The issue of electronics-challenged users connecting the output to both balanced and unbalanced inputs was always particularly vexing. The chain of inverters circuit emulates a grounded center-tapped transformer. When used with an unbalanced input by connecting the low-side output to ground, the chain of inverters circuit, injects current (usually distorted) into your ground system. If everything is free of pin 1 trouble, then this is probably OK. If the unbalancing happens at the load, then this ground current is flowing through your snake sys tem, which often causes trouble. This unbalancing act also severely taxes the circuitry driving the grounded output. Another problem this circuit has is a ‘free’ 6 dB gain. The cross-coupled output emulates a floating transformer winding. Unlike the transformer, this circuit demands that it be unbalanced at the source. Heaven help you if there is a pin 1 problem designed into the unit’s output connector (a “pin 1” problem results from pin 1 of input and/or output connectors being connected to signal ground rather than earth ground). Unbalancing at the load causes instability. The good news is that the gain is constant, balanced or unbalanced. All of this is exacerbated by most manufacturer’s omission of any discussion of output stage topology in their manuals. Of course, if they were considerate enough to include a schematic (not often), then anyone with a bit of circuit smarts can figure this out at a glance. Some manufacturers now include a discussion of balanced and unbalanced interface issues along with specific recommendations for their unit. I believe that they should also describe their unit’s input and output circuit topologies. As an equipment designer, I’ve agonized over this problem for some time now. One solution is to simply provide two connectors: one marked balanced, and one marked unbalanced. This works, and doesn’t really take a rocket scientist to figure out (good when the user refuses to read the manual). On the other hand, it requires two connectors along with more PCB space and panel space (both cost money, and panel space may be limited). I’ve also gone so far as to insist on using TRS connectors for the unbalanced outputs, wiring the ring contact to circuit ground. This way, connecting the gear using a TRS cable into a balanced input works just fine. The people at Mackie Designs have played with a scheme that they called “Impedance Balanced” for phone jack outputs. At first, I gasped and sputtered. But after thinking about the problem, I’ve come to embrace this as the best solution, especially when you don’t have dual output connectors. Simply stated, you use a TRS jack for the connector, and you connect the ring contact to ground through the same resistance as used for the build-out resistor on the tip contact. From the line’s standpoint, the circuit is balanced, driven from a grounded center-tap source. Yes, only half of the circuit is driven from an audio stand point. It doesn’t matter what sort of plug the user inserts, either way is optimum for that case. For the unbalanced case, no ground current flows.
From the balanced input’s standpoint, there are equal impedances from both sides of the line to ground. This results in the best common-mode rejection (CMR) performance, especially if close-tolerance resistors are used at the output side. The circuit noise is actually 3 dB less than either of the other two balanced line drivers. There is no level difference if used with a balanced or unbalanced input. Finally, you should note that both AKG and Neumann use this circuit for their transformerless microphones. If it’s good enough for them at microphone signal levels, I’m comfortable with it at line level. Rick Chinn is a consultant with Uneeda Audio in Redmond, WA. Formerly employed by Mackie De signs, he was instrumental in the design of the SR line of live mixing consoles. |
