Speaker impedence

[Sammich]

If you have a stereo that is only built to handle 8 ohm speakers, can you wire two 4 ohm speakers in series and use them on one bridged channel provided the stereo has enough power to drive them like that?

[Cal Weldon]

Hi Sammich and welcome to the forums.

Yes you can use the two in series to attain the 8 ohms but are you asking if it's OK to run them on only one channel or bridging them? If you are able to bridge them, you will require a 16 ohm load.

[Sammich]

Well, I have a couple of 4 ohm subs that I want to wire to an older amp that is bridgeable.

I was wondering if I could wire both subs in series to the bridged channel without hurting anything.

[Cal Weldon]

I would say no. I think you need to give the amp 16 ohms in bridged mode.

[AndrewT]

Hi,
stop a moment and think.
Take a hypothetical 100W + 100W into 8ohm amplifier that is bridgable.
It will give 200W into 16ohm, if it is designed properly.

Now look at what you have, a pair of 4ohm speakers, in single mode they have too low an impedance and in bridgable mode the series pair have too low an impedance.

Let's take your thought processing a bit further.
Assume you have two 8ohm speakers.
The stereo amp will drive them to 100W + 100W. The total power delivered to the speakers is 200W.
Wire the amp in bridge mode. Wire the speakers in series.
Now when you drive the series pair in bridge mode you find the amplifier delivers 200W to them. You have gained (and lost) exactly nothing, except your time.

Don't wire your 4ohm drivers into an 8ohm amplifier.

[Sammich]

Originally posted by AndrewT
Hi,
stop a moment and think.
Take a hypothetical 100W + 100W into 8ohm amplifier that is bridgable.
It will give 200W into 16ohm, if it is designed properly.

Now look at what you have, a pair of 4ohm speakers, in single mode they have too low an impedance and in bridgable mode the series pair have too low an impedance.

Let's take your thought processing a bit further.
Assume you have two 8ohm speakers.
The stereo amp will drive them to 100W + 100W. The total power delivered to the speakers is 200W.
Wire the amp in bridge mode. Wire the speakers in series.
Now when you drive the series pair in bridge mode you find the amplifier delivers 200W to them. You have gained (and lost) exactly nothing, except your time.

Don't wire your 4ohm drivers into an 8ohm amplifier.

That makes no sense to me.

Hypothetically, lets say I have two 4 ohm 200 watt speakers, and a two channel amp capable of 100 watts rms per channel into 8 ohms. Lets say this amp is also capable of 200 watts rms into 8 ohms, bridged.

The math all adds up, to me.

4 ohm + 4 ohm in series = 8 ohms, and would split current equally since the resistances are equal on each voice coil, so why wouldn't it be the same at an 8 ohm 400 watt speaker?

I don't understand why you guys are saying in bridged mode the amp has to drive a 16 ohm load, because all the amps I have are capable of 8 ohms stereo and bridged mono.

One of them also says something about "maximum load bridged 4 ohm pair" but I'm not sure what that means.

[Cal Weldon]

I'm afraid I have to bow out here as I'm not familiar with an amp that needs 8 ohms stereo and is stable with 8 ohms bridged. Mine aren't like that.

[Sammich]

I have a few Sansui power amps and recievers, some NAD amps and a kenwood amp and the two or three that are bridgable say on the back "min impedance 8 ohm min bridged impedance 8 ohm". Except for the one that says "min bridged impedance 4 ohm pair"

[AndrewT]

capable of 100 watts rms per channel into 8 ohms. Lets say this amp is also capable of 200 watts rms into 8 ohms, bridged.

this sounds wrong.
The usual rule is double the power into double the impedance when in bridged mode.

[CBS240]

Originally posted by Sammich


That makes no sense to me.

Hypothetically, lets say I have two 4 ohm 200 watt speakers, and a two channel amp capable of 100 watts rms per channel into 8 ohms. Lets say this amp is also capable of 200 watts rms into 8 ohms, bridged.


I don't understand why you guys are saying in bridged mode the amp has to drive a 16 ohm load, because all the amps I have are capable of 8 ohms stereo and bridged mono.

One of them also says something about "maximum load bridged 4 ohm pair" but I'm not sure what that means.

I isn't complicated, you can't get more power from the amp than it is capable. If it is a 400W amp than great. But if it is a 200W amp, than it will try to produce 400W, then the outputs will burn up. Bridging is like putting two channels in series, hence the load impeadances must be in series also. 2) 8 Ohm loads in series is 16 Ohms. If the amp is capable of driving 4 Ohms per channel, 8 Ohms bridged will work. BUT if it is 8 Ohms per channel, you are still putting a 4 Ohm load onto an 8 Ohm channel by bridging 8 Ohms(4 + 4), and the magic smoke may escape from the outputs.

The math all adds up, to me.

4 ohm + 4 ohm in series = 8 ohms, and would split current equally since the resistances are equal on each voice coil, so why wouldn't it be the same at an 8 ohm 400 watt speaker?

Yes, but in a series circuit, current is the same for both resistors and voltage is split. AKA 2 series resistors is known as a voltage divider. Likewise, 2 parralell resistors is a current divider.