What is a good amp for JBL 4208 monitors?

[apolloson3]

Hi - I recently bought a pair of JBL 4208 monitors and I am wondering what a good amp would be to power them. I am looking to spend under $500 - preferably much less, but I don't want a garbage amp. Any suggestions?

[DAS]

A lot of people seem to like the Alesis (http://www.sweetwater.com/store/detail/RA300/) amps for the money. They make it in several power ranges. If you can find an old, used Hafler it would be good, provided you find one in good working order.

[apolloson3]

what power range would be the best? I know it is possible to have not enough power - is it also possible to have too much power?

[DAS]

I still have a pair of 4208's in one of my studios. I use a Hafler amp with about 120 watts of power per channel on them and it's plenty. I sit pretty close to them most of the time and could probably get away with much less power 90% of the time.

[apolloson3]

can you reccommend to me some specific model numbers? (hafler or alesis)

[DAS]

Click on the link above, in post #2 of this thread.

[apolloson3]

So - I read somewhere that the Alesis RA-300 is a good power amp for the 4208s, but a larger amp would provide more headroom. What larger amp would that be? Also, I'm unclear as to whether the monitors want 75 watts at 8 ohms per channel or 150 watts at 8 ohms per channel? What is the risk of overpowering/underpowering the speakers?

[DAS]

So - I read somewhere that the Alesis RA-300 is a good power amp for the 4208s,...

Yeah, that's what I linked to above.



...but a larger amp would provide more headroom. What larger amp would that be?

The big brother to the RA300 is the RA500. In my opinion it is probably overkill.



Also, I'm unclear as to whether the monitors want 75 watts at 8 ohms per channel or 150 watts at 8 ohms per channel?

I don't remember this either. It might be written on them somewhere.



What is the risk of overpowering/underpowering the speakers?

Overpowering - if you put too much power into the voice coils and crossover electronics something will fail. Not sure what "too much" is in this case. It will certainly depend on frequency content a lot.

Underpowering - the risk is that with too little power you may come up against the headroom limitations of an amp too much, which can cause clipping and other forms of distortion that are arguably more damaging to speakers than minor overloading with clean power.

So either way you can blow them up if you turn it up too loud.

[apolloson3]

Thanks for fielding all of these questions singlehandedly, DAS. I really appreciate it. So - on the back of the speakers, it says "power handling = 75 watts". But in the manual, the power capacity for the speakers is listed as 75 watts continuous pink noise, 300 watts peak pink noise, all at 8 ohms. Does that mean that power anywhere between 75 and 300 watts at 8 ohms per channel will work?
Looking at the different models of power amps: the Alesis RA500 lists these three power options: "250 Watts RMS per channel into 4 ohms, 125 Watts RMS per channel into 8 ohms, 500 Watts RMS bridged into 8 ohms." Does that mean that, if I find an RA500 for cheaper than the RA300, I could set it to 125 watts per channel at 8 ohms and that would work? I wonder what is the optimal wattage for the best sound. As you can tell, I am experiencing some confusion over the matter. Thanks again for your help.

http://www.sweetwater.com/forums/images/smilies/classic.gif

[yeahforbes]

The 75 versus 300 thing really has to do with different parts of the speaker failing for different reasons. Constant sound that causes more than 75W amplifier output for extended periods of time could cause the coil to heat up to the point of failure (although the cone wouldn't be moving beyond its limits). Bursts of sound that cause an instantaneous 200W, 250W are fine, as long as it's just a waveform peak that happens infrequently. But exceed 300W on one of those, and you're causing your woofer to move farther in and out than it safely can, that's called over-excursion. Or it might be the limit of a crossover component, or anything else. This is a very simplistic way of thinking about it, but should give you an idea of why the huge difference in wattage figures.

This is why you want an overrated amp (300W) for that headroom, but don't assume you can crank it up near its clip point with a heavily compressed source.

[dpd]

Grab a Hafler DH-200 or DH-220 and 'POOGE' it! (the original mod sequence defined by Walt Jung and Dick Marsh in the mid-80's). There are gobs of resources on the net for the Hafler amp. eBay always has them listed for cheaps.

I POOGED mine way back then, but the right channel FETs finally died. Unfortunately, the power FETs are no longer available from Hitachi. You can find some on eBay or go with the Exicons. I'm currently re-modifying my old DH-200 into a pair of mono blocks, replacing the FETs with the Exicons and adding some AC line RFI filtering. Dual-mono rules - WAY more dynamic than the stock Hafler power supply and cleaner due to less distortion and crosstalk.

Warning - you can go crazy with these mods and spend a ton of $$$ (and get killer sound in the process.) See the Musical Concepts (http://www.musicaldesign.com/mus_con.htm) guys, for one. So, set your budget and decide if you want to do mods or just buy an 'appliance'.

Some good, moderately-priced amps with pretty stout power reserves you can find used (don't know your budget): Rotel 1080, Parasound Halo A21 or A23 (John Curl design!), Adcom GFA 5500, NAD C272.

PM me if anyone wants details

[apolloson3]

OK - so I finally got a power amp for my monitor speakers. The power amp is a Hafler transnova p1500. My speakers (as the topic suggests) are JBL 4208s. Now I need some help putting everything together. I am unclear as to how everything is connected (i.e. signal flow). It's my assumption that the output from the power amp goes into the speakers via banana jack, correct? Where does the input into the power amp come from? I use an Digidesign MBox 1, which has two 1/4" outputs which I have used in the past to feed audio signal directly into my powered monitors (I had a pair of Behringer truth b2031's). How does this work with the power amp in the equation?
Also - when I do get it set up, how far should I turn up the gain on the power amp? Each channel knob ranges from -15dB to 0dB. The power amp is rated at 170 watts, which I think is 85 watts per channel. On the back of my monitor speakers, it calls for 75 watt power handling at 8 ohms. Where should I set the gain on the power amp knobs to be at an optimal, non-dangerous level?

[dpd]

I'd recommend a level controller in between the Mbox and the amp so you have a convenient volume control, otherwise all you have is a ProTools fader to set volume.

One way to set master volume is to get a -20 dB FS noise file (I think you can find one at Bob Katz' web site). Play it back and set the amp volume control settings for a good - but not maximum - volume (Bob recommends 83 dB SPL).

[DAS]

Depending on which Mbox you have you should have a workable control you can use for volume, or as dpd suggests you can find a separate box to do that. I'd start with the amp at the minimum setting (-15), which will probably still be more than enough. Connect speaker wires in polarity between the amp's outputs and your speakers. Nothing too complicated here.