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Preamp noise level ?

HP8903B

Member
2010-06-06 8:17 pm
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.
 

artosalo

Member
2010-02-16 9:00 am
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.
 
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.

Best regards

Thomas
 
Hi!

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?

Best regards

Thomas
 
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?
 

artosalo

Member
2010-02-16 9:00 am
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 ?
 

artosalo

Member
2010-02-16 9:00 am
......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...

I did not find the signal level from your calculations and it it more essential than the sole noise level. That is why I wanted to emphasize the importance of the S/N instead of the N-level only.

Just an imaginative example:
In the above case the measured noise level was 1,2 mV, but if at the same time the signal level at the output - which was not mentioned at all - is 12 Vrms, then the S/N is 80 dB. But if the output level instead were 1.2 Vrms, then S/N is 20 dB less.

So it is useless to speak about noise level only if other conditions are not known.
 
I think the 30dB power amp gain is causing a problem. With a nominal 2V RMS output from the preamp, a 30dB gain converts this to over 60V RMS at the speaker which, at 8 ohms, equates to 500 watts.

Looked at another way, a really good tube preamp might achieve an output noise of -80dBV or 0.1mV. Amplified by 30dB this becomes -50dBV or just over 3mV which, in 8 ohms is a power of just 1.25uW. Your preamp appears to be 20dB noisier which means its noise power will be over 125uW.

As others have said, you really need to measure the preamp noise with the input shorted. You should also tell us what you use to measure the noise.

Cheers

Ian
 
Last edited:
I did not find the signal level from your calculations and it it more essential than the sole noise level. That is why I wanted to emphasize the importance of the S/N instead of the N-level only.

Just an imaginative example:
In the above case the measured noise level was 1,2 mV, but if at the same time the signal level at the output - which was not mentioned at all - is 12 Vrms, then the S/N is 80 dB. But if the output level instead were 1.2 Vrms, then S/N is 20 dB less.

So it is useless to speak about noise level only if other conditions are not known.

The problem that I'm having, like everyone else who tries to answer the question, is that manp111 has not provided enough information for a sensible answer. I fill in the gaps with assumptions, which may or may not be correct. My assumptions are that manp111's preamplifier is a thing with a volume control that is meant to drive a main amp and that the noise level he measures is the output-referred noise at low volume settings.

If any of these assumptions are wrong, then my answer is indeed not applicable.