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Old 9th February 2013, 07:43 PM   #21
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The gain of an power amplifier is not always given in a distinctive ratio or 32dB.
Often the nominal level in the amplifier system where the power amp is to interact in what determines the gain of the power amp.
The required gain of the power amp is different where the nominal level in the system is 0,75V or 2,0V. The funny thing is this: A 50W amp will need more gain than a 25W amp, unless one make an adjustment to the signal level in front of the power amp.
And a 100W need more gain, and further more a 200W, 400W, 800W... Given the inpedance is the same all over.
The voltage gain for the 800W amp need to be 5,7 times larger than in the 25W amp, when feeding them the same signal for full output.

Things are never as easy as they seems.
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Old 9th February 2013, 07:57 PM   #22
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Quote:
Originally Posted by AKSA View Post
THX standard is 29dB (Av=28.5).

Hugh

Are You sure?
And what THX-standard do you have theese requirements written down?
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Old 9th February 2013, 08:04 PM   #23
JMFahey is offline JMFahey  Argentina
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In fact there are 3 basic sensitivity standards.

Actual gain will vary depending on amp used and will be whatever's needed to drive that particular amp to Max power.

On Hi Fi amps a much used standard is 1V RMS , a.k.a "0dBV" because it's referred to 1 V RMS without stating impedance (which is presumed much higher than 600 ohms).

Logic behind this says: "so any preamp will drive any amp without trouble".

On Studio and "big" PS amps, +4dB (1.23V RMS into 600 ohms) is the most used standard and it's often called "the Pro standard".
In fact, VuMeters are calibrated with red (or nominal 0dB set to actual +4dB).

Some real old school guys (such as BBC Engineers) prefer to use 0dB (0.775V RMS into 600 ohms)

But, wait a minute: what about their VuMeters?
Do they follow a different norm than the rest of the World?
Well, it's not a problem .... because they don't use VuMeters.
What???
True, ..... because they use PPM meters, a very different kind of animal.
Click the image to open in full size.

Last edited by JMFahey; 9th February 2013 at 08:08 PM.
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Old 9th February 2013, 08:04 PM   #24
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by TANDBERGEREN View Post
And a 100W need more gain, and further more a 200W, 400W, 800W...
so if the 800watt is built with 'ordinary' 30db gain, and you still only have a buffer/unity gain 'preamp', you wont really have 800watt at all
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Old 9th February 2013, 08:45 PM   #25
wahab is offline wahab  Algeria
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For amplifiers the 30db gain is for a power that is barely 100W.

Best is to stick with a given input sensitivity for max power ,
a value between 775mV and 1V is a good range that should
match any valuable preamp output level but the downside
is that the more the gain the higher the amp noise.

Old preamps from the 70/80s have generaly too much gain
for recent digital source since they were matched for sources
that had a few hundreds mV output at most.
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Old 9th February 2013, 10:11 PM   #26
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generally amplifier sensitivity "standards" vary between 1Vrms to 2Vrms for the amp to be at full power. a gain of 28-31 is about average for 100W amps, reflecting the use of about 1Vrms as the most adhered to version of the "standard". for differing amounts of power, the gain will be changed. so if you own a 100W Pioneer amp, for instance, you know you hit clipping when the volume control on your preamp hits "7" on a scale of "10". you want more power, so you go and buy a Pioneer 300W amp. you will still hit clipping at "7". however if you bought a Yamaha 300W amp, you might not hit clipping until you get to "8" because a different manufacturer may have settled on a different "standard" sensitivity figure, and so the gain is different than in the Pioneer.
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Old 9th February 2013, 10:43 PM   #27
tinitus is offline tinitus  Europe
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and this is part of the reason why bigger power amps usually have many more devices in the preamp/driver stages ?
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Old 10th February 2013, 12:36 AM   #28
JMFahey is offline JMFahey  Argentina
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Quote:
and this is part of the reason why bigger power amps usually have many more devices in the preamp/driver stages ?
Maybe, but not necessarily.
In general they add more devices to make the amps symmetrical, or use CC loads instead of plain or bootstrapped resistors, more linearity, better phase characteristics, less distortion, etc.
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Old 10th February 2013, 09:55 AM   #29
RCruz is offline RCruz  Switzerland
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My amp has a spec sensivity of .75v and 30Watt output.

I use a DCB1 pre buffer and can get really good sound preassures when using a CDP, tuner or my riaa preamp builds.

From what I read, in average, a preamp can have 16x gain and the power amp around 30x so total gain should be 46x right ?
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Old 10th February 2013, 10:04 AM   #30
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,75V for 30W output gives a gain of 20X
30W in 8 Ohm gives 15,5V, 15,5/,75 = 20.

16X + 30X is not 46, but 480X. Not easy things like this.
Would actually mean that the sensitivity of the input to the preamp is 32mV.
For 30X gain in the power amp related to 0,75V sensitivity would give 22,5V at the output, equals 63W in 8 Ohm.
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