speaker amplification

[Tripp]

so ive been deciding on the drivers to use in my MTM 2.1 speaker build and im getting my head around all the finer points of speaker building when it dawned on me that i dont know much about the amplification side of this project.

As ive never owned a real amp before i wasnt sure where to start looking, i have decided that i would like to invest in pre/power amp seperates for my 2.1 setup. i have no intention of upgrading to surround sound until i own my own house and i will be using it for music 60% of the time so a nice stereo setup will satisfy me atm.

I know i will need a system with pre outs to hook up an active sub BUT when it comes to wattage i come a bit unstuck. obviously i dont want my first amp to underpower my shiny new built speakers.

my MTM design will be consist of - Vifa XT25TG30-04 1" Dual Concentric Dome Tweeter & Usher 8945P 7" Carbon Fiber/Paper Woofer w/Phase Plug (x2)

What im wanting to know is if there is a formula to working out how much amplification will be required per speaker so as to run them and be satisied with the sound at high volumes...say 90-110db.

I will be wiring the ushers in parallel (totaling 4ohms)

If anyone has any recomendations for any pre/power amps that may suit my needs please feel free to mention them

Cheers

[BoomieMCT]

Your speakers should be reasonably efficient so you don't need a whole lot. Off-the-cuff and depending on the size of your room you could probably get by with as little as 65 watts per channel.

"Reference level" is something like 105dB peak at your listening point. If you model your speaker in Unibox it will tell you the modeled efficiency with 1W/1m (not the voltage one). If you take the base efficiency and subtract 6dB for every doubling of distance (-6 dB @ 2m, -12dB @ 4m, etc.) and add 3dB for every doubling of power (+3dB for 2 watts, +6dB for 4 watts, etc.) you can find out how much power it takes to get 105dB at your listening point. This is assuming the drivers can take the power thermally AND that they have the displacement to hit that SPL at the lowest frequency they are expected to play.

So, if these are 90dB efficient at 1W/1m (I made that number up) and you listen 4 meters away you'll need 512 watts to get to "reference level" with one speaker (which would probably damage your drivers so listen closer). Please note I may not have my numbers right (doing this from memory) and this is assuming an ideal situation (perfect amp, no line losses, etc.).

One thing to consider is that a good multi-channel setup can be run in stereo mode but not the other way around. Depending on whether you'd rather pony up more money up front or have to buy another pre-amp when you want to add surrounds you may want to get a multi-channel pre-amp now. Also - if you need to buy smaller amps now they could make fine surround channel amps in the future.

[Tripp]

Ah thanks, thats the kind of working out i was looking at, appreciate the help

You do raise a good point with using a multi channel preamp and expanding the number of power amps later for possible multi channel speakers, that would probably be wiser though i imagine the prices for such processors would be huge.

Know of any cheap multi channel preamps?

[BoomieMCT]

Please note I'm not saying you need 500 watts / channel. That was a quick back-of-the-envelope calculation for one speaker. If you had two in the above example you'd only need 125 watts/channel. I'm not sure if the 105dB "reference level" peak has to be generated by each speaker or all the speakers in the system together.

Can someone else verify that what I'm saying is true?

[BoomieMCT]

Tripp wrote:

Know of any cheap multi channel preamps?

Wasn't Sonnie looking for one a while back?

[drf]

BoomieMCT wrote:

Please note I'm not saying you need 500 watts / channel. That was a quick back-of-the-envelope calculation for one speaker. If you had two in the above example you'd only need 125 watts/channel. I'm not sure if the 105dB "reference level" peak has to be generated by each speaker or all the speakers in the system together.

Can someone else verify that what I'm saying is true?

I believe this level needs to be met by each channel individually.

On a side note, if cost is an important factor, or you can't get reference levels from your design/driver selection, keep in mind that 105dB is actually quite loud. Most people do not listen to music or watch movies with their system turned to reference level. Having it peak at reference simply gives you the headroom you need for movies with bigger sonic dynamics. As for music, these days most recordings are so compressed that headroom is almost non-essential.

Good luck.

[mayhem13]

Outlaw audio 990 is not that cheap overall but considering what it can do, it's a steal. I am one for using headroom in my system. I like having loads of power on reserve to minimize introducing distortion into a system. I can't control the distortion already present in the recorded material nor inherent to the speaker design but by having lots of power, i certainly won't be adding any. I really enjoy the dynamics this provides as well. The Outlaw has balanced xlr outputs for amplifier connections and as Boomie stated, you can mix up the power amps depending on budget and speaker sensitivity. Don't skimp on the sub amp though as IMO is the foundation of any .1 theater system along with the center channel which is far more forgiving.

[thewire]

There might be also be such thing as too much headroom. I seem to have 104dB peeks at 70dB -30dB FS with them set to large so something is certainly right going on here. I think if you are going to have alot of power you will want to be able to control the trim level on each amplifier. Before when I was doing this I had no issues when set to small or large. I tried playing some music on my front left and right speakers on a CD like this and with 360W per channel on two 250W speakers the bass would fade on and off to softer levels. I switched the left and right to my rear speakers where I put my other pair of speakers (two towers each channel for more difficult to pinpoint sound similar to dipole bipole etc) more recently that I used to use for 7.1 when I had a longer HT. So I'm playing two pairs of towers in the rear of my room which I have setup for 70dB at -30dB FS with a CD called Epic The Essential Heart. They are 250W capable speakers each powered by 125W and 120W. I walk out of the room and get my hearing headphones for protection and turn on the meter in my listening position. I then read avearge 100dB and 111dB peeks with the meter set to C weighting. I then think to myself this is

[thewire]

Correction: I was watching "The day after tomorrow" in DTS on DVD and the speaker began to fade in and out again. I turned on the lights and took a closer look and I had forgotten to hookup my other speaker wire back up. I was testing the difference between single 12 AWG versus double 12AWG and I left it undone. Music and movies work fine now sorry. Carry on with high power amplification as you were.

additonal note: these were hooked up to an unknown guage pair in my wall that go in a spiral (for some reason). They looked like 14AWG and it about a 50ft run to the bridged mono amps.

Edit: Found the source of the problem. Cannot use more than a single 12AWG on the amplifiers themselves whether in use or not. These were leftover from once using bi-amplification for additional power. Strange since I used to power subwoofers with 10AWG on them just fine.

[DasBot]

If you have $50 to blow, go for a cheapie Sure Electronics full range class D amp. You only need an outboard power supply. It's got four channels to work with too so you can bi-amp