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[email protected] joe@mich.com is offline
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Default overvoltage on audio circuits

On Sat, 2 Nov 2019 16:28:47 -0700 (PDT), Phil Allison wrote:

wrote:


** 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.


Since you are an audio expert, I am surprised that you did not know ----

That shot noise is white noise
That the " High frequency noise that completely dominates" is known as thermal or Johnson noise
That either shot or thermal noise or both can be important noise sources depending on the application. Just because shot noise is not a factor in
your microphone case does not mean it's never important.

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.


The dominant noise source in this app is the parasitic base resistance of the input biplor and is not a function of bias current.

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.


I'm sure you know that you want the diaphram to move as little as possible to keep the response as linear as possible and thus distortion low, thus it
will be putting out a small volatge.

I designed a decent low noise op amp a few years ago, the OPA1662


** 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?
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.
Is that news to you ?


Yes it is. I've never known anyone to do that before, let alone think it means anything. I could put a Darlington on the inputs, get the shot noise
down 0.005na/rtHz. Would you then come up with 3.3Mohms as meaning something?

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


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


Note the fairly high supply current.


I'm glad we're back on point. I originally said that high currents and large devices were not needed for low noise in bipolars. You said I was wrong
and pointed to the SSM as proof. I suppose "high currents" and "large devices" are relative terms, so I'll say more. Shot noise increases with current
and is independent of manufacturing process and device size, period. Thermal noise is dominated in a bipolar by the parasitic resistance in the base
of the input device. Even with best layout geometry, the resistance can always be made smaller by increasing the emitter width (larger device).
However, in the 40V analog processes I'm familiar with, and all 40V processes will have similar resistivities to get the breakdown voltages, a bipolar
with a 1nv/rtHz thermal noise can be designed with dimensions about 200u x 200u. This is not a minimum size device by any means, but it's not exactly
huge, on the OPA design two of these devices together would take up 15% of the chip area, and it's a small chip. Had this size been used, the OPA
would have a 1nv/rtHz noise density spec. However, our customers told us 3nv/rtHz was quite adequate and they preferred lower Ips, better distortion,
and better AC. The OPA comes in a dual, and each amp uses 45mW, far lower than competing devices with similar specs. I suppose very old manufacturing
processes would have larger feature sizes, but even 15 or 20 year old processes wouldn't give monstrous devices.

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.


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


I very, very rarely do that, and I could have said much more. I did so in this case to show the absurdity of your repeateded statements that I knew
nothing about audio. I don't feel the need to broadcast anything. My plaques and patents are in boxes, and my walls instead have posters. I also don't
feel the need to belittle anyone or call them an idiot.

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.


For the OPA I get 0.49uV input noise over the audio band, for the SSM I get 0.27uV. Both of these numbers give excellent S/N ratios, and it is likely
the better distortion of the OPA compensates for the extra noise so that the signal error of each is close. Be that as it may, the OPA was not
designed as a special purpose amp for this application, it is a general purpose op amp. Nevertheless, Shure buys it as a microphone amp for battery
powered applications.

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.


And I won't. I am under a non-dosclosure agreement. I probably said more than I'm supposed to already, I sure as hell am not going to have someone see
my name in a newsgroup and report that I am blabbing away secrets. Anyway, I could list any name and how would you know if it was real or not?

As far as "attacking" you goes, I said you had an attitude and often called past posters idiots when they were looking for help. You have several
times told me I know nothing about audio. I question what drives someone to be like that.