(Attn Planet 10!) active xo & 2.5 way back to back Vifa P13 mids - questions

[paulspencer]

This is in particular directed to Davd (Planet 10), but I'm putting it in here for others to have input and it might interest others as well ...

Regarding the idea of a 2.5 way design where the 2nd 0.5 midwoofer faces back. Suppose I were to do this with in a 3.5 way design with Vifa XT tweeter and P13 on the front and one facing the rear. Peerless XLS 10" downfiring. Suppose I wanted to make this an all active design:

1. how far should it be placed out from the rear wall? Also how much is this likely to affect the BSC required?

2. how would you deal with an active xo which is 3.5 way?

Suppose I have the P13 mids actively crossed at 100 Hz to the XLS woofer, but one of them has a first order passive filter at say 600 Hz for BSC. Or should this be done with active filters? If I do it actively then it means a different amp for each woofer, which is perhaps not so bad if they are simple chip amps.

[planet10]

The push-push part of the system (the P13s) won't have any baffle step. To keep energy/power distribution fairly flat you'd probably want to roll the back off -- but it becomes much less critical in terms of getting it right.

When i'm doing it i;ve usually got something pretty extended as the "mid" covering at least 100-10k so i'll often just let the back driver run all the way up -- my room likes bipoles.

Since your back driver is rolled off, you probably don't need to pull them any further out than you normally would for decent imaging.

If you have the amps 1 for each driver is a good plan... the low pass on the back driver can simply be a cap to ground. A cermet pot in series with an appropriate series input resistor would make it adjustable to get this XO point at the right place for your baffle shape.

dave

[Timn8ter]

You keep getting younger and younger Dave

[paulspencer]

Dave,

Does the back driver have to be the same as the front? I notice the P13 rolls off early - fairly sharply below 150 Hz. Do you think it would work well to use the P17 on the back? Otherwise I think I would have to live with a dip where the mids cross to the woofer, or use another pair of P13s.

[planet10]

Originally posted by paulspencer
Does the back driver have to be the same as the front?

If you use 2 differenent drivers you open up a whole can of worms. 2 drivers loading the box differently -- and if you split the box in 2 you still have to worry about the phase differences as the 2 drivers roll off at a different place, not to mention efficiency differences. And you lose the close to perfect reaction-cancellation of 2 identical drivers push-push.

Possible but real tricky -- if you are worried about the bass, might as well just add the active wooder.



response to 60 cycles or so isn't bad for a 5" (top trace is the bipole) -- and i don't think the above enclosure has been optimized for bass.

dave

[paulspencer]

Which driver is shown in that plot? ... I only want the P13 to get down low enough to cross to the active XLS 10" woofer, which is down firing, so I assume that I need to be able to get down to 120 Hz with the P13.

[planet10]

Originally posted by paulspencer
Which driver is shown in that plot? ...

A pr of P13s -- in the TLb



dave

[leadbelly]

Hmmm, in my uninformed mind, I would have thought that it would be preferable, if doing a 3-way push-push, to make the woofer push-push, and cross over to the mid at some point above the baffle step freq, so that the extra woofer in the back would more effectively make use of room gain. In this case, the push-push mid takes care of baffle step, but the subwoofer does not gain any of the push-push benefits. Or did I miss something?

[planet10]

Originally posted by leadbelly
In this case, the push-push mid takes care of baffle step, but the subwoofer does not gain any of the push-push benefits.

You are correct ... my preference is to always use woofers in pairs (ie 4 for stereo)

dave

[paulspencer]

leadbelly,

ideally yes, but in this design its a bit different considering both the extra complexity and cost.

1. cost - the woofer has high output and is the most expensive driver - making up half the driver cost

2. complexity of getting it into the box

The box has curved sides 210mm wide at the front and back baffle, 400mm deep and 270mm wide at the centre to allow for the downfiring woofer. I did consider it but in this case I prefer the simplicity of one woofer.

Then again, perhaps if I used the Peerless CSX, it would be a bit more affordable ...