8 ohm vs 4 ohm loading

[DJ Higgumz]

I am gonna build another 8 ohm t39 (lab 12 is 6 ohm driver, pressure of horn increases impedance) and I want to run it in an amp already running my other t39. Few questions

If I run it in bridge mono at 8 ohms, let's say it puts out 500 watts, but in 4 ohms it goes to let's say 900. Would it really be putting out 400 watts more power? Or with less resistance It would nullify power gain?

Would it be quieter with 16 ohm? Or would just adding another sub just split the power originally going into the first sub, so it generally wouldn't be any louder

Just soo confused from different sources telling me different things

[Bill Fitzmaurice]

Watts don't matter. Volts matter.

Just soo confused from different sources telling me different things

Sources that talk about watts rather than volts should be ignored, they know not of what they speak.

[DJ Higgumz]

Watts don't matter. Volts matter.

Just soo confused from different sources telling me different things

Sources that talk about watts rather than volts should be ignored, they know not of what they speak.

Yea I know that.. But still which way would I get the most out of an amp? And the max spl of a speaker, would it change with the impedance?

[Dave Non-Zero]

If you run two cabs in parallel, they each get the same voltage as the single cab did.

[Bill Fitzmaurice]

Yea I know that.. But still which way would I get the most out of an amp?

Calculate the voltage swing capability of the amp from its specs. Never bridge unless your amp can't otherwise deliver adequate voltage swing.

[Israel]

what you get:
by adding a second sub in parallel you will get + 6db of output (3db from doubling speaker count and another 3 from halving resistance) if you wanna see it in watts if you get that 900w there'll be 450w per cab. Voltage on other hand ill be the same for both cabs ie 900w @4 ohms = 60v 500w@8 ohms = 63 volts
what you loose:
well you may fry your amp by using it at 4 ohms bridged also sound quality will be affected to some extent.

[DJ Higgumz]

what you get:
by adding a second sub in parallel you will get + 6db of output (3db from doubling speaker count and another 3 from halving resistance) if you wanna see it in watts if you get that 900w there'll be 450w per cab. Voltage on other hand ill be the same for both cabs ie 900w @4 ohms = 60v 500w@8 ohms = 63 volts
what you loose:
well you may fry your amp by using it at 4 ohms bridged also sound quality will be affected to some extent.

Yea that makes sense, I am probably lookin at a used crown ce1000 or other crown/qsc/crest etc amps, not gonna try 4 ohm witha lesser brand

How do I calculate voltage swing? How much voltage would a lab 12 handle?

[Gregory East]

What an amp puts out and what it's rated for can be very different.
Bridging is generally not recommended, specially not if you aren't familiar with the details.

You are mixing information about your setup with completely vague questions about bridging. I'm surprised nobody pointed you to the education section. I don't know enough to set you straight.

[doncolga]

Since being around here I also learned to calculate my amp needs around volts and was very surprised to find how small of an amp suited my Tuba 30's with 3012LF's.

Those take 50 volts absolute max so here's what I did:

600 watts * 4 ohm load = 2400
Square root of 2400 = 48.98 volts

So my amp needed to be about 600 watts for that 4 ohm load. Granted I don't have much headroom, but I really try to stay away from sending my T30's more than 40 volts anyway. At that point its suggested to add more boxes, which I'm doing now. I can run my amp wide open and not have to completely squash the mixer signal with the limiter so I don't exceed my voltage. My limiter starts engaging at about 45-47 volts.

The smaller amp is very nice. It's lighter, don't blow circuit breakers any more, and combined with properly placed T30's, it sounds amazing and is plenty for lots of my jobs.

Donny

[Bill Fitzmaurice]

How much voltage would a lab 12 handle?

It's in your plans. What do you have it limited to now? You do have it limited, right?

[SirNickity]

This is a tricky topic. I thought I understood it, and got myself into some trouble here recently.

All else being equal, an equivalent model that comes in 4 and 8 ohm versions (say, an Alpha 6a vs. 6c) should have exactly the same SPL given the same voltage, right?

If that's true, higher impedance would be easier on solid state amps right? So why are there not more 16, 24, 32 ohm speakers? Difficulty meeting other T/S specs? Current requirements to overcome the permanent magnet with electromagnetic force?

Bill exposed my ignorance when he said that amps deliver the same voltage despite varying impedance (which I knew, at least in voltage amplifiers), and that's why the output level doesn't vary (which I had assumed it did.) But Israel says you get 3dB from halving impedance. Do you? Or do you just generate twice the heat from the increased amperage / wattage?

Also, a respected engineer (Rod Elliot, from ESP) has a page on building a variable output impedance circuit meant to tack onto an amp. He says there are minor variations in output level on a loudspeaker caused by its varying impedance. I would be shocked to find out he's wrong, so I'm trying to find a way to correlate the two points of view. Would this loss in level be caused by the changes in current through the passive components between the amp and the speaker then?

Finally.. What's with the negative opinions on bridging? Assuming the amp is designed to allow it, the user knows how to connect the input and load properly, and the load impedance is high enough that the amp doesn't go into protection, what's the harm?

I hope it's clear, but I'm going to be explicit. I'm just trying to understand this better. I'm not trying to start an argument. Also if this is off-topic (I don't think so, since I think these answers would help the OP make an informed decision), please let me know and I'll edit this out and start a new thread.

[Gregory East]

This is a tricky topic. I thought I understood it, and got myself into some trouble here recently.

All else being equal, an equivalent model that comes in 4 and 8 ohm versions (say, an Alpha 6a vs. 6c) should have exactly the same SPL given the same voltage, right?

If that's true, higher impedance would be easier on solid state amps right? So why are there not more 16, 24, 32 ohm speakers? Difficulty meeting other T/S specs? Current requirements to overcome the permanent magnet with electromagnetic force?

Basic fail leads to more confusion

Bill exposed my ignorance when he said that amps deliver the same voltage despite varying impedance (which I knew, at least in voltage amplifiers), and that's why the output level doesn't vary (which I had assumed it did.) But Israel says you get 3dB from halving impedance. Do you? Or do you just generate twice the heat from the increased amperage / wattage?

Also, a respected engineer (Rod Elliot, from ESP) has a page on building a variable output impedance circuit meant to tack onto an amp. He says there are minor variations in output level on a loudspeaker caused by its varying impedance. I would be shocked to find out he's wrong, so I'm trying to find a way to correlate the two points of view. Would this loss in level be caused by the changes in current through the passive components between the amp and the speaker then?

Finally.. What's with the negative opinions on bridging? Assuming the amp is designed to allow it, the user knows how to connect the input and load properly, and the load impedance is high enough that the amp doesn't go into protection, what's the harm?

I hope it's clear, but I'm going to be explicit. I'm just trying to understand this

Swype is doing my head in nearly as much as you. The 4 ohm driver reaches xmax with half the voltage, case closed.

better. I'm not trying to start an argument. Also if this is off-topic (I don't think so, since I think these answers would help the OP make an informed decision), please let me know and I'll edit this out and start a new thread.

[Rune Bivrin]

This is a tricky topic. I thought I understood it, and got myself into some trouble here recently.

All else being equal, an equivalent model that comes in 4 and 8 ohm versions (say, an Alpha 6a vs. 6c) should have exactly the same SPL given the same voltage, right?

Not exactly. It's ultimately a design decision, but generally the will have the same SPL given the same power.

If that's true, higher impedance would be easier on solid state amps right? So why are there not more 16, 24, 32 ohm speakers? Difficulty meeting other T/S specs? Current requirements to overcome the permanent magnet with electromagnetic force?

Following that reasoning, if you make the impedance infinite you would no current and therefore have the same SPL with no power at all. Just ain't gonna happen. See Perpetuum mobile

Bill exposed my ignorance when he said that amps deliver the same voltage despite varying impedance (which I knew, at least in voltage amplifiers), and that's why the output level doesn't vary (which I had assumed it did.) But Israel says you get 3dB from halving impedance. Do you? Or do you just generate twice the heat from the increased amperage / wattage?

You get 3dB by halving the impedance and thus doubling the power.

Also, a respected engineer (Rod Elliot, from ESP) has a page on building a variable output impedance circuit meant to tack onto an amp. He says there are minor variations in output level on a loudspeaker caused by its varying impedance. I would be shocked to find out he's wrong, so I'm trying to find a way to correlate the two points of view. Would this loss in level be caused by the changes in current through the passive components between the amp and the speaker then?

There is no conflict. A regular amplifier has near zero output impedance, and so the impedance variations in the driver will mean little as the driver is designed to function as a voltage controlled device.
A musical instrument amplifier can be designed with non-zero output impedance to let the variations in driver impedance affect the voltage across the driver and thus the output.
The passive components in the cross over will have a similar effect, and that is one of the reasons they have to be low resistance = thick wires = expensive. This is a primary driver for using active cross overs.

Finally.. What's with the negative opinions on bridging? Assuming the amp is designed to allow it, the user knows how to connect the input and load properly, and the load impedance is high enough that the amp doesn't go into protection, what's the harm?

That's just a lot to assume, and often bridging happens for the simple reason that the user believes that this is a way to get the most out of his amp and speakers. Turns out to often be the most smoke, and nothing else. Most amps out there are powerful enough that their available voltage swing is enough to drive the speakers to their excursion limits. Anything more than that is just risk, nothing else.

[Bill Fitzmaurice]

Finally.. What's with the negative opinions on bridging? Assuming the amp is designed to allow it, the user knows how to connect the input and load properly, and the load impedance is high enough that the amp doesn't go into protection, what's the harm?.

Bridging isn't about watts, it's about volts. You only bridge if the voltage swing of one channel is inadequate. If that's the case bridging doubles the voltage swing. That's fine if you need it, but is a recipe for blown drivers if you don't. Unfortunately manufacturers only use bridging as a means to hype unrealistic power ratings to drive sales, and don't educate their customers in the inherent dangers of doing so.

[Tom Smit]

Aaaaaand, if you run hard too close to the minimum load too often, you shorten the life of said amp.