|Passives, equations and a state of benign confusion
On paper, the passive preamp makes a lot of sense. In actuality, however, the concept has more or less come and gone. Simply consider present availability and perception. Sparse and negative. McCormack's passive went active. So did Emmanuel Go's FirstSound unit. In fact, that one went valve. With passives, there are impedance changes with volume control settings. Those can cause audible frequency domain aberrations by interacting with the connected cables. There's insufficient drive over long interconnects. There's a lack of additive body, dynamics and drive. Alas, that last sentence is a matter of perspective. Detractors will accuse passives of being subtractive. They believe that they remove body, dynamics and drive which actives "leave alone". The counterpoint argument is naturally that actives add over and above what's on the recording and passives are simply neutral. But let's not go there. None of us except mastering engineers know what's on the recording. Ever. Nor would we know neutral if it bit us on the arse.
A few years ago, some Brits doing business under the Stevens & Billington flag generated word-of-mouth in the underground DIY scene. Shortly thereafter, Bent Audio issued a production passive that incorporated the British transformer attenuators. Voilà, the birth of one of the early commercial TVCs. That's short-hand for transformer volume control. A variation on the theme is the AVC or single-coil tapped autoformer volume control. That kind does away with the air-gap in the core of an attenuation transformer with discrete secondaries but is very similar otherwise. In either case, transformer/coil taps become the click stops on the volume control to determine how many dBs of attenuation separate one step from the next. TVCs can offer 6dB of step-up gain but are otherwise pure attenuation devices. They operate at or below unity gain of whatever output voltage the connected source provides.
The rationale for TVCs is that traditional attenuation in active preamps is a matter of loss. The preamp generates gain that's completely redundant in the context of high-output digital sources, hi-gain amps and reasonably efficient speakers. For normal listening, most that gain needs to be strangulated. The way that's done is to throw it away as heat via resistive action. After all, that's what volume pots are - resistor-based devices. The pro-TVC thinking considers this counterproductive and champions current/voltage conversion. Why pay for gain only to throw it away? And what else do you throw away while you're throwing out gain? Resolution perhaps?
That's a very good question indeed. Reviewers are whores. I'm a reviewer. Hence I've enjoyed plenty of sack time now with the Music First Passive Magnetic and superior examples of the active breed. Think ModWright SWL 9.0SE or the mighty Melody SWH 1688 II for example. The actives sound a tad fuzzy, blurry and indistinct by comparison. They seemingly lack articulation and definition. It's the old case of "you won't know it until you've heard it." But once you have heard it, you can't fail to conclude that these actives are simply less resolved than this particular TVC in its modest half-width box. That box doesn't look like much, doesn't weigh much and altogether inspires little confidence in Trophy HiFi eyes.
I'm still undecided on the additive/subtractive equation. I'm leaning to thinking that active preamps in fact are additive. It only makes sense. Since their gain is often entirely superfluous, why design actives if they don't contribute something else that's welcome and not about to be thrown overboard? In my mind, the best actives become dynamic expanders, tonal deepeners and spatial explosives.
|Here's my dilemma. If that's so, then how come this bloody passive sounds better? It's clearly more resolved. More resolution doesn't just mean more detail. More detail can clearly become enhanced spatiality since that depends on ambient micro-detail recovery. More micro detail, more ambience. That recovery is also linked to S/N ratios. Less noise, more minuscule signal. By definition, a passive circuit that's not connected to the AC line should be quieter than an active (unless the active is DC via batteries). Gain itself is noisy since it amplifies not just signal but whatever noise components ride on it. This is true even if you attenuate the amplified signal thereafter. The noise has already gotten into it. Passives without gain are dead quiet on two counts. They don't suffer operational self noise and they don't amplify upstream noise.
Tone is related to THD. Especially tube preamps alter the THD spectrum for arguably more satisfactory results. As Roger Modjeski points out, however, THD may sound alluring on very simple material but once the going gets complex, the THD that rides on every single tone that's occurring simultaneously adds and intermodulates in ways that undermine clarity and realism. THD then becomes IMD - intermodulation distortion. Not good.
All this by way of circumambulating an issue I've been wrestling with: how to understand what, exactly, the Passive Magnetic is doing -- or not doing -- that makes it sound "better" than highly regarded actives even on solid-state amplifiers like the groundbreaking First Watt F3. At the bottom of that issue lies what we mean by resolution. It's popular to refer to resolution or resolving power as magnification of fine detail. That focus can quickly degrade into detail über alles. It sets up the opposing pole of musicality. Intrinsic in that world view is a proposed conflict. The conflict is between resolution and musicality. If resolution and detail are assumed to be somewhat synonymous, think detail vs. musicality. It's as though beyond a certain point, more detail meant less musicality. It's as though this axis was a see-saw that had to be balanced or degrade into an either/or of detail or musicality but not both (at least not to a high degree for either).
What is that really saying? It's saying that you could have too much detail. Where the hell is that detail coming from? Does a component invent it out of thin air? Interpolation by whim and fancy? Surely not. Detail is on the source software plain and simple. Hence, there can't ever be too much detail. Why? By definition, there can't be more detail than what's recorded. Surely, you'd want to hear all of that. However, all of us have heard systems that sounded as though they had too much detail. What's that mean? We just reasoned that it couldn't possibly have been too much detail. What was it then that masqueraded itself as too much detail or, more succinctly, as a lack of something else?
Where this sweet pill of a passive is concerned, the "more detail = less musicality" equation simply ain't true. That's why it's a vexing issue. More detail clearly means more musicality (if by musicality we mean an open door of intelligibility and access that allows the listener to walk through and meet the music). That means that more detail does not equate to less musicality. So what do we mean by detail? Does it include dynamics? Does it include speed? Does it include transient precision and decay visibility? Does it include coherence?
Do you see where I'm headed? Audiophile review lingo uses words that, upon closer inspection, create confusion and conflicting messages. These terms seem neatly divided and catalogued only when we take them at face value. Only when we don't think deeper about what they mean and how they relate and connect to one another do they make sense. Get down deeper and gaps appear.
A more holistic and synthesizing/synergizing approach is needed. Supersede the analytical dissective method so I can understand why I currently prefer this S&B TVC to very good actives. Understanding is very often conceptual. In order to see, grasp, absorb, process and identify, we need a bigger perspective. Opposites must become complementaries whose polar presences become each other's ground of being.
I'm still wrestling with this subject. What does the passive do -- or not do -- vis-à-vis what the actives do or not do that makes it preferable to my listening biases? Not sure yet. What I can already say? This small box by Music First Audio makes life for the full-bore actives really tough. Their complexity of output transformers, tubes, power supplies etc. needs to justify itself. Thus far, that justification hasn't presented itself in ways that seem truly sensible and compelling. Mind you, I'm not saying that a passive is the silver bullet for all seasons and occasions. I distinctly prefer my AudioSector Patek chip amp with a good tube-based active pre. Conversely and so far, there hasn't been a single tube amp that doesn't sound significantly more resolved and simply better when the passive precedes it.
|Then the First Watt F3 appeared. Rather than signing on the dotted line of "tube pre + transistor amp = bliss", it favored the passive preamp in my book. That's when my evolving theory on these matters got set back to zero. I'm nearly starting to hate the passive for making a mess out of my concepts, beliefs and pet theories. Hey, just kidding. That's a good thing, ain't it? Learning only happens when you know that you don't know. For now I only know that this component is the hammer for a guy like me who likes high-sensitivity speakers and tube amps to drive 'em with. Gain is always an issue by being too much and too noisy and controlled THD is already provided by the tube amps. What technical reason is responsible for|
|having me prefer the passive on the JFET F3 is currently still beyond me. But I'm working on it and the new Hyperion Sound remote-controlled tube preamp is part of that picture to hopefully contribute further data and understanding...|