abbey road d enfer

Re: Mic input transformer grounding questions
« Reply #20 on: June 27, 2019, 09:57:58 AM »
I haven't gone back to look, but I seem to recall the Jensen mic splitter transformers have separate shields per winding with 1:1 or 1:1:1 or 1:1:1:1 connections.
Indeed, their JT-MB-CA "Microphone Input Transformer1:1 FOR ULTRA-LOW NOISE AMPLIFIERS" has dual electrostatic shield, and their recommended connection is pri shield connected to "earth/chassis" and sec shield connected to audio reference. I believe the 1:1 ratio leaves enough space in the bobbin for an extra shield.
Does anybody know how their shields are constituted?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


Re: Mic input transformer grounding questions
« Reply #21 on: July 15, 2019, 11:42:51 AM »
Although I registered for this forum on 7/1,  I was just "cleared" (must have been a very thorough background check) and I'd like to post more detail to issues raised in this thread.  Please forgive me if I'm "style clumsy" at first.  Here goes:

1. Shield Connections:
Faraday shields (inaccurately called "electrostatic" shields) are used to prevent capacitive coupling of AC electric fields. In an input-type transformer, they're intended to make ALL the coupling magnetic. Because the shield has capacitance to both primary and secondary windings, it matters whether the shield is tied to the incoming signal's ground or the SRG (signal reference ground) for the amplifier that follows. Most input transformers are driven by a balanced line, therefore the primary-to-shield capacitance is intentionally balanced as much as possible. Similarly, the output (secondary) is designed to drive a single-ended (ground referenced) amplifier. Therefore, no attempt is made to balance secondary-to-shield capacitances (it would generally compromise other design aspects. That's why it's important that both the designated "low" side of the secondary, as well as the shield, be tied to SRG for the amplifier. For typical non-inverting op-amp circuits, this point is the grounded end of the shunt resistor in the negative feedback. For typical vacuum-tube circuits, it's the grounded end of the cathode bypass capacitor (or resistor, if no bypass is used).

That being said, the balanced input signal to the transformer may have common-mode voltage (noise) with respect to SRG. The transformer is specifically designed to "reject" this noise (evidenced by its CMRR or common-mode rejection ratio figure). Therefore, connecting the Faraday shield to the signal source ground reference (pin 1 for an XLR connection) would result in the source's ground noise (relative to SRG) being capacitively coupled to the secondary winding. But connecting the shield to the amplifier's SRG makes the noise appear between primary and shield - where it can be rejected. This is the recommended connection.

For a typical mic input transformer, the Jensen JT-115K-E, the capacitances are: primary-to-shield = 475 pF (effectively 238 pF at each end); secondary-to-shield = 205 pF; and all windings and Faraday shield to case = 34 pF. While tying the can to SRG is ideal, the can must not touch the chassis as might happen via mounting to a metal chassis. SRG should be tied to other grounds at only one intended place. Therefore, connecting or mounting the case to chassis ground is generally recommended. It's very unlikely that the tiny voltage difference between SRG and chassis will cause noise. Remember that any noise between case and SRG would couple to the secondary through less than 34 pF - making the noise an order of magnitude lower than that caused by connecting the Faraday shield to chassis! Don't "float" the case connection - this disables it's ability to act as an external Faraday shield by allowing nearby electric fields (such as from the "hot" side of AC power) to couple to transformer windings inside. Although the can is a magnetic shield intended to route magnetic fields around the core/windings inside, why not make use of its electric field shielding, too?

2. Faraday "Shield-per-Winding" Feature:
To make "mic split" transformers more noise-immune, Jensen pioneered the "shield-per-winding" design. In some applications (live events where a mic must feed both a "house" and "remote truck" mixer), system ground-voltage-differences can be very high (often tens of volts). With a single shield between windings, there is no way to prevent this voltage from appearing between shield and one of the windings. Putting a shield on each winding now puts the voltage between shields, with each winding shielded by a "ground" voltage equal to its destination mixer's "ground" voltage. It neatly solves a pesky problem! It also drives home the point that ground is not ground is not ground.

3. The Pin 1 problem:
When system equipment is interconnected with cables, substantial currents can flow in the ground conductors of thos cables ... and, much more important, through the connector ground contacts at each end. Such currents have been observed at 100 mA or more - they are created by small voltage differences (quite normal) between the grounds of AC power outlets (I've written a paper about this, so I won't go into detail here). The current (noisy hum or buzz) flows in shield pins on balanced I/O of the equipment. Like all currents, it will flow back to the source of voltage that created it - in this case AC outlet ground. But, if these large, noisy currents are allowed to flow in wires or PCB traces that are part of internal equipment "ground," the resulting tiny voltage drops will inject noise into the signal path. Therefore, pin 1 (shield) for every XLR connector must have an exclusive, independent pathway to the chassis (where UL requires that the AC power ground be tied).  In general, connecting pin 1 to any wire or PCB trace used by amplifier circuitry is a recipe for noise disaster! This problem exists in a lot of commercial equipment ... it's called the "Pin 1 problem." Pin 1 has absolutely nothing to do with a balanced signal!  I'll spare the rant here ... but the misconceptions (even the very definition) of balanced interfaces has been a cause celeb with me for over 25 years!

4. About Ground Symbols:
I use ground symbols as specified by IEEE (Institute of Electrical & Electronic Engineers) Standard 315-1975 section 3.9 "Circuit Return." You can find this entire document at
https://www.ee.iitb.ac.in/~spilab/Tips/ansii_graphic_symbols_for_electrical_and_electronics_daigrams_1993.pdf
This standard has been adopted by: American Society of Mechanical Engineers, the American National Standards Institute, Canadian Standards Association, and US Department of Defense. The "chassis" symbol (as well as the "general" symbol using a stack of lines in a triangular shape) is also part of IEC standard symbols. The "triangle" symbol, as used in Jensen diagrams for an "isolated" signal ground, is one of the IEEE symbols "To be used when identically annotated common-return connections are at the same potential level."

Overall, as I always note in my seminars, the use of the words "ground" and "earth" to describe common circuit returns is unfortunate because many, if not most, electronic circuits are not connected to actual soil (battery-powered, aircraft, spacecraft, etc.). Soil connections are all about lightning and, secondarily, fault mitigation in high-voltage electrical distribution networks. It would be so much better if electronics engineers used the word "return" instead! It would encourage them to think in terms of current flow - instead of thinking of a soil connection as an infinite sink for all things noisy. My colleague, the late Neil Muncy, referred to this as the "sump theory of noise control."

5. I'm New Here:
I've been an analog circuit designer since about 1970 - serving as chief electronics engineer for Quad-Eight and Capitol Records, among others. In his will, Deane Jensen left me his company upon his death in 1989 and I served as owner/chief engineer until I sold it in 2014. Deane and I worked on many projects together before his tragic death ... and he inspired me to "dig deeper" into everything. I now do consulting out of my home in Ventura, CA (I can't allow myself to really retire - how boring!).

With some reservations, I  joined this forum just today. The only other forum I've been able to deal with for many years is Syn-Aud-Con ... many others just seem like "opinion fests." But I'm optimistic about participating here ... I just hope my post isn't "too much information" and that it answers more questions than it raises. Flame suit on!!

Bill Whitlock, AES Life Fellow, IEEE Life Senior
Whitlock Consulting, Ventura, CA

JohnRoberts

Re: Mic input transformer grounding questions
« Reply #22 on: July 15, 2019, 12:30:33 PM »
Hi Bill and welcome back.. I hope you find our community better behaved than most around the internets.

We look forward to hearing more of your wisdom.

JR

PS: If you do a member search using your name you will find your old account, and 5 posts you made in Aug of 2004. I ASSume that was you in 2004, the posts sound like you. If you can't remember your old password we could delete the old account, or just leave it.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

abbey road d enfer

Re: Mic input transformer grounding questions
« Reply #23 on: July 15, 2019, 12:32:40 PM »
Faraday shields (inaccurately called "electrostatic" shields)
Care to elaborate? As far as I can see, only Faraday cage has a proper definition, by which the xfmr's case would be it.
Electrostatic makes sense to me, since it essentially takes into account the electrostatic part of electromagnetic radiation. I certainly can live with it.

BTW, welcome aboard.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

squarewave

Re: Mic input transformer grounding questions
« Reply #24 on: July 15, 2019, 03:14:01 PM »
Hi Bill. Nice read. Thanks for posting. Just ignore the "Brewery" and you'll be good.

electrostatic shields
« Reply #25 on: July 15, 2019, 06:29:00 PM »
Frederick Terman, Sc. D,  Professor of Electrical Engineering Stanford University, in his text "Radio Engineering" discuses both magnetic and electrostatic shielding, pages 44 to 47.

"Elements of Sound Engineering" by Frayne and Wolfe,  mentions electrostatic shields on page 189.

All classic coil winders including Western Electric, UTC, Triad, Stancor, etc. refer to the "electric" shield as electrostatic.


Re: Mic input transformer grounding questions
« Reply #26 on: July 15, 2019, 06:49:12 PM »
Thanks guys for the kind words!  Rigorously speaking, it's the "static" in "electrostatic" that bothers me, despite its colloquial popularity to include AC electric fields.  Static literally means unchanging.  Both electric and magnetic fields can be static (DC) or dynamic (AC) ...  a Faraday shield stops electric fields of both types.  A minor point, I know, but I like rigor, it's what makes science work.  Perhaps I'm biased because I'm a huge fan of Michael Faraday ... mainly because (like me) he thought in terms of concepts rather than equations. Because of this, he was considered "less worthy" by the math snobs of his day. Interesting that Einstein had a picture of Michael above his desk. To this day, Maxwell gets the credit for most of Michael's brilliantly insightful work.

On a related note, dozens of textbook authors are provably wrong about what makes a balanced interface work, too, just parroting other authors. But this is a subject for another thread. Bottom line: I don't take what I read, even in textbooks, at face value.

I may visit the "brewery" briefly, but likely won't have a drink  ;)

Bill

JohnRoberts

Re: Mic input transformer grounding questions
« Reply #27 on: July 15, 2019, 07:28:29 PM »
Thanks guys for the kind words!  Rigorously speaking, it's the "static" in "electrostatic" that bothers me, despite its colloquial popularity to include AC electric fields.  Static literally means unchanging.  Both electric and magnetic fields can be static (DC) or dynamic (AC) ...
7 : of, relating to, or caused by radio static

The colloquial definition is in conflict with the nominal definition, but I had to go all the way down the list to find that.
Quote
a Faraday shield stops electric fields of both types.  A minor point, I know, but I like rigor, it's what makes science work.  Perhaps I'm biased because I'm a huge fan of Michael Faraday ... mainly because (like me) he thought in terms of concepts rather than equations. Because of this, he was considered "less worthy" by the math snobs of his day. Interesting that Einstein had a picture of Michael above his desk. To this day, Maxwell gets the credit for most of Michael's brilliantly insightful work.

On a related note, dozens of textbook authors are provably wrong about what makes a balanced interface work, too, just parroting other authors. But this is a subject for another thread. Bottom line: I don't take what I read, even in textbooks, at face value.

I may visit the "brewery" briefly, but likely won't have a drink  ;)

Bill
The brewery is less linear than technical discussions in other forums, so caveat lector.  But more fun than latest clone dissection. Politics and economics as found in the brewery do not strictly obey the rigid laws of physics... This forum is better behaved than most. It's actually against the rules to be a flaming a__hole here, but some still test the limits.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

PRR

Re: Mic input transformer grounding questions
« Reply #28 on: July 15, 2019, 09:40:06 PM »
> the use of the words "ground" and "earth" to describe common circuit returns is unfortunate

Bears repeating.

the use of the words "ground" and "earth" to describe common circuit returns is unfortunate

> because many, if not most, electronic circuits are not connected to actual soil (battery-powered, aircraft, spacecraft, etc.). Soil connections are all about lightning and, secondarily, fault mitigation in high-voltage electrical distribution networks.

And it comes up in utility power distribution *because* overhead and exposed utility power systems are dirt-bonded in self-defense against *lightning*. Only the highest-voltage distribution lines can fault-clear on ground current. (Go back to 1915. NEC theorists argued for un-grounded systems. Utility operators knew that would not work.)  So it is ALL about Zeus.

Ah, one more use of dirt. In low-frequency radio a good antenna is cheaper if you use some existing large conductive object for half of the antenna. Dirt is frequently handy. However on the LF bands atmospheric noise limits useful sensitivity, and for about 70 years we have been doing good with "poor" antennas and an extra stage of amplifier.

JohnRoberts

Re: Mic input transformer grounding questions
« Reply #29 on: July 15, 2019, 09:58:52 PM »
Lies, damn lies, and audio ground.......  8)

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.


abbey road d enfer

Re: Mic input transformer grounding questions
« Reply #30 on: July 15, 2019, 11:45:02 PM »
From Wikipedia:
"Electrostatics is the branch of physics that studies the phenomena created by static electric charges for the observer. The laws obtained can be generalized to variable (quasi-electrostatic) systems provided that the distribution of the charges can be considered as being in equilibrium at each moment. "
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

boji

Re: Mic input transformer grounding questions
« Reply #31 on: July 16, 2019, 01:28:28 AM »
Quote
Hi Bill and welcome back.. I hope you find our community better behaved than most around the internets.

We look forward to hearing more of your wisdom.

+1!

Re: Mic input transformer grounding questions
« Reply #32 on: July 16, 2019, 02:11:21 AM »
Regarding the dis of classic textbooks, it takes a bit of hubris to claim they are all wrong.

Back to where and when to strap an electrostatic shield:
seen plenty of vintage and not so vintage mic preamps, shield always tied to chassis.
If someone knows of a respected circuit otherwise, please share.

CJ

Re: Mic input transformer grounding questions
« Reply #33 on: July 16, 2019, 04:03:37 AM »
sometimes experimentation with grounding points can lead to the lowest noise on a system of permanent location, and having the theory can help with this experiment,

i just wanna know how they got the perm so high on the lams for the JT-10K61 line out,
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Mic input transformer grounding questions
« Reply #34 on: July 16, 2019, 07:04:40 AM »
Nice to meet you Bill ,

ruffrecords

Re: Mic input transformer grounding questions
« Reply #35 on: July 16, 2019, 05:20:29 PM »
It is both a pleasure and a privilege to have you as a member here Bill. We have recently started a sub-group named Magnetics which you might find of interest, if only to dispel yet more myths.

Cheers

Ian
« Last Edit: July 17, 2019, 07:16:01 AM by ruffrecords »
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Re: Mic input transformer grounding questions
« Reply #36 on: July 16, 2019, 08:44:05 PM »
We hear plenty of reasons not to use transformers and chokes from certain quarters   ;)
MrCMRR might help  balance the argument    :D
As far as RF hash ,honk and buzz goes its a more hostile environment than ever now , that means the usefulness of transformers in balancing stray external fields is more relevant than ever .
Compared  to 'transformerless' bandwidth is hindered  , but its the 'quality' of the distortion imparted , in harmonic terms is the other great plus of transformers  for me.




john12ax7

Re: Mic input transformer grounding questions
« Reply #37 on: July 17, 2019, 02:14:55 AM »
Very informative thread. Thanks to all for sharing their knowledge and experience.

Re: Mic input transformer grounding questions
« Reply #38 on: July 22, 2019, 02:57:20 AM »
Sorry guys but I got too busy to even check this site last week!

First, in answer to CJ's question about the Jensen 10K61 output transformer, its core is an 80% nickel alloy but the "secret sauce" (which shall remain a Jensen trade secret) is in the custom profile of the heat treatment (annealing) for the core material. It both increases magnetic permeability (inductance, indirectly) and lowers THD at low signal levels (the magnetic hysteresis part of the curve, as opposed to the magnetic saturation part at high signal levels).

Regarding the "anti-transformer" camp, I could go on and on about this but I'll keep it brief.  Transformers, if properly designed and fabricated, are amazing performers.  But, as the late Deane Jensen used to say, "first I'll tell you why you need a transformer, then I'll tell you why you need a Jensen."  With poor transformer designs, I'm often tempted to say "If you can't afford a good one, you should probably do without!"  Jensen isn't the only source of pretty good audio transformers, but I know of no others that do just about everything extremely well.  The best aspect of a transformer is, of course, galvanic (electrical) isolation. Since the windings float electrically, the transformer has no idea what "ground" means. That gives it excellent noise rejection when used at inputs and an "I don't care what you connect me to" attitude at outputs (will drive a balanced input or an unbalanced (grounded) one with equal ease and no risk of damaging the output line driver stage.
Now comes the bandwidth issue. The worst shortcoming of most other transformers is high THD at low frequencies and high levels (this is where most of the energy in contemporary music is, in fact). The others do it because physics tells us that high levels of low frequencies is what creates the strongest magnetic flux the core material will ever see ... and it simply takes more core material (a bigger core stack) to do that ... no escaping that. So a transformer barely bigger than a postage stamp (like that in the IL-19, for example) simply will have double-digit THD at 30 Hz and +20 dBu signal peaks.
Then there's phase distortion. As Deane wrote many years ago, phase shift is not necessarily phase distortion. Phase shift, as long as it changes linearly with frequency, is no different that moving your head a few inches toward a sound source. But phase shift that isn't linear with frequency IS phase distortion, because different frequency components of complex music will have different arrival times ... and change the timbre of the sound. All that being said, some may find it surprising that the most serious phase distortion occurs at low frequencies.  Marshall Leach wrote a paper back in the early '80s about this. He concluded (and I agree) that to keep LF phase distortion low at 30 Hz, the -3 dB frequency of a system (including the speaker and mic) needed to be over a decade lower. It affects the timbre of kick drums for example ... and explains why I personally dislike bass-reflex speaker systems (they behave as 4th-order high pass filters, while sealed boxes behave as 2nd-order high-pass filters - and, in general, the steeper the filter, the worse its phase distortion).  Getting to the point here ... it's why Jensen transformers have what seems to most over-designed low-frequency response. A typical Jensen design will have a low-end -3 dB point around 0.2 to 0.5 Hz and the reason is ultra-low "deviation from linear phase" or DLP as it shows on the data sheets. These parts have an incredibly tight and clean bottom end.
Regarding HF bandwidth, I'll simply say that more is rarely better. Going back to the Leach paper, at high frequencies. the shape of the roll-off is crucially important to minimize phase distortion. A class of low-pass filters called Bessel have the most linear phase/flat group delay response. Therefore, the top end of a Jensen design is made to conform to a 2nd-order Bessel response. As expected, such filters have near-perfect square-wave response. You can see this on the data sheet's DLP plot vs frequency, staying typically within 2-degrees of linear phase from 20 Hz to 20 kHz.  It's also the reason that the recommended load on mic input transformer secondaries, whether just a resistor or an R-C network be followed precisely.  All that being said, I much prefer listening to a clean, phase-distortion-free bandwidth of 20 kHz rather than a 100 kHz bandwidth. And there's another reason to intentionally restrict HF bandwidth ... it's called "spectral contamination" in a Deane Jensen/Gary Sokolich AES paper of that name. It describes a complex inter-modulation distortion that is made much worse when ultrasonic components are included in the signal applied to any active device (transistor, op-amp, vacuum-tube, etc.). These components are most often harmonic distortion products of previous active devices in the signal chain, DAC clock residue, or even stylus chatter in LP playback.  This would mean that the cleanest possible signal chain would include an intentional low-pass filter just ahead of any active stage.  I could go on further, but I'd like to end my "pro transformer" rant with an observation from 25 years of talking with Jensen customers.  Jensen's "transformers in a box" product, ISO-MAX, are widely used to fix ground loop issues in very high-end audiophile and home theater systems. At least half the customers I talked with say something like "Not only is the buzz gone, but my system has never before sounded so "clean" with "open space" and "air" that lets me identify each instrument ... it's amazing."  I usually just said thanks rather than trying to explain that spectral contamination literally clutters the audio spectrum with non-harmonically-related new spectral lines ... it's very hard to describe but some call it a "veil" that's lifted when the transformer is inserted in the signal chain.  And, for what it's worth, the most effective place to insert one is just ahead of the power amplifier (which makes sense to me since those amps have, in general, more inherent open-loop non-linearity and lower slew rates than low-level amplifiers.  Anyway, I have to stop myself ... but don't give all transformers a bad rap just because a lot of crappy ones are in wide use. Overall, a good transformer can be a tremendous benefit.

And please accept my humble thanks for the warm welcome I've received here. Maybe I'll get to meet some of you in person someday. Incidentally, I'll be teaching a workshop on power and ground issues for live performance venues (and possibly another class aimed at equipment designers about internal grounding of power supplies, shield connections, decoupling, etc.) at the AES convention in NYC this October.  Kind regards,  Bill Whitlock

ruffrecords

Re: Mic input transformer grounding questions
« Reply #39 on: July 22, 2019, 03:14:51 AM »
Bill, I wish I had you by my side back in 1973. I was fresh out of university working at Neve. I went on an APRS studio engineers course and one of the speakers was Clive Green who was MD of Cadac, probably Neve's biggest rival at that time. One evening over a few beers the conversation turned to transformers. Cadac were at the vanguard of the then new transformerless pro audio trend and waxed lyrical about how bad transformers were. I was young, inexperienced and had no answer to his arguments. Of course I now know better but it would have been fun to have you there too.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


 

Related Topics

  Subject / Started by Replies Last post
4 Replies
1615 Views
Last post October 21, 2004, 09:43:13 PM
by
3 Replies
1653 Views
Last post November 12, 2005, 09:14:12 AM
by maxime
41 Replies
9930 Views
Last post December 15, 2008, 01:24:01 PM
by Andy Peters
5 Replies
2335 Views
Last post January 10, 2009, 10:42:44 AM
by Rob Flinn