While building a Trimodal Amplifier, I decided that it must have balanced inputs as well as unbalanced. But as pointed out in Douglas Self's book
"Small Signal Audio Design" and the fifth edition of "Audio Power Amplifier Design Handbook", a standard electronically balanced 5532 circuit such as the one pictured below with 10k resistors will degrade the noise figure of the amplifier by 17.58dB.
This basic circuit block gets used in many devices from "pro" gear to DIY - anywhere a low-cost electronically balanced input is desired. We use a balanced input ostensibly to reduce noise in a system, but ironically the typical electronically balanced input is by its very nature tens of dB noisier than an unbalanced one!
Using a specialized device such as those offered by THAT, Burr-Brown etc. offers no great noise advantage either. THAT gives the noise output of the 1203 and 1243 balanced input chips of -105 dBu.
Of course there are solutions to this problem. One is to use an input transformer at a great increase in cost and weight (though not so much a factor when paired with a large power transformer in a power amplifier). An input transformer offers great common mode and RFI rejection, but at a cost of increased distortion even for high quality units. The Cinemag CMLI-15/15 datasheet gives a distortion measurement at 20 Hz, +4 dBu of 0.02% which will swamp the Trimodal's 0.001% distortion.
Mr. Self shows that we can reduce the noise of an electronically balanced input by lowering circuit resistances, adding buffers to drive this lower resistance and by paralleling devices to partially cancel the uncorrelated noise. This way the benefits of low distortion, weight and cost can be retained.
The full circuit described in the aforementioned books using 5532 dual op amps gets the noise down to -117dBu or only 2dB noisier than a typical unbalanced input. Douglas Self points out that the 5534A single op amp is even quieter than its dual counterpart and could be used to decrease the noise even further. But he dismisses this notion due to the board space needed for double the number of IC packages as well as the additional compensation capacitor needed for the single op amp per package version. He goes on to describe the use of other, much more expensive op amps to further decrease the noise.
Signal Transfer Company offers a PCB of the low noise balanced input card with slightly higher noise levels than the one described in the book using conventional through-hole components. According to the page linked, the board measures 62 by 76 mm.
Using surface mount components I have managed to fit the needed 12 5534A single op amp packages, the compensation capacitors plus all the other components required onto a PCB measuring around 63mm x 63mm - slightly smaller than the Signal Transfer version. As shown, they can accommodate both 0.2" and 0.1" terminal headers without modification.
Samuel Groner was kind enough to measure the completed board on an AP SYS-2722 and found that with a source impedance of 49.9 ohms within a 22 Hz-22 kHz bandwidth the noise output of the surface mount version is -118.5 dBu or only 0.5 dBu noisier than an unbalanced input. This without resorting to exotic and expensive premium op amps; 5534As are around $1 in quantities!
Unfortunately, the double sided PCB design and numerous vias make it impossible to etch the boards at home. On the upside, professionally made solder masked boards are very quick to build - I find I can knock one off in under a hour as there are no leads to bend and cut.
This is due in large part to the Johnson noise generated by the relatively high value resistors needed for a basic one op amp balanced input. These resistors set the input impedance of the balanced input circuit. The measured noise output of such a configuration is -104.8 dBu with the input terminated by 50 ohm resistors on each input to ground.
