Some random questions

[augerpro]

Hi guys. I need some info please...

1) If you had two 8 Ohm Nom. woofers wired in series but had 8 Ohm resistors shunted across each terminal you would end up with an 8 Ohm nominal load. Disregarding the effect on Q, what other concerns if any are there to doing this? If anyone has Speaker Building 201 there is a D'Appolito design that does this. Seemed like a neat idea.

2)When you have parallel notch (in series with driver) for say a woofer how do you determine the wattage rating for the resisitor? The potential for large current flow is pretty high I would think. I had this exact thing on my BB centerchannel. I ended up using multiple parallel resistors to get higher wattage capability while still maintaining the correct resistance.

3) Kind of similar to 2, but for series notch filters to ground.

4) if you had a 10" driver crossed to WG loaded tweeter at 2k LR4 and the ctc distance indicated combing at 1.5k would you be concerned? Kind of a complicated question probably...maybe I should just go with an 8"...anyone want to buy some Selenium 10PW3 drivers :B

[JonMarsh]

No time for definitive quantative answers, but I didn't want you to feel "ignored".


1) I can imagine reasons for doing this and reasons for not doing this, but keep in mind the 8 ohm resistors should have power handling similar to the drivers- think 50-200W. The chasis mounted alluminum cased ones should be good to go. If you're doing an LP crossover at low frequency, it will help tame the interaction with the crossover due to the LF resonance, but it will suck some watts- normally, the impedance rise in the LF lowers the current draw on the amplifier.

2) Depends on how wide the notch (area under the curve). If you consider worst case slow sine sweep testing, conservative practice might dictate a fairly large resistor wattage - 50-50W, perhaps.

3) Depends on the input to output difference- get one of the cheap/free circuit simulation demo programs and analyze the power dissipation in the resistor. LT has one, SIMetrix (Catena software in England) has a great one for Linux and Windoze.

4) Ya know, this is something a lot of folks just ignore and it always comes back and bites 'em in the @ss. Recommended max CTC at 2kHz is about 7". Gonna be mighty tough to hit that with a 10" woofer and any kind of waveguide that works clean down to 2 kHz. The side effect will be comb filtering off axis, and ragged crossover region power response. With say, a 10" midwoofer and a 10" waveguide, I'd recommend a crossover frequency no higher than 1400 Hz for optimum results. This may be tough for a waveguide that small, as an oblate spheroid that size will typically have it's low end HOM at around 1-1.2 kHz, and will work better with a crossover closer to 2 kHz, say, 1.7-1.8 kHz. Then you're looking at an optimal max CTC around 8", which works out at best with a 10" waveguide and 6-7" midwoofer.

I'd also be considering something steeper than an LR4, given the proximity of the WG roll off, perhaps LR6 as a starting point.

Just my $0.02. Doesn't mean you might get something sounding pretty good most of the time taking a different tack...



~jon

[augerpro]

Thank you Jon for the excellent info! ;x( This lack of usable WGs is starting to to get old. We need someone to start a little fabrication for the group or something.

I'll check out that software Jon, thanks again.

[Feyz]

On one, I think it is also to address the differences in T/S parameters of the drivers, which is more critical when in series than in parallel. 8 ohm resistors should dampen the impedance of the drivers, so I would think it would dampen the differences in T/S also, though I haven't really looked at it, how much of an affect it would have. But I had once a couple years back had looked at how different T/S parameters would cause when using two drivers in series vs in parallel. Below is one result of the calculations done using LTSpice that Jon referred to also:


Note in series case at tuning frequency one driver complete stops moving , and the other does all moving of air with extra excursion. Parallel drivers are not that bad, more immune to T/S differences in drivers.

I don't remember which T/S parameters I had changed and how much in these simulation, sorry it has been a while.

The simulations were done assuming the drivers share the same volume. When using separate volumes, results were better, but still not as good as the parallel case.

The extra shunt resistors I assume would remove the excursion differences between the drivers, but that is just an assumption without any real analysis and test on my part.

[augerpro]

That's very interesting Feyz! I would never have thought about that angle. Do you remember if the changes you made were representative of two different drivers, or of small spec differences we would see in a production driver? They look like they must be substantial.

I really need to do some measuring with some resistors and inductors hooked to a driver just to get an idea of what changes really happen.

[Paul W]

Though I don't have SPB 201, I saw a Joe D crossover using that approach. As I recall, the stated reason was "to balance minor differences between drivers". I used it as "good practice" with a pair of series connected D76's to reduce sensitivity but didn't notice any difference (other than the obvious difference to the crossover).

For the large mid/large WG crossover, I'm using LR8 to minimize overlap.

Here is a comparison between the DDS-1 WG and the 12" WG from Music Supply Center, both with a BMS4552ND. I don't recommend the MSC for screw-on drivers because the throat doesn't fit properly. For bolt-on, their plastic screw-on/bolt-on adaptor can be bored to exactly 1', glued in, a gap in the throat filled with microballoons or similar, then carefully filed for a smooth transition. Modification takes only about 1 hour for two waveguides.

DDS:



Modified MSC:

[augerpro]

Overall the MSC looks better yes? Did you have a cap on those or is that the natural rolloff of the WG?

[Paul W]

I have both WGs and the MSC is the pair I listen to the most...and it does quite well. It isn't quite as smooth as the DDS, but holds directivity a little better. Considering the price and DDS (un)availability, it does a great job.

Yes, there was a protection cap...about 30 as I recall, so it wasn't a huge influence. A good share of the low-end rolloff is the compression driver.

[Feyz]

Do you remember if the changes you made were representative of two different drivers, or of small spec differences we would see in a production driver? They look like they must be substantial.

I dug my archieve and found the sim files. I had changed the Fs and Mms of between the drivers, rest of the T/S params are same. One driver has 40Hz Fs and 19g Mms, the other 52.3Hz Fs and 17.7g Mms.

And I did a sim with the added 8 ohm shunts, as expected the difference between driver excursions are reduced.



The power requirements for the resistors are as mentioned before. At the impedance peaks of the ported box, most of the current passes through the resistors.I would assume decent power handling resistors.

Just using different volumes with no shunt resistors can be a fix, though not that good:

[Dennis H]

About using 2 drivers with resistors to adjust the impedance, going by Feyz's charts and an similar study by John K, I'd say the best thing is to run the drivers in parallel and use a series resistor to increase the impedance. That's better than drivers in series and parallel resistors to reduce the impedance. Same components, same net impedance, less interaction between the drivers.

[augerpro]

Is the effect on Q the same though?

[Feyz]

Is the effect on Q the same though?

Yeap Qes and so Qts gets changed (probably significantly) and also sensitivity of the system is dropped by 6db