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Help with amp and sub combo

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Help with amp and sub combo

Postby S.I.N » January 12th, 2016, 10:34 pm

Got an alpine pdx mono, regulated power supply making the same power at 2 and 4 ohms. Getting a nice dual 2 sub to match power.
1. Since making same power at 2 and 4 ohms, would a sub wired at 2 ohms have any advantages or disadvantages over a sub wired at 4ohms ? (Excluding THD and damping)
2. Would a dual 2 sub work to max potential with the amp wired at 4 ohms?

Thanks in advance guys

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Re: Help with amp and sub combo

Postby nervewrecker » January 13th, 2016, 6:02 am

What part of same power between 2-4 ohms you don't understand?
Wire it any way you feel like once it falls within that range.
No advantages or disadvantages.

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Re: Help with amp and sub combo

Postby Sully » January 13th, 2016, 1:20 pm

If you have a dual 2 ohm sub being wired to a monoblock, you cannot get a 2 ohm load at the amplifier.

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Re: Help with amp and sub combo

Postby nervewrecker » January 13th, 2016, 6:13 pm

Okay... let's look at the big picture, which I think will clear up most of the confusion.

RIPS has two parts... supply regulation and load impedance optimization. Supply regulation is what allows the amp to maintain rated power even when supply voltage drops to 12V or below. It doesn't let the rails sag when the voltage dips.

Load impedance optimization is what everyone seems to be confused about, so let's discuss it more specifically.

If you ask an engineer to design a "500 Watt amplifier", you are likely to get a few questions back, but the first one will likely be "at what impedance load would you like the 500W to occur". The answer to this question will determine several design parameters of the amplifier and power supply circuits.

If the answer is "1 ohm", then the amplifier must be designed as a "high-current" design, which means it will make its 500W at 1 ohm (meaning a rail voltage of 22.3 V and 22.3A of current capability).

If the answer is “I want 500W at 4 ohms.”, then the design needs to have a higher rail voltage and less current capability (44.7V and 11.2 A). In either case, we’re designing a “500W amplifier”, but certain aspects of the power supply and other parts of the circuit need to be adjusted depending on the load and whether more voltage or more current is required to make 500W.

The high-current version might be a little more expensive to make, but the two amps (1 ohm or 4 ohm) are going to be close in cost and similar in design.

Now, inevitably, a ‘creative thinker’ comes along and wonders what happens when we take the amp designed to make 500W at 4 ohms and connect it to a 1 ohm load! Well, since it has 44.7V rails and is not a sentient being, this amp will try to deliver 2000W into a 1 ohm load, which requires 44.7A of current… and here we have a problem as the designer never intended it to be able to do this and the amplifier gets really pissed off and hot and either shuts down to protect itself, blows a fuse or catches on fire.

Now, most car amplifiers are designed to produce optimum (highest stable) design power at 2 ohms, so if we follow that approach for our “500W” amplifier, we would specify a rail voltage of 31.6V with 15.8 A of current capability. If we try to load the amplifier down to 1 ohm, it will run out of current, so we put in a protection circuit to shut it down or mute if someone tries to do that. If you put a 4 ohm load on the amp, the 31.6V rails translate to 250W and the current demand goes down to 7.9 Amps. At 4 ohms, the amp is cooler than the other side of the pillow and cruising well below its capabilities, and we only get half the power output that it is truly capable of.

Now, the marketing guy says… “Perfect! We will call this a 250W amplifier and then tell people they can DOUBLE THEIR POWER to 500W at 2 ohms… woohooo, it’s like a 2 for 1 sale!” The engineer might say… “Well, at 4 ohms, you’re really not getting the power the amp is designed to produce, but go ahead and rate the amp any way you want, you silly marketing twit.” << and this is how we’ve all become accustomed to understanding amplifiers and how they behave into different loads. We’re all told that power increases at lower impedances, but it’s actually more correct to say that power output decreases at higher impedances (taking the optimal design power as the baseline figure).

Okay, so hopefully that explains the basics of an amplifier with a singular rail voltage.

Now, somebody who thinks a little deeper comes along and says. “You know, it sure sucks that we have to load the amp down to get all the power that was designed into it. It would be great if we could tell the amp what the load is so that it can always make its full power.” A well-meaning engineer overhears this reasonable request, and says… “We can do that! We’ll simply put a switch marked “2 ohms or 4 ohms” and let the user select the appropriate position for his application. The switch will lower the rail voltage at the 2 ohm position so the amp is still 500W and doesn’t catch on fire. (Soundstream and Sony did this a while back).

Seems like a really smart, reasonable solution to give the customer the 500W he paid for every time, right? Only one problem: the amp gets louder when you put it in the 4 ohm switch position and connect it to a 2 ohm load, and someone posts that on DIYMA and everybody starts to do it, and pretty soon everyone is complaining that the amp is a P.O.S. because it catches on fire.

So, then another engineer comes along and says “We must make this function automatic and transparent to the user.” and the ancestor of RIPS is created. (PPI 2350DM in the early 90’s). This amplifier had two settings (2 ohm and 4 ohm) that it selected automatically by detecting the load’s impedance through a current measurement on the output of the power supply at clipping. This clever piece of engineering cost some dollars to implement and took some time to tweak and perfect, but in essence gave this amp an automatic version of the “2 ohm or 4 ohm” switch, so that people couldn’t select the wrong position and burn the amp up. So, this unique amp was rated to produce 350W x 2 @ 4 ohms or 2 ohms. It should be noted, that during operation the amp will never switch back and forth from the “2 ohm” to the “4 ohm” rail voltage… it picks one and stays there, behaving just like a regular old fixed rail voltage source amplifier.

Fast forward a few years, and a few company upheavals later, and the engineer who designed the 2350DM ends up working for JL Audio and is tasked with designing an all-new line of amplifiers (the original Slash series). His name is Bruce Macmillan and he really wanted to take the lessons learned in the 2350DM and take them to a higher level of refinement across an entire line of products. We gave him the freedom to do so and he came up with the technology that we call RIPS. This is a more sophisticated version of the “2 ohm or 4 ohm” approach, actually involving 3 or 4 “switch positions”, depending on the model. The detection of the load is also far more sophisticated than the original 2350DM design.

I really can’t go into any great detail about its operating parameters, because it’s proprietary stuff, but I’ll give enough info to convey how it works in general terms.

A) Amp turns on and wakes up in the 4 ohm setting (highest rail voltage)

B) If the user never turns it up loud and never clips the amp, it will stay in the high-voltage setting (yes, with craploads of headroom if you’re running a low impedance).

C) When the user cranks it up gets near clipping, a circuit in the power supply control circuitry monitors current output and, based on that, makes a very intelligent decision about the connected load, and either leaves the rail voltage at maximum or reduces it to one of two or three lower positions (depending on the model). This position is now held (latched) until the amplifier is turned off.

Now, it should be noted that the amp makes an assessment based on real world current demands at clipping and the circuit has been tweaked to allow the amplifier to deliver the most rail voltage that is safe into a given load. The amp doesn’t “know” the specs of the speakers, it measures its output to determine what it can safely give and gives you that. In many cases, this results in significantly more “real-world” power than with amps that have fixed, singular rail voltages. What you think might be a “2 ohm load” based on speaker specs may actually never dip below 3 ohms, and the RIPS system will recognize that and give you full power at the higher real-world impedance. The circuit logic is also sophisticated enough not to be fooled by reactive or transient signals. There is also no gain change when the “switching” occurs. The output voltage remains stable.

It should also be noted that if you load a RIPS-equipped amp down to 1.5 ohms and then disconnect the 1.5 ohm load (without turning the amp off and on again) and replace it with a 4 ohm load, the amp will remain in the low voltage position and produce reduced power at 4 ohms (just like a normal amplifier). Once it latches down, it stays there until the next power cycle.

The last point I will make in this rather long-winded response, is that RIPS is by no means a dynamic circuit. The rail voltage will not fluctuate once the amp has determined the optimum voltage. It behaves like a simple voltage source amplifier with the right voltage to make the amplifier’s rated power into that load.

Hope that answers the questions and doesn’t raise too many more… but feel free to ask.

Best regards,

Manville Smith
JL Audio, Inc. ... -work.html
Last edited by nervewrecker on January 13th, 2016, 6:31 pm, edited 1 time in total.


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