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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default overvoltage on audio circuits

wrote:

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** FYI so called "shot noise" is not relevant to input devices operating
over the full audio band. High frequency noise completely dominates.



That's a surprise to me and many others as well, since shot noise is white noise and broadband, extending orders of magnitude beyond the audio range.
What is this "high frequensy noise" if it is not shot wde band noise or thermal wide band noise?



** Whatever name you give it, the noise is essentially white and so rises with frequency being proportional sq.rt of the bandwidth.


Not sure why discreet amps would use high currents for low noise.


The idea is to get the best NF impedances down to around 150ohms for use with dynamic mics.


What do you mean here by NF?


** Noise Figure.

Those are ratios, not impedances.


** BJTs have noise figures in dBs that vary with the source resistance and current. There are optimum values for both giving the lowest NF.

Surely you have seen the curves published on data sheets.



A microphone generates a small voltage and can tolerate very little
current draw without distorting.


** Dynamic mics can deliver signals of a volt or so while condenser types output several volts.

Dynamic mics have a source impedance that is mostly restive and a typically couple of hundred ohms. The load needs to be a few times higher for best results.

A good mic pre-amp needs to have input voltage noise less then that of a 200ohm resistor when operating at high gain, like 1000 times.



I designed a decent low noise op amp a few years ago, the OPA1662, 3.3nv/rtHz noise densisty. -124db distortion, total noise+distortion 0.00006%,
22MHz GBW, 22V/uS SR, Sig/noise 95db, voltage gain 114db, and ..... 1.5ma power draw total.


** Nice part - but with about 1uV of input noise.
With 3.3nV and 1pA per rtHz of input noise, the best impedance is 3.3kohms.


How are you coming up with these numbers?


** You are asking some very strange things.

They certainly do not agree with either the design results,
application characterization, or with the feedback from customers.



** There is no disagreement with the published data, at all.


Did you get them by dividing the input voltage noise density by the input current noise density? If so, what do you think that gives you?


** It gives you a good guide to the source resistance that gives the best NF.

Do the same for a JFET op-amp and you get about 1Mohm, cos the current is so low. 1uV of noise in the audio band is many times that of a 200ohm resistor.

Is that news to you ?


Though obsolete, this op-amp set a bench mark for low noise audio.

https://www.analog.com/media/en/tech...016SSM2017.pdf


Well let's compare your "benchmark" with the OPA1662.



** The SSM part beats it easily cos it is actually dedicated mic pre-amp, with a single resistor for gain control.

Did you not read the heading on the data sheet ?

You would need three op-amps connected as an instrumentation amplifier to do the same job and it would then behave like the SSM with input noise increasing at lower gains.


Note the fairly high supply current.



Indeed, but I don't think it has anything to do with low noise, in fact, the noise is actually quite hign for except for very high gain settings.


** Got news for you pal.

Having very low input noise only matters at high gain settings cos one adjusts the gain setting to get an output of about 1Vrms.

So the actual input signal is about 1mV at high gain and proportionally more at lower gains, defeating any input noise increases going on.


The fact that it goes down so rapidly at high gain settings has very little to do with bias currents and says that the noise gain loop has been tweaked.



** The common mode resistor that sets the gain ADDS to the input noise.

Unavoidable with that topology and not important in practice.

YOU are simply not very familiar with audio circuitry - as I suspected.



Let me say a little about that. I have designed audio circuits for 30 years.


** Makes you familiar only with your own designs.

Those which you have not seen remain a complete mystery.



One was selected by Electronics Design magazine as Product of the Year. I
have presented papers at the International Solid States Conference in NYC. I have 25 patents as individual inventor, not global group co-inventer as
some places do. My designs are in avionics,



** FFS - that is more than enough solo trumpet playing.

Mics and mic pre-amps are not your thing.

If you have not spent many years dealing with them, like I have, you are not going to know very much about them.


Please tell me about your honors and designs and explain why my
unworthy OPA1662 .........


** Where did I say anything like that ???

I said it was a "nice part", but with 1uV of input noise in the audio band.


You know what I think?



** Not very interested, thanks.

Because you are now beginning to sound like a raving lunatic.

Why are you posting on a pro-audio usenet group at all ??

BTW:

I use my real name and identity here, while you do not.

Attacking me from a position of anonymity is cowardly in the extreme.


( rest of this pig's misconstrued and paranoid garbage snipped )




...... Phil