I was wondering, I'm thinking of getting a 200W Gallien Krueger head, and was wondering if I would be able to play it through a vintage 1x15 Univox UB250 cabinet, but how do you determine the watt rating of the cabinet? I read that this Univox amp was rated at 20W RMS, so does that mean that's what the cab is as well? I wouldn't want to blow the speaker out... please help
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Double Bass How to tell a cab's watt rating?
- Thread starter dbass87
- Start date
It seems you're not asking about how you know an amp's wattage rating (see your thread title) but how you know the rating of a speaker cabinet. Knowing the rating of an amp that a cab is mated with tells you very little other than that it's safe to assume that the cab can handle at least the output of that amp. You can bet your Univox cab can handle 20 watts but it's likely the case that it could handle far more. If you knew what driver is in the Univox cab, then the ratings could be found. It's also the case that, unlike tweeters, woofers (bass drivers) will usually "complain" long before damage is done. As you push them too far, distortion will set in. After that, you will hear the voice coil "bottoming out." In the absence of driver ratings, you can simply give it a try. If you find that you're pushing it too far, then you'll need a different cab. The point is that you should hear the symptoms long before you blow it out. Fuses can also be added to limit the power to the driver but that's another chapter. 
Thanks for the advice... and you're right, there is a some ambiguity with the thread title, I should have worded it differently. Anyway, is there any minimum "standard" rating of 15 in. cabs, like a basic reference point? Just trying to figure if it's even practical to try this head/ cab combinationIt seems you're not asking about how you know an amp's wattage rating (see your thread title) but how you know the rating of a speaker cabinet. Knowing the rating of an amp that a cab is mated with tells you very little other than that it's safe to assume that the cab can handle at least the output of that amp. You can bet your Univox cab can handle 20 watts but it's likely the case that it could handle far more. If you knew what driver is in the Univox cab, then the ratings could be found. It's also the case that, unlike tweeters, woofers (bass drivers) will usually "complain" long before damage is done. As you push them too far, distortion will set in. After that, you will hear the voice coil "bottoming out." In the absence of driver ratings, you can simply give it a try. If you find that you're pushing it too far, then you'll need a different cab. The point is that you should hear the symptoms long before you blow it out. Fuses can also be added to limit the power to the driver but that's another chapter.
Nope, no minimum standard. It depends entirely on the parameter values of the driver. If you've made the decision to get the GK, then you've got nothing to lose by trying it out. Worst case, you'll need a different cab.
oh ok... Actually I haven't made the decision to get this head yet, just was looking into it... I was hoping to be able to use it with the aforementioned cab I already have, but if I might have to buy a cab anyway, I'm debating with just looking into a nice combo instead... Also i'm trying to stay within a reasonably low price range
Oddly enough, one's on eBay with a picture that shows the back open.
http://cgi.ebay.ph/VINTAGE-UNIVOX-UB250-15-ALNICO-GUITAR-CABINET-USA-/320613877418
I am going to guess that this speaker did not have a rating much above 25 Watts just based on when it was made -- probably around 1970. One of the reason for multi-driver bass speakers in that time frame was the relatively low power handling of the drivers.
http://cgi.ebay.ph/VINTAGE-UNIVOX-UB250-15-ALNICO-GUITAR-CABINET-USA-/320613877418
I am going to guess that this speaker did not have a rating much above 25 Watts just based on when it was made -- probably around 1970. One of the reason for multi-driver bass speakers in that time frame was the relatively low power handling of the drivers.
I agree with drurb's comments; Under full power, yes, the driver's voice coil will blow like a fuse, but as drurb said, usually you'll get plenty of audible warning before this happens.
Having said that, you *may* not reach that point if the driver is of sufficiently high 'sensitivity'; that is, the amount of sound pressure (dB) that it will impart, per watt of energy fed into it (the sensitivity standard is measured this way at a distance of 1m on-axis in front of the speaker cone, so sensitivity is generally quoted in the unit: dB/w/m). If it's a reasonably-high sensitivity speaker (97dB/w/m at a minimum), you may find that you'll never drive it with excessive watts, as the speaker is sufficiently 'loud' to begin with. ie, 1W will produce 97dB, 10W will produce 107dB, 20W ~ 110dB, 100W~117dB). Conversely, if the driver is of low sensitivity (88dB/w/m, like hifi speakers, or 83dB/w/m like car subwoofers), you'll find that you'll need to push more watts into the speaker to get the same levels, ie: 1W~88dB, 10W~98dB, 100W~108dB, 1000W~118dB. Of course, the frequency & power response of the driver-cabinet combination will also affect how 'loud' (or not) it will appear at various frequencies, which complicates the matter further, but let's assume relatively intelligent cabinet design for now & put those aside for this discussion.
Seeing as you seem to be on a learning curve here: alternatively, you can have a situation that is the opposite of yours: where the amp is underpowered compared to the driver. I think this is actually worse; sensitivity & frequency response factors aside, you run the risk of overdriving your amp in the chase for volume (ie, "Gee, that's not very loud; never mind, I'll just run with the amp turned all the way up permanently."). The danger here is that you risk your amp 'clipping'; that is, when the sinewave that you're asking your amp to produce is greater than the peak-to-peak limits of its voltage rails.
To explain this: conventional amps run off +/- power DC supplies, with a zero (ground) point in between, eg, +20VDC / 0V / -20VDC. In amplifying a signal, it uses this +/- range to produce the top & bottom halves of the sine wave that's trying to amplify, ie:
+V . . . . ._ . . . . . . . . . . ._
. . . . . . /. .\. . . . . . . . . /. .\
. . . . . /. . . .\. . . . . . . /. . . .\
0V. .. /______\_______/_______\_____
. . . . . . . . . . .\ . . . ./. . . . . . . \. . . ./
. . . . . . . . . . . .\. . ./. . . . . . . . \. . ./
-V. . . . . . . . . . .\_/. . . . . . . . . . \_/.
('Scuse the dots; multiple spaces are parsed out)
Output varies with amplitude (how 'tall' the sine wave is), ie:
Quiet (small/low signal):
+V. . . _. . . . _
0V. . /__\___/__\_____
-V. . . . . .\_/. . . \_/.
Loud (large/high signal):
+V. . . . . ._ . . . . . . . . . . . . . _
. . . . . . ./. .\ . . . . . . . . . . . /. .\
. . . . . ./. . . .\. . . . . . . . . ./. . . .\
. . . . . /. . . . . \. . . . . . . . /. . . . . \
0V. . /_________\________/________\_____
. . . . . . . . . . . . \. . . . . /. . . . . . . . \. . . . . /
. . . . . . . . . . . . . \. . . ./. . . . . . . . . .\. . . ./
. . . . . . . . . . . . . .\. . ./. . . . . . . . . . .\. . ./
-V. . . . . . . . . . . . .\_/. . . . .. . . . . . . . \_/.
(Incidentally, I'm ignoring the new generation of lightweight class-D amps like MarkBass does, because they run off a single polarity voltage rail eg +40VDC, and buggered if I know at the moment how they work).
The problem of an underpowered amp is as follows: If you ask a small (+/- 20V) supply rail to reproduce a large signal (+/- 40V), it will 'clip' the signal at the supply voltage limits, ie:
. . . . . . ._. . . . . . . . . . . . . ._. . . . . . . . . . . . . .desired +40V peak of the signal
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .____. . . . . . . . . . . .___. . . . . . . . . . . . .+20V limit of your amp .(clipping - the flat bit - occurs here)
. . . . ./. . . . \. . . . . . . . . /. . . . \
0V. . /_______\_________/_______\_____
. . . . . . . . . . . \. . . . . ./. .. . . . . .\ . . . ./
. . . . . . . . . . . . \.___./. . . . . . . . . \___/. . . . . +20V limit of your amp (clipping - the flat bit - occurs here)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . _. . . . . . . . . . . . ._. . . . . . .desired -40V peak of the signal
So, clipping is a metaphorical clipping of the top & bottom off the desired signal. Where the signal tops/bottoms out, those flat spots are pure DC voltage being applied to your speaker's voice coil. At best, it sounds horrible, but keep it up for loud & long enough, and eventually your voice coil wire becomes like a light bulb filament; it heats up and eventually fails (or worse, sets fire to the voice coil former that it's wound around and ends up burning the whole speaker to the ground). (Incidentally, this applies only to power amps; if this happens in the preamp stage (by turning the gain all the way up), you get the harsh distortion loved in rock/metal etc).
So (back to your original post): where does this leave you? You're best trying to match your amp & driver intelligently; having a decently sized amp with plenty of headroom (say, 100-150W at a minimum if you're playing small to medium gigs, 200-500W for larger but not massive gigs) with a speaker & cabinet that will handle AT LEAST that wattage is the safest option. Your amp is fine; it's the cabinet that you'll probably need to replace (sensitive or not, 20W doesn't give you a lot of room to move!). As drurb said, combos are great because they've done that work for you, whereas head/cab combinations require more careful thought. If you're uncertain, try to track down the driver specifications to give you an idea of whether it's a suitable match to your amp. And also note: multiple driver cabinets will not go any lower than a single driver cabinet (assuming the same model driver and a proportional scaling up of the cabinet size to suit the larger number of drivers); it will, however, go louder with less distortion (because each driver is moving less than the single driver would in order to produce similar output levels), and will also absorb more input power than the single driver.
Hope that helps.
Having said that, you *may* not reach that point if the driver is of sufficiently high 'sensitivity'; that is, the amount of sound pressure (dB) that it will impart, per watt of energy fed into it (the sensitivity standard is measured this way at a distance of 1m on-axis in front of the speaker cone, so sensitivity is generally quoted in the unit: dB/w/m). If it's a reasonably-high sensitivity speaker (97dB/w/m at a minimum), you may find that you'll never drive it with excessive watts, as the speaker is sufficiently 'loud' to begin with. ie, 1W will produce 97dB, 10W will produce 107dB, 20W ~ 110dB, 100W~117dB). Conversely, if the driver is of low sensitivity (88dB/w/m, like hifi speakers, or 83dB/w/m like car subwoofers), you'll find that you'll need to push more watts into the speaker to get the same levels, ie: 1W~88dB, 10W~98dB, 100W~108dB, 1000W~118dB. Of course, the frequency & power response of the driver-cabinet combination will also affect how 'loud' (or not) it will appear at various frequencies, which complicates the matter further, but let's assume relatively intelligent cabinet design for now & put those aside for this discussion.
Seeing as you seem to be on a learning curve here: alternatively, you can have a situation that is the opposite of yours: where the amp is underpowered compared to the driver. I think this is actually worse; sensitivity & frequency response factors aside, you run the risk of overdriving your amp in the chase for volume (ie, "Gee, that's not very loud; never mind, I'll just run with the amp turned all the way up permanently."). The danger here is that you risk your amp 'clipping'; that is, when the sinewave that you're asking your amp to produce is greater than the peak-to-peak limits of its voltage rails.
To explain this: conventional amps run off +/- power DC supplies, with a zero (ground) point in between, eg, +20VDC / 0V / -20VDC. In amplifying a signal, it uses this +/- range to produce the top & bottom halves of the sine wave that's trying to amplify, ie:
+V . . . . ._ . . . . . . . . . . ._
. . . . . . /. .\. . . . . . . . . /. .\
. . . . . /. . . .\. . . . . . . /. . . .\
0V. .. /______\_______/_______\_____
. . . . . . . . . . .\ . . . ./. . . . . . . \. . . ./
. . . . . . . . . . . .\. . ./. . . . . . . . \. . ./
-V. . . . . . . . . . .\_/. . . . . . . . . . \_/.
('Scuse the dots; multiple spaces are parsed out)
Output varies with amplitude (how 'tall' the sine wave is), ie:
Quiet (small/low signal):
+V. . . _. . . . _
0V. . /__\___/__\_____
-V. . . . . .\_/. . . \_/.
Loud (large/high signal):
+V. . . . . ._ . . . . . . . . . . . . . _
. . . . . . ./. .\ . . . . . . . . . . . /. .\
. . . . . ./. . . .\. . . . . . . . . ./. . . .\
. . . . . /. . . . . \. . . . . . . . /. . . . . \
0V. . /_________\________/________\_____
. . . . . . . . . . . . \. . . . . /. . . . . . . . \. . . . . /
. . . . . . . . . . . . . \. . . ./. . . . . . . . . .\. . . ./
. . . . . . . . . . . . . .\. . ./. . . . . . . . . . .\. . ./
-V. . . . . . . . . . . . .\_/. . . . .. . . . . . . . \_/.
(Incidentally, I'm ignoring the new generation of lightweight class-D amps like MarkBass does, because they run off a single polarity voltage rail eg +40VDC, and buggered if I know at the moment how they work).
The problem of an underpowered amp is as follows: If you ask a small (+/- 20V) supply rail to reproduce a large signal (+/- 40V), it will 'clip' the signal at the supply voltage limits, ie:
. . . . . . ._. . . . . . . . . . . . . ._. . . . . . . . . . . . . .desired +40V peak of the signal
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . .____. . . . . . . . . . . .___. . . . . . . . . . . . .+20V limit of your amp .(clipping - the flat bit - occurs here)
. . . . ./. . . . \. . . . . . . . . /. . . . \
0V. . /_______\_________/_______\_____
. . . . . . . . . . . \. . . . . ./. .. . . . . .\ . . . ./
. . . . . . . . . . . . \.___./. . . . . . . . . \___/. . . . . +20V limit of your amp (clipping - the flat bit - occurs here)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . _. . . . . . . . . . . . ._. . . . . . .desired -40V peak of the signal
So, clipping is a metaphorical clipping of the top & bottom off the desired signal. Where the signal tops/bottoms out, those flat spots are pure DC voltage being applied to your speaker's voice coil. At best, it sounds horrible, but keep it up for loud & long enough, and eventually your voice coil wire becomes like a light bulb filament; it heats up and eventually fails (or worse, sets fire to the voice coil former that it's wound around and ends up burning the whole speaker to the ground). (Incidentally, this applies only to power amps; if this happens in the preamp stage (by turning the gain all the way up), you get the harsh distortion loved in rock/metal etc).
So (back to your original post): where does this leave you? You're best trying to match your amp & driver intelligently; having a decently sized amp with plenty of headroom (say, 100-150W at a minimum if you're playing small to medium gigs, 200-500W for larger but not massive gigs) with a speaker & cabinet that will handle AT LEAST that wattage is the safest option. Your amp is fine; it's the cabinet that you'll probably need to replace (sensitive or not, 20W doesn't give you a lot of room to move!). As drurb said, combos are great because they've done that work for you, whereas head/cab combinations require more careful thought. If you're uncertain, try to track down the driver specifications to give you an idea of whether it's a suitable match to your amp. And also note: multiple driver cabinets will not go any lower than a single driver cabinet (assuming the same model driver and a proportional scaling up of the cabinet size to suit the larger number of drivers); it will, however, go louder with less distortion (because each driver is moving less than the single driver would in order to produce similar output levels), and will also absorb more input power than the single driver.
Hope that helps.
Nice job, swingin. You and others may wish to look here. Long before you joined Talkbass, folks around here were treated to my mantra, "Think SPL, not watts!". The link will lead you to my dissertations and explanations. Ah, if people would only use the search function...
It's also important to keep in mind that the amount of sound-pressure achieved for a given input is very much a function of the characteristics of the driver and the cabinet.
With regard to clipping, the issue really isn't usually so much that the voltage remains constant for short periods. Clipping the amplifier is especially dangerous for tweeters. Taking your own example, when you clip a sign wave, you splatter the energy to higher frequencies. So, when an amplifier goes into hard clipping, there is a dramatic increase in the power that ends up at the input to the tweeter. Before you get much of a chance to hear a problem, the voice coil will be fried. As we agree, not so much with low-frequency drivers that will serve warning.
Thanks to fdeck for the link. From looking at the picture of the cab with the back open, I'm quite confident that the driver will not handle much power at all. I base that on the size of the magnet and the nature of the cone and basket construction. I doubt that the cab/driver combination is very sensitive, especially where low frequencies are concerned, given the driver and that the cab looks to be an "infinite baffle" enclosure. Based on that picture, I'd say the OP should forget about using that cab.
BTW, an often overlooked factor is the excursion of the driver which, in turn, affects its dynamic range. One can have a very sensitive driver/cab system that, for example, can achieve 97 dB SPL @ 1 watt measured 1 meter from the driver on-axis. It's important to know at what frequency that specification is measured. Here's why. All other things being equal, for a constant input voltage, the distance the driver cone travels increases as the inverse square of frequency. This means that as you lower the frequency of the input, you will be causing to the cone to move over very rapidly increasing distances. While the driver may be able to achieve the specification at, say, 500 Hz, it may not be able to at 35 Hz. Now consider that as you increase the input voltage, you will again, be causing the driver to move over larger distances. Somewhere the limit will be reached.
What does all this mean? Simply put, one could have two speaker systems with identical sensitivity ratings. They could even have the same power handling ratings if the manufacturer chooses, say, 1000 Hz as the frequency at which to measure (many actually do!). Despite their seeming similarity, under real-world use, where the input voltage may rise for brief periods at low frequencies (transients), one cone may go "SPLATTTT" while the one designed with a greater excursion limit breezes right along.
=== Begin geekazoid section ===
Here, again, the design of the cabinet comes into play. It's possible to produce quite large (room- and house-shaking) sound-pressure levels at very low frequencies while the driver cone moves only a fraction of an inch. All you have to do is load the driver into a bass "horn." This makes the woofer cone function as a compression driver (like the drivers you typically see on the back of large mid-range and high-frequency horns of PA systems). The problem is that you need a HUGE cabinet (and sometimes the corner of walls to form the mouth of the horn). Google "Klipschorn" There are other ways to achieve this as well but, again, they require cabinets bigger than we're willing to haul around to gigs.
=== End geekazoid section ===
Finally, a word about "headroom." Headroom does not refer to having more watts than can be taken advantage of by your cabinets. Rather, headroom refers to the amount by which an amplifier can exceed its continuous wattage rating during short bursts. See here for a more complete explanation.
It's also important to keep in mind that the amount of sound-pressure achieved for a given input is very much a function of the characteristics of the driver and the cabinet.
With regard to clipping, the issue really isn't usually so much that the voltage remains constant for short periods. Clipping the amplifier is especially dangerous for tweeters. Taking your own example, when you clip a sign wave, you splatter the energy to higher frequencies. So, when an amplifier goes into hard clipping, there is a dramatic increase in the power that ends up at the input to the tweeter. Before you get much of a chance to hear a problem, the voice coil will be fried. As we agree, not so much with low-frequency drivers that will serve warning.
Thanks to fdeck for the link. From looking at the picture of the cab with the back open, I'm quite confident that the driver will not handle much power at all. I base that on the size of the magnet and the nature of the cone and basket construction. I doubt that the cab/driver combination is very sensitive, especially where low frequencies are concerned, given the driver and that the cab looks to be an "infinite baffle" enclosure. Based on that picture, I'd say the OP should forget about using that cab.
BTW, an often overlooked factor is the excursion of the driver which, in turn, affects its dynamic range. One can have a very sensitive driver/cab system that, for example, can achieve 97 dB SPL @ 1 watt measured 1 meter from the driver on-axis. It's important to know at what frequency that specification is measured. Here's why. All other things being equal, for a constant input voltage, the distance the driver cone travels increases as the inverse square of frequency. This means that as you lower the frequency of the input, you will be causing to the cone to move over very rapidly increasing distances. While the driver may be able to achieve the specification at, say, 500 Hz, it may not be able to at 35 Hz. Now consider that as you increase the input voltage, you will again, be causing the driver to move over larger distances. Somewhere the limit will be reached.
What does all this mean? Simply put, one could have two speaker systems with identical sensitivity ratings. They could even have the same power handling ratings if the manufacturer chooses, say, 1000 Hz as the frequency at which to measure (many actually do!). Despite their seeming similarity, under real-world use, where the input voltage may rise for brief periods at low frequencies (transients), one cone may go "SPLATTTT" while the one designed with a greater excursion limit breezes right along.
=== Begin geekazoid section ===
Here, again, the design of the cabinet comes into play. It's possible to produce quite large (room- and house-shaking) sound-pressure levels at very low frequencies while the driver cone moves only a fraction of an inch. All you have to do is load the driver into a bass "horn." This makes the woofer cone function as a compression driver (like the drivers you typically see on the back of large mid-range and high-frequency horns of PA systems). The problem is that you need a HUGE cabinet (and sometimes the corner of walls to form the mouth of the horn). Google "Klipschorn" There are other ways to achieve this as well but, again, they require cabinets bigger than we're willing to haul around to gigs.
=== End geekazoid section ===
Finally, a word about "headroom." Headroom does not refer to having more watts than can be taken advantage of by your cabinets. Rather, headroom refers to the amount by which an amplifier can exceed its continuous wattage rating during short bursts. See here for a more complete explanation.
Ah, if people would only use the search function...
Yeah, I know; but my geek persona (which is my main one, come to think of it!) took over before I could stop myself...
Drurb, I feel a strange affinity in our geekiness...
Great posts, man!
You too.
Give me your address and I'll send you off a pocket protector.
