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Chip Amps Amplifiers based on integrated circuits 

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7th April 2015, 09:24 PM  #1 
diyAudio Member
Join Date: Oct 2012

How do I calculate the inherent noise of this amplifier?
I'm getting barely audible static noise from my amplifier, but I don't know how I can find the source. I don't have equipment to measure levels this low, so I don't know how loud the noise actually is. Here's a quick circuit:
I don't know much about noise sources or how to calculate them, but I would like to be able to calculate the inherent noise created by the amplifier itself (while the input pot is turned all the way down). The only noise sources I know of is the inherent noise of the opamp itself and the thermal noise caused by the external resistors. Here's some figures from the datasheet of the opamp: I found a formula for thermal noise on wikipedia: v^2=sqrt(4kB*TR). For room temperature and 100kOhm (input resistor) value gives a result of 40nV/sqrt(hz). The datasheet gives an inherent noise density figure of 35nV/sqrt(hz) @ 10kHz. Adding these two figures together gives sqrt(40^2+35^2)=53nV/sqrt(hz). Since this is an audio amplifier my bandwidth is 20kHz so total noise is 53*sqrt(20000)=7495nV. With a gain of 3 the resulting output noise should be 22uV or are my calculations way of? I'm just taking a shot in the dark here, could someone help me out? :P Is 22uV of noise really audible? 
7th April 2015, 10:23 PM  #2 
diyAudio Member
Join Date: Apr 2011
Location: Upper midwest

That level of noise is not audible. Why just the 100k at the positive input  what's the source impedance?
Last edited by rayma; 7th April 2015 at 10:33 PM. 
7th April 2015, 11:36 PM  #3 
diyAudio Member
Join Date: Mar 2008

Use diodes instead of resistors for biasing your output. If resistors are used it's more sensitive to power supply noise  since you have just a voltage divider any variation in supply voltage would simply show up at the base of your transistors, giving you in this case a whopping 26dB PSRR.
22uV is 93dB relative to 1V, can be heard as a whisper on ear/headphones. Inaudible on speakers usually unless you put your ears against the tweeter. 
8th April 2015, 06:17 AM  #4  
diyAudio Member
Join Date: Oct 2012

Quote:
Quote:
I would like to know if my calculations are correct and if the two noise sources that I have included are the only ones? The noise is really really low and I'm testing with headphones that has a sensitivity of 109dB SPL/V. 

8th April 2015, 09:18 AM  #5 
diyAudio Member
Join Date: Mar 2008

This picture gives a better... picture... than words:
Source 26dB is based on openloop. With feedback the amp will try to correct it and greatly reduce the output error, assuming the amp and feedback loop is fast enough. Usually for audio frequencies it is fast enough, but RF not always. Anyway, consider the openloop case, with your opamp output at ground. We make an assumption here: Output voltage = Voltage at Base of transistor minus Vbe. If you're biasing using voltage divider, then voltage at Base = Power supply voltage multiply by (some value). Meaning any fluctuation in power supply causes the Base voltage and subsequently output to fluctuate at 1/(some value). That (some value) is 26dB based on those resistor values. Now, one interesting thing about this noise, is that it goes away if the same noise is on both + and  supply rails. If the noise is only present on one rail (say the +), the higher Base voltage of one transistor cause more current on one transistor compared to the other  this causes output voltage to shift from ground. But if both transistors get higher Base voltage at the same time, current through both is increased, the voltage changes are symmetrical, so output voltage is unaffected. With most audio power supplies a centertapped transformer is used so the charging cycle for both + and  rails happen at the same time and this noise cancels out. So I've mentioned why the 50Hz hum isn't audible, but random noise behaves differently and hence can still show up. One, if random noise is on both + and  rails, it will not cancel out like in the previous case, but would instead add together. Second, random noise is across all frequencies including RF  which can modulate some way some how and affect the audio frequencies, and may not be filtered away by your feedback and etc. Well that's my hypothesis. 
9th April 2015, 10:11 AM  #6 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

There are many more noise sources around the first stage opamp, than you have allowed for.
I recently stumbled up a Ti pdf that gave a very thorough, but understandable, analysis for predicting the noise output of their opamps. It was linked for us from a Thread here. I'll see if I can find the copy on my HDD, but that won't have the link to the original. However, if you have ONE dominant noise source and use that, the error in your prediction as a result of ignoring all the other non dominant noises, is very small. With 100k tacked onto your opamp, then I would guess you have correctly identified the dominant noise. But like the other Member I have to ask, why 100k, what's wrong with 6k666? (to match 10k20k0) Would that be the dominant noise now?
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regards Andrew T. 
9th April 2015, 11:04 AM  #7 
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Join Date: Jul 2004
Location: Scottish Borders

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regards Andrew T. Last edited by AndrewT; 9th April 2015 at 11:07 AM. 
9th April 2015, 11:51 AM  #8  
diyAudio Member
Join Date: Oct 2012

Quote:
Also, how do I determine what noise is caused my the amplifier itself and what comes from the outside? With input cables connected and volume turned all the way up the hissing is intense, but disconnecting the input cables removes the hiss, but I'm not sure what this means? The internal input wires run from the back of the box, past the mains transformer and to the front amplifier PCB, but how do I isolate the noise that this cable is picking up (if any)? Last edited by Plecto; 9th April 2015 at 11:56 AM. 

9th April 2015, 12:32 PM  #9  
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

if the input cable is open ended then you are hearing/measuring the noise of the smallest resistor across the input. If that resistor is 1M (to discharge the leakage of the DC blocking cap) then you have a lot of noise.
If the vol pot is the smallest resistance then you have 100k (10dB quieter than the 1M). If the input to the full open vol pot is shorted or connected to a Rs=100r, then that short becomes the smallest input resistor and the noise disappears below audibility. Quote:
One never has an open ended input with a "connected" system, provided the Source is powered ON. Your 109dB/V headphones when fed with a noise of 93dB below 1Vac will reproduce ~16dB SPL at your ears. That may not be audible. It depends on the background noise, how recently you listened to louder noises, how fast/hard your heart is pumping, etc.
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regards Andrew T. Last edited by AndrewT; 9th April 2015 at 12:37 PM. 

9th April 2015, 02:55 PM  #10 
diyAudio Member
Join Date: Mar 2009

No drivers for the output? It might be current starved. Of course I don't know what it will be driving or the supply rails. What about thermal stability?

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