Another Ohms and Watts Question

[jaxstarke]

Have a 300 watt GK Backline 600 and a 500 watt 4 ohm 4x10 cab and a 300 watt 4 ohm 15" cab. The lowest impedance the head will go is 4 ohms so I'm going to run the 2 cabs in series for an 8 ohm total load. How many watts will this be pushing to each cabinet? The manual says the head puts out 180 watts at 8 ohms so would that mean 90 watts to each cabinet? Thanks

 

[Smooth_bass88]

OK, explain what you mean by "run the 2 cabs in series".

 

[jaxstarke]

Building a series box to run one output from the head to the series wiring box and from there one output to each cab.

 

[B-string]

It will actually put out more like 200 at 8 ohms, but yes the two cabs will share the wattage equally.

 

[jaxstarke]

B-string. Helping me out once again. Thanks.

 

[Smooth_bass88]

Building a series box to run one output from the head to the series wiring box and from there one output to each cab.

OK, cool. B-string has it right I believe.

 

[supershifter2]

each cabinet will use 90 watts. amps dont put out watts they put out volts. watts is the usage but for simplicity sake and so the general public can comprehend watts is the measure of power thats used. i worked for 20 years as a commercial electrician.

 

[Thor]

And that's a bit of a problem. Assuming each cab gets 100 watts, also means the 15 gets a 100 watts and each 10" gets 25 watts. A much better solution would be to slave a line level outpot signal to a second amp, power amp only would be fine, so you can run the cabs up to their full power handling capabilities. Possibly with a little power reduction to the 15 and the 4x10 could probably take some extra juice.

I'm sure the combo will look cool, but I am not sure halving the available power output by reducing the loads on the amp (increasing the system impedance) is a step in the right direction. I will avoid the mixed speaker debate. I always shoot to have the maximum available the system is designed for, you can always turn down.

And for the sake of clarity, amplifier circuits at the output stage have 3 components. The voltage, current and resistance. With the voltage constant, the increasing the resistance DECREASES the current in the circuit, as stated in Ohm's Law V = IR. As noted above, amps do not deliver power per se, but the amp produces Voltage at some available Amperage and depending on the load you provide, you can affect that. However, the Voltage and amperage are related to Power in watts. That equation is P = I x V. And since V = I x R Power in watts can be expressed in terms of Amps and Resistance only as P = I x I x R or P = I2 x R. (superscript doesn't display, not sure why...)

So as an example, if the power available is 300 watts and the impedance is 4 ohms, then the current is 8.66 amps. Theoretically, if we increased R to 8, then 6.1 Amps would be the draw. But we can't ignore the voltage drop and the designer tells us that P = 180 , using the equation we find that the current is now 4.7 Amps at 38 Volts as opposed to 8.66 amps at 34 volts.

So, basically you want to use half the current at a negligibly higher voltage to drive the system. Go to plan B. You'll be a lot happier.

 

[jaxstarke]

Any suggestions on power amps? I've never dealt with them so it's a whole new animal to me. I did some Google searches but it keeps coming up with all sorts of results most of which aren't related to bass guitar at all.

 

[Zooberwerx]

Any suggestions on power amps? I've never dealt with them so it's a whole new animal to me. I did some Google searches but it keeps coming up with all sorts of results most of which aren't related to bass guitar at all.

Personally, I'd just run the 4 ohm 4 X 10 and be done with it...I don't think you're gaining that much territory by adding the 1 X 15. However, if you're hell-bent on it, I'd suggest checking your local CraigsList for something like a used QSC RMX 850 or 1450. Older technology lead-sleds and relatively inexpensive. Your selection may vary but you get the idea. A used PLX 1602 would work and I've seen these for $275-300...the list goes on.

Riis

 

[jaxstarke]

Was checking this out. Looks pretty cool except I'm not so sure about the connections on the back. [Invalid or Expired Link Removed]

 

[oldfclefer]

Personally I'd run one or the other cab with your backline to see what gives the sound closest to what I'm looking for until I found another amp to drive the second one. If you buy something other than an extra bass head, say a Crown or a QSC, check out the specs esp. the low end response and make sure they go to at least 40hz. Most of them do. For that price, I'd check into trading or selling the Backline for a bigger bass head that goes to 2 ohm. Most Peaveys do.

 

[B-string]

If you locked yourself into one cab with a 4 ohm one, you can pick up a second BL600 pretty cheap. Run from the effects send on the first to the effects return on the second. The preamp on the first controls all but the master volume on the second. 2 X 300 watts= a 600 watt rig.

 

[Munjibunga]

each cabinet will use 90 watts. amps dont put out watts they put out volts. watts is the usage but for simplicity sake and so the general public can comprehend watts is the measure of power thats used. i worked for 20 years as a commercial electrician.

Amps produce volts that give rise to current. Volts times amps is watts (power). It's safe to say that amps put out watts - that's why they're rated by the number of watts they can produce.

 

[supershifter2]

Amps produce volts that give rise to current. Volts times amps is watts (power). It's safe to say that amps put out watts - that's why they're rated by the number of watts they can produce.

nope amps do not put out watts. watts is a unit of measure for power used. i'm a certified journeyman electrician. heres the deal > the amp produces voltage. when a speaker is connected and a signal is put into the amp current(voltage) flows(amps) and the speaker uses that current(watts) as it produces sound. its the exact same thing that a 100 "watt" light bulb does. you turn on the light switch and the current flows and the light bulb uses 100 watts to produce 1750 lumens(the brightness) of light. you can go research this in electrical educational books and you will see what i say is so. also in order to have a circut thats not a direct short(KABOOM) you must have resistance in the circut. the light bulb filiment is the resistor. the speaker voice coil is the speaker. the reson some amps cannot handle a 2ohm load is because its close to a direct short and when you short some amps the go KABOOM. some amps have a protective circut that prevents the KABOOM.

 

[B-string]

Well keep learnin'. If you have 34 volts available to a 8 ohm IMPEDANCE (not resistance BTW) load with zero current no work will be done. You cannot separate Voltage (pressure) and current (flow) to a load if you want work performed. Voltage with no current = no work from the load, voltage drops to zero. Voltage cannot be sustained without current flow. That was the way it was when I was in engineering classes and I have yet to see the world stop turning.

Incandescent bulbs would also not work without the current to sustain the voltage. You can't do any work with just one of them (Voltage X Amperage = watts). Have you not learned the importance of protection devices yet (fuses, breakers).

Questions for your training: Why are power plants rated by their mega WATT output? Why do power plants not output at wall voltages?

 

[supershifter2]

Well keep learnin'. If you have 34 volts available to a 8 ohm IMPEDANCE (not resistance BTW) load with zero current no work will be done. You cannot separate Voltage (pressure) and current (flow) to a load if you want work performed. Voltage with no current = no work from the load, voltage drops to zero. Voltage cannot be sustained without current flow. That was the way it was when I was in engineering classes and I have yet to see the world stop turning.

Incandescent bulbs would also not work without the current to sustain the voltage. You can't do any work with just one of them (Voltage X Amperage = watts). Have you not learned the importance of protection devices yet (fuses, breakers).

just what are rambling about ? what i posted above is the way it works. and yes a speaker vioce coil is the resistance. its is measures in ohms. a wall receptical had 120 volts present. when you connect a light bulb(resistance/ohms) you cause the current to flow. i had to know this stuff everytime i took my journeyman recertificion test. as for breaker and fuses they only thing they do is protect the wire or the equipment.

 

[Munjibunga]

nope amps do not put out watts. watts is a unit of measure for power used. i'm a certified journeyman electrician. heres the deal > the amp produces voltage. when a speaker is connected and a signal is put into the amp current(voltage) flows(amps) and the speaker uses that current(watts) as it produces sound. its the exact same thing that a 100 "watt" light bulb does. you turn on the light switch and the current flows and the light bulb uses 100 watts to produce 1750 lumens(the brightness) of light. you can go research this in electrical educational books and you will see what i say is so. also in order to have a circut thats not a direct short(KABOOM) you must have resistance in the circut. the light bulb filiment is the resistor. the speaker voice coil is the speaker. the reson some amps cannot handle a 2ohm load is because its close to a direct short and when you short some amps the go KABOOM. some amps have a protective circut that prevents the KABOOM.

Wow. First, electricity and electronics are two different fields. Second, current in not watts or volts, it's amperes. If the amp does not produce current, where does the current come from? It certainly does not come from the speaker. The amp produces voltage, current flows as a result of that voltage across the resistance of the speaker, and the amp is therefore producing the power. Your perception of your understanding is much greater than your understanding.

 

[B-string]

just what are rambling about ? what i posted above is the way it works. and yes a speaker vioce coil is the resistance. its is measures in ohms. a wall receptical had 120 volts present. when you connect a light bulb(resistance/ohms) you cause the current to flow. i had to know this stuff everytime i took my journeyman recertificion test. as for breaker and fuses they only thing they do is protect the wire or the equipment.

Resistance is a Direct CURRENT measurement, impedance is an Alternating CURRENT measurement. You seeing a pattern here? If you "plug" a 100 watt light bulb into a circuit that has NO current capability there will be NO light and no "100 watt" power consumption for light production or waste heat.
Amplifiers are rated in watt output, "x" amount of volts and "x" amount of sustainable current into the specified load impedance (not resistance as there should be no DC at the amp's output.

 

[B-string]

Wow. First, electricity and electronics are two different fields. Second, current in not watts or volts, it's amperes. If the amp does not produce current, where does the current come from? It certainly does not come from the speaker. The amp produces voltage, current flows as a result of that voltage across the resistance of the speaker, and the amp is therefore producing the power. Your perception of your understanding is much greater than your understanding.

E/IxR is a rousse folks!