John Curl's Blowtorch preamplifier part II

[simon7000]

Originally Posted by PMA

If there is anyone interested, here is the complete measurement of opamps + schematics.

Very Nice!

[SY]

Originally Posted by elektroj

I haven't seen preamps where line stage has 40dB of gain lately.
What happens when you reduce the gain to 20 or 10dB? Still no 7th?

Best ask Pavel- he did the experiments. I use a unity gain line stage, and no discernable 7th.

[PMA]

Thanks for this post, John:
John Curl's Blowtorch preamplifier part II

What I thought is that here we could exchange our experience and results, and we may leave marketing to marketing people and market places.

As we are getting older, our experience and results are the most valuable things we may share. Everything else is passing.

[PMA]

Originally Posted by SY

Best ask Pavel- he did the experiments. I use a unity gain line stage, and no discernable 7th.

We may quite well predict the distortion much less for gain 0dB compared to 40dB. It is almost 40dB lower then, for many opamps. But difficult to measure. Yes, no 7th for 0dB. The 7th is easier to see at higher gain - like OP27 in my measurements.

[jneutron]

Originally Posted by magnoman

George
At least for the current crowding effect being discussed wouldnt the co-ax symmetry prevent it all together?

Thanks
-Antonio

No. Details below.

Originally Posted by DF96

Coax symmetry is not the problem. The issue with coax is that the spacing between inner and outer is large enough that normal skin effect dominates. Now I believe that skin effect and proximity effect are merely auto and mutual versions (respectively) of the same thing so will behave in the same way, but I am not sure everyone agrees so demonstrating nothing strange with skin effect might not convince those looking for strange proximity effects.

Coax exhibits two effects.

First, as frequency rises, the core wire current will skin to the outer surface of the core, and will skin to the inner surface of the shield.
Second. If the core wire is not centered (offset) within the coax, or the shield is asymmetric (oval for example), the shield current will redistribute on the shield so that it's centroid will coincide with the centroid of the core current. This is proximity based.

Originally Posted by DF96

You would probably need to setup something where conductor spacing is smaller than skin depth so proximity effect dominates, as may sometimes be the case for audio. Maybe an air-cored inductor or transformer, but with well-encapsulated windings so no physical movement is possible?

Done it all, measured same. Measured air coils of the same physical dimensions all wound identically using single conductor magnet wire, two conductor magnet wire, and now some awful litz stuff with 133 individual filaments, 15awg IIRC..and all potted in a two part alumina filled epoxy. The litz is difficult to connect to.

Originally Posted by Esperado

That all the irony of all this passionating discussion: The distortion levels we are talking about are so low that we can doubt of their exixtance, and there is hundred places in all our systems, where we can improve distortions of 100X this levels.

Ed did not try the resistors I mentioned...low value ones. The magfield drive currents in his test were rather small..

Originally Posted by RNMarsh

Comments -- skin affect and proximity have the same affect... but are caused by different means. The lowering of inductance is what causes the currents at higher freqs to flow in that region of the 'skin'. But what causes that to happen? The wire(s) geometry causes electromagnetic fields to be concentrated more and less across the conductor. In a round conducting wire the field is concentrated -denser- at the center(actually, that is backwards...the self induced magnetic field intensity within a round conductor is zero at center and increases linearly out to the surface of the conductor.) ... thus the center has the higher inductance. Where ever the fields get concentrated, the inductance is increased. Proximity can reduce inductance but it is not evenly distributed or symetrical anymore..

It's best to think of it as the current is trying to take the path of least reactance. As frequency increases, the path becomes the lower inductance path. For a wire pair next to each other, that path is as close to the other conductor as is possible given the fields. for a coax, it is the surfaces closest to one another.

Thsi also occurs on PCboards with a full ground plane, and also within chassis when ground current is flowing through the chassis, and when equipment is connected via more than one ground.

Originally Posted by Esperado

Because the inductances of the wires differ ?

The physical arrangement of the wires changes the inductance of the system. Also of concern would be if the entire foil construct is antiparallel current or parallel.

jn

[Esperado]

Originally Posted by Triodethom

It entertained the mass, and did no harm in epoxy it MAGIC . The buyer saw it as value no harm to them you will be forgiven.

I hope Mr Dolby will benefit of the same indulgence, with his "surround" process he imposed to the movie (tv) market, while he really harmed us with this poor and stupid standard, production side, for its own profit.

mylife.com:
We were obliged to get an official 'approval' of the studio equipment and tunings, to be "Dolby certified". The day the Dolby guy had finished the inspection of our brand new "Surround" movie mixing studio, he was writing the official certification paper.
When he handed me the certificate, I fell on my knees, in the position of the peasant about to be promoted to Knight. Everyone laughed (but he).

I remember too the story that happened to one of my technical manager friends. They just finished a fantastic movie mixing facility in a big theater in Paris. Sumptuous. The speakers where included in a very thick concrete wall across all the width ot the room, behind the screen. The guy from THX refused the certification, because, in the official requisites, the wall was described as to be "plaster panels" !!!!
They had to exchange letters with George Lucas, for this shame was arranged.

[Telstar]

Originally Posted by PMA

And finally, OPA637. Low distortion, low noise and low sensitivity to interference. My vote

No 7th!!

Or under 130dB which is your setup noisefloor. Edit: ok you raised the gain to show the harmonics above the noise floor, good work. So we can assume all values 10-20db lower than that.

BYW why dont you try with a 50 or 100hz tone and show us all the harmonic spectra? There may not be enough dissonant 7th, but there can be anything much higher than that.

[Telstar]

Originally Posted by Esperado

Many thanks, PMA, for sharing your measurements. Once again, it prove all those evil little black boxes works damn well !

'fraid it doesn't prove anything.
1k TDH and noise spectra is nowhere near a sufficient measurement of sound quality.

[PMA]

Originally Posted by Telstar

Or under 130dB which is your setup noisefloor.

Why dont you try with a 50 or 100hz tone and show us all the harmonic spectra? There may not be enough dissonant 7th, but there can be anything much higher than that.

1) -130dBFS is NOT the measurement noise floor. The lowest noise floor had the AD797 - you can check the measurements, it was -135dBFS. The noise floor of my measurement setup is below -140dBFS.

2) Why not to try 50Hz or 100Hz - for it shows nothing more and nothing new. The test condition would be softer compared to 1kHz (higher feedback factor at 50 or 100Hz). With 1kHz, you might see harmonics up to 47th, if there were any.

I made the measurements for the reason I want to stay accurate - I do not like speculations and unsupported claims.

[PMA]

Originally Posted by Telstar

'fraid it doesn't prove anything.
1k TDH and noise spectra is nowhere near a sufficient measurement of sound quality.

It proves that speculations about the 7th were fairy tales. But it takes much less time to figure out and tell the fairy tale then to prove it false by measurements.

If you could read the plots you might see the difference in sensitivity to interference. It clearly shows that JFETs are superior to BJTs in this.