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Working out/Learning about Voltage Gain

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Joshvdh View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Joshvdh Quote  Post ReplyReply Direct Link To This Post Topic: Working out/Learning about Voltage Gain
    Posted: 11 August 2020 at 11:53pm
I've Spent the last few weeks learning about Amp specs and what to do with the numbers.

I think I've started to get my head around input sensitivity and voltage gain but there is still parts i don't understand. the following may make absolutely no sense as I've been teaching myself the maths and have no idea if I'm on the right path. there's a few questions about what I'm trying to understand but I'm also just writing down what i think is correct and hoping someone can tell me if I've got it all wrong!

I'm trying to understand the relationship between voltage gain and the load on the amp. i.e does the voltage gain change depending on how the amp is loaded. (most amp specs only state the voltage gain at a given load, usually 8ohms). My understanding is this is true if the amplifier has a fixed input sensitivity but I'm not 100% sure.
Also, how can you find out if an amplifier had fixed input sensitivity or fixed voltage gain?


I'm trying to find correct limiter settings for various speaker arrangements 

using the Crown XTi 4000 as an example:

Rated output 1600w @2ohms , 1200w @4ohms , 650w @8ohms
voltage gain is listed as 34.2dB @ 8 ohms
input sensitivity listed as 1.4V (5.13dBu) - (i believe the XTi's have a fixed input sensitivity as all the amplifiers in the XTi range are all listed as 1.4v input sen)  

Driving one 150w AES 8ohm speaker would mean limiting the output from the LMS to 0.675volts (-1.2dBu) correct??  (Speaker RMS Voltage = squareroot of 150 x 8 = 34.64volts. Gain factor @8ohms = 10^(34.2/20)=51.28. voltage in = 34.64/51.28= 0.675v=1.2dBu)

if, however, the amp was driving four 150w AES 8ohm speakers in parallel, a 2ohm load on the amp, how would this affect the voltage gain and therefore the limiter settings?

i assume that because the input sensitivity is fixed then the voltage gain would be 32.1dB at 2ohms?(1600w @ 2 ohms = 56.568 volts. 20xlog(56.568/1.4 = 32.1))

Gain factor would = X40.27 (10^(32.1/20)=40.27)

using these numbers i would need to set the limiter to 0.869volts (1dBu) ?


why don't manufacturers publish voltage gains at different impedance's if it is fixed input sensitivity and why don't they publish input sensitivity at different impedance 's if the voltage gain is fixed?




 

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DMorison View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DMorison Quote  Post ReplyReply Direct Link To This Post Posted: 13 August 2020 at 2:15pm

Hi,

Generally, voltage gain is fixed for a given amp.

Amps are by design Voltage amplifiers, and the power rating is the result of the amount of current demanded from the amp when that voltage is applied to the speaker load.

Because real world amps usually don’t have big enough power supplies, they usually can’t source the current needed to fully double it every time impedance is halved, which is why we see ratings like the XTi.

That means that if we apply an input signal strong enough to get to full power at a higher impedance while connected to a lower impedance, the amp will run out of current sooner, and thus internally limit/clip before getting to that full output voltage.

The amp therefore has a different input sensitivity for each impedance, even though voltage gain stays the same.

If the output “power” you desire for your drivers is within the amp’s rating at each impedance, you don’t need to change the limiter setting at all as it will still limit at the same voltage whatever the load impedance.

HTH, I know that as I’ve done nothing for 4 months my attention span has become approximately that of a goldfish with severe dementia, so if I’ve missed something, sorry.

Cheers,

David.

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Sonic the hedge View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic the hedge Quote  Post ReplyReply Direct Link To This Post Posted: 15 August 2020 at 6:36am
I would add, that when a real world amplifier is pushed close to clip point, into a low impedance, the amplifiers supply rail voltage will start to sag, due to the high current demand.

So approaching clipping, the voltage gain will reduce slightly, in proportion to the supply rail voltage sag. But there would obviously be no sense in increasing the input signal, at this point, because the amplifier has no more power to output. 

Assuming that, to protect your drivers, you are limiting somewhat below the maximum amplifier output, supply voltage sag should not be an issue. So as David said the voltage gain, can be assumed to be the same, regardless of load impedance. 

But be careful if you are aiming to limit the amp just below clip. Because the supply voltage sag, may cause the amplifier to clip the output, before the  theoretical maximum voltage gain is reached. 

This will depend on how conservative, the amplifier manufacturer has been, WRT output rating at each impedance, and the crest factor of your source material (i.e. Muzak). So YMMV.




Edited by Sonic the hedge - 15 August 2020 at 7:41am
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