I have a customer that has two pair of 4ohm rock speakers in their backyard. Each pair has a 60w volume control in line for attenuation. The amp is a crestron 16x60 (16 channels by 60w). There are two channels bridged for the rock speakers which crestron specs to push 8ohm @ 220w. I have replaced the volume controls once already but I believe I will have the same problem in a few months if I use the same 60w volume control again, the amp is getting too hot and shutting down. Any suggestions?

You know 1 4ohm speaker + 1 4ohm speaker equals around 2 ohms of resistance right? Not 8. Your amp is heating up because it needs 8 ohms...not 2

I'm not running the speakers in parallel therefore they maintain their 4ohm load. Crestron says that this amp will push 4ohms @ 90w when it's not bridged but when bridged they only give specs for 8ohms.

Highest rated V/C I have seen is Sonance at 100 watts - though I would be curious to know why in the world you are using a V/C with a Crestron system (yes, attenuation I know, but I would drive the speakers direct off the amp and use a keypad to adjust volume on your multi-room preamp). You need to find out if the Crestron amp is stable at 4 Ohms when bridged. That could be your problem.

This message was edited by QQQ on 04/20/04 15:54.

True...I was just making sure you knew that.

It may not handle a 4ohm load then...I'm not sure though. I don't know what volume ctrls you're using, but if not, try impedence matching controls.

The volume controls are for sub zones QQQ.

If you are using a Rockustics speaker or the like thaty are pretty efficient. I would unbridge the amps and see if you continue to have a problem.

Well, this bears some thought! If you are not running them in parallel, then you are running them in series, in which case they look like eight ohms, or you are running them off of separate amplifiers, in which case they look like 4 ohms to each amp. There is no way you can run two 4 ohm speakers off of one amp and have them look like 4 ohms.


That is the way it goes with bridged amps; they can only feed their power to an impedance twice that of the unbridged version.

This is a Multi channel amp, no series, no parallel. I'm going to take QQQ's advice and remove the bridging modules and push each speaker with a single channel and hope I maintain a decent SPL. The homeowners are outdoor people with a big backyard and they like to throw parties so sufficient volume is a concern. I hope it works. Thanks everybody for your input and expertise.

Dan,

Just an FYI many amps can not drive a lower impedance load when bridged. That *may* be what's happening here.

It is common for amps rated at 4ohms to be rerated to 8 ohms when bridged. As far as volume controls go you can use impedance matching volume controls to protect the amp. Audioplex makes 100, 200 and 300 watt volume controls if you need them.

If current is that critical, ditch the VC and figure out a way to control it via line level.
And yes, if the amp is stable at 4ohms stereo, it most likely will not be stable at 4ohm bridged.
If it were stable 2ohm stereo, then you could run a 4 ohm load to it in bridge mode.

I was wondering when someone was going toget around to recomending the Audioples VC's. Definatly what you need.

BTW.. the Creston/ATI Amplifiers are pretty solid. THey bridge into a 4 Ohm load just fine with enough ventilation/cooling.

Okay, so I think I see what the confusion was here. A couple of posts after mine also seemed to be confused.

You write that you have two pairs of speakers, each pair has a volume control, and there are two channels bridged for the rock speakers.

Then you stated that they were maintaining four ohms.

You must have meant that each rock speaker runs off its OWN two channels in bridged mode. That way, the impedance is maintained, it sticks with your comment that two channels are bridged for the rock speakers (but you meant for EACH rock speaker), it all looks correct, and your volume controls are SMOKIN!

I haven't used the Audioplex, but you need two things here; an eight ohm load, like the amp specifies, and a volume control that can handle the power.

By the way, 90 watts times 2 = 180 watts, which is only three decibels louder than 90 watts; 220 watts is less than fifty percent more than that, so it is probably four or four and a half dB louder than 90 watts. Are you SURE that 90 watts, at fullish volume, won't cut it? Because if it won't, then 220 watts could be way too low.

You need to figure in the distortion factor also. When you try to force that much current into a V/C, it is going to cause a lot of distortion, which in its self will really wreak havoc with the whole setup. You are probably losing a great deal of your power as unusable distortion and heat. Honestly, if the sound is that important to them, get them into a keypad controllable preamp (even if its only 2 channels) and ditch the volume controls, you will get a lot more gain and way better fidelity while only needing half of the power you are fighting with now.