Another sealed cab question... Sealing a badly tuned ported cab!

[Bigwan]

I know there's another sealed cab thread running but I don't want to muddle it.

I recently bought an old Peavey 410TX cheap. It has had the 4 original drivers replaced with 4 (older model) Eminence Beta 10 drivers. There's been no attempt to retune the cab for the replacement drivers, and it was originally a 4 ohm cab, but the new drivers have been configured to 8 ohm. I always run the cab with the tweeter all the way off. The cab sounds... mediocre, although all I have to compare it too is a Barefaced 2x10 which is unfair in the extreme.

My question is, rather than going to the time (which I have little of) and effort of retuning the cab for optimum ported performance, could I take out the crossover and tweeter, patch the cutouts for the port and the tweeter, add some stuffing to the cab and use it as a sealed cab?

[stevie]

That's how those drivers sound in that cab and there's probably very little you could do to the porting to change that. The Beta 10s are suitable for used in sealed cabs. How well they will work in the Peavey depends on its internal volume. Why not simply close up the port by stuffing some cloth inside, and have a listen? If it sounds promising, take it from there.

Sealing the cab will reduce the amount of low bass. If the problem you are experiencing is excessive, boomy bass, sealing the cab could be a good solution.

[Beer of the Bass]

Could there be some weirdness going on from running 8-ohm drivers with a crossover designed for 4-ohms? I don't know much about crossovers, but it seems like it could move the crossover frequency around. IIRC these Peaveys use a proper crossover which low-passes the woofers, so would still affect the sound with the attenuator turned down.

Edited September 30, 2016 by Beer of the Bass

[stevie]

Very good point, BoB. I agree, there will definitely be some weirdness going on if it's a proper crossover (you don't come across those very often). Better to try it with the crossover bypassed first. To convert the crossover to 8 ohms, double the size of the inductor and half the size of the capacitor going to the LF drivers. In the unlikely event that there is a resistor in the LF circuit, you can probably leave it as it is.

[Bill Fitzmaurice]

[quote name='Bigwan' timestamp='1475237660' post='3144278']
My question is, rather than going to the time (which I have little of) and effort of retuning the cab for optimum ported performance, could I take out the crossover and tweeter, patch the cutouts for the port and the tweeter, add some stuffing to the cab and use it as a sealed cab?
[/quote]If you seal it you'll lose about 4dB of sensitivity below 100Hz, and 12dB of maximum SPL at 50Hz, which is huge. If you don't use the tweeter you should pull the crossover. If it's not currently lined with damping material it should be.

[stevie]

That's not correct, Bill. You're assuming that the driver is matched to the cabinet, which it isn't. A QB3 cabinet for a Beta 10 is nearly 100 litres. Even Eminence suggest using a 64-litre cabinet per driver, and it still peaks then.

To try to prevent this from becoming really confusing, I've modelled the driver using 90 litres as the internal volume for the Peavey cab - that's probably not too far out. This is what the frequency response looks like:



The yellow curve is the sealed box and the pink one is the vented one. Personally, I'd take the sealed curve anyday, but some people like the peaked response and that's their choice.

I also modelled power handling. The sealed cabinet reaches the driver's 3mm xmax at 50 watts input, the vented cabinet at 75 watts. So the choice is between a 200-watt sealed box or a 300-watt vented box.

Having said all that, Bigwan needs to take it one step at a time, and the first thing to do is to connect the drivers directly to the input, bypassing the crossover. That could be the real problem.

Edited September 30, 2016 by stevie

[Bigwan]

Thanks folks!

I roughly figured about 4.7 cubic feet going by external dimensions minus 3/4" of material per face and recessed grill. Didn't go as far as to account for bracing (probably minimal!), volume of port tube or drivers. I'd guess from that that it's maybe slightly more than 90 liters, but probably not much more than 100, 110.

I'll unhook the crossover first and take it from there as suggested.

[Phil Starr]

Does anyone know whether the 'crossover' is simply a high pass filter to the tweeter or a proper crossover. If the former it will work to protect the horn and the impedance of the horn won't be an issue but the problem will come because the betas have a healthy midrange peak which may add to the horns output and be a bit too much to sound good. If we can ascertain how the crossover is set up (photos would help) we might be able to give some advice.

I'd advise setting up with the whole horn/crossover out of circuit however, it might sound quite nice as the Betas look like they have a nice response curve for bass.

[stevie]

BoB said earlier that they have a "proper" crossover and not just a high pass filter. If it's a 2nd-order crossover designed for 4 ohms, connecting it to an 8-ohm load will cause a nasty peak in the midrange and pull the impedance down below nominal. If it's just a 1st-order (inductor only), it will lower the crossover frequency and depress the mids. Either way, not good.