common value of output stage noise in uV? - diyAudio
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Old 13th March 2014, 05:41 PM   #1
diyAudio Member
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Default common value of output stage noise in uV?

what is the common noise value observed in the output of the amplifier in uV?

for the following conditions..

With the regulator
without regulator
just with toroidal transformer
with R-core transformer

I feel R-core transformers are much better than the toroidals in terms of psu noise floor reduction.
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Old 13th March 2014, 05:55 PM   #2
AndrewT is offline AndrewT  Scotland
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D.Self quotes around -115dB ref maximum power.
Some, or maybe that should be many, manufacturers also achieve >110dB ref maximum power.

-120dB ref 100W into 8ohms would be 28.3uVac
-115dB would be 50uVac
-110dB would be 90uVac

If that really quiet Power Amplifier, had a gain of 28times, and had -120dB ref 100W into 8ohms, was converted back to input referred noise, it would be ~7.1nV/rtHz.

Most amps are around 10 to 30nV/rtHz.
To get better than 8nV/rtHz is quite an achievement, but doable.
I suspect around 4nV/rtHz is near some practical limit for a power amplifier.
regards Andrew T.

Last edited by AndrewT; 13th March 2014 at 06:03 PM.
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Old 13th March 2014, 06:11 PM   #3
sreten is offline sreten  United Kingdom
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Location: Brighton UK

And its way lower than any practical preamplifier source.
In other words essentially meaningless and nothing
to do with the power supply much at all.

rgds, sreten.
There is nothing so practical as a really good theory - Ludwig Boltzmann
When your only tool is a hammer, every problem looks like a nail - Abraham Maslow
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Old 13th March 2014, 08:55 PM   #4
tvrgeek is offline tvrgeek  United States
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Noise is mostly the feedback resistor in a typical amplifier design. Surprised the heck out of me when I modeled my design and did noise analysis on everything. Then I was reading Self or Cordell and they discussed this as well.

When I had old dried out caps in my Hafler, I had power supply noise. I repaired the amp, and that was that. Standard E-core with belly band. I hear no noise sitting at my desk between tracks. Whatever that level is, is sufficient.

If you have an issue where you think noise needs to be lower, look into class D amps. That is the one thing I can say for them, quiet.
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Old 14th March 2014, 09:00 PM   #5
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Originally Posted by tvrgeek View Post
If you have an issue where you think noise needs to be lower, look into class D amps. That is the one thing I can say for them, quiet.
On average it rather seems to be the other way round though. Many practical D amps manage a dynamic range of no more than about 103 dB(A), sometimes no more than 95 even. A quiet preamp would typically be good for about 110 (and noise floor would be even lower if you found one with a 4-gang volume pot). Of course there's the Hypex NCore amps at >120 dB, but those are not the norm.

While AB power amps spec'd at 103 dB(A) do exist, 110 to 120+ dB seem to be quite common and actually are not particularly hard to hit.

But as stated, once you hit a halfway high value, chances are that preamp noise will dominate. 4 nV/sqrt(Hz) times (16+26)=42 dB equals 71 V @20 kHz or 112 dB ref. 100W / 8 ohms, no way around that. And that's a fairly low-noise preamp with lowish total gain (45-46 dB are not entirely uncommon, since that's what you need to hit 100 W / 8 ohms at a standard 150 mV of input sensitivity). Take an average preamp at 10 nV/sqrt(Hz) and 45 dB of total gain, and we're at 101 dB ref. 100W / 8 ohms.
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