@audiokinesis "So it is not obvious to me that series resistance reduces the voltage ratios that a driver sees, and therefore I do not see how it would reduce the dynamic contrast. The volume level being reduced is not the same thing as the dynamic contrast being reduced."
@erik_squires "Resistors are the most linear parts in the entire audio chain, except wire."
While I’m trying to keep my posts as brief as possible, I’m glad the topic of resistors came up. I also used to think like the points raised. As I built speakers, I spent an incredible amount of time on crossovers, listening and auditioning, tweaking values and configurations, then doing it all over again. Moving up in complexity did impact dynamics, to the point of losing the ideal match between amplifier and loudspeaker because of it. I also noticed the damage resistors did to the sound, and it left me perplexed. Even the best resistors I found sounded surprisingly and disappointingly bad (though not nearly as bad as capacitors) in comparison to nothing. Later, I began building amplifiers, and noticed how even more profound the effect of resistors really are.
Along the way, I was fortunate to work as a material scientist / R & D chemist during the time when electronic materials needed reformulation because of the environmental legislation (CE, ROHS, etc) requiring the elimination of materials such as mercury, lead, cadmium, etc. Among our many products, my company provided the resistor materials to CTS, Dale, General Motors, Ohmite, TRW, Vishay, and others that manufacture the resistors such as get used in HEA, although our audio world is like a bucket of water in the ocean. It proved the most exciting time in the industry since the 1960s when the original formulas were developed. I was incredibly lucky to be at the right place at the right time to do such work in going back and essentially getting to start from a clean sheet of paper, and to learn how such technologies impacted our hobby.
Knowing what’s actually inside quickly proves thinking those resistors coming anywhere close to the copper and silver wire we employ in our cabling and internal wire as simply and fundamentally wrong. It opened my eyes to why resistors sound so poor, with some sounding much worse than others.
One of the fundamental characteristics of resistors is temperature coefficient of resistance (TCR), which you can find in most resistor product / sales literature. TCR quantifies how much the value of the resistor changes in relation to temperature, down to -55C and up to 125C. As most of the world (especially, the military) considers it imperative a resistor not change value in relation to temperature, a neutral (0 PPM - parts per million) TCR is the goal of most companies producing resistors. And in fact, most modern (unlike the old carbon composition) resistors do exceedingly well with that, carrying a TCR of +/- 50PPM or even +/- 25PPM. Not getting too technical, except to say that means the resistance in relation to temperature remains quite stable. On the other hand, we know the resistance of metals typically behave quite the opposite of that, as resistance rises (substantially, in fact) with temperature, which we refer to as positive TCR. For example, the resistance increase of copper is 0.393% for every increase in degree Celcius, and vice versa. As I said, it’s substantial. That’s exactly what Duke described in terms of thermal compression of a driver, which definitely trumps crossover design and implementation - when it comes to dynamics and everything else. At some point, the resistance of the voice coil due to its temperature rise precludes it from behaving as it did previous to that in handling the signal passed to it.
How do we produce resistors with the impressively neutral TCR specifications I mentioned? The formulations, logically, include materials that have substantially negative TCR profiles to balance / shift things from positive toward the desired 0 TCR. What sort of materials possess negative TCRs? Semiconductors (materials that only conduct electricity under certain conditions) or non-conductors / insulators such as ceramics (bismuth trioxide most famously, but also compounds like titanium dioxide, barium titanate, boron triphosphate, calcium oxide, and on and on and on) and infinitely (literally) varied glass formulations. Although these materials may not conduct electricity themselves, they can exert a strong enough influence on the resistor as a whole to bring the TCR down to the desired level. Glass provides the fusing material for the resistor to attach (literally melts and fuses) to the substrate as it goes through the especially violent sintering process in a high (1100F - 1600F) temperature furnace, as well as the means of forming the conductive (quite far away from what "wire" looks like) matrix of the metallic conductive components, as well as often (and, hopefully) benefiting the TCR situation. In fact, as the old timers used to say, "It ALL comes down to the glass." The main driver for formulating new products was the elimination from lead and cadmium from the glass. Lead oxide, which as in other fields such as leaded crystal and decorative glass, yields very low temperature melting, wonderful performing glass, and provided the extremely high-performing resistors that came out of the 60s. The presumption of most around the world was the new "green" materials would never come close to the ones they were replacing due to the removal of lead and cadmium. In fact, most of our competitors green products looked downright scary. Through a two-pronged approach, but mostly from the grace of God, I was fortunate to actually end up with resistors that were superior to the old products, to the astonishment and disbelief of my boss who developed our products in the 60s. Our testing showed them to be the finest, highest performing low resistivity (0.01 Ohm/square - 10 Ohm/square) resistor material in the world.
Wirewound resistors fare no better, as instead of using the sort of wire materials we might expect in HEA, they consist of alloys of base metals such as nickel and chromium, again with negative TCR compounds as part of their proprietary formulations to produce the desired neutral TCR specification. I mostly avoid the religious wars on cable, fuses, etc., but my experience makes me to allow for the possibility of yielding positive results by employing much improved conductive materials. And instead of a 0.5" or 1" wire in a fuse, consider the very, very many feet that makes up a wirewound resistor.
As one can imagine, reducing the TCR of such heavily positive materials down to 0 or 25, 50, 100, or even 500PPM, typically requires substantial amounts of these negative TCR materials. To anyone who has chased audio cable with increasingly pure levels of copper or silver, say from 99.9% to 99.9999% through the reduction of compounds (what we call impurities) like oxygen in the quest for improved sonic performance, this admittedly must come as more than a shock. Conductive content as part of the overall (conductors, sintering aids, glass, additives, dopants) end product could even drop as low as 5 or 10%, though it’s typically far higher in order to produce a reliable conductive matrix. Products get sold as a series, normally each product representing a decade of resistance 10X higher or lower than the next in the lineup. More conductive products contain more of the conductive material. Less conductive (more resistive) products contain less, up to a point, that is. At some point, a lack of conductive material no longer supports a conductive matrix, and one moves up to a different series, featuring a higher resistance element as its conductor.
It may sound less pristine and "right" than we imagined, yet while still leaving out a lot of the details, that’s how resistors get made. Look at electron micrographs of the end result, and some will leave you to wonder how any HEA component using them could ever allow the purity and beauty of the music to pass through without looking like a car wreck.
Does anyone still think resistors are close to "wire"? And is it now even just a little easier to think the pinball trip through these resistors could possibly negatively impact dynamics, or by multiplying their number or complexity of the circuits that employ them could result in same?
