Suggestions needed for 8 ohm DIY speakers

[Torn212]

So I just ordered a Marantz SR7005 AV receiver for my HT system. I previously had the SR5003 and wanted to upgrade for both networking and the new HDMI 1.4a since I have a 3d TV and BD player.

So the issue is the SR7005 has 7.1 channels at 125 watts @ 8 ohms. Check out the specs here:

http://us.marantz.com/Products/3230.asp

I've been looking at a bunch of DIY speaker kits on Zaph, Madisound, PE, etc. But most of the designs seem to be down in the 4 ohm range. I'd like to build a 5.1 system- although the subwoofer will be powered so not so much an issue. (I'm using the surround rears as zone 2 so won't be moving to 7.1 any time soon.)

What do people recommend for DIY surround speakers (fronts, centers, rears) that would work with the SR7005?

Or should I be looking into an amplifier to compliment the receiver? (Again suggestions welcome.)

[---k---]

These were designed to be very amplifier friendly and are 8 ohm:

http://www.htguide.com/forum/showthread.php4?t=35905

The TriTrix and Aviatrix are 8 ohm friendly.

I'm sure there are other projects that I'm not familiar with.

[Undefinition]

So the issue is the SR7005 has 7.1 channels at 125 watts @ 8 ohms. Check out the specs here:

http://us.marantz.com/Products/3230.asp

That looks like a pretty beefy amp to me. If it doesn't play "4 Ohm" designs, I'll eat my hat!

addendum: I just looked at the specs. It's spec'd to 6 Ohms, which means you're safe at 4 Ohms. Why home audio amps rate at 6 amps, I don't know; but it DOES mean they will play the 4 Ohm designs.

[numberoneoppa]

That looks like a pretty beefy amp to me. If it doesn't play "4 Ohm" designs, I'll eat my hat!

Yup!

[dar47]

So I just ordered a Marantz SR7005 AV receiver for my HT system. I previously had the SR5003 and wanted to upgrade for both networking and the new HDMI 1.4a since I have a 3d TV and BD player.

So the issue is the SR7005 has 7.1 channels at 125 watts @ 8 ohms. Check out the specs here:

http://us.marantz.com/Products/3230.asp

I've been looking at a bunch of DIY speaker kits on Zaph, Madisound, PE, etc. But most of the designs seem to be down in the 4 ohm range. I'd like to build a 5.1 system- although the subwoofer will be powered so not so much an issue. (I'm using the surround rears as zone 2 so won't be moving to 7.1 any time soon.)

What do people recommend for DIY surround speakers (fronts, centers, rears) that would work with the SR7005?

Or should I be looking into an amplifier to compliment the receiver? (Again suggestions welcome.)

I have the SR7002, and had the MM8003 amp at 140watts. The SR7005 is receiver power and the MM8003 was a wimp. It went into standby at 103db with my 4 ohm wwmt's. I sold the amp and got a Emotiva XPA 5 channel. The SR7002 just does the processing and the amp is worth every penny. If you have the money use the SR7005 to run just the rears and get an amp, then build one of the very nice designs from here.

[BobEllis]

Page 5 of the owner's manual suggests minimum nominal impedances. If you use front A and Front B you should use 8 ohm speakers --> the front channels can handle a 4 ohm load. Recommended loads are 6-8 ohms. You should have no problem with most of the 4 ohm designs here if you allow plenty of room for ventilation.

[---k---]

You should have no problem with most of the 4 ohm designs here if you allow plenty of room for ventilation.

Except for maybe CJD's big RS-3ways with one.

[looneybomber]

Jed's Dynamic 4T's stay at or above 5ohms.


*edit*
I'm starting to sound like a 4T cheerleader now. Oh well, at least he's a forum sponsor.

[5th element]

Also the phase angles at the impedance minimum are quite low, easing how hard they are to drive.

[looneybomber]

Why is that?

[mischmat]

I'm driving the 4T's with Harman Kardon amp and it's handling them just fine. Quite satisfied with them. I still double check if I have my sub on or not.

[5th element]

Why is that?

If you're driving a purely resistive load with a sine wave, the peak of the sine wave corresponds with the peak in the current draw. In this way when the voltage is swinging positive the +ve side of the output stage delivers all of the current. Also when the voltage swings negative the -ve side of the output stage delivers all of the current.

However if we are looking at a reactive load this isn't the case. The peak in the current draw does not coincide with the peak of the voltage swing.

See the attached picture for a second. There are two separate graphs here. Both show an amplifier driving a sine wave, as you can see the magnitude of the sine waves is identical.

In the first graph also shown is the current draw of the negative side of the amplifier. As you can see the current maximum corresponds with the negative voltage maximum too.

The next graph shows the same thing, except now the amplifier is driving a highly reactive load. A 2 ohm resistor in series with a 20uf cap, giving an impedance of 8ohms @ 1khz with a phase angle of around 65 degrees.

As you can see how the current maximum has shifted considerably!

Lets look for a second at where I've added in the black arrow. Here the voltage swing is positive, but the current is being drawn through the negative side of the amplifier.

Say the voltage rails are at +- 40 volts and the positive voltage @ the black arrow is at +20 volts. The negative side of the amplifier has to deliver the current however, so it has to swing from -40 up to +20 at a drop of 60 volts. In the worst case scenario the negative side of the amplifier will have to deliver the current maximum to the voltage positive maximum. In other words, due to the reactive load the negative side of the amplifier has to swing 80 volts @ the required current. Rather then 40 volts at the required current into the purely resistive load.

Quite clearly driving say 2 amps at a voltage drop of 80 volts is a lot more demanding of an output stage then driving 2 amps at a voltage drop of 40 volts.

[Torn212]

Thanks everybody. SR7005 arrives on Tuesday! I'll get back to choosing a speaker design and stop fretting over whether the receiver amp will drive it.