Positive Feedback ISSUE 65
january/february 2013



The following submissions are for the 'Readers Who Want to be Writers' Contest. The authors are not Staff members of Positive Feedback.


The Iron Theory – Stereo System Inductive Devices and Impact on Listening Preference
by Jeff LaFortune

Building an Audio System to Meet your Personal Preference – The Iron Theory (IT) Approach

This paper describes a new approach to building an audio system based on your holistic, emotionally-based listening preferences. This theory will put you on the right path to a more satisfying musical experience, whether you are a pure music lover, golden ear, or someone just entering the hobby. This theory can be used to simply replace a single component in an established system or assembling an entire system from scratch. This theory can be combined with and is complementary to current thinking, observations and experience.

The theory's premise is based on a simple concept and a few subjective parameters. The theory focuses on the amount, quality, and implementation of "iron" in your audio system. "Iron" is characterized by any element, passive or active that relies on magnetics to perform its function. "Iron" influences the music signal through the following ways:

  • Production (transducers - phono cartridges and loudspeakers)

  • Manipulation (transformers - input, isolation, output, auto and crossover components)

  • Transmission (speaker cables and interconnects)

  • Influence (indirect - power supplies, conditioners, power cords, etc.)

  • Interaction (iron is considered within the context of the entire system)

This paper reveals subtle but significant ways iron impacts what you hear, and more importantly, how to apply this new understanding to better satisfy your individual listening preference. From a macro perspective, iron modifies overall tone, naturalness, and sonic underpinnings that help delivers music that "stirs the heart" to you. If this feeling does not stir within you when listening to your system, it won't deliver all the satisfaction that is possible.

Iron signatures are unique and distinct from other sonic characteristics. For sake of argument and simplicity, IT lumps all direct and prominent sonic features into a broad class called "surface texture." Passages below were pulled from Positive Feedback equipment archives and offer three examples of how surface texture affects the sonic picture and how it is generally communicated:

 "…had a good top end but a muddy, inconsistent bottom end. The soundstage was nothing to write home about either, although vocals were quite good if you could overlook the brilliant highs and not-so brilliant lows..." - Loudspeakers

"…has some subtle but definite classic tube characteristics, including a bit of fullness in the upper bass to lower midrange and a bit of added sparkle in the upper midrange to lower treble region." - Preamplifier

"…in its stead is a far more neutral and accurate sound (in the positive sense), particularly from the upper bass through the upper frequencies." – Turntable

The one thing all three passages above have in common is that they all report on "the what." For instance, the loudspeaker has "brilliant highs and not so brilliant lows." In other words, the loudspeaker is overly bright and has sloppy bass. Readers immediately get that. But, they also would like have an idea on how the reviewer "felt" about the component too. Although we all know preference (sometimes called bias) is subjective and personal, the review will always be more complete with it included. The two passages below include an aspect of preference:

 "They are easy to listen to, without losing the bite of triangle strokes or the magic shimmer of cymbal overtones." - Interconnects

"If you enjoy short bursts of moving-coil type thrills and chills, you'll be better served by another cartridge."

The first comment mixes "the how" (easy) with "the what" (shimmer). 'Easy' connotes comfortable or stress-free listening and is a term which expresses feelings toward a component. Describing how a reviewer feels about a component can convey more about a component in one word than a dissertation of the surface textures alone. Obviously the reviewer has had some prior experience with easy cables in the past that did not convey details like these cables did. Unfortunately, we don't know if the cables provided that detail on their own (intrinsic) or as a result of complementary interactions with the rest of the system. This question usually is not addressed and leaves the reader at a disadvantage in making upgrade decisions since partial information is only available. IT fills in these missing gaps and allows for a priori predictions on component compatibility in your system... based on your preference.

In the cartridge review above, the writer is differentiating between moving coil and moving magnet cartridges expressly using emotional metaphors. Reading further into it, the reviewer is implying that if you want a moving coil signature, (this) moving magnet cartridge will not deliver it. As you will discover in this paper, IT clearly reveals "the why" behind this truism and expressively states that any moving coil cartridge will never sound like a moving magnet cartridge… in any system it is used. IT will show how your choice of cartridge plays a much greater role in delivering personal satisfaction than previously realized.

To reiterate, most professional reviews focus on surface textures when describing, contrasting, and comparing components. This is what readers generally use to help inform auditioning and purchasing decisions. Surface textures are reported because they are intrinsic to the specific component, fairly prominent, and can be described in relatively concrete terms. Excluding or minimizing emotional aspects is completely understandable as it may reveal a bias' the reviewer does not want known. More importantly, the reviewer cannot predict how the component will perform in your system, nor do they have insight into your individual preference needs.

Roger Skoff just touched on this in his recent Positive Feedback article, "Two Things You Can Rely On." In it, Roger states, "There ARE two things that you can rely on absolutely to help you in making your equipment purchase decisions: Your ears! Using them, you are just exactly as much of an expert as anyone else. Even more of one, in fact, because you, more than anyone else, know exactly what you like; how you think music ought to sound; and how the product or system you are listening to compares to everything else that you have ever heard." Roger also shows through a quick calculation that trial an error is a futile means to get what the listener is seeking, and this the challenge that this theory attempts to tackle.

Iron Theory Explained

There are three important aspects to consider:

1.) An honest understanding of your holistic personal listening preference and the ability to articulate this preference to yourself (and to others).

2.) An awareness and working knowledge of "iron" in your system

3.) Application of IT principles and anticipated results.

"Iron" is defined as any element that interacts (directly or indirectly) with electrical signals via magnetic means. Typical iron containing elements include output and step up transformers, cartridge motors, cross-over chokes, and power supplies. We could even regard some Class D (switching) amplifiers as having iron. For these amplifiers, there is usually a series inductor at the speaker output to reduce switching artifacts, reclaim the analog signal, and provide circuit stability. To show the importance of iron even in a Class D amp, take a look where the ink has been spilled. It has not been the active modules per se, but the impact of the output inductor and interaction with the loudspeaker!

The induced iron signature discussed here is related to non-linear distortion common to nearly all inductive devices—hysteresis and specifically, "remanance." Rigorously defined, remanance is the net magnetic field remaining within any material after the magnetic field strength has changed. Since the magnetizing field is generated by changing electrical current, the resultant signal is a combination of the original musical signal, and any distortive induction artifacts. This resultant signal is a dynamic interplay of both itself and magnetics that are too complicated to analyze as constituent parts. To keep this aspect of IT simple and actionable, the listener will only require an appreciation of iron in the music system and application of a couple guidelines. You do not have to become an electrical engineer to apply the tenets of IT to satisfy your preference goals.

In some cases it is possible to replace the inductive device with a better one (loudspeaker cross-over networks are a hot area). For nearly all other areas, it is not recommended to "open the covers" and make physical changes. IT only needs an inventory of the inductive devices in your system and most decisions are based on this. In addition to purchasing individual components based on surface texture features alone, the informed listener will also determine if iron is to be added, removed, or replaced in the context of the entire system.

Just to be clear, inductive devices are not immune to surface texture artifacts. The degree of surface texture (as with all other components) is usually a conscience design choice and is directly related to quality of the iron. Let us consider an inexpensive tube amplifier with an output transformer having a large ultrasonic peak. Although in-band hearing response may not be negatively impacted, this peak will result in leading edge/transient sharpening as evidenced by spikey square wave shape in measurements. This ultrasonic peak will is manifested in the mind as an increase of apparent detail and transparency when in fact it is nothing more than a disfigured harmonic envelope pretending to be an improvement in detail and transparency.

Framing Your Personal Listening Preference

The IT concept is framed around a listener preference and what is valued in the reproduced presentation. This preference may be very general, but it sets the foundation around which the system can be assembled. General preference can be stated simply as, "I like my music romantic, with non-fatiguing, realistic vocals and a deep soundstage." At the other extreme, listeners may value detail, "I like transparency, clean transients, dynamics and one that is truthful to the recorded performance."

The awareness and strength of preference also differs; preferences may be known and easily articulated, or may be partly known and not well articulated. Your preference may also be in the camp of the "unknown-unknown." The unknown-unknown is not knowing your preference, nor being able to articulate it to yourself or to others – the root of most Audio Nervosa symptoms. You may have never taken the time to fully articulate your preference to yourself or anybody else. Verbalize your personal preference - it may be harder than you think.

If the listener does not know and/or cannot articulate their listening preference, the emotive subconscious will help nudge them in the right direction. This subconscious flow creates a strong undertow that listeners will naturally be swept into, but why not acknowledge these yearnings and use iron signature as a practical way to obtain this end state with less trial and error.

Basically, after a new component is inserted into a system, it will take the listener in one of two directions—either toward or away from fulfilling preference. Some say this is a simple yes or no answer. For some, this may be a simple yes / no answer. For others it might be a struggle. IT is silent on how simple this answer is, but is adamant that intellectual (surface texture) and emotional (iron signature) elements both be considered and included the final analysis. The best decisions are made when both the head and heart are engaged.

Iron Theory Explained

Inductive influence can easily be discerned after knowing what to listen for. In general, greater inductive signal manipulation (either directly or indirectly) is manifested at the listening seat as increased color (as opposed to "colored"), smoothness, and a natural portrayal of the recording. Sometimes this is described in the press as being engaging or romantic. A reduction in system iron shifts the general perspective in the opposing direction, towards the drier, analytic, more precisely accurate to the source. Notice that none of these are derogatory descriptions because personal preference will determine the iron signature is more or less satisfying to you. Iron signature is consonant with the human listening experience—where is your system today with regard to iron?

In taking an iron inventory, let's start with a dynamic two-way loudspeaker crossed over at three-kilohertz. If this speaker happens to employ a third-order low pass filter, the woofer circuit will contain two inductors in series with the musical signal. The total inductance will be about 1mH (milli-Henry). Likewise, a fourth-order filter will have the same total inductance, but connected in parallel with the musical signal. If we just look at the number of loudspeakers models available for public consumption and then factor in different drive units and all possible crossover topologies, we see why it is necessary to take the simpler, lumped approach for determining iron. In this example, both 3rd and 4th order crossovers have roughly one milli-Henry of inductance. Yes, the inductors are implemented differently, but total value used is roughly the same in either design. In this case we would inventory it as 'some iron' contained in the loudspeaker crossover. Iron content in the crossovers is fairly independent of speaker sensitivity, as similar designs (crossover points and slopes) have similar inductance values. The difference shows up in the drivers—more power capability means additional iron and a greater iron signature. From an IT perspective, examining the crossovers covers only one half of the iron equation.

The balance of the analysis needs to factor in the drivers themselves. A general rule is that there will be more iron in lower sensitivity drivers than the higher efficiency types. For example, let's compare the iron parameters of an eight-inch high-efficiency Dayton Audio PS220-8 full range speaker against a lower sensitivity Dayton Audio DC200-8 woofer. Both have primary resonance in the 45 to 50Hz range and are of similar Q. From an IT perspective, their inductive specs are significantly different comparing the two Dayton drivers; we see that the low efficiency woofer has over three-times the voice coil inductance of the full-ranger and a seventy-percent more powerful magnetic field. Considering the example of a 4th order crossover at three-kilohertz, the woofer contributes two and one-half times more iron than the crossover. Add a couple of large woofers, midranges, and a tweeter or two and you can see how the iron signature can be impacted.

All things equal, a low sensitivity, high order multi-way speaker system will have a decidedly different and more pronounced iron signature than a full-ranger operated without any crossover. Here is a typical situation; a listener decides to replace his high-efficiency speakers with a low sensitivity multi-way design. From theory, this replacement speaker boasts much higher iron content and a more substantial iron signature. If the system was tipping into the romantic and colorful already, these speakers will intensify the iron signature which may or may not be what the listener wanted. The listener may ask, "Do these new speakers satisfy my personal preference for better or worse?" If the answer is yes, all is well. If it is no, perhaps the listener may want to consider reducing iron elsewhere in the system to restore the iron signature balance. The next section will tell you how to do this.

Now let us consider the front end of the stereo system with the source being vinyl. To convert mechanical energy into electrical energy, a phono cartridge employs induction. In comparison to other components in the system, the cartridge delivers a lot of it.

Examining a few popular cartridges, we see a wide range of values depending on the generator type used. Moving Magnet (MM) cartridges are known for their relatively "high" inductance specifications. To get some sense of proportion, let us compare a couple cartridge specs to the loudspeaker crossover values mentioned above.

Two very popular MM cartridges, the Audio-Technica 440MLa and Ortofon 2M Black have inductances of 490mH and 630mH, respectively. These values are up to five-hundred times the value found in a typical loudspeaker crossover. For Moving Iron (MI) cartridges, inductance values are much lower. The Grado Statement cartridge has an internal inductance of 2mH, roughly the same as a typical crossover. Moving Coil (MC) cartridge inductances are much lower - typically in the micro-Henry (µH) range for the low-output variety.

One would think that adding "quality iron" at the cartridge position would be more beneficial (and also more impactful) than inductor replacement in the speaker crossover. Why? Here we are dealing with a transducer that creates the entire music signal, not a downstream element that may manipulate only a portion of it.

Have you ever wondered why "straight in" MC cartridges are typically favored for their delicate, airy, more ethereal (some say analytic and precise) presentation? Conversely, do you ever wonder why MM cartridges are generally touted for their overall power response, gravitas and weight? More importantly, how can a listener increase the iron signature of an MC cartridge and bring along the positive aspects of the higher inductance MM signature? Based on IT, this can be simply achieved by inserting a Step Up Transformer (SUT) between the MC cartridge and phono amplifier. The SUT not only introduces the required gain, but also introduces a strong iron signature that may suit the listener's preference goals better than a direct connection to the phono preamp.

To put this in perspective, step-ups typically have a primary and secondary inductance of approximately 1 and 200H, respectively which is "whole unit Henry" inductance, orders of magnitude higher than anything discussed above. Mutual inductance between the coils is in the range of 10 to 20H—again, very high even compared to high inductance MM cartridges or loudspeaker crossover elements. As such, a MC-SUT combination is an enormous source of iron and its signature. Returning to the low/high speaker sensitivity example above, if the listener thought the new low sensitivity speakers were a bit too "colorful," he could reduce system iron by replacing the MC/SUT combo with a high output MM cartridge of moderate inductance, thereby restoring the system back to the more enjoyable listening experience.

Since a SUT can be the primary contributor of iron in some systems, iron quality becomes very important (all music flows through the SUT). Better quality step-ups have lower mutual inductance, since coupling is more efficient between coils. Inductor quality keeps detail and transparency intact, keeps bass tight and defined without veiling the performance with surface texture.

Now we'll turn to a less obvious example that involves digital playback. Let's assume we purchased a bargain DAC that comes with a Switching Mode Power Supply (SMPS)—a generic wall wart. You have heard that by replacing the wall wart with an aftermarket linear power supply, the DAC will perform better. But why should it? Although the two power supplies provide essentially the same output per specification (for instance, 12VDC @ 0.5A). Between the two power supplies, there will be dynamical and temporal voltage and current variations directly attributable to the iron signature. Changes made to the power supply signal will always superimpose itself some way on the music you ultimately hear.

Compared to the wall wart the linear supply will probably have a higher rated power transformer as well as smoothing and isolation chokes for noise suppression. By replacing the SMPS wall wart with this new supply, you have just boosted the "iron" content of your system and altered how the DAC will interact with the rest of your system. It is expected that the DAC with the beefier supply will sound more fleshed out, natural and relaxed, exactly the improvements touted by the manufacturers. Although intrinsic surface texture of the DAC remains, it may be presented in a manner that suits your preference.

Wrapping up this section, it is intriguing that many audiophiles only associate iron with massive forty-pound toroid that manufactures often tout in advertisements. Unwittingly, some listeners take iron to the extreme. Some will make a power amplifier purchase decision solely based on the VA power rating without knowing what it really means! On the surface it sounds like good advice, but how can this decision be effectively made without first appreciating and understanding how it will behave within the system it is placed? IT shows that you cannot make decisions like this in vacuo and always expect an improvement. We need to recognize the impact of the new transformer in relation to other iron in the audio system - only then can a better, informed decision be made.

How Iron Signature is Manifested – Documented Examples

Before we begin, no assumptions or judgments are made regarding the inferiority or superiority of one type of component versus another. It is the interaction of that component in context with the rest of the system that is of importance. For instance, an "engaging or romantic" character does not automatically infer lower transparency, detail or clarity compared to "accurate or analytical" systems. Vice versa, the best "analytical" systems show great nuance and color without losing essentialities of speed, dynamics, and power.

No one should rely solely on equipment specifications for system purchasing decisions and here is good example why. Anyone can take their analytical Solid-State (SS) power amp and make it sound warmer. This is easily accomplished by inserting a power resistor in series with the speaker load. (Carver did this a while back with their SunFire amplifiers and called it "Current Source" output.) For those of us old enough to remember, both Hafler and Carver demonstrated something similar in the past as well. By tweaking and tuning their solid-state power amps, they could make them emulate the sound characteristics of tube amplifiers to the point that experienced ears could not discern between them. Don't be fooled between designer tuning and a true iron signature!

To see iron theory in action, let's go the May 2012 issue of Stereophile, where Art Dudley simply states, "Virtually every MC cartridge I've tried has sounded better—more dramatic, more impactful, more nuanced, more colorful—when loaded with a step-up transformer, compared with being used to drive an active phono preamp alone." He didn't explain the reason behind it, only that he likes it better. Art seemed not to care how the SUT does its magic and neither should he. Art knows from personal listening experience that quality "iron" in the phono signal path makes his system sound more pleasing. Applying IT principles, Art knows 1.) his personal preference, 2.) knows that adding a SUT satisfies his preference better, but 3.) misses the realization that this same concept can be applied across his entire system, not just the SUT. This example demonstrates how elements of iron unexpectedly make their way into equipment reviews and impressions of particular components.

Here's another example how IT can take some of the mystery out of what is heard and why. In the October 2012 of The Absolute Sound, there was an excellent review by Robert Harley on the Rowland Corus preamp and 725 monoblock amplifiers. He reports, "Their sound is not one that calls attention to itself as "spectacular" hi-fi, but rather one that subtly conveys musical expression." He then follows with, "The sound was simultaneously lush and highly resolved…" and, "Again, the combination of lushness and resolution is a powerful one; the lushness invites physical involvement by allowing your ears to more fully "open up," and the resolution invites intellectual involvement." Notice that he uses the term "lush" in his review several times. What would make these components sound so lush? Could it be the 3D CNC machined chassis from a solid billet of aluminum? Possibly yes, due to its inherent excellent EMI and RFI protection and rejection. How about the state-of-the-art switched-mode power supply? Like the chassis, this is a possibility too, but both do not seem to fit well with what was heard and described in the review. What could really be contributing to what Robert heard?

Let's look at signal path itself. Here, things start to get interesting. These are unusual designs that employ a number of isolations transformers to manipulate the signal. Each 725 employs an input isolation transformer. The Corus preamp doubles up by employing both input and output transformers. Include the companion Aeris DAC and another transformer is added. Configured as a complete system from DAC through power amp, the music will "interact" with four individual high iron content devices on its way to the speaker. Even more intriguing is that two transformer pairs are directly wired in series with each other using cables the listener provides. Obviously the massive implementation of iron and their enormous signatures resulted in a sound as reported. Much of the Rowlands' sound is anticipated based on IT principles: ample iron ↔ "lush" and quality iron ↔ "resolute." Kudos to Rowland if this was what they were trying to deliver because it appears they were very successful. A system with this strong of an iron signature will appeal to a subset of listeners and not so much to others, as we will illustrate later in this paper.

Putting it All into Context

To put these component descriptors into better context, let us continue with the last example above but look at it from a different perspective. A more common definition of lush is, "growing vigorously especially with luxuriant foliage." Using this imagery, let's portray a system's sound presentation as a huge deciduous forest being viewed from some distance away. As nature has it, the forest is a living and changing entity. During the spring, most trees are semi-bare, perhaps with small buds. As spring turns into summer, the trees become more active and start unfurling their leaves, but these leaves tend to be of a similar color, green. As autumn approaches, the green is transformed by an explosion of vibrant color hues and increased complexity. To many, autumn is considered the most beautiful time of year to appreciate the forest. Using this analogy, it seems Art Dudley is more of a late-summer listener, but others may prefer spring or early-summer better. There is no right or wrong—it is entirely personal preference at play here.

One may ask, is one season's nuance and color more artificial or unnatural than another? The answer is an emphatic no because each season is as natural as the next and is exactly the way nature intended it to be. If you happen to like the autumn sound, there is no reason why you shouldn't consider adding iron to your system. Likewise for summer or spring listeners, go ahead and remove or replace iron until you achieve satisfaction. For those that prefer the winter forest, excessive foliage is a hindrance. For these listeners, a dryer, less verdant presentation is more satisfying and entirely appropriate based on what the listener prefers.

Differences in listening preference should not be judged, as recently played out in the press. In the February 2013 issue of Stereophile, Jason Serinus in the "As We See It" column compared two speaker cables in a system and asked the listening group which cable they preferred. The group selected the lower priced, "inferior" cable over the one Jason thought should provide a more accurate and emotional experience. In the column, Jason states, "After the Mahler, I was dismayed to find some people preferring the lower-priced cable's brasher, less-refined presentation of the horns and strings, and an overall more limited palette of colors for this music." The last part is quite telling, "…overall more limited palette of colors."

Obviously, the group preferred a slightly less verdant sonic presentation than Jason. The group did not care that the expensive cable had, "more refined highs, tighter bass, and exceptional transparency." What they really preferred was the holistic totality of the musical performance delivered by the "inferior" cable. Simply put, the group preferred early-summer over Jason's autumn. What this experience teaches us is that your personal preference is the only one that matters, not some reviewer's. The fact that Jason was somewhat surprised shows he does not completely understand how people listen. Jason goes on, "Might it be the case that, because we often spend the bulk of a review discussing certain musical elements to the exclusion of others, we give short shrift to how the totality of the musical experience affects us, and have thus led our readers astray?" The answer is yes, but not deliberately.


The Iron Theory approach shows us how important it is to listen for ourselves. It provides a novel method to interpret and understand what we hear and gives guidance for upgrade decisions in ways not previously appreciated. By controlling subtle but significant iron signature contributions in shaping your system's sound, a more direct path to personal satisfaction can be achieved.

Capacitor Context

Over the past twenty-plus years, there have been recommendations to use top quality 'caps' throughout all your audio circuits. It seems anywhere a better 'cap' can be substituted, do it. Experience with capacitors has shown increased refinement and smoothness coupled with improved clarity and tone across the board. Over the past few years, a cottage industry has even been built catering to this crowd.

The ideal capacitor is a purely reactive device, containing zero resistive or power dissipative effects, but in the real world, nothing is so perfect. However, capacitors have the virtue of being closer to the idealized function than inductors and are relatively easy to substitute. Try replacing an EI power supply transformer with a toroidal type having similar tap-outs and voltage configurations—it is an involved process out of reach of the general audiophile. Compared to capacitors, inductors don't usually invite "plug and play" experimentation.

Generally, capacitor improvements walk systems up the quality ladder in a linear, predictive way. Inductors behave differently and cannot be "stacked up" with the expectation that each iron element added will provide an incremental improvement. Inductive interactions are more complex and in most cases more may not always be better—but that depends on you.