2-way MTM Active DSP project, number of amps, impedance, damping factors?

[lofgrenmeister]

Hi all!
I'm in the process of starting my first speaker building project, I plan on taking the "easy" route of using one Hypex AS2.100 DSP plateamp on each speaker, With ScanSpeak Relevator midwoofers and Vifa/Peerless XT19 drivers.

It'll be floorstanders with a narrow front baffle, and I'm at a point where I can't decide if I want a 2-way MTM or a 2-way TM.

The idea with a MTM is to wire the 2 midwoofers in parallel (18W/8531G00 - 8ohm version), sharing one amplifier channel, and the the XT19 tweeter on the other channel.

As i understand it one of the main benefits of active crossovers and dedicating an amplifier channel to a driver is direct control over the driver by the amp, in terms of damping, impedance variations etc.

My main concern with connecting two midwoofers in parallel is that this control is somehow reduced, by perhaps by small factory variations in the woofers, do you think this is something to be concerned about?
I know the impedance will be effectively halved to 4ohm when connecting the woofers in parallel, and that I can expect 6dB increase with twice the electrical increase (i.e. 6dB+ for 2w over 1w), or 3dB increase in acoustical output per 1w, but are there more things I should be concerned about?

I've noticed several active studiomonitors in a 2-way MTM configuration usually has 3 amps, with only one crossover point, with both amp channels on the midwoofer producing the same signal.
But other than increased power to the more power-hungry bass/midrange, do you think there are any other reasons they do this?

Sorry for the noob questions, I should probably buy a book on the subject! But I'll be grateful for any hints/tips, thanks

[bunkie]

One advantage of using this approach is that simply by adjusting input level to each channel, you can control the relative levels of the tweeter and woofers, so I wouldn't worry too much about the parallel woofers output level.

One disadvantage is that tweeters require far less current than woofers, so dedicating 50 watts to it is a bit of overkill.

[lofgrenmeister]

Yeah I know what you mean, the tweeter even gets 100w into 4ohms
Perhaps i'll go the minidsp+chipamp approach instead

[spintonik]

I have experience from Hypex AS2.100 DSP in MTM configuration, and from Hypex Filter designer perspective, it does not make any difference if you wire woofers in series or paraller. What you cannot do, is to have separate XO filterfor each woofer + tweeter, as ths would need 3 amp channels.

(Actually, DSP has 3 channels, one is meant for subwoofer, so you COULD do it, with external amp).

I used Hypex as a fast prototype platform before staring to fight with analog xover, and was very happy to results.

But regardless your wiring, you're stuck with same stuff as allways:

1. Build Box and connect speakers
2. Measure each speaker (in your case both woofers combined)
3. design filter
4. ponder why it does not work
5. re-design filter
6. Repeat parts 3-5 until happy to results

If you insist on building MTM instead of TMM, You should have a look at Joseph D'Appolito's White paper about lobing errors and directivity. IMHO MTM is not "first easy and funny project" to do, as I am now harshly finding myself too (I stumbled to this thread because I was browsing through the forums to find solutions for my issues with MTM lobing errors.)

Good luck!

-Mikko

[Piersma]

hi lofgrenmeister,

Did you already simulate the suggested driverset in the MTM configuration?
I would provide valuable setup information.