Can someone please explain rails?

[Imshirazy 10+ year member]

I know that amplifiers are supposed to follow ohms law, but of course many amplifiers seem to go around it

ex:

ohm's law standard

4ohms = 500rms

2ohms = 1000rms

1ohm = 2000rms

yet most amps would do something like

4ohm = 500rms

2ohm = 700rms

1ohm = 1000rms

I'm told that it's like this because amplifiers use some kind of "rail?" Can someone explain this to me, or link me to some indepth info about amplifiers?

I'd greatly appreciate it, thanks

-Ivan

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[helotaxi 5,000+ posts]

They all follow Ohm's Law. It is a law after all.

What are typically referred to a voltage rails are the power supply outputs of the amp. One is positive and the other is negative. They are at least the RMS output voltage multiplied by sqrt(2). In good amps they will be something higher than this. Along with the voltage they must be able to flow the current required for the load impedance at that voltage. Ohm's Law. The amp uses its output devices to modulate this DC waveform to match the input signal. The output current has to flow through the output devices. They have a limt on how much current they can flow usually limited by the amount of heat they can shed. They have a resistance when "on" and resistance and current means voltage drop and voltage drop and current mean heat. This voltage drop also reduces the amount of voltage going to the load and lower voltage means lower current through the load per Ohm's Law. Lower voltage and lower current means lower power. The answer you are looking for is that the relatively high on resistance of the output devices in some amps causes a voltage drop in the output stage that keeps the power from doubling as the driven load halves. The amps that you see with advertised doubling of power as the load is halved usually has output devices with really low on resistance, multiple sets of output devices in parallel to lower the final on resistance and/or are underrated into the higher impedances in orderto make rated power after the voltage drop at lower ones.

Now the is the answer for single rail power supplies. There are amps out there that have multirail and/or "smart" power supplies. The PG Xenon and JL Slash amps are the most known examples. These amps actually monitor the output voltage vs. the output current and adjust the rail voltage to maintain a constant power. My understanding is that the JL amps do it somewhat real time and the Xenons do it only at power on. Because of the circuit in the Xenon, you could "trick it" into thinking that there was a higher load on it than there really was using a timer circuit on the outputs and then it would stay with a higher rail voltage and produce more power into a lower load (within limits of cooling).

Clear as mud?

 

[wvufootball 10+ year member]

What are some other lesser known amps that do the same thing as the slash and xenon series?

 

[helotaxi 5,000+ posts]

The older JBL mono amps worked a bit similarly. The new Alpine PDX are full range Class D amps that deliver consistant power regardless of load. The Precision Power 2350 DM worked similarly, but they quit making those beasts about 15 years ago.

 

[Imshirazy 10+ year member]

so more or less, as the resistance increases the amps make up for it by supplying a higher voltage, or by reducing the resistance?

 

[helotaxi 5,000+ posts]

Resistance is fixed. The resistance causes a drop in voltage as the current increases with a drop in load impedance. The more current, the more voltage drop and the more heat generated. That's with a fixed rail (normal) amp.

The "multi-rail" amps drop the voltage in the power supply rather than the output stage. Since they don't up the voltage as much there is less loss in the output stage, at least in theory. For some reason though the Xenons were not hugely efficient (though not horribly inefficient either) my guess would be high on resistance in the switching devices inthe power supply. The JL amps aren't known to be too efficient either.

 

[oxygen65]

jbl bpx amps

 

[Trey803 5,000+ posts]

Any benefits to this style of amp? What about the alpine pdx series is their any benefit to that

 

[helotaxi 5,000+ posts]

The only real benfit is that you get the same power regardless of the load on the amp. Opens up choices (especially with subs) for changes without having to change amps every time you chang sub configuration.