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[Kevin Conlon]

Like the old Peavey CS800 that people either love or hate.

You could drive it into a dead short on the outputs and the amp would not die.

In fact that was the final test after a repair.  Short out the output terminals and drive it into full clip until it shut down.  Then remove the short and if it still worked, the repair was good.

That's a test alright! Like sending 60hz to an amp and making a pot of coffee with it. Thanks to all for your input. Luke, The amps are at the speakers with only 10 or so feet of #10 wire, so DF should be fine. Ivan, yes all the cabs ohm a little different, all rounded off, like cc's in an engine. I have seen the freq/ z plots for the drivers and most of time, except kick, the impedance is much higher than 4 ohms. Thanks again, i will use two amps at 4 ohms. The BGWs will be in the trailer just in case. I am trying to not move so much weight if i can do with less. These are what i have, and none are light.

[Luke Geis]

In high stress situations I worry less about impedance and more about the stress. The heat that will build up in the voice coil will raise the impedance seen at the amp as it gets hotter and hotter. Hence power compression.

The power compression that ensues is usually followed by giving it more gas, which simply creates more heat and less performance ( more power compression ). This may go on until the operator is into clip and out of room to go any further.

This is not any different at any other impedance, but if you're already near the end of the line, running down to a 2 ohm load won't help. So as you run out of room you are simply working the amp and the speakers that much harder to essentially go nowhere.

This particular case is a perfect example of why you don't need to bother with going down to 2 ohm's. The speakers in stock form are rated for 800 watts RMS and 3,200 watts peak. That puts this amp pretty much spot on for these speakers. Since there is only a 300 watt difference between 2 and 4 ohm rating on this amp, there is no real benefit to go down to 2 ohm's. The 300 watts will get you nowhere.

[lindsay Dean]

CS800 still brings a nostalgic tear to the eye,
what a dependable, heavy beast. and the
infamous project one,
man what a day.

[John Roberts {JR}]

cs800 still brings a nostalgic tear to the eye,
what a dependable, heavy beast. and the
infamous project one cabs.
man what a day

cs800 was a light weight compared to cs1200...   

JR

[Kevin Conlon]

In high stress situations I worry less about impedance and more about the stress. The heat that will build up in the voice coil will raise the impedance seen at the amp as it gets hotter and hotter. Hence power compression.

The power compression that ensues is usually followed by giving it more gas, which simply creates more heat and less performance ( more power compression ). This may go on until the operator is into clip and out of room to go any further.

This is not any different at any other impedance, but if you're already near the end of the line, running down to a 2 ohm load won't help. So as you run out of room you are simply working the amp and the speakers that much harder to essentially go nowhere.

This particular case is a perfect example of why you don't need to bother with going down to 2 ohm's. The speakers in stock form are rated for 800 watts RMS and 3,200 watts peak. That puts this amp pretty much spot on for these speakers. Since there is only a 300 watt difference between 2 and 4 ohm rating on this amp, there is no real benefit to go down to 2 ohm's. The 300 watts will get you nowhere.

The reason I was thinking of 2ohms was to carry just one amp. You have changed my mind about that, besides with only one i would need a long run of speaker wire, just easier to use two. Thanks for pointing out all possible drawbacks, this is why i asked, thank you very much.

[Frederik Rosenkjær]

I know that Powersoft encourages people to run both their K and X-series amps in 2 ohm, not just as a plan B, but that the amps actually "like it there".

[John Roberts {JR}]

I know that Powersoft encourages people to run both their K and X-series amps in 2 ohm, not just as a plan B, but that the amps actually "like it there".

It is good to design amps that are up to the stresses that customers routinely expose them too.
 
I just spent some time on the powersoft website and couldn't find any mention that they like 2 ohms, only that they are capable of 2 ohm loads (4 ohm bridged). I suspect it's the customers who like 2 ohms because they think they are getting something for nothing.

If the K series likes operating at 2 ohm why do they spec the frequency response at 1W and 8 ohms? Hint, because the amps like 8 ohms and low power even more. 

JR

PS: That "likes 2 ohms" sounds like something a rep or salesman might say... While I won't anthropomorphize about what inanimate amps like, I don't like operating gear at the limits of their capability. 

[Matt Lillie]

IIRC, the 4050HD was designed to handle 2 ohm loads all day every day, it's in memory as a "down-spec'd" RMX 5050. Certainly could be wrong....

Just as an aside, Adamson's T21 sub is loaded with two 2 ohm 21" drivers. You have to run an amp channel per driver at 2 ohms.   

I have never been a fan of 2 ohm loads, so close to a dead short. I have been reading and people say 2 ohms on a "modern" amp is ok. What i have is a QSC 4050HD, discontinued for the A model i think. QSC gives an output rating for 2 ohms, like 1600 watts per side. I want to use it on 4 ev mtl2b cabs (dual 18 manifold design for the youngsters). each cab is 4 ohms and i have been driving them with 4 BGW 620a mono amps ( no one has been able to id this mono model yet) and they do very well. I have 3 bgw amps x2 racks, 2 in use and a spare on each side, running on 240v. As i said they sound great but before i use the QSC i just want some feedback on using it at 2 ohms on a all day event. Next big thing i need the subs for is bluegrass and accustic stuff leading up to wide open rock for the closer at an outdoor festival. Is this 4050 modern enough for the task? I have access to another one( one serial # apart) for one channel per cab if this would be better. Thanks for any input!

[John Roberts {JR}]

IIRC, the 4050HD was designed to handle 2 ohm loads all day every day, it's in memory as a "down-spec'd" RMX 5050. Certainly could be wrong....

I don't know about "all day every day" but indeed if you drop the rail voltage, for a given heatsink and output stage you will reduce dissipation at low load impedance, but for the "no free lunch" file, the reduced rail voltage means less peak power into 8 ohms. So a trade off favoring low Z loading.

Just as an aside, Adamson's T21 sub is loaded with two 2 ohm 21" drivers. You have to run an amp channel per driver at 2 ohms.

Modern amps are increasingly comfortable driving low z... The customer is always right yadda yadda...

Modern saturated switch (class D) output stage amps don't suffer the old linear output stage dissipation issues, so have a different thermal performance envelope.

I stand by my advice but do what works for you... I suspect adamson wouldn't design such a sub if modern amps couldn't drive them reliably (remember 2 ohms is a nominal impedance rating not a fixed resistance so it may be an easy 2 ohms, not a hard one).

JR

[Jeff Bankston]

I have the really old 1980's , non-modern , very antiquated QSC 3800 amps for woofers(I dont do subs). Each channel is rated for a 2 ohm load. I have run the 3800's in bridged mono for many many years driving 4ohm woofer loads. Each channel is seeing a 2 ohm load or somewhere in the 2 ohm range. I never had a problem. The punch is incredible and the sound is great. I have know of Crowns to explode when driving a 2 ohm load. A friend was using Phase Linears to drive 4 ohm loads and kabloom ! It all depends on the amp and the QSC series 3 was overbuilt. I have heard the QSC 4050HD is a heavy duty amp. If it has thermal protection it will go into protect mode if it gets too hot.

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