receiver @ 8 ohm w/ power amp running at 4 ohm?? (1 Viewer)

[Antlue]

I have a Yamaha RXv-3000 - running at 8 ohms right now. Will be getting Martin Logans for front L&R which requires amplification @ 4 ohms. although the Yamaha has a switch selector for both 4 & 8 ohm loads - I am not comfortable running the Yamaha at 4 ohms (I have 8 ohm B&W rear surrounds and rear centre) - I hear the current draw from the larger ML's might be too much for a receiver. So does anyone out there have a similar setup where you are using a receiver @ 8 ohms and an outboard power amplifier from the mains preout running @ 4 ohms (if that is possible). Some body help!!!!!! Is there a problem with this setup??
thanks.

 

[Chris PC]

Lots of people here do exactly that. If you set the receiver to 4 ohms, you will get less power from it, but you will prevent clipping and blowing your amp so your thinking is about right. All you need is a robust stereo amp. Regardless of current, even if your speakers were 8 ohms, you'll get improveed sound quality at least somewhat above the receivers amp sound quality.

Sure, get an outboard 4 ohm capable stereo amp that has at least the watts per channel of the Yamaha 3000. Say a 100 to 150 watts per channel into 8 ohms amp that handles 4 ohms.

 

[jason celaya]

I do the exact same thing, i have a denon 3802 w/ a 2 ch denon 5200. all three of my fronts are 4 ohm M&K, It works great , however I do wish I bought a three channel amp in the front just for piece of mind.

 

[Arthur S]

If you are saying you will not be using the front amp channels of the Yamaha to actually drive speakers, but just using the mains pre-outs, I would leave the switch at 8 ohms so their is ample juice for the speakers that are being driven by the Yamaha. The pre-outs will be the same whether you have the switch at 8 or 4 ohms. I am using a somewhat similar arrangement with my receiver only driving the center and perhaps the surrounds. My mains are connected to a 450 watt per channel amp. I have always left the 8-4 ohm switch in the 8 ohm position. Why cripple the receiver if it is well ventilated and doesn't get real hot?

The Martin-Logan 'stats can be a difficult load. You will want an amp that is very stable into 4 ohm loads with at least 150 watts per side.

Artie

 

[MichaelGomez]

If you set the receiver to 4 ohms, you will get less power from it, but you will prevent clipping and blowing your amp so your thinking is about right.

This doesn't make any sense. By going from 8 ohms to 4 ohms, you are almost doubling the power. As for clipping, I haven't seen any way to not do this on a receiver. It is usually caused by improperly set gains. The impedence on the source and the amp need to match. This is why there are gain adjustments on amplifiers for car audio equipment. That is because there are too many different head units with different voltage preouts. So manufacturers put gain knobs on amps so you can match them with the HU.

 

[Pete Gia]

I agree with most of the other posters. Leave the switch at 8 ohms if you`re going to use an outboard power amp for your 4-ohm speakers;and I respectfully submit that there is NO WAY the cheesy amps that are most receivers will double their power into 4 ohms.-----Pete

 

[jeff lam]

If you are using an external amp, the ohm switch on the Yamaha means nothing. I would suggest using seperat amplification for the ML's anyway. The ohm switch has nothing to do with the pre-out's on the receiver, only the power amp section(actual speaker driving section of the receiver).

 

[Wayne A. Pflughaupt]

Michael,

This doesn't make any sense. By going from 8 ohms to 4 ohms, you are almost doubling the power.

You are correct. However, the purpose of the switch is to prevent this, by limiting the current output from the amp that the 4-ohm speakers are demanding. This is a “band-aid” or stop-gap for the amplifiers that are not robust enough to run 4-ohm speakers.
Jeff,
I think Pete meant if you were using an external amp for some but not all of the speakers. In this case the switch would still be usefull.
Regards,
Wayne A. Pflughaupt

 

[MichaelGomez]

Pete-here is how it works. Notice I said "almost double your power." In a hypothetical situation, it would double. It is Ohms law.

Where P=power, E=voltage, I=current, R=resistance:

P=EI

E=IR

You can use simple algebra to derive all sorts of neat relations like:

P=V^2/R

Now, for our purposes, an amplifier works like a voltage source; that is, it varies the current going through the load in order to keep the voltage constant. The input signal into the amplifier sets the output voltage. The exact relation is: Input Signal Voltage * gain = Output Signal Voltage.

Inside the amplifier there are two voltage "rails," both centered around zero (ground). These represent the maximum output voltage swing of the amplifier. Say an amplifier's power supply has a +25V and a -25V rail (that's 25V above 0 (ground) and 25V below 0). So the maximum voltage ever to be seen across the load is going to be 25V (the - speaker connector is connected to ground).

Now, let's connect our hypothetical amplifier to a speaker. Let's say that it's an 8 ohm speaker, and it's a purely resistive load and its impedance (resistance) doesn't vary with frequency.

Assuming the amplifier is putting out an AC signal, we need to get the RMS value of that output voltage (+/- 25V) for power calculations. The RMS value of a sine wave is 0.707 times the peak voltage (25V), so here it's about 17V. Using our P=V^2/R relation, we find that the power into the speaker is about 36W. A quick calculation also shows that our RMS current is I=V/R, or about 2A.

Now let's hook up a 4 ohm load to the amp. Assuming the amplifier can double its power when we do that, we'll get about 72W, double. But now our current is 4A.

In the real world, of course, there are some limitations to exactly how close the amplifier can approach the behavior of an ideal voltage source. The current the amplifier can source is limited by its power supply and its output transistors.

Now let's assume the amplifier can only handle 3A before the power supply rails start sagging. Now when we connect that 4 ohm load, we're not going to get double the power, we're going to get less. More than the 8 ohm load, sure, but not double.

Wayne-I thought that the point was so that the amps in the receiver went through a different circuit. One that is more 8 or 4 ohm friendly. I don't know much about this part since I have never oppened a receiver.