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Discussion Starter #1
...over the weekend i acquired a power amp for my stage monitors. problem is, it puts out 4 ohms, and my speakers are rated at eight ohms.

will i encounter problems?

thanks,

-dh
 

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Not sure if it is a problem or not.
You could put both 8 ohm monitors on the same channel and that will give you a 4 ohm load together.
 

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Running an 8 ohm load on a 4ohm amp will not damage anything, you just won't get the full power potential. 2 8ohm speakers though will show your amp a full 4 ohm load.
 

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david henman said:
...over the weekend i acquired a power amp for my stage monitors. problem is, it puts out 4 ohms, and my speakers are rated at eight ohms.

will i encounter problems?

thanks,

-dh
David, is it a solid state power amp?

Solid state amplifiers are much more forgiving than tube amps. They don't really have a specific impedance like 4 or 8 ohms. Rather, the lower the load the more power they put out. If the load is too low the amp can burn itself up. So the designer sets a lowest limit load impedance for the power the amp can handle. It could be rated 100 watts at 4 ohms and as long as the load is never lower then the amp will run just fine. If you plug in an 8 ohm cab the amp will just put out a little less power. There should be no differences in tone, just power.

Tube amps as you know are a bit more fussy but even then a 2:1 mismatch shouldn't bother the tubes all that much, although the tone will change and you'll have to decide for yourself if you like it or even notice it.

Just FYI.

Wild Bill
 

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Discussion Starter #5
...thanks, guys. its a solid state stereo (alesis) amp. i'll try running it in bridged (mono) mode. one speaker out will send the signal to my own monitor. the other will go to the bassist's monitor and then "through" (series, i think) to the drummer's monitor. the monitors are all rated at eight ohms.

-dh
 

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Hi David,

3 8ohm monitors will give you a 3.3Ohm load. If your amp states that it will only handle 4 Ohms bridged, you might run into problems if you bridge the amp at this load.

The through connection is usually a low level line signal that is buffered to allow you to drive a second amp's input - but you should check your manual on that one since it is not that common of a feature.
 

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David, my buddy's band has an Alesis RA300 which they use for practice PA.

It has 4 8-ohm monitors attached to it, 2 per side. This gives 4 ohms load into each channel. If they use only 2 monitors (one per side), then it still works fine but at less power.

You could put two monitors on one side (4 ohm load) and one on the other (8 ohm load) but I wouldn't guarantee that it's not gonna hurt the amp. (Comments from more experienced sound monkeys welcome.)

I definitely wouldn't bridge the amp into 3 (or more) speakers. The specs on this amp seem to indicate that it wants a MINIMUM load of 8 ohms when bridged.

Can you find one more monitor and run 2 monitors per side?
 

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Discussion Starter #8
buckaroobanzai said:
David, my buddy's band has an Alesis RA300 which they use for practice PA.
It has 4 8-ohm monitors attached to it, 2 per side. This gives 4 ohms load into each channel. If they use only 2 monitors (one per side), then it still works fine but at less power.
You could put two monitors on one side (4 ohm load) and one on the other (8 ohm load) but I wouldn't guarantee that it's not gonna hurt the amp. (Comments from more experienced sound monkeys welcome.)
I definitely wouldn't bridge the amp into 3 (or more) speakers. The specs on this amp seem to indicate that it wants a MINIMUM load of 8 ohms when bridged.
Can you find one more monitor and run 2 monitors per side?
...no. i can't see carrying around an extra monitor just to have a "dummy load".

i find it very difficult to grasp these concepts. for example, you mention "per side". when the amp is bridged, that is in mono mode, does this not eliminate the stereo/per side factor?

also, when you guys say "less power", what do you mean? how much less?
 

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I'm not the leading expert on this (Milkman?), but from the amounts of transistor-smoke I've generated over the years -

If an amp is rated at, say, 200 watts into 4 ohms, you'll get roughly half power (100W) at 8 ohms.

Most monitors have 2 jacks on the back, wired in parallel, so as you add speakers by plugging one speaker into the next, you are halving the ohmage.
(2 8-ohm monitors plugged one into the next = 4 ohm load to the amp)

When you bridge a stereo amp to mono, it requires twice the load as the 2 individual stereo channels did. ( minimum load for a stereo channel ( for this model amp, others are different) is 4 ohms, so the minimum load in bridged mode is 8 ohms.)
I have, however, read specs on some amps that say they'll work in bridged mode down to 2 ohms, so I guess it's not a hard-and-fast rule. However, all the specs I can find on Alesis amps say bridged mode has an 8-ohm minimum.
(If you run a solid-state amp into too small a load, you'll fry the main power transistors.)

After poking around the 'net some more, it looks like you'll be safe running 2 monitors off one side of the amp and one off the other. If the amp has separate volume controls for each channel, you should be able to crank up the channel with ONE monitor on it (it's 8 ohms, the channel with 2 monitors is 4 ohms) to compensate. Watch the clip lights.

I guess the bottom line is, wire it up to 3 monitors with the amp in stereo mode and see if it's loud enough. You won't damage the amp. I wouldn't run it in bridged mode into 3 monitors until you find out for sure if the amp can take it.
 

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Hi again David,

You will want to connect 2 monitors to one side of the amp and one to the other - forget about bridging the amp alltogether unless you know for sure that it will handle a 3.3ohm load.

You get more power out of the side that will have 2 monitors on it, but that power is distributed between the two monitors. The other side will put out less power, but it is only into one monitor. In the end, all three monitors will have similar volume levels, but the single monitor will not be driving the amp as hot. If there is a slight difference in volume between the two sides of the amp and it is not working for you, just attenuate the louder side untill everything is matched.

The only thing left to consider is how many monitor sends you have from your console, and if you need two seperate or just one mix.
 

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Discussion Starter #11
...i'm still confused:

1. mono source

2. alesis ra300 power amp

8 ohms - 300 watts rms bridged (mono) mode
8 ohms - 90 watts per channel stereo
4 ohms - 150 watts per channel stereo

3. three eight-ohm monitor speakers

i sent a letter to alesis tech support, and to a friend who is a sound tech.

-dh
 

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No need to be confused....

- Monitor Out from your mixing board needs to go into both channels of the amp (hopefully there are 2 Monitor Out jacks on your mixer - if not, a Y cable is required)

( like this) http://www.directproaudio.com/product.cfm?directid=23399

- leave amp set for Stereo

- connect 1 monitor to one channel of the amp

- connect the other 2 monitors together and connect them to the other channel of the amp

- position the monitors where you need them

- adjust the volume controls of the amp as required

- Rock out!
 

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Discussion Starter #13
buckaroobanzai said:
No need to be confused....
- Monitor Out from your mixing board needs to go into both channels of the amp (hopefully there are 2 Monitor Out jacks on your mixer - if not, a Y cable is required)
( like this) http://www.directproaudio.com/product.cfm?directid=23399
- leave amp set for Stereo
- connect 1 monitor to one channel of the amp
- connect the other 2 monitors together and connect them to the other channel of the amp
- position the monitors where you need them
- adjust the volume controls of the amp as required
- Rock out!
...thanks, bro'!

there's only one monitor out, so i'll have to fashion some sort of "Y" adaptor.

but i'm curious...why wouldn't i run everything mono?

-dh
 

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Wild Bill said:
Tube amps as you know are a bit more fussy but even then a 2:1 mismatch shouldn't bother the tubes all that much, although the tone will change and you'll have to decide for yourself if you like it or even notice it.

Just FYI.

Wild Bill
David, sorry for the small hijack but I'm always confused by this stuff too.

Bill, if you think back to my concern with my Allen amp with the extension jack wired in series. Are you saying that if I add an 8 ohm cab to the combo, resulting in a total of 16 ohm load, I should still be OK with the ohm switch on the combo set @ 8 ohms? I only have a 4 or 8 ohm option on the combo.
 

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david henman said:
...thanks, bro'!

there's only one monitor out, so i'll have to fashion some sort of "Y" adaptor.

but i'm curious...why wouldn't i run everything mono?

-dh
You will be, you just need a way to get your single monitor send input into both sides of the amp. The switch on the back of your amp switches from stereo (dual mono) and bridged mono (2 sides of the amp working as one amp in a push-pull configuration), but does not have a mono input option (according to the manual on the manufacturer's site). You need some way to get the signal into the second channel, and the "Y" cable is the simplest means to accomplish this.

You could also use an XLR input and jumper both sides together with a ¼" cable. If your monitor EQ (or whatever you are feeding the monitor send from) has an XLR output, this may actually be easier as you won't have to solder any cables.
 

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"Just the same only different..."

davetcan said:
David, sorry for the small hijack but I'm always confused by this stuff too.

Bill, if you think back to my concern with my Allen amp with the extension jack wired in series. Are you saying that if I add an 8 ohm cab to the combo, resulting in a total of 16 ohm load, I should still be OK with the ohm switch on the combo set @ 8 ohms? I only have a 4 or 8 ohm option on the combo.
Yeah, nothing's likely to blow up or anything. If you were to play the amp consistently at 8 or 9 the output tubes might lose a couple of months of life but that's about it.

I've forgotten our thread about your Allen amp. Is it like old Fenders, where you have a speaker and an extension speaker jack and just one speaker won't work if you plug it into the extension one? This arrangement works by putting any extension speaker in series with the regular speaker. That would make the speaker loads add up. If the jack is just an extra parallel connection and you can plug into either one this would put the loads in parallel and you divide the 8 ohms by the number of 8 ohm loads involved.

My memory worked fine before the kids came...
 

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Wild Bill said:
Yeah, nothing's likely to blow up or anything. If you were to play the amp consistently at 8 or 9 the output tubes might lose a couple of months of life but that's about it.

I've forgotten our thread about your Allen amp. Is it like old Fenders, where you have a speaker and an extension speaker jack and just one speaker won't work if you plug it into the extension one? This arrangement works by putting any extension speaker in series with the regular speaker. That would make the speaker loads add up. If the jack is just an extra parallel connection and you can plug into either one this would put the loads in parallel and you divide the 8 ohms by the number of 8 ohm loads involved.

My memory worked fine before the kids came...
Yep, they're in series. Our other guitar player figures it would be a snap to rewire them in parallel so I might do that. That would give a 4 ohm load which the is switchable to. Thanks for the info.
Once the weather clears a bit I'll be bringing my Zinky down to you to see if you fix what a local "tech" did a year or so ago.

btw the Busen will be getting a work out at the Richmond Tavern this Saturday afternoon.
 

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Discussion Starter #18
Hamm Guitars said:
You will be, you just need a way to get your single monitor send input into both sides of the amp. The switch on the back of your amp switches from stereo (dual mono) and bridged mono (2 sides of the amp working as one amp in a push-pull configuration), but does not have a mono input option (according to the manual on the manufacturer's site). You need some way to get the signal into the second channel, and the "Y" cable is the simplest means to accomplish this.

...i read the manual carefully last night. it says clearly that i should NOT send a signal to the second channel when the amp is in bridged mono.

just waiting to get the final word from justin taro at alesis.

the only concern i have now is power: will the 300 hundred watt alesis amp (solid state) have sufficient power for three large vocal monitors, each rated at 250-500 watts program?

-dh

-dh
 

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david henman said:
...i read the manual carefully last night. it says clearly that i should NOT send a signal to the second channel when the amp is in bridged mono.

just waiting to get the final word from justin taro at alesis.

the only concern i have now is power: will the 300 hundred watt alesis amp (solid state) have sufficient power for three large vocal monitors, each rated at 250-500 watts program?

-dh

-dh
Hi David,

You don't want to bridge the amp mono. You will be running a 3.3Ohm load if you do, and the amp won't take it. You should have 2 monitors on one side of the amp and one on the other and the switch should be set to stereo mode.

It will run your monitors, but you won't have allot of volume or headroom, so try not to be clipping the amp all of the time as it is bad for the speakers and the amp.
 

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david henman said:
the only concern i have now is power: will the 300 hundred watt alesis amp (solid state) have sufficient power for three large vocal monitors, each rated at 250-500 watts program?

-dh

David, your question implies some basic assumptions that might be hurting you rather than helping.

The speakers are rated at the maximum power rating that will not blow them up. This has nothing to do with how much power is needed to drive them. A half watt of power should still drive them!

Now, we have to assume that the monitors are reasonably efficient. If they are, a few watts will sound loud. You feed them more if you need them louder to be heard over the damn drummer! Different speakers and cabinets have different efficiencies. We can only guess about yours. If they do indeed need hundreds of watts I'd suspect they're actually blown! You should be able to turn the volume control up just enough to put a few 10's of watts into them and that should be enough.

Since power is shared among all the monitors no matter how it is split up with different speaker loads or whatever one would think that 3 x 250-300watts is more than safe enough. You've got a total of 300 watts available and you're gonna split it 3 ways. Even dialed to 10 there's no way each monitor will see more than it can handle.
 
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