Alesis RA500 power amps & ohms in speakers

Just opened boxes for two Alesis RA500 power amps sent me by Simon for very reasonable s/h price. Having had the gain controls removed (by Simon) the amps have 150 watts per channel or can be set to mono-block mode with 500 watts each. "They are basically as close to a blameless power amp as you will likely find."

My plan is to assess these as monoblocs, running in fully balanced mode from my Dax Decade and into epos es22 standmounts - or into some future speaker upgrade. If that doesn't work out then, the options are to give them as stereo gifts to offspring or sell them on.

I decided to sit and read the manuals while I listened to some music, especially the bit on setting them to mono mode. The manual has some nice features, readability being one. These include a final page entitled "Architect's and Engineer's Specifications". There are also the following passages about speaker cables and impedance, the first two amusing, the third, the subject of a question to them that knows:

1. "In recent year years, expensive audiophile cables of high current capacity have appeared. these have been somewhat controversial; some feel ... "

2. "Never use shelded guitar cords as speaker cables."

3. "... it is possible to use it as a mono amplifier with twice the power. This uses both sides of the amplifier in a push-pull configuration, so the speaker must be wired differently. .... However. note that the minimum load impedance is 8 ohms, instead of the the 4 ohms in stereo operation"

That last remark had me thinking.

The impedance for my speakers is 6 ohms; sensitivity 87 db. They have a 1 inch dome tweeter and twin 6 inch midwoofer and bass driver. This loudspeaker does not use a crossover, just a capacitor to protect the tweeter. The amplifier connects directly to the drivers.

Should I be concerned?

 

this issue with using them in mono is that each amplifier section effectively see's one half of your speakers impedance... e.g. in the case of yours some 3 ohms.. and thats pretty low. The amp wont be all that happy driving into that for long periods. You could try it, and i dont think you will do the speakers any harm, most likely the amps if anything.

Its why i have never liked bridging stereo amps.... in my opinion its a poor replacement for a proper monoblock and rarely gives the same power delivery of a single larger amp (due to the fact that the amp is struggling into lower impedances)

 

I think you will still get more power monoblocked... I would think that if it will deliver 150 watts bridged in to 4 ohms (same as a single channel in to 8 ohms) and 500watts in to 8 ohms then it should deliver somewhere between that to 6 ohms.

So far how do they sound in standard stereo operation?

 

I was not feeling short changed wrt the extra watts from bridging, and in fact was looking for the push/pull from bridging. It was the speakers I was concerned about.

Dont know a lot about impedance, altho I should add that I have Goertz speaker cable, which purports to be 4 Ohms.

Simon, am now using one in stereo, and it sounds very clean with a fair amount of punch and drive - with Ruben Gonzalez
http://www.amazon.com/Chanchullo-Rubén-González/dp/B00004X158/ref=pd_bxgy_m_img_b

 

Your speakers won't likely be damaged, if anything the amp should shut down, but it should really be just fine. Don't worry.

The cable has a characteristic impedance of 4 ohms, but thats very different form resistance, which the amp would see as a load. The cable will not do anything to help here. A good thing too, or damping factor would go out the window.

 

OK, just waiting for wife and daughter to go out, then its bridging time.

did you say that you had disabled the gain/vol control?

 

Yeah I bypassed the volume pots with some silver wire. If you want them back just cut the thin wire on the back of the pots, it should be clear what ones.

 

I'm owner of x2 Alesis RA300 and will be buying more RA300 at a later date along with x1 RA500 for sub bass and LFE.1 sub bass channel.

Question: does the Alesis RA500 that you have produce a mild humming sound as it is when powering it up, can you hear a mild transformer humming sound.

Thanks.

 

so, I have vol knobs up to max - except the right ones when playing in mono mode - right?

that's how its doing now, and they're doing just fine. big sound stage. I'll let them warm up a bit.

great value, no doubt.

I cant hear a hum just know but I'll listen out

 

Hmm.. did the manual say something about turning one channel down when you mono-block? I do kind of remember that. It might be an idea to remove the bypass if that is the case or its as though both controls are at max, no matter what position they are in. I always used them in stereo.

I used to notice a slight hum from the transformers but it got quieter when they were warm. I bought 3 amps in all, and sent one back for replacement when I bought it as the hum was louder than the others, and I used to sit very close to the amps.

 

Yeah do that because I brought a used Alesis RA300 from Turnkey last year and it sounded great. Then a few weeks later I brought a second Alesis RA300 from Digital Village and once I received it and powered it up, I noticed straight away a mild humming sound that was so off putting in the room.

It wasn't a ground loop humming sound being reproduced though the loudspeakers I do know the difference. This was the fault of the production build, the top plate is too close to the transformer which is located in the centre of the RA300 and no doubt the RA500.

Or then again maybe the RA500 is built differently over the RA300 anyway power her up, and listen in a quiet space and see if you can hear any transforming humming sound, even if it means turn it on then off while listening.

Thanks.

 

There certainly is the potential for speaker damage - nothing to do with the speaker impedance issue, but everything to do with being able to feed 500w into your speakers. Watch the volume!

Ian

 

It's not really about power it's about the ability t play loud without distortion. Just watch the levels as some source material can be very damaging, I'd get a loudspeaker management system and maybe a SPL limiter!

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hi Ditton, how are you getting on with the Alesis?