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Old 3rd February 2012, 11:03 PM   #20321
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Quote:
Originally Posted by Juergen Knoop View Post
I'm sorry my tin/lead ears have a hearing threshold...so I don't need a zero THD circuit.
Whatever you may need or not, unless there is zero THD there is nonlinearity, which enables modulating HF noise with the audio signal.

Quote:
Originally Posted by Juergen Knoop View Post
But why do audiophiles obviously prefer high THD circuits with consequently more modulation than less?
Who said they do?
If some of them do, sometimes, what is the relevance of it to the issue of HF noise modulating the audio signal?
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Old 3rd February 2012, 11:13 PM   #20322
Bonsai is offline Bonsai  Taiwan
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From janneman
"BTW You cannot 'kill' noise or 'take care of LF'. It is always a matter of more or less noise, or more or less LF. For a given set of parts or parts layout, some topologies may be better than others."

Of course - perhaps a bad choice of words on my side.
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Old 3rd February 2012, 11:22 PM   #20323
SY is offline SY  United States
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Quote:
Originally Posted by Johnloudb View Post
Okay, I don't see what you mean. For the LED the DC voltage doubles, and the dynamic impedance also doubles, right? That would be bad in my circuit.
That's right, but it isn't necessarily bad. If you're using the LED as a reference, the voltage doubles, but the noise voltage adds as power, so the noise voltage increases by 3dB because the noise between the two devices is uncorrelated. So you gain a 3dB advantage in signal to noise ratio by stacking two of them. For example, a common red LED has a Vf of 1.6V or so. The noise voltage is roughly 2uV (20Hz-20kHz). So the s/n of a single device is 20 log (1.6/2x10e-06) = 118dB. Stack two of them and the noise voltage is sqrt([2x10e-6]^2 + [2x10e-6]^2) = 2.83uV, but the reference voltage is now 3.2V, for a signal to noise of 20 log (3.2/2.83x10e-06) = 121dB.
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Old 3rd February 2012, 11:37 PM   #20324
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SY doesn't 'understand' me either, Joshua. '-)
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Old 4th February 2012, 12:47 AM   #20325
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Quote:
Originally Posted by SY
If you're using the LED as a reference, the voltage doubles, but the noise voltage adds as power, so the noise voltage increases by 3dB because the noise between the two devices is uncorrelated. So you gain a 3dB advantage in signal to noise ratio by stacking two of them.
Okay, thanks SY. I didn't think of a current source as an amplifier where the DC voltage was the signal, but I get it now.
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Old 4th February 2012, 01:01 AM   #20326
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Quote:
Originally Posted by Joshua_G View Post
This is so when the noise of the reference is proportional to its' voltage.
There is no 'the reference' in SY's statement. He's saying to use many. Here's a thought experiment -

Say you need a 10V reference and you wanna use TL431s. I can see two ways to go. First use resistors in 3:1 ratio to multiply up the 2.5V reference to 10V, using a single TL431. Second, use 4 TL431s in series with no resistors. The DC voltage is the same but the noise will be 6dB better in the second case. In fact the DC voltage will be more accurate on average in the second case because of lack of resistor tolerance and the power of statistics.

Its possible to mitigate the noise in the first case by bypassing the upper resistor with a suitable cap to reduce the noise gain.
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Old 4th February 2012, 01:03 AM   #20327
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Originally Posted by john curl View Post
Real-time designers, like Wavebourn and I, instinctively know, from experience and previous study, ....
I would call us just "Designers", as opposed to "Simulator Operators"
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Old 4th February 2012, 02:25 AM   #20328
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Quote:
Originally Posted by Juergen Knoop View Post
I'm sorry my tin/lead ears have a hearing threshold...so I don't need a zero THD circuit. But why do audiophiles obviously prefer high THD circuits with consequently more modulation than less?
Because HIGH THD is your personal number. "They" prefer LOW ENOUGH THD instead of sacrifice of other parameters they value in order to get EVEN LOWER THD ON 100 HZ ON ONSET OT CLIPPING.

LOUD ENOUGH FOR TIN/LEAD EARS?
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Old 4th February 2012, 08:00 AM   #20329
gerhard is offline gerhard  Germany
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Quote:
Originally Posted by SY View Post
OK, John, give a number. How much power supply noise is low enough to meet the numbers game for your market?
You should know him by now, that he never gives hard facts.
Doing so would in this case reveal the complete absence of any PSRR in his input stage.

After all, his business model is based on "Tales Of Mystery And Imagination"

regards, Gerhard

(with due respect for Alan Parsons)
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Last edited by gerhard; 4th February 2012 at 08:10 AM. Reason: misspelled psrr and added attribution to A.P.
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Old 4th February 2012, 10:24 AM   #20330
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by gerhard View Post
...complete absence of any PSRR in his input stage.
What input stage are you referring to?
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