Sharing speakers with two amps

Hey guys,

I am using a dual amp configuration. For stereo listening, I am using BHK 300. For 5.1 movies, I am using an Anthem MRX710. As of today, when I am going to use the BHK, I disconnect the speaker cable of the MRX710, and do the other way around when using the MRX710.

The point is: is it alright to leave the speaker cables to both amps connected at all times?

Of course, both amps would never be turned on at the same time. I would turn off the P10 outlet that feeds the BHK300s when using the MRX710, and vice-versa.

I am preocupied if leaving both speaker cables connected, even to a powered off amp, I would mess with the resistance, or any other electrical characteristic of the speakers or the amps.

Best if you could disconnect at the speaker instead of the amp, but if you disconnect at the amp all you get is the extra capacitance of the open wires going back to the amp. There’s no current flowing as long as at least one wire is disconnected from the unused amp, so there’s no impedance loss there. But there’d be a little capacitance loss. Should be no problem for the BHK, I’d think.

One way to find out: Do a test run where you disconnect the MRX710 at the speaker and listen to the BHK. Then just connect the MRX710 speaker wire without connecting it to the amp and see if there’s any difference. I’d think when you’re listening to the movies you don’t care if there’s a little loss from capacitance from the BHK side of things, so don’t bother to test that.

Let us know if there’s any difference. I’d bet there won’t be much. . . .

Best of luck.

Hey, thanks for your reply. I think I did not make myself clear. The cables would be connected at both ends (speakers and amps) at all times.

I would only disconnect the power from the amp not being used.

If I understood correct li y your response, I should face higher impedance in this scenario. Is that it?

Oof. The BHK when powered has an output impedance of 0.1 ohm–I have no idea what it would be when the power is off. But if it is that low, it would represent an equivalent of a full-blown short circuit to the other amp and no power would go through the speaker. All the MRX710 power would bypass the speaker and go through the BHK.

So, no, don’t do this. I’ll let Paul and Elk chime in to clarify, but you may want to think of a switch or some other mechanical override to prevent that from happening.

Do not do connect two amplifiers to any speaker such as you are describing! It will short out the amps. What pre amp are you using when you are not using your MRX710? There should be a way to send the front left and right preamp outs of your MRX710 through your preamp to the BHK and use that amplifier all the time for your front speakers.

That is exactly why I have not done it yet. And yes, I can route through my preamp. I always just looking for an alternative to maintain the same amp for all speakers.

But, for sure, without destroying my speakers or amps!

I have seen many people upgrade their systems like you are talking about. Mine is currently in a similar situation. What preamp are you using and I can suggest the best way to do it…

Thanks again for your kindness. I am using a musical fidelity primo. I believe i could easily use the left and right channel outputs from the MRX710 hooking them to an unbalanced input of the pre amp. The pre has a 6db gain button to compensate for the use of the unbalanced Input.

How does this sound to you?

OK, your preamp does not have a Home Theater mode, so you will have to remember what volume setting you use.

First, make sure all your equipment is turned off. Make sure the volume controls on the preamp and receiver are both turned all the way down before you power down.

Connect the RCA unbalanced input on your preamp to the Left and Right Main channel RCA outputs of your MRX710. For the purpose of this example I will just say it is the TAPE input.

Connect your Preamp to your BHKs with either RCA or XLR cables.

Connect your BHKs to your speakers via speaker wire.

Turn on your equipment, check for ground hum with the volume controls all the way down.

You can and will use the room correction on your Receiver to your advantage for this next part. If you have a sound level meter your can use that to set the initial volumes.

Using a source with constant level, like a test tone through all your channels, set the volume control on your Receiver to a low to moderate level. Now slowly turn up the volume on your preamp until the volume of the main speakers closely matches the other channels. Make a note as to the volume level on your preamp (you want it to be an easily remembered and number that you can repeatedly set when you want to listen to multi channel.) You can play with the 6 db correction if you want, but it is not necessary. If you end up with, for example 33 as the setting, I would choose to put the volume on 30 or 35 so you can remember and easily reference it. You do not want it to be a significantly higher level than a safe listening volume ( for example 70 to 80.)

Next run the room correction process on your receiver, and it should be able for any small difference in output to the front left and right speakers. You might want to run it more than once, and play with different settings to your preamp volume control to see what works best. The less correction your receiver has to make on the left and right front channels the better.

You might also want to run it on a couple of different listening volumes to see how well the receiver/preamp combination volume tracks the front two channels compared to the center and surrounds. Your center channel amplifier will be your weakest link. You might consider getting an M700 to run you center speaker. The amplifiers in your Receiver will be able to run a little stronger since you will no longer be using the left and right channels, reducing the drain on the internal power supply.

Now you will only have to turn on your Receiver when you listen to multi channel, unless you are feeding a particular source through it and not directly into your preamp. When you want to listen to multi channel, you will set your preamp volume on the number the process I just described reveals (you see why I said make it easy?) An d set the input to TAPE, or which ever you end up using. You will then use the volume control on your receiver to set the system level, and make adjustments for the source material.

Enjoy and let me know how it works out.

Hey man, thank you very much for the time you dedicated to help me in your post. I will follow your script for sure, and I am certain that I will be able to level match both amps.

Another way to do it, and simplify the level matching, would be to hook up the MRX710 directly to the BHKs. Since I use XLR for stereo listening, the BHK would receive XLR from my preamp and RCA from my receiver. But, then again, I would have two connections coming in to the BHK, and I am guessing this would do more harm than good.

I would not do that either, you risk feedback that could short something out.