Playing with my Polk DB651s set up - 2 ohm vs 4 ohm question

[swatski]

Be careful with the Polks. I am not recalling if it is the db series or the mm series or both but the true impedance on them is 3.7 ohms which when split in two gets dodgy for the amp. @Bruce made a good comparison to pushing things uphill. Impedance is resistance and I like to compare it to deadlifting. If you think you have 400 lbs on the bar but only have 200 lbs then exert 400 lbs worth of power that bar is going to come up with much more energy (watts in this comparison). Doing this multiple times as a human means you can do it quicker and your heart rate goes up quicker so you heat up. If an amp is not 2 amp and stable or there is too much load on a 2 ohm stable amp it will overheat or clip the signal.

I may need to do some testing, and observe which configuration is more prone to clipping.

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[swatski]

What boat do you have? Running the port speaker wires for me was actually pretty easy since I mounted the amp near the batteries. The starboard run was a bit longer and I ran it through the engine compartment along the front edge of the hatch.

That would definitely make the battery connections easier and save some $$$ for big power wires to the helm, but in the ARs there is no factory amp and the HU is in the helm - one still needs to run either long speaker wires or RCAs.

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[Bruce]

You have the wires purchased and run so I would not change anything. But for future reference higher voltage is better suited to longer runs. That is why the electric company transmission lines are thousands of volts that are transformed down to 120, 240, etc before entering our homes. The speaker output is much higher voltage than the 12 volt power supply so it is best to have the longer runs on the speaker side of the amp.

 

[swatski]

You have the wires purchased and run so I would not change anything. But for future reference higher voltage is better suited to longer runs. That is why the electric company transmission lines are thousands of volts that are transformed down to 120, 240, etc before entering our homes. The speaker output is much higher voltage than the 12 volt power supply so it is best to have the longer runs on the speaker side of the amp.

I did not think of it! Glad you pointed this out.
Indeed - I had to run new wires and some pretty long runs (swim platform) - I am happy to leave those in place - running wires in our boats is not much fun.

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[maboat]

That would definitely make the battery connections easier and save some $$$ for big power wires to the helm, but in the ARs there is no factory amp and the HU is in the helm - one still needs to run either long speaker wires or RCAs.

You definitely want to keep the RCA (input signal) wires as short as possible because longer wires pick up more noise and if the long wires are on the input side of the amplifier than you amplify the noise too.

 

[David Analog]

Be careful with the Polks. I am not recalling if it is the db series or the mm series or both but the true impedance on them is 3.7 ohms which when split in two gets dodgy for the amp. @Bruce made a good comparison to pushing things uphill. Impedance is resistance and I like to compare it to deadlifting. If you think you have 400 lbs on the bar but only have 200 lbs then exert 400 lbs worth of power that bar is going to come up with much more energy (watts in this comparison). Doing this multiple times as a human means you can do it quicker and your heart rate goes up quicker so you heat up. If an amp is not 2 amp and stable or there is too much load on a 2 ohm stable amp it will overheat or clip the signal.

Don't get the 2.7 ohm MM/UM series confused with the 4-ohm dB series.
If the dB speaker measures 3.7 ohms, that is simply the DC resistance, and not the true AC impedance. The impedance is frequency dependent and non-linear. Most all speakers will have a narrow dip slightly below the nominal impedance rating, which is essentially what you are measuring....they also will be above the nominal impedance over a majority of the frequency spectrum. So if you test a DCR that is only 90% of the rated impedance, don't get concerned that this translates to an amplifier stability issue. It's still a 4-ohm speaker.

When halving the impedance/resistance, by connecting two speakers in parallel, you have reduced resistance to the flow of current, thus more current flows, and you have more wattage to get the work done. However, unlike a simple DC light bulb, an audio amplifier is far more complex. The amplifier has a DC to AC to DC switching power supply designed to create higher voltage so that you can make more audio power. But, unlike the enormous current reserves in a battery for example, this power supply is a fixed & limited source of current. And nothing much good happens when you approach its limitations.

The audio output section of an amplifier is more of a variable electric valve, although it does have power dissipation limitations, it's usually the stronger link in the chain. The power supply section has to actually recreate power as it transitions from low voltage to high voltage, and this constitutes the biggest bottleneck.

A 2-ohm load is appropriate for most mobile/marine amplifiers. You are more likely to test the amplifier limitations, thereby exhausting the power supply capacity, when you load down all channels of a multi-channel amplifier to the permissible minimum. So if you have more than one amplifier, do your best to evenly distribute the workload.

 

[swatski]

Don't get the 2.7 ohm MM/UM series confused with the 4-ohm dB series.
If the dB speaker measures 3.7 ohms, that is simply the DC resistance, and not the true AC impedance. The impedance is frequency dependent and non-linear. Most all speakers will have a narrow dip slightly below the nominal impedance rating, which is essentially what you are measuring....they also will be above the nominal impedance over a majority of the frequency spectrum. So if you test a DCR that is only 90% of the rated impedance, don't get concerned that this translates to an amplifier stability issue. It's still a 4-ohm speaker.

When halving the impedance/resistance, by connecting two speakers in parallel, you have reduced resistance to the flow of current, thus more current flows, and you have more wattage to get the work done. However, unlike a simple DC light bulb, an audio amplifier is far more complex. The amplifier has a DC to AC to DC switching power supply designed to create higher voltage so that you can make more audio power. But, unlike the enormous current reserves in a battery for example, this power supply is a fixed & limited source of current. And nothing much good happens when you approach its limitations.

The audio output section of an amplifier is more of a variable electric valve, although it does have power dissipation limitations, it's usually the stronger link in the chain. The power supply section has to actually recreate power as it transitions from low voltage to high voltage, and this constitutes the biggest bottleneck.

A 2-ohm load is appropriate for most mobile/marine amplifiers. You are more likely to test the amplifier limitations, thereby exhausting the power supply capacity, when you load down all channels of a multi-channel amplifier to the permissible minimum. So if you have more than one amplifier, do your best to evenly distribute the workload.

Wow - thank you!

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[Mainah]

Don't get the 2.7 ohm MM/UM series confused with the 4-ohm dB series.
If the dB speaker measures 3.7 ohms, that is simply the DC resistance, and not the true AC impedance. The impedance is frequency dependent and non-linear. Most all speakers will have a narrow dip slightly below the nominal impedance rating, which is essentially what you are measuring....they also will be above the nominal impedance over a majority of the frequency spectrum. So if you test a DCR that is only 90% of the rated impedance, don't get concerned that this translates to an amplifier stability issue. It's still a 4-ohm speaker.

When halving the impedance/resistance, by connecting two speakers in parallel, you have reduced resistance to the flow of current, thus more current flows, and you have more wattage to get the work done. However, unlike a simple DC light bulb, an audio amplifier is far more complex. The amplifier has a DC to AC to DC switching power supply designed to create higher voltage so that you can make more audio power. But, unlike the enormous current reserves in a battery for example, this power supply is a fixed & limited source of current. And nothing much good happens when you approach its limitations.

The audio output section of an amplifier is more of a variable electric valve, although it does have power dissipation limitations, it's usually the stronger link in the chain. The power supply section has to actually recreate power as it transitions from low voltage to high voltage, and this constitutes the biggest bottleneck.

A 2-ohm load is appropriate for most mobile/marine amplifiers. You are more likely to test the amplifier limitations, thereby exhausting the power supply capacity, when you load down all channels of a multi-channel amplifier to the permissible minimum. So if you have more than one amplifier, do your best to evenly distribute the workload.

As usual what @David Analog said .

 

[kthrash]

simple answer.. buy another amp.. whats another $100?

 

[swatski]

simple answer.. buy another amp.. whats another $100?

Oh, no - that would be way too simple! LOL
But seriously - I considered various options - at the end having those two micro XCs driving 10 speakers (2 pairs on one amp at 4 ohms, and 1 pair at 4ohms plus 2 pairs at 2ohms on the other) - is pretty close to the ideal configuration, all things considered.
With all the integrated ballast I'm squeezing in w/controls for reversible pumps, trim tabs, cruise control, stereo/sub etc. my helm compartment/ battery space/capacity is getting very precious!

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[kthrash]

2 pairs on one amp at 4 ohms, and 1 pair at 4ohms plus 2 pairs at 2ohms on the other

exact same setup I did...

 

[swatski]

2 pairs on one amp at 4 ohms, and 1 pair at 4ohms plus 2 pairs at 2ohms on the other

exact same setup I did...

I had to ask here before I rewired it that way, but it does work great!
And still have your fade/zones on Connext same way as factory. It is a great setup.

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[FloJet]

Go 4 ohm if you can. 2 ohm just means more power draw. To be honest I would never go 2 ohms in a boat.

 

[swatski]

Go 4 ohm if you can. 2 ohm just means more power draw. To be honest I would never go 2 ohms in a boat.

Well, we will see how it does. Keep in mind it is ONLY 2 out of 4 channels in one of two micro amps (that I am running wired for 2ohms), the other 6 (out of 8) channels are running in 4 ohms. I bet you my power draw is very favorable as compared to most systems. IDK.

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[Bruce]

In contrast I prefer to run at 2 ohms for more sound out of the same equipment.

 

[Tad Steinberg]

That statement is true of ALL speakers. No "4-ohm" speaker is fixed at 4 ohms. Impedance changes with frequency. Speakers are labeled with "nominal" impedance which is an approximate average resistance over the frequency range of the speaker. Any speaker's resistance in ohms is lower at lower frequencies and higher at higher frequencies with the nominal impedance being the approximate middle.

Well put.....

 

[swatski]

Well put.....

To me this whole thread has been pretty educational.
The XC1410 has high pass filter, so that is what I'm using to prevent possible (lower frequency) damage especially in the 2 ohm (nominal) configuration.

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[Tad Steinberg]

In contrast I prefer to run at 2 ohms for more sound out of the same equipment.

I tend to agree as well. There are so many variables to consider, power draw, heat, efficiency and the like. I think the small Class D amps are perfect for a jet boat application like ours. They are usually 2 ohm stereo stable and usually have much better efficiency than a class "A" amp, but usually aren't as "Clean" as the Class A's. but in a Boat???? one really doesn't need that high of sound quality do they, well I don't anyway, I want volume. I want to be able to have my head in the wind at near 50 MPH and still hear and feel the music.

Just to pile on to all the goodness in this thread, I'll share my setup. I too have the small class D amps. I went the Marine/UTV route by PowerBass. They make UTV/Marine micro amplifiers (XL line) that do really quite well. I have 2 of the Powerbass micro monoblock amps running two Orion XTR Pro 12" subs which are dual 2 ohm woofers, wired to a one ohm mono load, the amps were designed for just that purpose so a perfect fit. Had them running two seasons and no issues with overheating, shutting off and the like. New for this year is another 5 channel Micro amp from Powerbass to run mid ranges in the dash for that "In your face" musical experience and likely another freeair sub, haven't decided on that as of yet. I also picked up a JL audio 4 channel from a garage sale (seller didn't know what they had) for $50. It is running all 8 cabin speakers at 2 ohm stereo - had 6 and installed two extra in the mid cabin area (no tower) Ebay takeouts from another Yamaha. Perhaps in the future I'll make the JL jump, just too expensive for me today. To be able to use the stereo when parked, I now have 4 batteries. Three (3) house batteries and one Start battery all on isolation. Two mounted port and two mounted starboard - all 4 in one area made the boat list. I've also mounted a battery tender for those times it sits in the garage. I did run a 4 AWG Power and ground from the battery compartment to the cabin area behind the helm. With distribution blocks - down to 8 AWG going to each of the amplifiers. Pictures will follow once the cover comes off for the season...