Here's the inside of my BG250.

[Stinsok]

The grill is held in place by wood screws. The speaker fasteners are machine screws, The tweeter isn't crossed over but is wired from the 15", with a capacitor.

[mystic38]

there are a number of photos of the bg250 in its own thread, so this may be a good addition?..

fyi a capacitor is a crossover...

Originally Posted by Stinsok

The grill is held in place by wood screws. The speaker fasteners are machine screws, The tweeter isn't crossed over but is wired from the 15", with a capacitor.Attachment 338262Attachment 338263

[Stinsok]

Yeah, but not of "MY" BG250.

[B-string]

So not impressed.

[AstroSonic]

I am sure that all of the obvious details were carefully considered by the manufacturer. Still, that's an awful lot of acoustic insulation in that enclosure. If there is no unobstructed path between the open space (what little there is) behind the driver and the interior vent opening the bass reflex loading will be highly impaired. If the driver is underdamped, this could be a good thing. OTOH, you might be surprised and pleased with the change in tone resulting from substantially reducing the amount of acoustic insulation. I would try none, then an inch on the back, one side and the top, then start tucking more in the corners, trying it out at each step. It's a modification that is easily tried and evaluated. Regardless, just make sure that there is an unobstructed path between the vent and the back of the driver.

The tweeter is a piezo. The cap is the crossover, but it is minimal, especially in this case (piezo). You would need 2 power resistors and a (different) cap to optimize the crossover, but it might be better to simply replace the piezo with a decent horn and crossover. OTOH, this is only worth while if you otherwise are pleased with the unit and plan on keeping it.

[Steve Dallman]

A cap is used with a piezo tweeter to attenuate the piezo, not as a crossover.

[Stinsok]

At least the speaker wasn't held on by wood screws like the GK MB's I have owned. The grill is metal instead of plastic. I tried a Carvin PS15B in it (just because.) It added about 7lbs to the amp.

[drpepper]

Originally Posted by Stinsok

At least the speaker wasn't held on by wood screws like the GK MB's I have owned. The grill is metal instead of plastic. I tried a Carvin PS15B in it (just because.) It added about 7lbs to the amp.

I don't recall seeing any reports of problems with various combos due to the use of wood screws (although I wasn't looking for them). There's no general intent for the end-user to be taking it apart.

Good that you're happy with the construction. Weight wise, the BG250 seems to be in the middle of a pretty tight pack, but the Carvin MB15 appears to be the lightest of them all, by a few pounds.

[Stinsok]

There wasn't anything "wrong" with wood screws for speakers, but I prefer that extra step by using T-nuts, etc.

[AstroSonic]

Originally Posted by Steve Dallman

A cap is used with a piezo tweeter to attenuate the piezo, not as a crossover.

This apparent disagreement could be just a matter of terminology. However, in my book, an audio frequency attenuator provides a level of resistance that is constant with (or independent of) frequency. A cap will present an impedance that decreases as frequency goes up (and increases as frequency goes down). Since piezos present a capacitive load to the amplifier, a series cap will act to decrease that capacitance (series caps 'sum' like parallel resistors). That total capacitance acts as a single smaller capacitor, which more strongly attenuates the lower frequencies. Functionally, this has decreased (or attenuated) the amount of signal at lower frequencies (relative to the tweeter alone), effectively raising the frequency at which the tweeter begins to output appreciable sound (essentially raising the crossover frequency). A power resistor would be more appropriate, but more expensive. The Motorola application note that used to come with the piezo drivers discusses all this. With an optimal piezo crossover, an L-pad can be used to provide a variable level of attenuation.

[B-string]

A resistor is used with Piezo to provide stability for the driving amp. A purely capacitive load can cause instability (destructive self oscillation) with some designs.

[acmebass]

Originally Posted by AstroSonic

This apparent disagreement could be just a matter of terminology. However, in my book, an audio frequency attenuator provides a level of resistance that is constant with (or independent of) frequency. A cap will present an impedance that decreases as frequency goes up (and increases as frequency goes down). Since piezos present a capacitive load to the amplifier, a series cap will act to decrease that capacitance (series caps 'sum' like parallel resistors). That total capacitance acts as a single smaller capacitor, which more strongly attenuates the lower frequencies. Functionally, this has decreased (or attenuated) the amount of signal at lower frequencies (relative to the tweeter alone), effectively raising the frequency at which the tweeter begins to output appreciable sound (essentially raising the crossover frequency). A power resistor would be more appropriate, but more expensive. The Motorola application note that used to come with the piezo drivers discusses all this. With an optimal piezo crossover, an L-pad can be used to provide a variable level of attenuation.

A piezo tweeter presents a load equivalent to a resistor in series with a capacitor. I've tested a few...

To attenuate one, with no change in response characteristics, would require series and/or parallel components showing the same impedance phase angle at all frequencies, which would also consist of resistors in series with capacitors.

So neither a series resistor or capacitor makes a whole lot of sense as an attenuator or a high-pass filter, but a resistor can be used as a sort of highly imprecise stabilizing element, as mentioned above, but as I said, piezos are already somewhat resistive. At low frequencies, of course, their resistance is negligible, because of the capacitive reactance.

A Zobel network consisting of an inductor in series with a resistor can be built, but doesn't make much sense, because it would generate useless heat. I'm not a big fan of Zobels anyway. If you wanted to use a conventional resistive L-Pad attenuator on a piezo, it would be necessary to add these extra components, if you wanted it to work right, in addition to conventional filter components.

A precise HPF can be designed, as can accurate attenuators, but the arithmetic's a little complicated. Usually, piezo tweeters are just used as a cheap, easy alternative to something more precise, but in fact, they can be used in a precise way, it's just that nobody understands them very well, if they haven't taken the time to understand their impedance characteristics.

Interestingly, misunderstood as they are, their impedance is actually simpler than that of moving-coil drivers, not more complicated. It's just that if you resort to a purely resistive model for design of filters, the piezo completely falls apart, whereas the moving-coil driver only partially does!

FYI...

Thanks,
Andy



Andy

[BassmanPaul]

Originally Posted by B-string

A resistor is used with Piezo to provide stability for the driving amp. A purely capacitive load can cause instability (destructive self oscillation) with some designs.

And that is exactly my thought too. That said, I have used piezo tweeters before in some vocal monitors with a 0.1µF cap in series. I've not encountered any stability problems. I did eventually change them out for regular horn tweeters plus a crossover and much preferred the result.

[acmebass]

Originally Posted by AstroSonic

I am sure that all of the obvious details were carefully considered by the manufacturer. Still, that's an awful lot of acoustic insulation in that enclosure. If there is no unobstructed path between the open space (what little there is) behind the driver and the interior vent opening the bass reflex loading will be highly impaired. If the driver is underdamped, this could be a good thing. OTOH, you might be surprised and pleased with the change in tone resulting from substantially reducing the amount of acoustic insulation. I would try none, then an inch on the back, one side and the top, then start tucking more in the corners, trying it out at each step. It's a modification that is easily tried and evaluated. Regardless, just make sure that there is an unobstructed path between the vent and the back of the driver.

The preferred methodology would be to block the port, and then test the Qtc externally. If it's around .5 to .7, the stuffing ain't much of an issue. The degree to which the operation of the port is impaired can also be determined using impedance curves, but in my experience, doesn't seem to be as big a problem as one might expect. Think of a passive radiator...

Thanks,
Andy

[AstroSonic]

Originally Posted by B-string

A resistor is used with Piezo to provide stability for the driving amp. A purely capacitive load can cause instability (destructive self oscillation) with some designs.

+1

Some (but not all usually) of the bad rap piezos get is due to amplifier instability into strongly capacitive loads. Resistors are also useful for creating a stabile, resistive load for both the amplifier and the crossover (see Andy's post below).

[AstroSonic]

Originally Posted by acmebass

The preferred methodology would be to block the port, and then test the Qtc externally. If it's around .5 to .7, the stuffing ain't much of an issue. The degree to which the operation of the port is impaired can also be determined using impedance curves, but in my experience, doesn't seem to be as big a problem as one might expect. Think of a passive radiator...

Thanks,
Andy

My assumption was that the OP has neither the tools nor the expertise to check port obstruction via measurement. While Qtc can be measured, it returns a non-unique answer. A low Qtc can be due not only to an obstructed port, but also to a leaky cab joint, surround, driver seal, terminal plate or dust cap. When there is an issue with port obstruction, removing the obstruction is quite audible. If you don't hear a audible change it wasn't a big enough issue to worry about. After working with a lot of ported cabs, I've concluded that it's just good practice to avoid potential obstructions. If it looks like it might be obstructing the port, just move it out of the way.

[AstroSonic]

Originally Posted by acmebass

A piezo tweeter presents a load equivalent to a resistor in series with a capacitor. I've tested a few...

To attenuate one, with no change in response characteristics, would require series and/or parallel components showing the same impedance phase angle at all frequencies, which would also consist of resistors in series with capacitors.

So neither a series resistor or capacitor makes a whole lot of sense as an attenuator or a high-pass filter, but a resistor can be used as a sort of highly imprecise stabilizing element, as mentioned above, but as I said, piezos are already somewhat resistive. At low frequencies, of course, their resistance is negligible, because of the capacitive reactance.

A Zobel network consisting of an inductor in series with a resistor can be built, but doesn't make much sense, because it would generate useless heat. I'm not a big fan of Zobels anyway. If you wanted to use a conventional resistive L-Pad attenuator on a piezo, it would be necessary to add these extra components, if you wanted it to work right, in addition to conventional filter components.

A precise HPF can be designed, as can accurate attenuators, but the arithmetic's a little complicated. Usually, piezo tweeters are just used as a cheap, easy alternative to something more precise, but in fact, they can be used in a precise way, it's just that nobody understands them very well, if they haven't taken the time to understand their impedance characteristics.

Interestingly, misunderstood as they are, their impedance is actually simpler than that of moving-coil drivers, not more complicated. It's just that if you resort to a purely resistive model for design of filters, the piezo completely falls apart, whereas the moving-coil driver only partially does!

FYI...

Thanks,
Andy



Andy

+1

It's always a bit more complicated and one never knows quite where to stop.

The impedance of a cone/dynamic driver or horn loaded compression driver is more complex, but usually more benign with respect to amplifier stability. The phase angle of a piezo is usually pretty nasty at the low end of their range (more capacitive, as you said).

I have had very good results with a few of the Motorola piezos, but generally avoid them as I usually desire more performance than they can deliver.

They are nearly always used for tweeters in inexpensive cabs, and the implementation is inexpensive (poor?) as well.

[acmebass]

Originally Posted by AstroSonic

My assumption was that the OP has neither the tools nor the expertise to check port obstruction via measurement. While Qtc can be measured, it returns a non-unique answer. A low Qtc can be due not only to an obstructed port, but also to a leaky cab joint, surround, driver seal, terminal plate or dust cap. When there is an issue with port obstruction, removing the obstruction is quite audible. If you don't hear a audible change it wasn't a big enough issue to worry about. After working with a lot of ported cabs, I've concluded that it's just good practice to avoid potential obstructions. If it looks like it might be obstructing the port, just move it out of the way.

A safe assumption, regarding test equipment.

A non-unique answer? I have no idea what you mean by that, unless you're getting into power-related non-linearities again.

Well, yeah, obviously if you don't hear an audible change, it wasn't something to worry about.

Thanks,
Andy

[acmebass]

Originally Posted by AstroSonic

+1

It's always a bit more complicated and one never knows quite where to stop.

The impedance of a cone/dynamic driver or horn loaded compression driver is more complex, but usually more benign with respect to amplifier stability. The phase angle of a piezo is usually pretty nasty at the low end of their range (more capacitive, as you said).

I have had very good results with a few of the Motorola piezos, but generally avoid them as I usually desire more performance than they can deliver.

They are nearly always used for tweeters in inexpensive cabs, and the implementation is inexpensive (poor?) as well.

Absolutely.

BTW, I don't think Motorola has made any in a long time. I don't know if the clones are comparable or not.

Andy

[vin*tone]

Looks like it's made from ply. That's a lot better than others in its class.