I'm aware of the CM i/p Z problem of convential audio difference amps and how the system CMR gets blatted by cable Z mismatch.
Can you explain the shortcomings of using e.g. INA317, its inputs buffered by 2 x op-amp stages like OPA1642 (to make a classic instrumentaion amp), plus say 4.7MOhm resistors to gnd on each op-amp input for bias?
We may have to agree to disagree on that one. For the record, I wish the RCA to XLR adapter didn't exist because it encourages folks to use an RCA cable (which intrinsically couples ground noise directly to the signal) for the hookup - and this is true regardless of how the adapter is wired (whether pin 3 is grounded or left floating). I was mistaken regarding my Muncy reference comment - it was another blog at another website! Sorry.
I've never heard that arrangement of bias resistors called a "T-circuit" but, in audio, it seems everything gets a pet name. Even if just two high-value resistors to ground supplied bias, I doubt they'd contribute significant noise because it will be seriously attenuated by the differential source impedance - somewhere in the 150 to 200 ohm range for a mic preamp. Also remember that i-noises of op-amp inputs are uncorrelated with each other. The i-related voltage noises in R1 and R2 will not be nullified by the following diff-amp. The two i-noises will add as the sq-rt of the sum of their squares in R7, creating a common-mode noise component that will be nullified in the diff-amp. Mic preamps are a bit of a special case. In the case of a passive (dynamic) mic, the source's CM impedances are extremely high, making conversion from CM to DM (noise into signal path) much less of a problem. With a phantom-powered mic, CM impedances are roughly comparable to those at the mic preamp (with phantom power turned on), making conversion more likely. But it's a rather moot point because ground voltage differences tend to be trivial, since the mic (source) is rarely, if ever grounded anywhere but (via the shield of the mic cable) the mic preamp itself. However, mic preamps driven from "mic splitter" outputs can sometimes be a problem (generally only when cheap split transformers without Faraday shields are used - Jensen models use dual Faraday shields for exactly that reason). It should also be obvious that a mic preamp using an input transformer requires no capacitors in the signal path, no transient phantom power protection (diodes, series resistors, ad nauseum), and no RFI filtering - since the transformer provides it all, as well as a match to the amplifier's optimum source impedance for minimum total noise.
As a point of reference I recently bought a Roland Quad capture USB-powered interface for use in instrumentation. I was quite disappointed to learn that the combo XLR/TRS would only accept levels on the XLR from -60 to -6 dBu. The TRS "line" input accepts -50 to +4 dBu. +4 was not the nominal, it was, unfortunately, the clip point. Though I wasn't surprised by the XLR range, I would have been much happier with it had the line input had greater than +10 dBu capability. Unfortunately, headroom is shrinking along with supply voltages.
"As an aside, I'm curious why my (late) friend and colleague Neil Muncy has his 1995 AES paper listed in your references and not mine (in the same issue of the AES Journal)?"
Not sure I follow you there. I didn't cite Muncy.
"I honestly don't believe that a balanced input should be required to perform with only one input leg connected. The fact that there's little or no output should alert the user that he's done something wrong (you wouldn't expect your toaster to work if you plugged in only one wire of its power cord, would you?)."
That may represent a difference in philosophy.
The fact is that the majority of active line inputs do produce output with an open leg, kinda sort of. Or, should I say in the -6dB case they work "halfway."
The fact that there's no output with a transformer or high CMZ input does let you know something's wrong but not necessarily where it's wrong since you don't often know that the cable you're holding in your hand has actually got signal on it to begin with. So in the interest of the show starting or the performance of a lifetime being recorded (and being the support person carrying a beeper) my hunch is that it's better to get signal under any circumstances at a reasonably consistent level no matter which port you're stuffing signal into. The use of combo TRS/XLR connectors in particular and wrongly-made adapters seem to almost require it.
With regard to the toaster example I'm pretty sure Sunbeam gets those calls.
Bill - Thank you for your comments. An example of a "medium" CMZ "T-bias" circuit used in a mic preamp can be found in figure 5 of the THAT1510/1512 datasheet. R1 and R2 are the bias resistors, R7 a CM resistor. I provided that circuit to Rosalfonso Bortoni and he performed measurements with R7 up to 100K with no measurable increase in noise from it being 0R. The advantage of having the higher CM impedance is that it reduces the matching requirements of the input caps. http://www.thatcorp.com/datashts/THAT_1510-1512_Datasheet.pdf
For a relatively high CMZ line input using something with as "poor" Ibias and Inoise as a bipolar input 5532, one can scale R7 to very high values and allow both the Ibias Vos and Inoise to develop in common mode across R7. The CM rejection of the following stage removes both the DC-component and noise component. One can build a AC-coupled line input with modest value film caps that has virtually the same LF CM as mid-band and without significant noise penalty.
I'd be very interested to learn more about your high CMZ circuits. Although classic instrumentation amps have extreme input impedances, there's the pesky matter of bias currents in a practical circuit. XLR inputs can't simply be tied to an IA input because many signal sources have no DC path for bias currents (transformers or capacitor-coupled outputs, notably). So the downfall of most IA implementations is the resistors from inputs to ground for bias. These now set the CM input Z ... and generally at a value far too low. I honestly don't believe that a balanced input should be required to perform with only one input leg connected. The fact that there's little or no output should alert the user that he's done something wrong (you wouldn't expect your toaster to work if you plugged in only one wire of its power cord, would you?). Any adapter that leaves either pin 2 or 3 of its XLR floating (I know of none) is simply built wrong. As an aside, I'm curious why my (late) friend and colleague Neil Muncy has his 1995 AES paper listed in your references and not mine (in the same issue of the AES Journal)? His paper deals almost exclusively with the "pin 1 problem" while mine specifically deals with the issues being discussed in this thread. BTW, that issue of the Journal, June 1995, has become the largest selling issue ever printed by the AES. If you don't want to spend the $5 for it, I'll e-mail a copy to any interested reader. I'm firstname.lastname@example.org. - Bill Whitlock, president & chief engineer, Jensen Transformers, AES Life Fellow, IEEE Life Senior
Thanks for your very kind words about our transformers! As a related note, Jensen is celebrating its 40th year in 2013. First, I never recommend using a center-tapped transformer at an output (or an input for that matter). One of the problems with the simple diff-amp is that there's an intractable tradeoff between resistor noise and common-mode input impedance. Input buffers, as in the classic 3-op-amp "instrumentation" amp, eliminate this tradeoff and very low-noise designs are possible. Since we don't specify any of our transformers for CT operation (except with high-Z secondary loads in a few cases), I have no hard data to answer your question. That being said, small amounts of DC offset ("small" in this context depends on many things like winding DC resistance, number of turns, etc. so it will vary considerably among transformer types) will cause a "redistribution" of transformer distortion products. Without DC, transformer distortion at low levels is almost entirely 3rd harmonic. As DC is gradually introduced, there is an increase in 2nd (and sometimes a slight reduction in 3rd). At higher DC flow, 2nd harmonic dominates (and is responsible for that "warm" bass coloration typical of vintage class A vacuum-tube designs ... Jensen doesn't cater to that market BTW - our designs are as audibly transparent as we can make them). When it comes to inputs, we need to stay aware that a "pro" line-level input must keep its input Z at 10 k-ohm or more. The de-facto standard for mic inputs is 5 to 10 times the source (assumed to be a 150 to 200 ohm mic), making it 1 k-ohm or more typically. Consumer inputs must be 22 k-ohm or more, again by IEC standards. These impedances are *differential*. For balanced inputs, there are huge advantages to making the *common-mode* impedances much, much higher (for a typical input transformer, they are some 50 M-ohm at 60 Hz).
Yes, "floating" or ungrounded equipment is problematic. But this problem, as well as the common-impedance coupling that's responsible for hum and buzz in unbalanced audio cables can be avoided in all but the most extreme electrical environments by simply making the correct interface cable. Caution - most commercial ready-made cables are made WRONG, so check them out before you buy one. A proper cable to feed a balanced input from an unbalanced source should always use a 3-conductor (twisted-pair plus shield) cable. The two pieces of equipment (the consumer source and the pro destination) have their chassis grounds tied via the cable shield, thus eliminating the "floating" CM voltage problem. The signal is now carried on the twisted pair and no noisy ground current flows in either signal wire, avoiding the common-impedance coupling of hum and buzz. A proper cable will have normal +, -, and shield connections at the balanced end and will tie - and shield to the RCA outer contact and the + to its center pin. This will, depending on the CM input Z of the balanced input stage, produce typically 30 to 40 dB of noise rejection compared to using an RCA cable (two conductors) and an RCA to XLR adapter. In extreme cases, you may need to add a Jensen (or other quality Faraday-shielded "input") transformer at the balanced end to improve CMRR of the existing input stage.