Speaker design, X-Over, and MTM Questions - Please help a noob!

[planet10]

I have to agree with Hisatugo. You need to choose drivers 1st (unless it is a case of what you are gifted with). I like to choose drivers that are very extended, i usually start with a "full-range" to do the middle since i prefer not to have any XO points between 300 & 5 K and further out is better (80-150 at the bottom, and 6-15k at the top (if needed)). If the "mid" speaker is listenable without any help at either end, i consider that a good start.

I also like to get as high efficiency as i can, unfortunately with my favored 4" in the middle, this usually limits me to 90-93 dB.

dave

[Hisatugo]

Sbolin,
oh, you are welcome...

So let's show you what I did, as Dave did. So you can learn collecting experiences...

Drivers are Dynaudio 21W54 / 15W75 / D-260 (out of sales)

The last (8th) speaker have LP at 360 Hz, 3rd order electrical (4th acoustic) BP at 360 Hz/ 2600Hz 2nd electrical (4th acoustic) and HP at 2600 3rd electrical. I'm serious thinking on active as soon as my gain clones are ready. Passive in this case are very difficult to realize as I have 6 drivers each channel (two of each one) in WMTTMW config. Response has several small dips that I think comes from the room response and reflections I am dealing with.

Unfortunately I can not stay out of crossing in the 300 to 5K as Dave recommends (good advice). The response curves of the drivers do not permit or would make circuit a real challenge.

Regards..

Hisatugo

[Jennice]

Dave,

I'm really starting to like your concept of the push-push woofers for "active" BSC.

There's still one important thing (probably mre things later ) which I haven't figured out.

Obviously I need to figure out the formula for calculating the BS frequency, and based on that I need to find the appripriate frequency for the LP filter for the compensating Mid/woofer.
BUT: Is it a simple 1st order phenomenon, requiring just an inductor in series with the compensating driver, or what lind of filter would it need?

The units I had in mind are:
Tweeter: Scan-Speak D2905/9300000
Woofers: Peerless HDS 6¾" 850439 (I'll need two for each side then).

My plan was to make an active XO for a 4 ch.amp, maybe with 2nd order, crossed at 3.5kHz. One channel for each tweeter, and one for each mis/woofer section in each side. Then I'd still need the passive filter for the BS compensating woofer.

Would this be good, or can you see any "obvious" mistakes you'd make fifferent?

My reference (for comparison)sound at the moment is the B&W 309 floorstander, and I'd like to make it better than that.

Jennice

[planet10]

Originally posted by Jennice
Obviously I need to figure out the formula for calculating the BS frequency, and based on that I need to find the appripriate frequency for the LP filter for the compensating Mid/woofer.
BUT: Is it a simple 1st order phenomenon, requiring just an inductor in series with the compensating driver, or what lind of filter would it need?

luckily the frequency you need to roll the back one off at is right near were a woofers impedance curve is usually its flattest so an inductor is often all you need. Sometimes a zobel to flatten the impedance rise might be needed.

The f=4560/w (in inches) gets you in the ballpark. "The Edge" simulator or the FRD BDS will get you even closer.

The beauty of the 0.5 woofer on the back is that as long as you err on the side of too high, because it is in the shadow of the cabinet you still get perfect baffle-step compensation, you are only worried about extra total energy in the room coming from the back speaker. You could even start out by just letting the back speaker run ragged and see how it interacts with the woom/

My plan was to make an active XO for a 4 ch.amp, maybe with 2nd order, crossed at 3.5kHz. One channel for each tweeter, and one for each mis/woofer section in each side. Then I'd still need the passive filter for the BS compensating woofer.

Thst works -- you could also use 3 amps/side (an advanatge if you are trying to get max level out of 2-8W tube amps)

dave

[Jennice]

Ho Dave,

Thank you for the description and and the links.

I have 150W x 4ch at my disposal, so I don't think I'll run out os juice.
My only thought is, if it's overkill to make this bi-amp thing? If I could use a reasoably simple passive filter, maybe it would be just as good?
Then again, tweaking filter values and volume levels is a lot cheaper when talking about small signal components.
Speaking of which...
I have been thinking, whether an LC-based low pass filter or two RC filters after each other gets me the best result. Theory says I should get 2.nd order from both (IIRC), but what does real life say?

Jennice

[planet10]

Originally posted by Jennice
if it's overkill to make this bi-amp thing? If I could use a reasoably simple passive filter, maybe it would be just as good?

Anything more complex than a single cap on the tweeter, go active

Speaking of which...
I have been thinking, whether an LC-based low pass filter or two RC filters after each other gets me the best result. Theory says I should get 2.nd order from both (IIRC), but what does real life say?

Not many LC active XOs get made since RC is easier.

With that 4 k peak on the HDS, 3.5 kHz is going to be interesting.

dave

[Jennice]

Originally posted by planet10

With that 4 k peak on the HDS, 3.5 kHz is going to be interesting.

dave

Hmm... Somehow I don't like that word... "Interesting".

Where would you cross it? The tweeter data sheet says that I should go no lower than 2.5kHz with a 2nd order filter, but I was thinking of using the drivers roll-off.
I don't like going too close to the 1-2kHz either.

The beauty of the 0.5 woofer on the back is that as long as you err on the side of too high, because it is in the shadow of the cabinet you still get perfect baffle-step compensation, you are only worried about extra total energy in the room coming from the back speaker. You could even start out by just letting the back speaker run ragged and see how it interacts with the woom/

I don't think I understand this part... or the consequences.
What do you mean by "letting the back speaker run ragged"?

Jennice

[planet10]

Originally posted by Jennice
What do you mean by "letting the back speaker run ragged"?

Don't use anything to roll it off at the top.

I wonder if there is a "mechanical way of killing that 4 k peak?

dave

[Jennice]

Hmm.. interesting thought.
If there's a rise at 4kHz, then why can't the summed (with the tweeter) SLP be corrected reasonably well by letting the tweeter HP start a little "too high". If the woofer had a flat response, it would cause a fall in the total SPL, but maybe the peak could compensate?
If this approach was to be used, shouldn't I try to create a filter for the tweeter, which has a similar steep curve, to match the woofer roll-off, giving a constant SPL in total?

I always thought it was a sin to let a driver be pushed to it's natural roll-off, but is that only for tweeters then (dur to damage with LF signals)?

Jennice

[planet10]

Originally posted by Jennice
If there's a rise at 4kHz, then why can't the summed (with the tweeter) SLP be corrected reasonably well by letting the tweeter HP start a little "too high". If the woofer had a flat response, it would cause a fall in the total SPL, but maybe the peak could compensate?
If this approach was to be used, shouldn't I try to create a filter for the tweeter, which has a similar steep curve, to match the woofer roll-off, giving a constant SPL in total?

The peak is probably from ringing in the cone which has more problems than just the FR

I always thought it was a sin to let a driver be pushed to it's natural roll-off, but is that only for tweeters then (due to damage with LF signals)?

A midbass that rolls off smoothly on its own is a good thing IMO. This is a natural 2nd order rolloff. This is the kind of driver i'm always looking for.

Arguments against just using the natural rolloff is that the off-axis response gets pinched just below where the tweeter comes in... choice of tweeter can minimize this, and the rest is your choice of compromizes. I like as few reative components as possible between amp & speaker.

Going active gives you a lot more choices.

dave