John Curl's Blowtorch preamplifier part II
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scott wurcer
diyAudio Member

Join Date: Jan 2004
Location: dorchester ma
Quote:
 Originally Posted by a.wayne What , I thought all IC guys were Vegan ......
You mean sheep are not vegetables?
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."

vacuphile
diyAudio Member

Join Date: Apr 2011
Location: Seaside
Quote:
 Originally Posted by scott wurcer If you have an input resitor and drive the other end with a replica of the input voltage so the resistance appears much larger to the driving signal I call it bootstrapping as it is in general use.
Well, my point is that you don't see a replica of the input voltage to lower the resistance to the driver signal, so there is no bootstrapping there.

Let me refer back to you post #32541

Quote:
 The signal gain is one and since the 10 Ohm resistor is bootstrapped by the Aol of the op-amp (divided by the feedback factor), the input resistance is very large, any bench generator has no trouble driving this +-10V. ALL errors refered to the op-amp input noise, CMRR, distortion, etc. have a gain of 101 to the output. Do you really not see that? Simply consider a nearly ideal op-amp put 10V on the + input the op-amp forces the - input and the output to 10V by feedback i.e. there is no current in either resistor, the circuit acts like a gain of 1 follower, and the input resistance is very large. If need be say the op-amp has an Aol of 10^6 so 10uV appears across the 10 Ohm this is all the generator needs to supply i.e. the input looks like 1Meg Ohm.
Since I am following the threat on a discrete opamp with interest, and have learned a lot of your insights there, it is with great trepidation that I go against your statement here.

1. Both + and - inputs of the opamp will be at the same potential, regardless of the Aol of the opamp.

2. Therefore, the 10 Ohm resistance will carry no current, this with a perfect opamp.

3. With an imperfect opamp, only the error signal will develop over the 10 Ohm resistor.

4. This error signal is all the signal generator will see, this plus the input impedance of the opamp.
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But from now on I will be discrete about it.

martin clark
diyAudio Member

Join Date: Jul 2003
Location: Bath, UK
Quote:
 Originally Posted by a.wayne What , I thought all IC guys were Vegan ......
Lean bias ?

simon7000
diyAudio Member

Join Date: Nov 2008
Location: Oakmont PA
JN

You are a grrfter nerzzog frakidle! Attached is a picture of the test setup and the FFT with the wires close a shown and then pulled back. The third harmonic is identical and extremely low. That would be from resistor distortion not quite being the same. There is a clear spike on the second harmonic with the wires close. It is greater than the thermal distortion.

As this took me 4 hours I am not inclined to do much more.

But the peanut gallery as usual is arrogant and wrong.

ES
Attached Images
 Skin Effect.JPG (638.4 KB, 184 views) Skin Effect FFT.JPG (53.6 KB, 185 views)

Banned

Join Date: Jul 2011
Location: Portugal
About pots, what did-you think about PGA23XX ? With no contact noise, no wear, and a HD+n of ~0.0002%, i think we can consider those as "quality" parts despite the low price ?
Quote:
 Originally Posted by simon7000 As this took me 4 hours I am not inclined to do much more.
Thanks a lot for your effort, Ed. It was very interesting.
It would be great, if you have some more time, to compare various film resistances brands, to figure out if any measured real difference ?

Last edited by Esperado; 13th January 2013 at 12:10 AM.

scott wurcer
diyAudio Member

Join Date: Jan 2004
Location: dorchester ma
Quote:
 Originally Posted by simon7000 JN You are a grrfter nerzzog frakidle! Attached is a picture of the test setup and the FFT with the wires close a shown and then pulled back. The third harmonic is identical and extremely low. That would be from resistor distortion not quite being the same. There is a clear spike on the second harmonic with the wires close. It is greater than the thermal distortion. As this took me 4 hours I am not inclined to do much more. But the peanut gallery as usual is arrogant and wrong. ES
No you are wrong and don't even know it.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."

simon7000
diyAudio Member

Join Date: Nov 2008
Location: Oakmont PA
Quote:
 Originally Posted by scott wurcer Ed I'm not here to teach EE101 end of this chapter. If the equations are obvious why can't you write them?
A few reasons, it is a pain to type equations, when I present EE100 level questions almost no one gets them, those folks who do try are ridiculed by the peanut gallery.

To recap for the peanut gallery, this exchange started out by George and I discussing resistor distortion, when I mentioned that opamps today can be limited by the passive parts and cited the LM4562. I then mentioned I thought that it was mismeasured it went off on a tangent.

My feel is that they got an AP and used it to produce lots of wonderful pictures but did not quite have a grip on it's limits. It seems that when you measure THD it looks at the frequencies above the fundamental. As a result for the same level of distortion as the frequency rises the displayed result drops. The second issue is the base noise level of the AP when in the THD mode. When a human looks at THD today they should be using an FFT with adequate resolution and often averaging to reduce noise. They can then pick out distortion from noise. Using autopilot may not do that and be misleading. The final issue exactly which circuit did they use at first to make measurements. Scott points out that AD does it correctly and presumes other do also. (Which I think is our only area of disagreement.)

However in the LM4562 data sheet I found a wonderful curve, Phase vs Gain. If accurate this device would do wonders for some types of signal processing. But I think they really meant phase margin.
Attached Images
 Phase Gain.JPG (38.3 KB, 176 views)

simon7000
diyAudio Member

Join Date: Nov 2008
Location: Oakmont PA
Quote:
 Originally Posted by scott wurcer No you are wrong and don't even know it.
Scott,

We already determined you are not in the peanut gallery. So what error do you see?

scott wurcer
diyAudio Member

Join Date: Jan 2004
Location: dorchester ma
Quote:
 Originally Posted by simon7000 Scott, We already determined you are not in the peanut gallery. So what error do you see?
That circuit with the 10 Ohm and 1k Ohm resistor, it works fine there is no problem with NS's or TI's numbers at all.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."

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