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Old 18th February 2005, 01:20 PM   #1
keyser is offline keyser  Netherlands
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Default Input sensitivity

Hi,

I've got 2 poweramps. One has input sensitivity of:

Input sensitivity (for rated output into 8 ohms) 1.1V

The other has input sensitivity of:

Input sensitivity 0.77V


With the second amp no load is mentioned. What does the manufacturer mean?
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Old 18th February 2005, 01:53 PM   #2
Netlist is offline Netlist  Belgium
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That would be the minimum needed input voltage to get the maximum output power.

/Hugo
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Old 18th February 2005, 08:18 PM   #3
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well...
Input sensitivity shouldn't be much dependent on load as audio amps are voltage amplifiers, aren't they
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Old 18th February 2005, 11:39 PM   #4
anatech is offline anatech  Canada
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Hi darkfenriz,
The output voltage may drop depending on the damping factor. Not very much at all but this is an exact answer to your question. In practice with an average amp - not that you will notice, so yes.
-Chris
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Old 19th February 2005, 06:24 AM   #5
PRR is offline PRR  United States
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> Input sensitivity (for rated output into 8 ohms) 1.1V
Input sensitivity 0.77V
With the second amp no load is mentioned. What does the manufacturer mean?
Input sensitivity shouldn't be much dependent on load as audio amps are voltage amplifiers


Yes, but they are not perfect voltage sources.

Rated output voltage varies with load.

A typical amp may be rated 160 Watts in 4 ohms, 100 Watts in 8 ohms, 60 watts in 16 ohms.

In voltage, these are 25V in 4 ohms, 28V in 8 ohms, 31V in 16 ohms.

The power supply sags as the load resistance decreases. A 20% sag is not at all uncommon.

The amplifier voltage gain is probably constant over this range of loads. If damping factor is 1,000, then the voltage gain drops 0.01%, not enough to measure easily.

Gain might be 20 (+/-0.01%).

So the input sensitivity is 25V/20= 1.25V in 4 ohms, 28V/20= 1.4V in 8 ohms, 31V/20= 1.55V in 16 ohms.

The range from 1.25V to 1.55V is measurable but probably not very important.

One maker choose to say what the test load was, the other didn't. This is often NOT a designer decision, but some manual-writer who just barely knows what an amplifier is.
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Old 19th February 2005, 01:32 PM   #6
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I think the good modern amp should have (almost) double power on 4 ohm compared to 8 ohm. Otherwise it means output devices are on the threshold of Icmax/Idmax (=poor linearity). Emitter resistors aren't so crucial.
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Old 19th February 2005, 03:08 PM   #7
anatech is offline anatech  Canada
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The older mainstream amplifiers used to almost double the output power on 4 ohm loads (ie Marantz 500). Unless you spend $$$ on a new amp, forget it. Power transformers and heatsinks add weight and cost money. You will therefore find the minimum (or lower) capasity installed in newer avaerage consumer grade amplifiers.

Methinks you are idealistic there darkfenriz. I've said it before. Early on, we consumers voted for crap with our dollars, now that's about all there is in the mainstream. Yeah us! And that's what DIY is all about.

-Chris
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