DIY balanced interconnects

I'm a little curious. Let's go with Ralph's theory for a bit. If I own an MP-3 preamp and M-60 amps, then logically, the cable between these should not be an issue with regard to cable artifacts since the MP-3 conforms to the 600 ohm standard. However, let's assume I put my DAC into the equation which in balanced mode via it's XLR outputs has an output impedance of 100 ohms. In this case cable artifacts could be an issue running from the DAC to MP-3. Do I have this correct?

From my perspective I thought the key aspect of a balanced design was that it is differential with the circuit balanced throughout. BAT and Rowland meet this criteria, but not the 600 ohm standard. If I read Kirkus' post correctly, the 600 ohm standard should not be relevant.

I'm more confused than ever.

Clio09, there are two common meanings of the term "balanced" in high-end audio these days.

First, there's "balanced" as it applies to interconnects, where the idea is that there are two signal-carrying conductors, each with equal impedance (though not necessarily voltage) to ground, the signal being defined as the voltage between the two conductors. Both the driving source and the receiving equipment are responsible for maintaining this impedance balance, and the receiving stage is responsible for separating the signal voltage that appears between the two conductors (the "differential mode" voltage) from any noise voltage that happens to develop equally between both conductors and ground (the "common mode" voltage). The performance of the receiving equipment in performing this task is usually expressed as "common-mode rejection ratio", which is the difference in sensitivity between the same voltage, applied common-mode vs. differential-mode.

"Balanced" or "differential" as it applies to circuitry inside the equipment usually refers to the fact that there are actually two equal (voltage and/or impedance) and opposite-polarity signal paths inside. It is possible to have an unbalanced input feeding a differential circuit, or vice-versa . . . and ditto on the output side.

It does seem that a huge percentage of high-end audio manufacturers are unaware of the distinction between these points, as it's common to build a "balanced" preamplifier by simply building two non-differntial circuits, and connecting one each to pins 2 and 3 of the input and output XLRs. Equipment designed this basically takes all the incoming noise, sometimes amplifying it, adding some noise of its own, and "passes the buck" to the next piece of equipment in hopes that it may have some common-mode rejection capability. Frequently, that next piece of equipment ends up being the speaker.

From past threads, I think that Atmasphere and I are both in agreement about the need for equipment to have good common-mode rejection. We're also in agreement about the need for balanced line output stages to have a low output impedance, and excellent performance into low-impedance loads. Where we differ is in the specifics of how to design a balanced input stage.

My main problem with the 600 ohm terminating resistor is that it places a very high current demands on the preceeding electronics, which in all likelihood will have degraded performance into a 600 ohm load. It is relatively ineffective at reducing the effects of cable reactance - this is determined mainly by the source impedance.

The 600 ohm resistor may show a slight improvement on the common-mode rejection ratio, but the same or better results can be obtained by raising the common-mode impedance instead of lowering the differential-mode impedance . . . without affecting the performance of the preceeding equipment. And the only argument left is that of transmission-line effects . . . which is irrelevant for typical (<100 feet) lengths in a voltage-transfer system.

Tvad, what you are missing is the output impedance is not the spec. For example, the Modright might have a 100 ohm output impedance- if the output winding is designed to drive 600 ohms then it would work fine. Ditto Roland and Wadia. The output impedance is something very different from what load the circuit will drive.

Its a good idea for an electronic circuit to have about 1/10th the output impedance vs the load it has to drive, however there are some exceptions where the output impedance can appear to be much higher, yet it will drive the load just fine. For example, my Neumann microphones are set up to drive 150 ohms. What this means is that if you don't load them at 150 ohms (if instead you have a load of 1000 ohms or higher), the output transformer will express the inter-winding capacitance rather than the turns ratio, and you will get coloration and no bass.

If we take the example of the Modwright, a similar situation exists- its measured output impedance is one thing, the load it drives (and is optimized for) is another. I suspect it has that load built-in, much like the old Ampex tape machines did, so that their output transformers would be properly loaded.

The Cary has sufficiently low enough output impedance to drive 600 ohms, but if it employs a coupling cap, its likely that you will get a low frequency roll-off if you try to do it. IOW the only tube units that will drive 600 ohms properly will have:
1) a low output impedance (well withing the range of several of the units already mentioned) and
2) will either employ
a) an output transformer, or
b) a very large coupling cap, or
c) be direct-coupled.

It turns out a large coupling cap is impractical, so you can see how the realities of trying to do this limits the field.

In the world of transistors, its quite easy to get semiconductors to drive 600 ohms, so there should be lots of the examples there.