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Old 2nd February 2013, 05:02 AM   #1
HP8903B is offline HP8903B  United States
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Default Preamp noise level ?

I want to know what is acceptable noise level (mV AC) of preamp with the input connected but with no signal.

I know for amplifiers, 2mV AC or less is pretty quiet.

I designed a preamp circuit and the output noise level is ~1.2 mV.
I am wondering whether this is quiet. I have seen a preamp with ~0.4 mV.
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Old 2nd February 2013, 07:16 AM   #2
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Usually the acceptable noise level is determined as a S/N ratio (in dB) not in noise level only, because this depens on the preamp gain.
Acceptable S/N depends on the sort of preamp.
With line level preamp more than 90 dB S/N is achievable, but with low level mic or phono preamps even 60 dB is good.
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Old 2nd February 2013, 07:35 AM   #3
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Hi!

What is acceptable depends on the sensitivity of power amp and speakers and mostly on your tolerance of hum. Some people don't mind a little hum as long as it is not noticeable from the listening position. Others don't accept any hum with their ears right at the speaker.

Most amps have gain figures in the region around 20dB. So the hum at the pre output gets amplified by that. 2mV hum at the power amp output might be tolerable with a 90dB speaker but surely not tolerable with a 105dB speaker.

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Old 2nd February 2013, 08:02 AM   #4
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In my opinion the acceptable S/N of a preamp must be possible to determine without mystifying this with power amp or the sensitivity of the speakers.
And I was solely speaking about hiss, not hum.
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Old 2nd February 2013, 08:25 AM   #5
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Hi!

My intention was not to mystify this. Just wanted to point out that what is acceptable depends on the context this gets used in. There are huge differences in the gain of power amps and sensitivity of speakers. Depending on the combination there can be a 20dB difference how audible the noise of the preamp is going to be.

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Old 2nd February 2013, 08:25 AM   #6
HP8903B is offline HP8903B  United States
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The noise level of my signal generator is 0.04mV.

With my signal generator connect to the preamp, the noise level of the preamp is 1.2mV.

How can I determine whether my preamp noise level is good ?
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Old 2nd February 2013, 08:30 AM   #7
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Hi!

Quote:
Originally Posted by manp111 View Post
How can I determine whether my preamp noise level is good ?
You can go with artisolos suggestion: Put the noise level you have at the output in relation with the achievable output amplitude. This gives a S/N ratio.

I would no measure this with a signal gen attached, but with the input of the preamp shorted.

Does your preamp generate audible noise in your system?

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Old 2nd February 2013, 08:44 AM   #8
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To give you a ballpark figure:

Suppose you have 30 dB main amp voltage gain, two 90 dB, 1 W, 1 m, 8 ohm loudspeakers, a room with 1 m reverberation radius and you want to keep the noise level below 0 dB(A). With one loudspeaker, you could then allow 10^(-90/10) W=1 nW of A-weighted noise, but as you have two of them, you need to reduce that to 500 pW per loudspeaker. In 8 ohm, that corresponds to an RMS voltage of sqrt(500 pW*8 ohm)~=63.25 uV RMS. At the input of the main amp, the allowable voltage level is 10^(30/20) times smaller, or 2 uV RMS A-weighted.

You measure 1.2 mV, which is 600 times as high as what I just calculated, giving you a noise sound pressure level of around 55.56 dB, which is quite high.

Maybe things are not as bad as they seem, because you don't specify whether you measured a RMS, quasi-peak or quasi peak-to-peak value and over what bandwidth you measured. If you haven't limited your measurement bandwidth, a large part of the noise may actually be ultrasonic and will only annoy your dog, cat or any bats living in your house.

About the generator: you can measure the preamp noise with input shorted, or with the input terminated by a resistor. Why would you need a generator?
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Old 2nd February 2013, 09:04 AM   #9
HP8903B is offline HP8903B  United States
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I just connected my 2A3 amp to the preamp circuit.
My 2A3 amp, with the signal generator connect, put out .6mV.
With the preamp circuit connected to the 2A3 amp, the amp put out 6.5mV.

I my system, I need the amp to put out less than 1.5mV.
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Old 2nd February 2013, 09:42 AM   #10
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Quote:
Originally Posted by manp111 View Post
The noise level of my signal generator is 0.04mV.

With my signal generator connect to the preamp, the noise level of the preamp is 1.2mV.

How can I determine whether my preamp noise level is good ?
The noise level solely do not tell if your preamp is good or not, but the signal to noise ratio tells this.

What is the gain of your preamp ?
What is the purpose of your preamp; line level preamp or something else ?
What is the typical input level you feed into your preamp ?
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