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Old 28th November 2010, 05:16 PM   #7041
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Quote:
Originally Posted by AndrewT View Post
A flow of charge (current) into a capacitor changes (gradually) the voltage stored between the plates.
A constant current into (or out of) the cap gives rise to a straight line slope of voltage with time.

The current does not need to change for there to be a change in voltage.
The current being ON or OFF is the criteria for a change in voltage across the cap.
The current value changing gives rise to a change in voltage vs time slope. Note: slope of Voltage vs time, not absolute change in voltage.
Andrew, I know you understand these things. My point was that there is NO delay between the current into and the voltage across a cap, despite the phase shift. Phase shift is NOT delay.

The fact that a sine current into a cap gives a sine voltage 90 degrees shifted is not due to delay. We know that a cap voltage is the integral of the current. The integral of a sine current is a cosine voltage, which is a sine but 90 degrees shifted....
Soooo, you put a sine current into a cap and out comes a cosine voltage... which looks like a delayed sine. But looks deceive, big time!

jan didden


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Old 28th November 2010, 05:19 PM   #7042
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[snip]. Even real results would not convince them.
[snip].
That's what amazes me every time. People that say 'feedback can't work because it is always too late' exist because feedback works. They use feedback as a matter of fact in their daily lives, and STILL they say 'it can't work'. Amazing .

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Old 28th November 2010, 05:20 PM   #7043
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Currents at the summing node are same (resistor current and capacitor current), and capacitor voltage is an integral of the current.
(explanation - I speak about true integrator results of which I showed several pages before. RIAA network is not a true integrator, it has 3 freq. breakpoints - just for the reason that someone wanted to say i was inaccurate).

They seem not to understand, no background. I give up, this is a loss of time. I will free the space for stories.
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Last edited by PMA; 28th November 2010 at 05:28 PM.
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Old 28th November 2010, 05:43 PM   #7044
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Quote:
Originally Posted by janneman View Post
I don't think this is a correct way to look at it. Even if you have phase shift in the feedback loop (which you will get if you get high enough in freq) there is no 'delay' as implied here.

For instance, it is known that the voltage across a cap is shifted 90 degrees wrt the current into the cap.
If you use a sine wave current, the voltage is also a sine wave, 90 degrees phase shifted. It *looks* like a delayed input wave but it isn't.

Think about it: every minuscule change in the input current gives an *immediate* minuscule change in cap voltage. There is NO delay!

I tried to explain here:
http://www.diyaudio.com/forums/blogs...-all-time.html
Let me know if I succeeded

jan didden
i think there is a delay, but measured in picoseconds
[feedback loop, not caps]
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Old 28th November 2010, 05:50 PM   #7045
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yes, I fell into that trap.
I got confused by folk referring to phase shift as if it were signal delay.

The loop gain must not send back a signal that causes instability. That instability is caused by the sum of the circuit delay plus the phase shift.
It's just fortunate that in Audio the phase shift predominates and the circuit delay is trivial.
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Old 28th November 2010, 06:11 PM   #7046
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Quote:
Originally Posted by myhrrhleine View Post
i think there is a delay, but measured in picoseconds
[feedback loop, not caps]
Agreed. it is not something that looks like the phase shift you sometimes see in scope pictures. It is of no consequence to the working of feedback in audio.

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Old 28th November 2010, 07:34 PM   #7047
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Of course, depending on the opamp or discrete topology used, its SR, GBW etc. But, it has nothing to do with 90 phase shift reasoning. And even vinyl mistracking signal is not that fast for us not to be able to make a choice of appropriate opamp. It was a problem 25 years ago, but is no problem today. I have made a choice of 2 types of opamps with SR 55V/us that work great in RIAA phono pre. The circuit is not as simple as opamp only, but opamps may be very well utilized in RIAA preamps.
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Old 28th November 2010, 08:08 PM   #7048
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Regarding Bob's question on higher harmonics in vinyl distortion, maybe it is also a question of distortion of the vinyl measurement record cut (and of course a question of cm/s), here is another plot, with harmonics only up to 4th:
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File Type: png phono 80Hz.PNG (69.6 KB, 200 views)
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Old 28th November 2010, 08:21 PM   #7049
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One way to design around the probem of high slew rate garbage at the input of the phonostage is passive EQ of the 75usec BEFORE it enters the amplification stage. I designed a passive inductive RIAA for ETF that does just that. By coincidence i even use the ADA4898 at the input. The reason i use it is, that it can swing 40mA clean so a feedback resistor of 300 Ohm can be used to limit the gain in the first stage. As funny as it looks, this stage works fine with the ceveat that the impedance of the cart. is known. In my case the 6 Ohm of my Titan i ( including losses in the connectors ) are tumed with a series resistor of 4 Ohm to have the required 75usec with a 0.75mH coil.
I put the coils in an oversized Mu-Metall can for low hum. Again this is a low Z design with 4 Ohm DC impedance. AC impedance goes up in the treble though so this is not exactly a zero ohm design.
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File Type: pdf ETF Inductive RIAA.TSC - TINA.pdf (42.4 KB, 72 views)
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Old 28th November 2010, 08:23 PM   #7050
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The real values are L3 = 0.75mH and R10 = 4 Ohm
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