Ok, this is kind of a simple question, but what would that exactly look like in practice? Would a person need to put together a custom speakon cable to make that work properly?
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Peavey IPR 1600 - INTERNAL PHOTOS
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Ok, this is kind of a simple question, but what would that exactly look like in practice? Would a person need to put together a custom speakon cable to make that work properly?
billfitzmaurice
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You should have paid attention in 10th Grade physics class.Count me into that category. Amps are marked in watts, as are cabs. How the hell am I supposed to know anything about volts? I'm just a Joe Schmo for crying out loud!Seriously.
Few 4 ohm cabs can make use of more than 28 volts. Most 8 ohm cabs can't make use of more than 40 volts. And it doesn't matter how many cabs you have, one or a hundred, the voltage seen by one is seen by all. So if you bridge an amp to get more voltage than your cabs can use they won't go one dB louder. But they might just blow.
If you want to go louder you need to use a second cab, allowing the amp to deliver more current. That's easy, you just plug one cab into Channel 1 and one into Channel 2.
Some manufacturers do publish the voltage swing capability of their amps. All should.
You should have paid attention in 10th Grade physics class.
Few 4 ohm cabs can make use of more than 28 volts. Most 8 ohm cabs can't make use of more than 40 volts. And it doesn't matter how many cabs you have, one or a hundred, the voltage seen by one is seen by all. So if you bridge an amp to get more voltage than your cabs can use they won't go one dB louder. But they might just blow.
If you want to go louder you need to use a second cab, allowing the amp to deliver more current. That's easy, you just plug one cab into Channel 1 and one into Channel 2.
Some manufacturers do publish the voltage swing capability of their amps. All should.
I have never heard of any of this. Could you point me in the direction of some info on this topic?
billfitzmaurice
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Google 'Ohms Law'.
The interactions between amps and speakers involve voltage, current and power. Using only power (watts) as a measuring tool leaves two of the three elements out of the equation. Most speakers are damaged when fed with too much voltage, most amps are damaged when called on to deliver too much current, so knowing how it all works is a necessity if you want to get the most out of what you've got without breaking anything.
I meant how to determine the voltage a speaker can make use of. I've never heard of voltage ratings for cabs.
Don't most amps say something like "500W at 4ohms"? Given W = I2R, that means I = 11.2 amps. And since V = I * R, then V = 44.8 volts. So aren't they basically specifying the current and voltage rating of the amp?
Also, a speaker would be rated at "500W and 4ohms", so isn't that similarly rating the voltage and current also?
MX
Edit: I know that the speaker ohms are nominal, so there's probably some variation with frequency.
You should have paid attention in 10th Grade physics class.
Few 4 ohm cabs can make use of more than 28 volts. Most 8 ohm cabs can't make use of more than 40 volts. And it doesn't matter how many cabs you have, one or a hundred, the voltage seen by one is seen by all. So if you bridge an amp to get more voltage than your cabs can use they won't go one dB louder. But they might just blow.
If you want to go louder you need to use a second cab, allowing the amp to deliver more current. That's easy, you just plug one cab into Channel 1 and one into Channel 2.
Some manufacturers do publish the voltage swing capability of their amps. All should.
I'm a little confused with BFM or anyone using ohms law when "R" is not a constant? Is this "voltage limit" of 4 ohm and 8 ohm speakers based on the DCR of the speakers? But then I am not feeding DC to my speakers (my amp is working fine sending AC to my speakers). Feeding AC to speakers the IMPEDANCE varies wildly. Would not the voltage "limit" of a speaker also change with the variance of the impedance (not DC resistance).
billfitzmaurice
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Use Ohms Law to calculate from the power input the voltage input to reach xmax or Pe, whichever is lower, within the passband. If you have a hard limiter, standard gear in good PA rigs, you measure the voltage output of the amp to set the limiter. You can't set the limiter for a specified power output because you can't measure power output.
You don't use DCR, you use nominal impedance. It's not a perfect calc, but it is an adequate one, since the voltage output of an amp is constant irrespective of the impedance load.
Just so I am clear on what you are saying (as your posts are highly respected here).
The voltage of an audio amps output is constant.
The impedance (nominal) of speaker cab can determine the maximum voltage that speaker can handle (under ohms law) for a moving coil of wire in a magnetic field.
Only the current varies from the audio amps output due to the change in AC resistance (of the coil of wire in that magnetic field), the voltage is fixed?
Does that sum it up?
The voltage of an audio amps output is constant.
The impedance (nominal) of speaker cab can determine the maximum voltage that speaker can handle (under ohms law) for a moving coil of wire in a magnetic field.
Only the current varies from the audio amps output due to the change in AC resistance (of the coil of wire in that magnetic field), the voltage is fixed?
Does that sum it up?
billfitzmaurice
Commercial User
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Pretty much. The impedance doesn't have any direct bearing on the voltage limit that a driver can handle, but excursion is directly proportional to voltage. In most cases excursion is the driver's Achilles Heel, as once you exceed it most of any additional power input applied ends up as heat in the motor, which then can cause thermal failure. The 'autopsy' will reveal the driver died from overheating, but the underlying cause will as often as not be from pushing too many volts, sending the driver past xmax.
I tried this yesterday. I used a Max preamp to drive the power amp. The way I did it may not be the best way, but I was in a hurry.
I plugged into the channel 1 input, then used a jumper from the channel 1 through jack to the channel 2 input.
Next, I used 2-1/4" plugs for the output speaker cable (12 gauge wire). I soldered the positive side to the channel 1 positive output, and soldered the negative side to the channel 2 negative output.
I ran both gain controls at unity (all the way up) to make balancing the two sides easy.
I used this setup to drive an 8 ohm 410 cab. It seemed to work well. This configuration was giving me 1000 watts into 8 ohms, so I had pretty crazy headroom.
Using Speakons would be better, but I didn't have any spares on hand.
I have a 4 ohm, 1200 watt 412 cab. The next thing I will try is bridging into that.
Awesome, thanks for the reply. I would imagine Speakon would be better (or at least easier) for a soldering-challenged person like me since they are usually screw down connections.
Also, you seem to stress having the two channels at equal gain. What happens if a knob gets bumped and they are unequal?
Also, you seem to stress having the two channels at equal gain. What happens if a knob gets bumped and they are unequal?
When you bridge an amp, you are taking the difference between two opposite polarity channels rather than a solid ground reference for negative and an alternating signal for positive. For this to work properly the outputs need to be as equal as possible and out of phase. If they become unbalanced, the output signal can become asymmetrical and clip sooner on one side. If a knob gets bumped it's not the end of the world. It would probably sound weird.
Because of the design of the amp, this works out if you connect everything as I described a few posts up.
I don't want to describe the reasoning behind this because I don't know if I'm giving away something proprietary.
Sorry, I overlooked that... should have told me that you are running one entire channel out of phase
you can trust us, we are all friends here on Talkbass
Well, I'm gonna go give one a whirl this afternoon. I think I'm just going to take my pedal board and plug my VT Bass into it and see how that works out.
Wish me luck!
Wish me luck!
Calling attention to the voltage swing capabilities of an amp really reminded me of a time long ago and a neat trick we used with Phase Linear power amplifiers. They used transistors which *loved* to run at higher voltages (and the power supply of the amp allowed that). We used to run pairs of JBL L100 speakers in series on each side to present a 16 ohm load to the amp, and it would KILL. The readings on the big meters on the front of the amp wouldn't be useful any more (you could slam the meters against the right side way before you'd hear any clipping)... which would freak out the club owner (I was a club DJ at the time). Something about the transistors available in those days (we're talking early to mid 70's) being more designed for high voltage applications, like TVs or something I guess.
This thread has really helped me get much closer to pulling the trigger on this amp to lighten my gig rig... but how crazy is it that the SKB 4U "light" case into which this thing will be mounted outweighs the amp by ~80%?
Pretty crazy, I agree.
My 4U Flyer weighs more than all the gear loaded into it at this point.
