Zero Field Transformer With DC Servo

[bcarso]

[quote author="Samuel Groner"]Yep, they changed that.

Samuel[/quote]

Effective June 2000, according to my book.

 

[Bo Hansen]

I remember that Studer have design a interesting mike pre, in a very complex "zero field" configuration.
I think they start with this in the A-960 console, and they use this also in the later digital console system, D-19 MUX units.
(maybe you can find the schemtic on Studers`s FTP tech info sait)

Regarding the output amps,
This Audio Precision design you talk about, (I have this in my AP "Portable One Plus") it is a very nice design, but take also a look on Urei LA-10, 12, 22 compressor/limiter, they use a form of "mixed feedback drive" design that work very well.

Take also a look on:
Studer A-960 output amp,
Marantz Hi-Fi pre amp SC-22,
Cine Mag application paper AN-102 http://www.cinemag.biz/application_notes/AN-102.pdf
Lundahl tech paper "mixed Feedback drive circuits" http://www.lundahl.se/pdfs/papers/feedbck.pdf

--Bo

 

[PRR]

> I question the need to approach zero source impedance;

AP is lab-gear, not studio gear.

When I use my H-P 200AB oscillator, and need to pad its output to a specific impedance (as when testing transformers), I have to remember to allow for its 70Ω output resistance (DCR of the nominal 600Ω secondary winding). Need 600Ω exact? Find a 530Ω resistor. And while my needs are not so precise, some folks would notice the DCR drift a few ohms as the chassis warmed up. You can calculate-away most of these effects, but it may be nicer if you don't have to. (My Heath oscillator, I modified to have a nearly-zero-Z output, for similar operational simplification.)

Anyway, for what AP charges, you should expect every convenience for high precision.

And of course, the other reason AP "must" use NFB around the isolation transformer is to reduce distortion. THD of good iron is low, but not as low as the AP's THD residual. 0.1% iron-distortion at 20Hz may be inaudible, but people measuring transformerless gear want to test and boast their 0.01% THD, not be limited by iron distortion.

 

[TedF]

Point taken....
For my own lab gear I use parallel outputs from ICs to obtain a source impedance genuinely below 5 ohms; which seems to be good enough so far. Galvanic isolation? I float the whole lot... well isolated power supplies.

 

[SSLtech]

[quote author="TedF"]Point taken....
For my own lab gear I use parallel outputs [/quote]
[quote author="TedF"]Point taken....
For my own lab gear I use parallel outputs [/quote]
...and parallel posts, too! :green: {dupe deleted by moderator- PRR}

Is that to reduce resistance to the idea? :wink:

Keith

 

[TedF]

You must make allowances! I have a pre computer-age mind; I can only think of one subject at a time and the manouverings required to get a post to appear at all, strain my abilities..... I'm totally floored by the 'quote' box; I would love to use it but it doesn't come out right!

So I'll stick to simple comments; if they amuse or inform, then that's good. If I get too boring, just tell me to shut up. :?
(At least I can handle the Emoticons!) :wink:

 

[Samuel Groner]

I'm totally floored by the 'quote' box; I would love to use it but it doesn't come out right!

Text outside the quote-tags will appear as standard text; this post looks like this on my screen: [removed]

If you wait a few seconds after hitting the "Submit"-button, you'll see an "edit" and "delet" button at the right top of the post; they work as the name implies.

Samuel

 

[TedF]

If you wait a few seconds after hitting the "Submit"-button, you'll see an "edit" and "delet" button at the right top of the post; they work as the name implies.

Thanks Samuel! gotit!
(to amused onlookers.... some stupidly simple tasks are a challenge to an older generation.)

Now where were we? Ah yes, 'zero field', I was surprised that no-one came back to me on that one; Lars christened the current transformer 'zero field' and I questioned the name. Now it seems that others have been using the idea.
One of the great attributes is that because there is so little field, the quality of the core is almost insignificant; I have been experimenting with all sorts of core material with uniform success. A 'Current Transformer' (which I prefer to call it) is a beautifully docile device. 'Do it yourselfers' should try playing with inexpensive comms modem transformers and they will see what I mean. :razz:

 

[Samuel Groner]

If we use the negative input impedance to cancel the secondary resistance, the term "zero field" is IMO appropriate - perhaps a bit less appropriate than saying an opamp has "zero" output impedance, but it goes in the same direction. If used the way as in the St*der console, your terminology is surely more suitable.

Another Q popped up in my head: does the negative input resistance also cancel the noise contribution of the secondary? I doubt it does, but I don't see the mechanism right now!

Samuel

 

[bcarso]

[quote author="Samuel Groner"]If we use the negative input impedance to cancel the secondary resistance, the term "zero field" is IMO appropriate - perhaps a bit less appropriate than saying an opamp has "zero" output impedance, but it goes in the same direction. If used the way as in the St*der console, your terminology is surely more suitable.

Another Q popped up in my head: does the negative input resistance also cancel the noise contribution of the secondary? I doubt it does, but I don't see the mechanism right now!

Samuel[/quote]

No---nature cannot be fooled quite that easily. As well, there are noise gain issues when conjuring with negative impedances, but that's a bigger subject than time allows at the moment.