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Author Topic: 2 ohm loads on bottom end  (Read 16128 times)

Kevin Conlon

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Re: 2 ohm loads on bottom end
« Reply #90 on: September 24, 2016, 05:13:29 pm »

My storage unit has 8 feet of headroom.
I keep hitting my head in the trailer, do i need to put resistors in the wire harness?
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Mark Wilkinson

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Re: 2 ohm loads on bottom end
« Reply #91 on: September 24, 2016, 05:27:28 pm »

No problem good questions are good thing, I just get a little tired of answering the same ones over the years. Perhaps for some of the amplifier current limiting mechanisms (there are several in the output stages) but "transformer regulation" is a simple IxR winding wire loss, and in the limit magnetic flux saturation so either not enough copper (winding resistance) or not enough magnetic flux (iron and turns of windings). This is why bigger (old school) amps have heavier transformers.

Sorry I am not trying to blind you with science but transformer regulation is just one of the limits. Power transistors have a finite on resistance so will lose voltage based on another IxR loss, not to mention most old school amps used emitter degeneration resistors in series with power devices to force sharing.

A crude way to quantify or estimate how lossy the amp is wrt current is to compare rated power between 8, 4, and 2 ohm loads. In an ideal world (that we don't live in and only politicians talk about) a 200W @ 8 ohm power amp, would deliver 400W at 4 ohm and 800W a 2 ohms. Looking at any real power amp specification reveals significantly less than theoretical output at 2 ohms. This shortfall is partially from transformer regulation, losses in power devices, and in the extreme max current output. Old school amps follow a fairly predictable power vs load relationship but some modern class D amps get funny with with voltage limited or current limited extremes.

There are other wild cards in here, like reservoir capacitor ripple. For audio frequencies lower than 2x the mains frequency charging rate, the capacitors are tasked with supplying more current than at higher signal frequencies. While this is getting a little esoteric, at high frequencies the sine wave alternates between the positive and negative swing drawing half from the plus supply and half from the negative. At very low frequencies the signal draw remains always negative or always positive for a full charging cycle (only a few mSec) but long for the caps. You can parse this out if a amp specs more power at 1khz than 20-20kHz (but make sure THD is same for both too).

Again this is TMI for most users. Modern amps are so good that we can all relax and buy something light that won't break our back or the bank.  8)

JR

PS: I didn't tell you everything there is about amps yet,,, but I am trying to calm you all down not scare you.       

You really do help.  I jump into a book like one from Douglas Self and get immediately hopelessly in over my head.
You share info that is still over my head  :), but perhaps digestable with time...

I'm glad you brought up amps that truly double wattage as impedance is halved.  I had a Krell and a Meitner hi-fi amp, both of which could pull that off....IIRC the Krell could do it all the way to one ohm.  I think these amps might have started my speculation about amps being current starved when they sag.

Anyway, thx again....I'll keep my word with no more questions !
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Jeff Bankston

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Re: 2 ohm loads on bottom end
« Reply #92 on: September 24, 2016, 05:44:08 pm »

I keep hitting my head in the trailer, do i need to put resistors in the wire harness?
no you just need a cap with enough farads to fit your head that needs more room so you dont hit it.
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