Crossover question

[walterw]

Because power doesn't disappear. It just gets used in different ways. In the case of a variable L-pad on the tweeter, if you turn it all the way off, the L-pad is 'soaking' up the power that was meant for the tweeter in the form of heat.

That's really hard to believe, especially for any kind of professionally-built cab.

Doesn't the L-pad present a very high impedance to the circuit, like 50 ohms or something? That would "shunt" basically all the power to the 8 ohm speaker load instead.

 

[will33]

There must be a way to protect the crossover so the tweeter can be turned off like on the GK MB’s, etc - no?

In those and many bass cabs, the woofers still recieve a full signal, no lowpass so you could simply put an on/off switch BEFORE all the filter components, lpad and tweeter. Don't just unhook the tweeter.

 

[will33]

That's really hard to believe, especially for any kind of professionally-built cab.

Doesn't the L-pad present a very high impedance to the circuit, like 50 ohms or something? That would "shunt" basically all the power to the 8 ohm speaker load instead.

The lpad only works because it presents an 8ohm load or close to it throughout the range of the dial, otherwise it would be an attenuator, like a volume knob on your bass, which cuts volume by increasing resistance.

 

[BurningSkies]

The lpad only works because it presents an 8ohm load or close to it throughout the range of the dial, otherwise it would be an attenuator, like a volume knob on your bass, which cuts volume by increasing resistance.

+1 Not all knobs are the same. (loaded statement, I know)

Beyond that, very few "professionally built" cabs are truly crossed over. At best they have a high pass on the tweeter only, or just let the 'natural' off of the tweeter take care of things.

 

[BassmanPaul]

I'm sure Paul or one of the others here could do a better job of explaining it.

I think BurningSkies and your good self did just fine.

 

[1958Bassman]

That's really hard to believe, especially for any kind of professionally-built cab.

Doesn't the L-pad present a very high impedance to the circuit, like 50 ohms or something? That would "shunt" basically all the power to the 8 ohm speaker load instead.

One of the basic Laws of Physics states that "Power can neither be created, not destroyed". It can change forms, however. That's why things get hot when electricity flows. An L-Pad presents a fairly constant load to the amp- otherwise, some amps will have a problem with a high impedance load (tube amps, especially). Most L-Pads are rated at 8 Ohms- it's shown on the case. If you look at a fixed L-Pad circuit, you would see a resistor in series with the speaker load and another resistor wired parallel to the load, after the first one, but before the speaker. This is used to pad the level of a speaker by a fixed amount and results in a "normal" impedance load presented to the amp. These too, need to be rated at a high enough power level to dissipate the heat created by the resistance.

 

[1958Bassman]

Turning your high range driver off can lead to damage to your crossover and subsequent damage to your amplifier.

Any two cabinets will sound different than one single cabinet. The best choice for a second cabinet is one identical to the first.

Turning it off with an L-Pad? That shouldn't be a problem if the L-Pad can handle the heat. Otherwise, if the tweeter is removed, the crossover will be open-circuit and the amp won't see anything it can't handle.

 

[will33]

+1 Not all knobs are the same. (loaded statement, I know)

Beyond that, very few "professionally built" cabs are truly crossed over. At best they have a high pass on the tweeter only, or just let the 'natural' off of the tweeter take care of things.

The part values in the filter that blocks the low frequencies from the tweeter are based on being attached to an 8ohm load. If you put an attenuator there that would present say a 20ohm load when you turn it, the filter won't work properly and start blowing $#!+ up.

 

[Joe Louvar]

That makes sense - thanks Will, and BS.

 

[BurningSkies]

Why haven’t already tried it - and always do it already? No reply necessary.

Because it's not a priority. Because I'm a bass player in a band not a guitar player. Because when I use the amp, its not for gigging, just for home use, and it doesn't matter in a 15x20 foot room when I'm standing in one place. Because when I lend it to my lead singer/rhythm guitarist he doesn't know what the knobs on the face of the amp do, much less care whether the amp is horizontal or vertical. Because I tend to worry about things like comb filtering in the lows way more than in the highs.

 

[BassmanPaul]

Turning it off with an L-Pad? That shouldn't be a problem if the L-Pad can handle the heat. Otherwise, if the tweeter is removed, the crossover will be open-circuit and the amp won't see anything it can't handle.

Often the Lpad for the high range is a 50W rated unit. The largest easily available Lpad is around 100W. Some of these crossovers are connected to 500W and higher amplifiers.

Removing the high driver presents an open circuit to the crossover and not a typical 8Ω load. Once again problems occur.

 

[Joe Louvar]

BS, since I really don’t care, no reply was necessary and “because” would have been good enough, but thanks for the amusement.

 

[BurningSkies]

BS, since I really don’t care, no reply was necessary and “because” would have been good enough, but thanks for the amusement.

Also, my twin has those chrome legs on the side, and that means it doesn't like to be stable up on end, or at least I would assume, since as you know, I haven't tried.

I figured that if you didn't care you wouldn't have asked...

 

[HeavyRockBasser]

RDK, it's been several months since this thread went up, so you've probably already done whatever you needed to do. If not, I was just thinking - and this is coming from someone with no amp or electronic experience - but I was envisioning one very large square cab. With the speakers arranged like the five on the dice, with the 15" in the middle. You could probably keep the electronics the same. But, that still leaves all five speakers sharing the same air. Probably not a good thing. But what if you built an enclosure around the 15, inside the cab? I'm thinking the body of an old drum for instance?

Absolutely no idea what it would sound like, but it would be pretty impressive to look at, don't you think?

Maybe just a brain fart. If so, I'm sorry for wasting your time.

 

[Joe Louvar]

Well BS, I've tried it and it's not worth the trouble!!!

 

[Joe Louvar]

RDK, it's been several months since this thread went up, so you've probably already done whatever you needed to do. If not, I was just thinking - and this is coming from someone with no amp or electronic experience - but I was envisioning one very large square cab. With the speakers arranged like the five on the dice, with the 15" in the middle. You could probably keep the electronics the same. But, that still leaves all five speakers sharing the same air. Probably not a good thing. But what if you built an enclosure around the 15, inside the cab? I'm thinking the body of an old drum for instance?

Absolutely no idea what it would sound like, but it would be pretty impressive to look at, don't you think?

Maybe just a brain fart. If so, I'm sorry for wasting your time.

What would be the point - why would you want something like that?

 

[Joe Louvar]

Wait a minute, I was thinking of the driver pointing in different directions like on dice (dumb) - but what you are talking about is something like the old Mesa/Boogie Powerhouse 1000 and if so - it’s already been done.

 

[will33]

RDK, it's been several months since this thread went up, so you've probably already done whatever you needed to do. If not, I was just thinking - and this is coming from someone with no amp or electronic experience - but I was envisioning one very large square cab. With the speakers arranged like the five on the dice, with the 15" in the middle. You could probably keep the electronics the same. But, that still leaves all five speakers sharing the same air. Probably not a good thing. But what if you built an enclosure around the 15, inside the cab? I'm thinking the body of an old drum for instance?

Absolutely no idea what it would sound like, but it would be pretty impressive to look at, don't you think?

Maybe just a brain fart. If so, I'm sorry for wasting your time.

That would basically be a clone of the Mesa Powerhouse1000 cab. They didn't use a drumshell but you can divide the inside into 3 chambers, one for the top 210's, one for the middle 15 and one for the bottom 210's. Not the way to go about building a "good" bass cab, for the reasons we've all been talking about, but they have sold plenty of them.

 

[BassmanPaul]

Wait a minute, I was thinking of the driver pointing in different directions like on dice (dumb) - but what you are talking about is something like the old Mesa/Boogie Powerhouse 1000 and if so - it’s already been done.

Do a search and see how many threads there are from owners of the PH1000 asking about replacement 15s, EVs IIRC. I remember several.

 

[Joe Louvar]

Oh, I know Paul - I think I’m even in one or two of those threads. BTW, I live near Mesa/Boogie and I have some nice hand picked 12ax7’s.