There's so much to comment on here! Perhaps the most glaring omission from your discussion is the importance of extremely high common-mode input impedances (why do you think transformer inputs work so well when diff-amps perform miserably at noise rejection?). They are the key to consistently high CMRR in the real-world where signal sources have significant impedance imbalances (I convinced the IEC to change its CMRR test because testing with a lab-generated zero imbalance source proves nothing). The InGenius IC manufactured by THAT (and patented by me) overcomes this very serious issue. You, like many practitioners, obsess over the non-match of single-ended input impedance for the ordinary diff-amp. It is a trivial issue - what matters is that the common-mode input impedances match - and they do (drive both legs to the same voltage and you'll see that the input currents in the two legs match). The asymmetry of input voltage with non-zero source impedances affects only headroom and crosstalk if cabling is unshielded. Concerning single-ended (unbalanced) sources: a shielded twisted-pair cable, with diff-amp "low" grounded ONLY AT THE SOURCE and the unbalanced signal to diff-amp "high" will eliminate common-impedance coupling in the unbalanced cable. The "benefit" of being able to drive only one leg of the diff-amp input and ground throws away at least 30 dB of noise rejection. There's lots more to criticize here, but you should sit in on one of my free tutorials at the next AES convention (or CEDIA or InfoComm) or read the chapter I wrote in Ballou's handbook or just visit the Jensen website and read the generic seminar handout there. It may be a real "eye opener". - Bill Whitlock, President & Chief Engineer, Jensen Transformers, www.jensen-transformers.com, AES Life Fellow, IEEE Life Senior
Wow, good article, but also very good first reply! In simulation and theory you need to take things into account explicitely. Whereas in reality "unexpected" things just happen. And indeed the proposed methods are not as good in the presence of impedance unsymmetries as what THAT has done. On the other hand the well-known 3-opamp solution is also good, and using 2 buffers instead is not that much a saving, but all in all THAT offers a real good practicable solution.
Bill - Thank you for your comments. I'm well aware of the benefit of high common mode impedance and the superiority of transformers. I'm also well aware of InGenius and have also developed some high CM impedance three op amp "active" approaches that do not use InGenius bootstrapping using "T-bias." I'm also not particularly obsessed with having equal input impedance from each leg to ground though I note in references 1 and 2 there are people who do, most notably Birt in his work with the BBC. What I do concern myself with is predictability in the field when unbalanced connections are fed into balanced inputs under what are often non-ideal conditions with no time for troubleshooting or optimization. I have tubes of THAT1200s, 1203s and 1206s InGenius ICs in my inventory and I am quite familiar with them. I've used them to make some excellent AC-coupled line inputs where the high CM impedance reduces capacitor mis-match effects on LF CMR. (The T-bias input also permits this.)
InGenius, and other inputs with high CM impedance are the most "transformer-like" input and with non-zero and unequal source impedances will provide greater CM rejection than those with low CM impedance. I think we agree on that.
There are a number of issues with high CM impedance inputs when they are improperly connected to unbalanced sources. A floating port connection to a transformer input will yield very little (or no) output because there is no primary current flow. An InGenius input with an open port will also produce no (or a highly attenuated) output. OK, we know this, but in the field I might have to grab another adapter to pull this off quickly.
(Comments are limited to 2000 characters so this reponse will be in two or more parts.)
Bootstrapping an active input to achieve high CM impedance and make it transformer-like has limits due to finite power supply voltages. The CM channel can only bootstrap the input bias resistors as long has it has the headroom to do so. An Ingenius input with the (-) input grounded and the (+) input driven has one-half the headroom compared to an InGenius input driven differentially. This is because in single-ended situations Vcm is equal to 1/2 Vdiff. Though the THAT1206 can accept (with 30V supplies) inputs up to +27 dBu differentially, the limit for single ended inputs is around +21 dBu because the Vcm bootstrap channel starts to clip. The problem increases with lower supply voltages.
Another issue with high CM impedance active inputs occur when single-ended equipment having a "two wire" ungrounded AC power cord is connected to a high CM impedance input that develops leakage current into the high CM impedance. As you well know, potential differences (in 120V countries) can often produce chassis potentials that are 60V (RMS) above ground. In that situation - and I've received support calls about it - the CM channel of the InGenius input clips. The solution in that case - to prevent CM channel overload - is to bond the two equipment grounds. It's not the best solution but we often have to take what we can get to stop hum.
One aspect that has not been explored - and you could provide some insight into this - is the effect on unequal port impedances to ground when they are connected to CT transformer outputs or sources whose op amp outputs may be heavily loaded internally. As the need for lower circuit impedances arise in balanced inputs to reduce Johnson noise in the era of 24 bit conversion, what is effect on unequal transformer loading on second harmonic distortion? With inputs built using 10K resistors I doubt this is a problem. But what about inputs made with 2K resistors? I've never explored it but a heavily-loaded balanced source (CT transformer or ground-referred "push-pull" op amp output) having 2-3 times the load on one leg might give rise to second-order distortion. Cohen in his 1984 mic preamp article used 300 Ohm feedback resistors around an NE5532. For a 5532 this is heavily loaded. His output stage (cross-coupled differential) used 2K resistors. Did Cohen's use of a cross-coupled output reduce THD-2? It's a question worth asking.
If I may be blunt Bill there's no substitute for one of your excellent transformers. Having said that for a low to medium CM impedance active input the double-balanced approach simply uses less "stuff" and works better than the Birt topology or other two op amp approaches.
I went back and looked at some old data and the following statement isn't exactly correct regarding the entire 1200-series:
"An InGenius input with the (-) input grounded and the (+) input driven has one-half the headroom compared to an InGenius input driven differentially. This is because in single-ended situations Vcm is equal to 1/2 Vdiff. Though the THAT1206 can accept (with 30V supplies) inputs up to +27 dBu differentially, the limit for single ended inputs is around +21 dBu because the Vcm bootstrap channel starts to clip. The problem increases with lower supply voltages."
In a THAT1200 Vcm = 0.5*Vdiff with unbalanced inputs. Under the same unbalanced conditions a THAT 1203 and 1206 have Vcm = 0.354*Vdiff. The maximum output for a THAT1206 is +24.5, not +27 with 30V supplies which is 4 dB less than the 1246/1286 or cross-coupled circuit. At reduced supply voltages (think USB-powered products) unbalanced inputs produce significant Vcm bootstrap voltages and put the InGenius topology at a disadvantage with less headroom.
What is more important however, regardless of supply voltage, is the issue of AC leakage currents that can develop across the high common mode input impedance which saturate the CM bootstrap channel. So what high CM impedance giveth in terms of reduced source loading with unbalanced source impedances sometimes gets taken away due to AC leakage current. This is particularly true with consumer equipment having RCA outputs and two wire power cords.
I have been wondering if there was a "just as good" substitute for a good input transformer, now I understand thatbthe answer is "not really, for all conditions, but yest for some", which is probably useful.
For certain, though, this is one 9of the best and most educational postings that I have come across. Thanks for that!
I have seen some very small input transformersthat made me wonder if they are really intended for auidio. Little tiny toroid transformers, not more than a quarter inch in diameter. Are those the current high quality audio imput transformers?
In addition to Mr. Whitlock I wanted to thank the readers who have posted here. WKetel wrote: "I have seen some very small input transformers that made me wonder if they are really intended for audio. Little tiny toroid transformers, not more than a quarter inch in diameter. Are those the current high quality audio input transformers?"
Probably not. For examples of transformers intended for professional audio I'd visit Mr. Whitlock's site, Jensen transformers, or any of the manufacturers listed here:
The one outstanding attribute transformers provide that has not been successfully duplicated is galvanic (metallic) isolation. Though Wurcer and Kitchin wrote in their 1982 Design Idea (ref 7) that the cross-coupled circuit was an "electronic transformer" this wasn't exactly the case.
(The prior post is continued due to character limitations.)
I had previously discussed the low supply voltage performance of InGenius vs. the simple or cross-coupled differential stages and wrote: "The maximum output for a THAT1206 is +24.5, not +27 with 30V supplies which is 4 dB less than the 1246/1286 or cross-coupled circuit. At reduced supply voltages (think USB-powered products) unbalanced inputs produce significant Vcm bootstrap voltages and put the InGenius topology at a disadvantage with less headroom."
To begin with I meant to use the word "input," not output, for maximum levels.
I had a chance to measure the maximum input levels for both a THAT1206 InGenius and THAT1246 with +/-5V supplies. These low supply voltages (low for pro-audio applications) are typically available in USB-powered devices where the +5V supply is inverted using a charge pump or simple switcher to provide -5V.
The THAT1206 operating on +/-5V allows a maximum unbalanced input level of +10 dBu before clipping. The THAT1246, in the circuit of figure 1 or cross-coupled topology (figure 4) allows up to +16 dBu unbalanced input (twice as much) before clipping. The circuit of figure 3, not tested, could also operate at these low voltages if a different dual op-amp were used.
I don't see the real-world need for concern about "headroom" for the InGenius (or any balanced input for that matter) with an unbalanced source. The standard signal reference level for unbalanced (i.e., consumer) sources is 316 mV rms (that's -10 dBV). The +10 dBu (that's 2.45 V rms) maximum unbalanced input level accepted by a THAT 1206 on bipolar 5V supplies now represents 18 dB of headroom ... which is generously adequate (most program material will remain undistorted through a channel with 12 to 14 dB of headroom). Further, operating practically any input stage on such low supply rails would make it incapable of dealing with normal pro levels, whether applied symmetrically or not. This is a bit of a "red herring" issue, since unbalanced outputs (even in so-called "semi-pro" gear) very rarely operated at "pro" levels.