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Old 16th May 2012, 09:41 PM   #1
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Default Active preamp gain

Hi , my marantz 2226 has main amp , with sensitivity of 1.5V for full output .
I build for him active preamp with opamp with gain of X4 . is it to much ?
do you think I should lower it to X2 , since a normal DAC\CD PLAYER has output of ~2V ?
thanks .
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Old 16th May 2012, 09:52 PM   #2
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Math I can handle.
If the source is 2V, and the amp sensitivity is 1.5V, you don't need any preamp gain. There's already more than enough input signal.
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Old 23rd May 2012, 11:08 PM   #3
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A gain of 4 (12 dB) actually isn't a bad choice. Sensitivity would still be high enough for an MP3 player or the like.

Typical values for input sensitivity in commercial amplifiers range from 150 to 200 mV. This was chosen for the typical 500 mV of phonopres, with some room to spare.

Obviously the higher gain after the volume pot, the more important low-noise design becomes. That's why some preamp designs actually split gain (look at Rod Elliott's pre, 6 + 6 dB) or get even more fancy.

That said, with the kind of power amp we're looking at in the 2226, a standard 5532 based pre with some attention to resistor values (each OP input seeing 1 kOhm max, volume pot 10k-20k max) should easily give very low noise even at 12 dB gain. Total voltage gain is barely 33 dB here. Even high-sensitivity horn speakers should show very little hiss at best, which I don't think the original pre can claim (the 250k volume pot would be sort of in the way of that).
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Old 24th May 2012, 03:28 AM   #4
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Quote:
Originally Posted by sofaspud View Post
Math I can handle.
If the source is 2V, and the amp sensitivity is 1.5V, you don't need any preamp gain. There's already more than enough input signal.
As long as both devices use the same scaling. A source with an output of 2V p-p into an amp that runs 1.5 V rms will definitely need some gain to make it to full power.

G
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Old 24th May 2012, 03:45 AM   #5
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I can only go by the information given, and have no reason to believe it wasn't scaled.
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Old 24th May 2012, 07:18 AM   #6
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Originally Posted by sofaspud View Post
I can only go by the information given, and have no reason to believe it wasn't scaled.
In the past consumer audio gear - power amps in particular - almost all defined the inputs in V rms. When I got my first CD player in '83 the output was 2 V p-p max. Later gear is similar. I only point it out as something for the OP to be aware of.

G
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Old 24th May 2012, 07:41 AM   #7
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by WhiteRhino View Post
Hi , my marantz 2226 has main amp , with sensitivity of 1.5V for full output .
I build for him active preamp with opamp with gain of X4 . is it to much ?
do you think I should lower it to X2 , since a normal DAC\CD PLAYER has output of ~2V ?
thanks .
You can agonise and analyse the figures all you like, the real question is how it works and feels in practice and whether the volume control range "feels" correct.

On a practical level we can say roughly that if a CD player outputs around 2Vrms as an absolute upper limit then that is "around" 6 volts peak/peak. So with a gain of four you need a preamp that will swing at least 24 volts pk/pk. Allowing for differences in individual player specs and so on that means supplys of -/+15 volt for the preamp and that a gain of 4 really is as high as you should go.

And as if to emphasise that point I just looked at the spec of my Marantz Pearl-Lite and that quotes 2.4Vrms output. That's 3.4 volts pk/pk so a gain of 4 on -/+15 volt rails isn't really enough and you would need -/+ 18 volts and definitely no higher than a gain of 4.
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Old 24th May 2012, 08:34 AM   #8
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Originally Posted by Mooly View Post
And as if to emphasise that point I just looked at the spec of my Marantz Pearl-Lite and that quotes 2.4Vrms output. That's 3.4 volts pk/pk so a gain of 4 on -/+15 volt rails isn't really enough and you would need -/+ 18 volts and definitely no higher than a gain of 4.
Umm, your math is wrong. 2.4V rms is 6.79 V p-p. See how it happens? - and you know what you're doing.

The point to me is defining exactly what you want / expect.

G
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Old 24th May 2012, 03:28 PM   #9
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thanks for the replies ,
I tried gain of 2X , an it was too low
i tried gain of 3.2X and it was ok there is more then enough power ,
i think i stay with this .

Last edited by WhiteRhino; 24th May 2012 at 03:32 PM.
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Old 24th May 2012, 05:16 PM   #10
Mooly is offline Mooly  United Kingdom
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Originally Posted by stratus46 View Post
Umm, your math is wrong. 2.4V rms is 6.79 V p-p. See how it happens? - and you know what you're doing.

The point to me is defining exactly what you want / expect.

G
Sometimes I know what I'm doing
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