Concepts in Audio Signal Transmission Part IV image

Concepts in Audio Signal Transmission: Part IV

image credits: eatuppizza

Part IV: Balanced Signal Transmission

What are balanced inputs and outputs, how do they operate, and does it really matter for signal quality?  This post will review common practices used in audio circuits, and explain how Naiant products can help to bridge the gap between systems.

First, we’ll look at unbalanced connections.  Their main failing is they are prone to interference, and we live in an increasingly noisy world!  Consumer audio equipment is almost always unbalanced, and suffers from that frequently.  It’s humorous to see audiophiles—to pick on them for a minute—sometimes spending hundreds of dollars on fancy RCA cables, when just about any reasonable quality XLR cable in a balanced interconnection would be a huge upgrade!  But most home audio gear does not support balanced connections, so what can be done?  Although such connections are now becoming more common in recent home audio gear.

In the professional world, unbalanced connections have become rare, and thankfully so.  The last bastion of unbalanced professional gear is found in instruments!  Not much has changed in that area since I wrote a post on the topic back in 2009.  It’s hard to update when all of the most revered gear is vintage, I suppose.

This image from that earlier post illustrates the benefit of balanced connections, depicting a 20dB reduction in interference to the point of inaudibility:

Balanced vs. Unbalanced Guitar Noise Comparison

Balanced connections can have much more noise reduction than that—as much as 70dB CMRR, or common mode rejection ratio; a measure of the reduction in interference.

Let’s look at signal vs. interference in an unbalanced connection:

unbalanced interconnection

The cable shield is the only defense the unbalanced connection has against interference, and it’s often woefully inadequate.

Let’s compare that to a balanced connection.  Here there is not only the benefit of the cable shield, but the audio signal doesn’t have to be referenced to the shield; or in the event that it is, the shield’s interference signal is common to both signal leads.  Further, our balanced cable uses a twisted pair, which helps ensure that the interference in the signal leads is the same.  But wait, aren’t balanced cables immune from interference?  In fact, they are not, they probably are getting the same interference as the unbalanced cable.  The balanced circuit—at both output and input devices—enables that interference to be canceled!

Note that the twisted pair in our cable is of no benefit in an unbalanced circuit.  In some places I have read recommendations to use twisted pair cable in an unbalanced circuit.  Go right ahead, but don’t expect any miracles!

How do the input devices cancel interference?  They use a differential input, which is either a differential amplifier or a transformer.  The transformer only sees the difference in potential across its two primary leads, and reflects that potential across its secondary:

active balanced to transformer input

The differential amplifier performs the same trick, by using math.  Effectively, it subtracts the signal on its inverting input from the signal on its noninverting input.  Since the interference signal is the same at both nodes, it gets eliminated.

active balanced interconnection

Great, so that is easy, right?  Just always use balanced inputs and outputs, and everything will be fine!  Generally, that is true.  The complication arises when unbalanced and balanced circuits are mixed.

Unbalanced output to balanced input

Rane has authored an authoritative post on sound system interconnection.  The basic strategy they outline for connecting unbalanced outputs to balanced inputs is straightforward:  connect the unbalanced ground to the inverting input of the balanced receiver, and connect the cable shield at the balanced end only:

pseudo-balanced interconnection

This yields what I would call a pseudo-balanced circuit; it’s not fully balanced as the impedance to the balanced input reference is likely not the same.  Still, it can be useful to have the equipment chassis isolated, and there can be some rejection of interference.

A further improvement on that circuit is to match the impedance on the inverting input, which creates an impedance-balanced circuit (Rane calls this pseudo-balanced, to which I object as it is really actually truly balanced!).  This requires that we know the output impedance of the unbalanced output device; either by specification or measurement (which is accomplished by measuring the difference in output between an open circuit vs. a defined load).  Even though there is no audio signal on the impedance-balancing leg, the same interference signal appears on both legs, and thus gets canceled:

impedance balanced interconnection

If the impedances are closely matched, this becomes a fully balanced circuit.  There is a misconception that impedance-balancing is somehow not as effective as active balancing.  To the contrary, an active differential output with unmatched impedance (which one would hope doesn’t exist!) will have far lower CMRR than a well-matched impedance-balanced circuit.

Finally, an inline transformer can solve the unbalanced to balanced problem rather elegantly; when placed at the unbalanced output, it will allow the following interconnection to be fully balanced:

transformer balun interconnection

Balanced output to unbalanced input

Let’s now consider balanced outputs to unbalanced inputs.  Above we have listed three options for balanced outputs:

  • Impedance-balanced
  • Active- (or electrically-) balanced
  • Transformer-balanced

And two options for balanced inputs:

  • Active differential input
  • Transformer input

The good news for balanced connections is that all of the outputs will work into either type of input, without any concern to the audio technician.  The bad news is when connecting a balanced output to an unbalanced input, the type of output must be considered.  The issue is what to do with the inverted output—what we’ll call the “pin 3 problem” (as distinguished from the “pin 1 problem” discussed in the Rane note).  For an impedance-balanced output, pin 3 is typically the impedance-balancing leg, meaning that it carries no signal.  It will often just be a resistor (or resistor-capacitor network) to ground, and thus safely left unconnected.  It won’t even know it’s being ignored!

impedance balanced to unbalanced interconnection

 

Active-balanced outputs are one of two subtypes:  what I’ll call a standard active output, which has two (mostly) independent output amplifiers for each leg of the circuit.  They don’t really know what is happening to the other, so the main effect of leaving one of them unconnected is that the output signal level will drop 6dB.  That could even be helpful, since unbalanced inputs are frequently “consumer”-level, or -10dBV, which is 12dB below the professional +4dBu level.

active balanced to unbalanced interconnection

One twist on that approach occurs if pin 3 carries the signal, and the noninverted output (pin 2) is the impedance-balancing leg.  The standard output for the Naiant X-R system is wired this way, because the X-R capsule outputs are inverted.  Routing the signal to pin 3 enables the microphone to have positive polarity at the microphone preamplifier with respect to positive pressure at the diaphragm.  Where the X-R system needs to be used into an unbalanced input—for example where an external phantom power supply is used—the X-R amplifier has an active-balanced option, at the cost of twice the required phantom supply current, with both inverted (pin 3) and noninverted (pin 2) independent active outputs available.

With either active- or impedance-balanced output, there is usually no problem with leaving pin 3 unconnected (“floating”), as the circuit will perform in an unbalanced manner without termination.  However, terminating the extra pin to ground can have adverse consequences, as that can create a load on the device which in some circuits could cause distortion in the other leg of the output.   Many devices can tolerate that load without distortion, but the best practice is still to leave the unneeded pin floating.

The other subtype of active output is the not-as-often encountered “cross-coupled” output (see the appendix in the link), which is tolerant of grounding (or floating) either signal lead.  If you know your equipment has that output type (the manuals are likely to brag about it, since they have gone to the trouble!), then you have less to worry about.

Transformer outputs, on the other hand, require connection of both output pins.  When used into an unbalanced input, that means that pin 3 has to be terminated to ground:

This means that separate cable assemblies are potentially necessary for the different types of balanced outputs when connected to unbalanced inputs, which also means that the technician needs to catalog the equipment used for output types—information that is not always forthcoming in the manufacturer specifications!

An alternative to that process is to always use a transformer as a balun:

transformer balanced to unbalanced input interconnection

This approach has the advantage of working with any output type, as its input is balanced!  Also, the signal transfer will be balanced if the balun is located at the unbalanced input, at the end of a balanced cable.

Polarity

Polarity is another source of misunderstanding.  First, let’s call it polarity, and not phase, which is a different concept.  With reference to microphones, it should be simple:  does the microphone output a positive polarity signal (on pin 2, for an XLR output microphone) in response to positive acoustic pressure at the microphone’s diaphragm?  If so, we can call that a positive or noninverted polarity output.  As an alternative, for an XLR microphone, the microphone can output negative or inverted polarity on pin 3, to the same effect.  Either option for a balanced output XLR microphone offers the ability to invert polarity if required by the technician (for example, when used on opposite heads of a drum) simply by crossing pins 2 and 3 in the microphone cable, or in an inline barrel adaptor.

If the microphone is unbalanced output, or has an impedance-balanced output, polarity inversion is not possible when connected to an unbalanced input unless an inline transformer is used.

Naiant solutions for interconnection

Microphones

The Naiant design philosophy places small form factor, minimal power consumption, and low cost as important design specifications, of course in service of the primary goal of sound quality!  Thus, the default configuration for the X-R microphone system is an impedance-balanced output with pin 3 as the inverted signal carrier, which offers both low cost and the lowest possible current consumption while maintaining excellent CMRR.

However, if a noninverted output is required for connection to an unbalanced input, then active-balanced or transformer-balanced outputs are available as options.  Note that any of the three output options will perform well into a standard, balanced microphone input.

This table summarizes the characteristics of the X-R output options:

Output option

Specification

Impedance-balanced Active-balanced Transformer-balanced
Cost lowest low highest
Current draw 0.6mA 1.2mA 0.6mA
CMRR, typical
50dB 50dB 70dB
Frequency response 20Hz – 30kHz 20Hz – 30kHz 30Hz – 20kHz
Input to Output level 0dB +6dB 0dB
Unbalanced signal termination (after external phantom supply)
pin 2 floating or grounded,
pin 3 signal (inverted)
pin 2 signal,
pin 3 floating
pin 2 signal,
pin 3 grounded
Unbalanced signal polarity inversion (after external phantom supply)
not available pin 2 floating,
pin 3 signal
pin 2 grounded,
pin 3 signal

Note that the X-X lapel microphone has an active-balanced output, and the X-8S multipattern microphone has a hybrid active-/impedance-balanced output (depending on pattern selection) and also has a bidirectional-only transformer-output option.

Naiant inline devices

Naiant’s inline device range offers solutions for interconnection of balanced and unbalanced equipment:

The ITA inline transformer adaptor can be configured as a directional (balanced to unbalanced, or vice versa) or symmetrical (function determined by attached cables) balun, including ground isolation where desired, for a full range of interconnection requirements.

Custom cable assemblies can be made for any unbalanced to balanced format, including impedance-balancing networks inside the cable, as required.

The IFA and PFA inline amplifiers can provide low-impedance active-balanced outputs for any unbalanced input source, into any standard phantom-powered amplifier input.

The MPD attenuator can also be constructed as an attenuating impedance-balancing network for unbalanced input to balanced output.

Any of the inline device products can also provide polarity inversion for balanced input or output.

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