Speaker impedance

Donovan Puppyfucker

It's embarrassing for me to admit this, but I have gotten thoroughly confused.

If I had a stereo amplifier rated to drive 100 watts per channel at 8 ohms, and wired two sets of 8 ohm speakers in parallel to give 4 ohms, that is going to be a LOWER load on the amp, correct?

Then why is a manufacturer saying this:

Dynamic Power / Channel (8 / 6 / 4 / 2) ohms = 140 / 170 / 220 / 290 W

??

Doesn't reducing the resistance you push an amperage through also reduce the wattage?

I think it has been so long since I've had to do anything with Ohm's Law that I am completely mis-applying it.

Nova_C

Nope. Half the resistance means double the wattage. (Okay, this is purely off the specs for my brand new car stereo. The amp is rated 150 watts at 4 ohms and 300 watts at 2 ohms)

I think it's bad for an amp to drive a speaker that has a lower impedance than it's rated for, though that might be bad info - I was told that by a salesman and I've never verified it.

zerzhul

the simple answer is to look back at ohm's law.

current = volts/resistance

wattage = volts * current

voltage staying the same, resistance goes down, current goes up, wattage goes up.

As for what your amplifier can do, well I have no clue, but that's what the math says to sort of prove that out.

Donovan Puppyfucker

Thanks guys!

It's not my amplifier, I'm looking at things to get my Dad set up with a record player, amp, and speakers next year. Just sourcing prices and parts for the moment.

bsjezz

i'm no expert on this stuff but if you're worried the parallel speaker sets will be too much of a drain, you can get something like this (which i got mega cheap from seconds world) which has two separate amplifier channels at 100w each. also it has a phono input. i can vouch for sherwood stuff, i was so impressed with the turntable that i ended up replacing my amp, too. stereo 4 lyfe.

Donovan Puppyfucker

There's no link to a device in your post? The amp I was looking at is this:

http://www.amazon.com/gp/product/B0044779G8/

bsjezz

whoops. here it is. it's probably a tier below that yamaha one, but if your purpose is a simple phono setup with extra inputs and some options for multiple channels, it's good.

Donovan Puppyfucker

That looks like a neat unit, but it's not quite the kind of thing I'm going for. They get the 100w for stereo from bridging the 4 mosfet power circuits into 2 channels, which if I then try and overdrive by hooking up two pairs of speakers to each of the two channels, will probably have heat problems because they're already being overdriven from the bridging. It looks like a really good value for money unit, but the signal-noise figures and THD aren't quite at the level I'm looking for, because once you start messing about with amps things go downhill quickly, so you want to start from the best position you can. I'm looking to replace Dad's old HMV stereo setup he bought back in the 70s and used all the way up to the 90s, which was a really nice piece of gear with a Rega RP3, two pairs of SSF-7000 speakers (one for each corner of his room), and an amp to go between them.

BlindZenDriver

Since you have time I highly suggest looking for 2nd hand equipment. Lots of people have moved away from stereo and instead gone surround sound and/or picked something that interfaces with their phone, so there are loads of good deals to be made.

On driving two pairs of speakers then the best really to get an amp that offers that or by going with two separate amps, however the more powerful amps should be able to pull it off even when not specified for it. As a rule of thumb I'd say look for one that follows ohm's law rather than dropping off like the one you mention. Also I'd be careful with gear that uses terms like "dynamic power" or "Peak Music Power Output", but there is no rule without exceptions so google what gear you find.

Donovan Puppyfucker

That's why I've got my eye on that Yamaha, but I just don't know for sure which is annoying.

Second-hand stuff isn't really an option, unfortunately. I live in a city with a small population and not a lot of stores that carry that kind of stuff, and a shitload of vinyl nerds that have time and money that I don't.

BlindZenDriver

chrishallett83 wrote: »

That's why I've got my eye on that Yamaha, but I just don't know for sure which is annoying.

The Yamaha looks good to me. It clearly has support for two sets of speakers so I think it should good to go as long as you stay away from exotic speakers with challenging characteristics.

For comfort I found the manual for the Yamaha, here is a link: manualslib.com/manual/352345/Yamaha-R-S700bl.html#manual

chrishallett83 wrote: »

That's why I've got my eye on that Yamaha, but I just don't know for sure Second-hand stuff isn't really an option, unfortunately. I live in a city with a small population and not a lot of stores that carry that kind of stuff, and a shitload of vinyl nerds that have time and money that I don't.

My thinking with the 2nd hand thing was to pick something up at a garage sales or similar. Because most people have move away from stereo and vinyl there are some nice finds to be made, but that does require some luck and time and the Yamaha seems like a viable option to me.

One thought. Is there a chance that the setup of your fathers was really a quadrophonic setup!? If that's what you're trying to recreate then I suspect you main challenge will be the source material ie. vinyl in CD-4/Quadradisc recorded format and that going for a modern surround system will be a lot easier. Or maybe the way is simply to give up on the 4 speaker thing and go with a conventional stereo setup.

I had never even hear of the Quadro thing until I came across the inner sleeve of an old Carly Simon album, but here is some info: en.wikipedia.org/wiki/Quadraphonic_sound#CD-4_.28Compatible_Discrete_4.29_.2F_Quadradisc
Apparently one can get 4 way sound from an vinyl disc if one hooks up the four speakers in a special way and the disc has also been produced in a certain way.

Donovan Puppyfucker

Nah, he didn't have a quadrophonic setup. It's like stereo, but with two more channels.

The manual for the Yammy seems to indicate that running both sets of speakers at once will be fine because that ends up working out to a 4 Ohm load per channel, and they suggest that the amp will drive approximately 135 watts RMS per channel, which should be fine for the speakers I've picked as they have a recommended power handling of 200 watts RMS.