the MPA 3250) in my home theater system. This one weighs in at over one hundred pounds.

The manufactures description of this amp can be found here http://www.monstercable.com/productdisplay.asp?pin=1902. I have used this amp for a couple of years and I can tell you it really brought my RF-7s to life. Before adding these amps to my system, I just used the 125wpc provided by my H/K AVR. This sounded fine but my wife is a bit of a metal head and want her metallica LOUD. When she would listen with the H/K I knew she was clipping constantly which is really hard on my speakers. I bought these Monster amps more for protection (no more clipping) than anything else. But WOW what a difference they made. They drive the RF-7s to unbelievable volumes effortlessly and with a clarity which is constantly commented on by visitors to my home. The amount of bass your RF-7s will deliver when given all the power they want is nothing short of amazing.

As it happens I recently listed two of these amps (brand new still in the original boxes) on the garage sale forum (http://forums.klipsch.com/forums/t/119071.aspx). I also had them listed on Audiogon. Both amps sold but one of the buyers never paid so I still have one for sale. It is listed on Ebay (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MESELX:IT&item=230342162848).

If you are interested drop me a PM.

Steve

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I can't say I fully understand all of the technicalities... but read
that even if you have an amp with very high WPC it doesn't necessarily
mean that it is stable down to the low ohms like that. I read that the
Klipsch speakers can really put a load on an amp that isn't stable that
low. I originally wanted to get an Emotiva XPA-5 or a Rotel RMB-1095,
which I'm sure that either could easily do the job. It's just when I
started reading about the ohms thing, and neither amp mentions anything
about less than 8 ohms, I wondered if I shouldn't keep that as a
variable when choosing my amp....

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I also do not claim to fully understand the technical stuff. However, what my research has made clear (to me anyway) is that as the impedence drops the power (wattage) must increase proportionately. To me this means that if your speakers, under normal conditions, had an impedence of 8ohm and draw 2 watts of power but at some frequencies dipped to 4 ohms then your amp, in order to maintain the signal and not clip, must (at those dips) be capable of providing 4 watts of power. Also if you checked out the manufactures description (from the link I posted) for these monster amps you will see that they show power ratings for many different configurations (bridging, impedence differences, etc.). I know they list a 4ohm power rating but I do not know if they list a rating all the way down to 2 ohms.

On a separate but related note, these speakers (the RF-7s) are amazingly efficient (IIRC 102db/watt/meter). This means that they do not require much power to play REALLY LOUD. That said I have seen mine pull 350 watts each (peak) when my wife is "In The Zone" listening to Ozzy, Metallica or whatever. That 350 watt reading is according to the digital displays on the amps, which according to the manufacturer are labratory quality meters showing the actual realtime power being supplied.

Steve

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That dip in impedance with the RF-7 is insignificant -- I used to run my 7s with a 15 wpc tube amp. I ran them with about a dozen different amps over the years and never had a problem.

I do not want to highjack dbossa's thread but I have heard this from many both here on this forum and elsewhere. I have always wondered how this can be. As I say I see my wife regularly push mine to > 300wpc. How can a low power amp possibly compare (at least in volume). I understand that the db (sound pressure level aka volume) has a logarythmic (sp) relationship to the power (3db of spl increase requires double the power) but still 300wpc should result in > 12db increase over 15wpc.

Can anyone explain this?

Steve

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I'm running RF-5's, the little brother to the RF-7's, on a 2WPC single-ended triode and have no problems. They ran fine on the 8 ohm taps but sound a bit more balanced (less boomy, smoother) now that I re-wired the amp to provide 4 ohm outputs. I would assume that the RF-7's would be rather similar in terms of impedance dips. With a sand amp, I can't imagine that you would run into too much trouble, esp. if it can put out 100WPC or more.

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Best results to date with my RF-7's......I run a 500 watt pro-amp with lotsa balls to the LF's and a vintage 30 watt receiver to the HF's. Don't let the 30 watts fool ya because it cranks.

Gain controls on both amps enables me to blend it just right.

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There are two schools of thought here. One says that my speakers are so
sensitive that I could drive them with a 2 watt tube amp and they
would be fine. However, I've also read tons of posts from people who used to drive them with a receiver (like me),
upgraded to a high current amp, and that the results were nothing short
of spectacular.... that they simply could not believe the difference it
made. Currently, I find the sound from my RF-7's to be 'OK'. I just
sent in my crossovers to DeanG. The next step will be to start using an amp. I just don't
want to spend money on one only to realize that another model would
have been much better suited for my system.

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Best results to date with my RF-7's......I run a 500 watt pro-amp with lotsa balls to the LF's and a vintage 30 watt receiver to the HF's. Don't let the 30 watts fool ya because it cranks.

Gain controls on both amps enables me to blend it just right.

I found an excellent amp by Crown, the xTi 2000 with is 475 watt at 8 ohms, 800 watts at 4 ohms, and 1000 watts at 2 ohms and the amp is reasonably inexpensive for what you're getting. Only thing is that it is a pro amp which means noisy fans. Other than the fan issue I'm wondering if there is any other difference between a pro amp and an HT/audio amp.....

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Only thing is that it is a pro amp which means noisy fans.

The 2000 is the amp I use on the LF's. The fan is not an issue. They very rarely ever turn on.......only breifly when you're really cranking it.
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My personal experience with the Klipsch speakers I've owned, especially the RF-7's is that they don't need a lot of power to go LOUD.......but they sound better with more power and more importantly......the right power.

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Ah! Good to know! Did you used to run just this amp at one point for the whole speaker and add the tube amp later on? Did you really see that much of a difference with the tube amp?

The 30 watt amp I run on the highs is not a tube. I wish it was. It's a Harmon Kardon 430 vintage receiver that I paid $100.00 for. Some say it has "tube qualities" but I don't hear it.

After all the amps etc. I've experimented with RF-7's.......It seems that a lot of power to the bass and minimal "good" power to the highs with the ability to adjust the gain controls on both just works. Some CF owners have claimed the same thing.

And by the way.....yes, I did run the Crown on the whole speaker and the HF's were terrible. Does a great job on the bass, though.

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I originaly ran my RF-7s off my H/K AVR 7300 receiver. Now this receiver is no slouch with a respectable 125wpc with all channels driven. However, when I added these amps to the system the difference was phenominal. The bass and mid bass was much tighter and more controlled, it was just obvious that the RF-7s loved having the extra power when they wanted it.

Bottom line, the RF-7s are great speakers and sound great on very little power. However you can never have to much "GOOD CLEAN" power. The more power (headroom) these speakers have the better they sound...at least that is my experience.

Steve

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Ah! Good to know! Did you used to run just this amp at one point for the whole speaker and add the tube amp later on? Did you really see that much of a difference with the tube amp?

The 30 watt amp I run on the highs is not a tube. I wish it was. It's a Harmon Kardon 430 vintage receiver that I paid $100.00 for. Some say it has "tube qualities" but I don't hear it.

After all the amps etc. I've experimented with RF-7's.......It seems that a lot of power to the bass and minimal "good" power to the highs with the ability to adjust the gain controls on both just works. Some CF owners have claimed the same thing.

And by the way.....yes, I did run the Crown on the whole speaker and the HF's were terrible. Does a great job on the bass, though.

Eeesh.... I was hoping to get just the one amp that would power the whole speaker... and not sound like crap [*-)] For now I'd probably go something like an Emotiva XPA-5 or a Rotel RMB-1095.... unless someone can prove to me that isn't the way to go...

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