John Curl's Blowtorch preamplifier part II

[scott wurcer]

Originally Posted by a.wayne

What , I thought all IC guys were Vegan ......

You mean sheep are not vegetables?

[vacuphile]

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

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.

[martin clark]

Originally Posted by a.wayne

What , I thought all IC guys were Vegan ......

Lean bias ?

[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

[Esperado]

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 ?

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 ?

[scott wurcer]

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.

[simon7000]

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.

[simon7000]

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]

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.

[Kindhornman]

The term quality has many different definitions to different people. One person might say that a Toyota Camry is their idea of a quality car as it does it job well and stay together for the duration while another person would think anything less than a Mercedes or Porsche is utter garbage for the masses. It is all in your perspective and what qualities you are looking for or must have. John seems to be getting a bit condescending in some of these responses as if he has some secret about what is a quality electronic part here. Perhaps many here do not make their living off designing amplifiers and preamps and such but that does not diminish the fact that many here do seem to understand the differences between components. Sometimes the quality parts are known by many, sometimes by a few, but in this forum that is what is discussed and analyzed to death. Some of this is pure bs and some of this is great information. When it is only for the sake of naming names the information could be construed as biased, but there are times when a particular manufacturer has deservedly earned that reputation for quality. I have no problem taking advise from Scott, or George or Sy or RNMarsh and a handful of others who I can tell by the responses here have a very good idea of what a quality component is and why it is one. John I am sure has had to learn many lessons the hard way just as anyone else does with a lifetime of designing products for themselves or others. Unless you are a chemist I probably know more about polyurethane materials than most anyone on this forum. Does that mean that I have a lock on that information, no, but that is something that I have spent over thirty-five years working with and learning from experience. As long as that information is not going to damage my own personal business I gladly advise others in what I know and how to get the results that others are trying to achieve. I have always thought that giving away your knowledge never hurt anyone, it usually comes back in spades as goodwill and the favor is often reciprocated. I surely don't expect John to print a schematic for something he is working on presently but sharing what switches he prefers and what relays and attenuators would seem trivial in the realm of things, not something that is going to cause someone to change a design based on those simple parts, but just information that could help another while taking nothing away from the supplier of the information.