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

On Fri, 1 Nov 2019 15:02:56 -0700 (PDT), Phil Allison wrote:


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Increasing bias currrent increases shot current noise at the collector
and base, so low noise bipolar amps tend to be biased at lower currents.


** 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?

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? Are you referring to noise factor or noise figure? Those are ratios, not impedances. NF in my world is the SNRout/SNRin,
it's a measure of extra noise to the signal added by a component. Matching impedances gets maximum power to the load, but that is not what is desired
here. A microphone generates a small voltage and can tolerate very little current draw without distorting. The object is to amplify the microphone
VOLTAGE without loading it, not maximize power delivered to the amp. The last thing you want to do is put 150 ohm load on the microphone input. BTW,
the YBM data sheet gives the diff input impedance as 1Mohm or higher, depending on gain setting, which is what I would expect.

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

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. Conditions do not perfectly match, but I've gotten them close.

YBM OPA1662

Total Harmonic noise distortion 0.008 0.00006 % conditions
input voltage noise density 107 3.3 nv/rtHz 1KHz, G=1
" 12 2.9 " G=10
" 2 2.7 " G=100
" 1 2.6 " G=1000
input noise current density 2 1 pa/rtHz 1KHz input bias current
25 1.2 ua wc
input voltage noise density corner (1/f noise) 70 8 Hz
THD+ noise 0.007 0.0005 % G=2, [RL=5K YBM, 2K OPA]
Slew rate 17 22 V/uS
sm sig bandwidth 4 20 MHz G=1
Ips 10.6 1.5 ma

Now lets calculate input noise value using the YBM data sheet procedure and comparing to the YBM results -
From the YBM data sheet --------
For a microphone preamplifier, using a typical microphone im-pedance of 150 ohms, from the YBM data sheet,
using En =sqrt {(1 nVvHz)2 + 2 (pA/vHz× 150 ohms)2 + (1.6 nV/vHz)2} we get the input noise as

YBM OPA1662
noise source en = (for G=1,10,100,1000) 107,12,2,1 3.3,2.9,2.7.2.6
in = 2 1 pa/rtHz
RS = 150 ohm, microphone source impedance 1.6 1.6 nv/rtHz
et = 1.6 nV/vHz@ 1 kHz, microphone thermal noise
for G=1000 1.93 3.0 nv/rtHz
and for G=100 2.65 3.1 "
and for G=10 12 3.2 " and for
G=1 108 3.3 "

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. 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.
Noise gain is usually tweaked in the customer application to give the user more flexibility since it can impact other parameters, but is often tweaked
in the circuit as well. Having an order of magnitude reduced amplifier BW also reduces noise.

** Then 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. 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, satellites, medical equipment, and one is currently in every battery powered Apple product. However, I
will bow to an obviously superior master. Please tell me about your honors and designs and explain why my unworthy OPA1662 seems to be purchased in
the millions by these equally clueless customers such as Shure, Pioneer, and Marantz.

You know what I think? I've noticed in past posts when someone posts seeking help, and they've misunderstood their application or made a mistake,
rather than offering assistance, you call them an idiot. People like that are usually the C students in college and have limited accomplishments at
work, and use forums like this to bully those who know even less. Here, rather than offering transistor theory to prove your point, you pull out an
old data sheet designed by someone else and say, "see, see, it has high currents and low noise" without any understanding of the circuit. I would have
been impressed if you had offered theory and knowledge new to me, but didn't happen.

So, once again, please tell me how you came up with 3.3Kohms as input impedance for the OPA1662. The only resistance of the amp that affects noise is
the actual parasitic resitance of the base diffusion resistor of the input bipolar transistor, which is about 20 ohms. The dynamic input impedance is
set by the AC path, doesn't affect noise, and is not calculated by dividing the input voltage noise density by the input current noise density, if
that's what you did, then shows an appalling lack of understanding. Also, explain to me the mechanism of how the high current levels of YBM reduced
the noise. Also explain why, at 14% of the current draw, the OPA eq input noise bests in a major way or matches YBM for all gain settings below 100.
While you're at it, explain why you appear to want a 150 ohm NF input resistance for the amp.



..... Phil