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Discussion Starter · #1 ·
Story:

I have Polk LSi's all the way around. Right now they are driven off of my Kenwood VR-6070 rated at 100wX6 at 8ohms. The LSi's are rated for 20-250w at 4ohms. Right now the speakers sound good in my opinion, but my receiver does not have the high 'wattage' needed for those Polks.


Dilema:

I have a chance to get an ADCOM 7400 5 channel amp for $500. This would drive 125(I think)w at 4ohms to each channel.


So.. is it worth it? I can afford it, but that is a lot to dump on something I am not sure will help my system.


Also does anyone know how the adcom amps 'know' to drive at 4ohms?


Thanks for your time.. :)
 

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ost, I don't know much about your Kenwood receiver, but if it's like most other receivers out on the market its 100w/ch rating is way over rated and was measured a 100w/ch in a certain set-up enviroment. Your probably lucky if the Kenwood really puts out 1/2 that with all channels driven. There was a site that has these real power ratings but I've since forgotten, I'm sure someone else here knows. As for the Adcom 7400, being a true power amp, the 100w/ch @8ohms and 150w/ch @ 4ohms, are probably true to their rating, and yes you will notice an appreciable difference in sound and power. As for the $500, thats a great deal, and a definite steal. Go for it.
 

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Opinion number two: Stepping up from 100 watts to 150 is a small difference, even going from a receiver to a separate amp. If your speakers are really starving for more power, you should at least double power to make spending the money worthwhile. Anything less is barely audible.


As for amps "knowing" to drive a particular impedance, an amp puts out a given voltage for a given input voltage, the ratio being the 'gain' of the amp. The resulting power of this output voltage is a function of the speaker's impedance, or resistance, to current flow.


As long as the amp's power supply can handle the current demand, power will increase with decreasing impedance, because the reduction of resistance allows a given voltage to push more current, and power (P, in watts) is equal to EMF (E, in volts) times current (I, in amperes).


P = E x I
 

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Discussion Starter · #4 ·
Hmm. That is why I love forums, helpful people to educate.


My Kenwood spits out 100w at 8ohms, while my speakers impedance is 4ohms. Seeing that my Kenwood is not at 4ohms I am under the impression that I would need an amp that could drive at 4ohms.


Adcom 7400:

Watts per channel into 8 ohms: 100

Watts per channel into 4 ohms: 150


Kenwood VR-6070

Watts per channel into 8 ohms: 100

Watts per channel into 4 ohms: ??? (there is no switch to change to 4ohms)


I am under the impression in order to get the most out of my speakers I would need a high watt 4ohm amp. If going from 100w at 8ohm is not that different than 150w at 4ohm, then I should not buy the amp..... Or do the watts increase from the Kenwood due to the lower impedance of the speakers? (following Watt's law)


Or am I confused over the ohms.


Again, thank you for your time.
 

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Quote:
ost, I don't know much about your Kenwood receiver, but if it's like most other receivers out on the market its 100w/ch rating is way over rated and was measured a 100w/ch in a certain set-up enviroment. Your probably lucky if the Kenwood really puts out 1/2 that with all channels driven.
While I don't own a Kenwood receiver, I do recall a review that I read in Sound and Vision. The Kenwood was rated at 100 watts per/channel -- and while tested, it put out 92 watts to 5 channels. Certainly quite a bit better than the "half" that others might put out.
 

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If you crank it up and it begs for more power than I'd say yes.

Something else to consider.

I dealt with my Onkyo receiver for a while. I could crank it up loud and it was still great. It was only 65 watts * 5. I got into separates for the fact they are separates. My amp is 125 watts * 6. My electric bill went up about 5-10 bucks a month. It also puts out more heat. In the summer this sucks.

The sound rules! Plus if you enjoy this hobby, then it's a good start on a path of upgrades and decisions.


Doug
 

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ost - a simple suggestion for you. Hook up the ADCOM amp to your system and have a listen.;)


If this is not possible - go to your local audio retailer and borrow - or buy a good amp with a 30 day return policy. Again - have a listen.


My guess is that you may notice a small difference only when pushing your amp at very high levels on demanding soundtracks. I'd like to hear back from you after you demo and make a decision. Good luck.
 

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Discussion Starter · #10 ·
Thank you everyone. I think Mit07 has great advice. I will try to take it home and try it out. If my ears don't think the amp is worth the 500 plus electricity, then no go.


I also didn't realize that I would need to double my gain to be worth it.


Thanks!
 

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wje, I am curious to see the Sound & Vision the Kenwood Receiver was featured in, do you know the particular issue? I quoted the "half" rating because many if not most receiver manufacturers way over rate their power ratings and magazines like S&V and Home Theater Mag, etc.,etc.,etc., have found this to be true, not all receivers but a good amount of them. Sorry if I mislead you OST!!!
 

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Discussion Starter · #12 ·
No misleading was done. I know the Kenwood as an amp is not all that great. It does not have discrete amps or anything else notable besides a 100x6 sticker on the front.. :)


I too was intersted to see that there was a test done on that particular model. I know it was very popular due to cheap THX...
 

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OST, receivers with an impedance switch generally have low-current power supplies; the switch lowers the voltage to reduce overheating the output transistors when pushing low-impedance loads. You don't have to use the switch if you're careful with the volume control and monitor your receiver's temperature.


Any amplifier can be used with any speaker. What happens if you use a lower-impedance speaker is that, at the same volume setting, the amplifier will attempt to provide more current, which results in a greater power output. The limiting factor is the amplifire's power supply and output stage.


Note that speaker impedance ratings are averages; impedance varies all over the place at different frequencies, often as much as twice or half the rating. This is normal, since speakers have both capacitance and inductance factors, collectively known as reactance.


Impedance is a combination of resistance and reactance. Only a pure resistor has pure resistance. Speakers are given a simple number just for reference; some are rated as 'compatible with amps rated for X ohms', or something similar. Actual impedances are complex curves, except for planar speakers.


When an amplifier is rated, it means that it is capable of supplying X watts into an impedance of X ohms, the power being a product of voltage output and load impedance (think of impedance as AC resistance). In theory, any amplifier is capable of supplying twice the power into half the impedance.


Ohm's Law states that 1 volt will push 1 ampere through 1 ohm. If you halve the resistance, the current will (should) double. Watt's Law says that twice the current at the same voltage will produce twice the power. That depends on limitless power reserves.


If you want to find an amp that works well with 4-ohm speakers, find one that can provide twice the power into 4 ohms as into 8 ohms; most can't, because the power supply can't handle the load, or the transistors can't handle the heat.


Sunfire amps will double power into half the load until the supplying circuit-breaker trips. That's one reason I like them so much. I have two; a stereo for my mains and a 5-channel for the center, sides, and rears.
 
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