Need amp that's stable into 2 ohms

[nightflier]

I'm trying to help out a local school with the purchase of a new amp for their exercise room. They were originally using a Harman Kardon 3480 Receiver to drive 6 Infinity Primus 160 Speakers using a speaker selector box. Well after a year the amp in the receiver died. I considered these speakers pretty easy to drive (90dB/8Ω), but the HK amp couldn't do it, I guess. The space is about 2000 sq. feet, so they probably turned the volume up pretty high.

Anyhow, I gave them an old preamplifier I had and I am now trying to find them an amp that can drive the Infinity speakers. I presume the amp has to be stable down to 2 ohms to do this, but I really don't know. The problem is that they have a budget of around $200, which I know is very little. I've looked at pro amps like the Mackie FR800, but I really am not sure if this is what I should be looking at. Any help?

[02audionoob]

You could perhaps try an Adcom GFA-545 or 545II with that budget. Maybe you could even find an Adcom speaker switch. They have a 'protection' button for amps not designed to handle the low-impedance loads.

Edit: I didn't realize those commercial amps are actually fairly inexpensive. That does seem like a better idea.

[Kevio]

Yep, this is a commercial application blows that consumer right stuff up. I'd be looking for a commercial or MI amp. QSC GX3 might be a more robust choice.

[blackraven]

You could try find this amp used http://www.audiophileliquidator.net/...oducts_id=1206

or this one which sells for under $400 http://www.audiophileliquidator.net/...roducts_id=532

[Mr Peabody]

I don't know their price but Sherbourn makes some multichannel amps with switching designed for multiroom systems that would be perfect. I don't know if they are stable to 2 ohms but with this design it wouldn't need to be.

[blackraven]

Here's an 8 channel amp http://www.speakercraft.com/files/files/file/267.pdf

Any multichannel amp is going to cost a bundle.

[RoadRunner6]

I think you will find that with a reasonably decent switching box the impedence will not drop below 4 ohms with 3 sets of 8 ohm speakers (the impedence thru a switching box does not drop to the low impedence levels like it would if you had all six speakers hook up in parallel). That said, you would think that the HK receiver which has a fairly decent amp as far as stereo receivers go would be OK driving a 4 ohm load thru the swithing box.

On the other hand, these are not PA speakers. They are medium sized bookshelf speakers designed for homes. They should be OK at reasonable levels. However, they do not put out low bass. They probably start rolling off about 60-70Hz and are down to about -3dB's at 50Hz or so. I can imagine that in this circumstance somebody might have been boosting up the bass contol to try and get more bass from the speakers. This would play havoc on the amp in that receiver. I've seen this happen many times at parties, etc. where some idiot cranks up the bass or "loudness" control to get more bass. Of course, this will soon overtax the amp. If this is the case, I can see why it didn't last long with this type of abuse. In home speakers something much more efficient like Klipsch speakers would have been a better bet.

I suggest that the solution for this school would be to make sure they understand not to touch the bass control, leave it at the medium setting and to leave the loudness control off. Make sure the amp has adequate ventilation.

I would stretch the budget a little and go for a quality amp. This UPA-2 amp from Emotiva should have no problems at 4 ohms and is a steel for $299.

http://emotiva.com/upa2.shtm

The pro amps linked by Kevio might be the heavy duty way to go. Also, check that switcher box spec sheet and see how much power it will handle

RR6

[basite]

I'd say 'Get a Mcintosh', but that budget sorta limits options

[nightflier]

Thanks for the input. It looks like the only way I can squeeze this into their budget is to buy used. The QSC amp might be a good bet, but the specs say it is only good into 4 ohms (correct me if I'm wrong). The speakercraft amp is another option, but well over the budget I have for these. If I go used, any recommendations on places that are reputable (as opposed to eBay and that sort of thing)? Pro amps tend to suffer a lot of abuse already, so buying used is lot more risky. Adcom would be nice, but I don't know how well they would handle a 2 ohm load. And older NAD maybe, since they have decent protection circuits?

Regarding the bass, that's something that I can tell them, but eventually someone, at some point is going to turn up the bass, so I need an amp that can handle it. Sound quality isn't really an issue, especially with all the ambient noise in the exercise room. Maybe I can find an old sub that we can pass the signal through at a later date, but for now this is what it is. Yes, Klipsch speakers would have been better, but they're not going to swap out these speakers.

[markw]

that's a lot of room to fill with sound, even with six speakers. Unless they only want dentist's offict style background muzak, odds are that any consumer stereo amp will be driven pretty hard. I think smaller amps for each speaker would be a less volitale, more permanant soution.

I don't know how you're gonna do this for under $200, but sometimes you simply can't get there from here. Maybe three beater stereo receivers from garage sales?

If you shop carefully and they are somewhat flexible (realistic?) with their budget, three Audiosource AMP-100's would be ideal.

[RoadRunner6]

Regarding the bass, that's something that I can tell them, but eventually someone, at some point is going to turn up the bass, so I need an amp that can handle it.

Handle it???

Let's say for the sake of argument that you have an amp that puts out 150 watts per channel and it dies after a year (sound familiar?).

Now lets say that one of the little bastards turn up the bass and or loudness control to +12dB's. This is entirely possible. With a 12 dB boost in the bass frequencies you now need an amp that puts out aproximately 1500 WATTS per channel! all things being equal.

Find one of those for $200.

RR6

[nightflier]

Multiple amps. I hadn't considered that. OK, let's say I purchase a solid amp for four of the speakers and then add a second amp later. That does ease up on the requirements. What can I get for around $200 that will drive 4 infinity speakers? And yes, I do need an amp that will "handle" the occasional stupid kid turning up the bass?

I know this is a tall order, but times are tough and schools just don't have big budgets these days. I really would like to help them out, if I can.

[02audionoob]

I like the idea of the multiple AudioSource AMP-100 amplifiers.

Back on the Adcom idea, though...I downloaded the manual for the GFA-545II just now and it was clear in saying it could drive two or more sets of speakers and referred to loads like this. I also checked the manual for their speaker switch and it indicates the protection button is for amps other than Adcom, which might not be able to handle the load. It seems to be saying an Adcom can handle it.

That said...I've seen AMP-100 amps going for less than $100 new on Amazon.

[Feanor]

Thanks for the input. It looks like the only way I can squeeze this into their budget is to buy used. The QSC amp might be a good bet, but the specs say it is only good into 4 ohms (correct me if I'm wrong). ...

Yep, you're wrong. Or at least, the majority of QSC amp series come in 2 ohm versions. Check out their website.

[Mr Peabody]

I was checking Amazon for Sherbourn just to see how much they were and oddly enough I didn't see any product. However, I did see a bridgeable AudioSource starting used at $70.00.

[Kevio]

Yep, you're wrong. Or at least, the majority of QSC amp series come in 2 ohm versions. Check out their website.

The GX series are the bottom of the line (and the only thing that comes close to fitting the budget). Corners have to be cut to meet these price points. The GX amps are only spec'd at 4 and 8 ohms. On the bright side, the specs don't specifically prohibit lower-impedance loads.

The reason amps have a hard time with low impedance loads is because they require a lot of current to drive them. It is current, not voltage that generates heat and fries things. Amps that can drive low-impedance loads have overbuilt power supplies good cooling and protection circuitry.

I'm sure the QSC amp will be an improvement over the Dennon because at least the QSC has a fan. The GX amps do not have appear to have overbuilt power supplies. They have an overdrive prevention circuit but do not appear to have thermal protection. If the former is well-designed, the latter may not be strictly necessary. A real commercial amp would have both though.

Other suggestions: Use a preamp without tone controls or build the system without a preamp. Use a fancy speaker selection box that will connect multiple selected sets of speakers in series rather than in parallel. It won't be optimally loud or sound great with this configuration but neither will anything blow up.

[Feanor]

The GX series are the bottom of the line (and the only thing that comes close to fitting the budget). Corners have to be cut to meet these price points. The GX amps are only spec'd at 4 and 8 ohms. On the bright side, the specs don't specifically prohibit lower-impedance loads.

The reason amps have a hard time with low impedance loads is because they require a lot of current to drive them. It is current, not voltage that generates heat and fries things. Amps that can drive low-impedance loads have overbuilt power supplies good cooling and protection circuitry.

I'm sure the QSC amp will be an improvement over the Dennon because at least the QSC has a fan. The GX amps do not have appear to have overbuilt power supplies. They have an overdrive prevention circuit but do not appear to have thermal protection. If the former is well-designed, the latter may not be strictly necessary. A real commercial amp would have both though.

Other suggestions: Use a preamp without tone controls or build the system without a preamp. Use a fancy speaker selection box that will connect multiple selected sets of speakers in series rather than in parallel. It won't be optimally loud or sound great with this configuration but neither will anything blow up.

'Flier's problem is that he has a "professional" application for which a pro amp is the best solution. Hooking the speakers in series will keep the impedance under control but cause people to crank the volume control with the result that the amp will still be strained.

A new QSC ISA280, which will do 2 ohms, can be had for $460 plus shipping. Maybe he can get one used. But when you start talking about multiple amps or the likes of an Adcom 555II -- a nice amp that will do 2 ohms -- you're getting into that price range.

[Kevio]

'Flier's problem is that he has a "professional" application for which a pro amp is the best solution. Hooking the speakers in series will keep the impedance under control but cause people to crank the volume control with the result that the amp will still be strained.

I agree, it is a pro application but there's a decidedly no-pro $200 budget. You can go ahead and say it can't be done but I'm trying to put some options on the table.

Yes, you'll definitely have to turn the volume control higher to get the same loudness driving a higher impedance load. But, so long as you stay below the clip point (the GX amp has clipping protection circuity and indicators), at the same power output level (loudness), an amp will be less strained driving an 8 ohm load than 2 ohms. Less strain means less heat, less distortion, lower opportunity for component failure.

[Rich-n-Texas]

10% off all in-stock items at www.emotiva.com

[Feanor]

I'm trying to help out a local school with the purchase of a new amp for their exercise room. They were originally using a Harman Kardon 3480 Receiver to drive 6 Infinity Primus 160 Speakers using a speaker selector box. Well after a year the amp in the receiver died. ... I presume the amp has to be stable down to 2 ohms to do this, but I really don't know. The problem is that they have a budget of around $200...

It's a skimpy budget. Could be this is as close as you'll come, new, for a 2 ohm amp ... Behringer EP2500 for $270 plus shipping: 1200 wpc into 2 ohms(!)

Or a little less power for a little more money, the Mackie FR-800.

[Kevio]

It's a skimpy budget. Could be this is as close as you'll come, new, for a 2 ohm amp ... Behringer EP2500 for $270 plus shipping: 1200 wpc into 2 ohms(!)

Or a little less power for a little more money, the Mackie FR-800.

Nice options!

[markw]

I'm just curious how, working with three pairs of eight ohm speakers, you arrive at a two ohm impedance.

I can envision a configuration where 5.3 ohms overall is possible.

[nightflier]

Mark, the Ohm measurement is variable, so it's just a guess. Since the HK amp was able to drive 4 ohm loads, I presumed that the impedance during loud passages would have dipped well below that to fry the amp, as RoadRunner suggested.

Feanor, I like the Behringer amp, especially with the bass cut-off options. It may be just what I'm looking for. Regarding the inputs, I'll be using RCA to TS connectors. The manual mentions that the 1/4" jacks can take unbalanced inputs (i.e. TS 1/4" connectors), so using mono RCA-TS converters shouldn't be a problem, correct?

[markw]

Mark, the Ohm measurement is variable, so it's just a guess. Since the HK amp was able to drive 4 ohm loads, I presumed that the impedance during loud passages would have dipped well below that to fry the amp, as RoadRunner suggested.

Feanor, I like the Behringer amp, especially with the bass cut-off options. It may be just what I'm looking for. Regarding the inputs, I'll be using RCA to TS connectors. The manual mentions that the 1/4" jacks can take unbalanced inputs (i.e. TS 1/4" connectors), so using mono RCA-TS converters shouldn't be a problem, correct?

Dunno about the mechanics of the amp involved. I'm just doing the math.

Three eight ohm speakers in parallel is a nominal 2.34 ohms but impedances generally dip at low frequencies. Note the word "nominal".

[Feanor]

...

Feanor, I like the Behringer amp, especially with the bass cut-off options. It may be just what I'm looking for. Regarding the inputs, I'll be using RCA to TS connectors. The manual mentions that the 1/4" jacks can take unbalanced inputs (i.e. TS 1/4" connectors), so using mono RCA-TS converters shouldn't be a problem, correct?

Should be no problem with RCA to unbalanced 1/4" TS -- see the manual, page 11.