A Patchel of Passive Pre-Amplifiers by Herbert Reichert

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January 2002

A Patchel Of Passive Pre-Amplifiers
Review by Herb Reichert
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Why do we call metal boxes with knobs that are filled with little do-dads like audio-taper volume controls and input selector switches, "passive pre-amplifiers"? The adjective "passive" is a misnomer when it is intended to modify the noun "pre-amplifier" because in audio, nothing is truly passive. Same goes for the word "pre-amplifier". If the metal box has no gain stages why do we call it an "amplifier"? Where is the amplification? And "pre"? What is it going before? It goes after the source and before the amplifier. So isn't it really an, "in-between"? Why not be accurate and call these things "audio control units" or some such? I use one of these audio control units most of the time, but I am against them in principle. I believe every audio stage (including volume and selector controls) should be optimized and every filter element buffered and impedance matched. Why not just make integrated amplifiers? Or basic amplifiers with bypass capable selector and control functions?

One of the four audio control units I will discuss here is one made by yours truly in my own shop a long time ago. It selects from four different sources and provides an optional first order variable-frequency high pass filter function for use in simple bi-amplified systems. I designed it as a cost-no-object project that in hopes it would serve me well in a wide variety of systems over the long haul. This was done back when a dollar was almost still a dollar... and even then, the unit I made cost me more than $900 just for parts!

Even with all this money, my own unit still might roll off the highs a tiny bit. It might choke the music a bit. Therefore, be aware, passive pre-amplifiers are a flawed idea right from the start. It is nearly always better to use a properly designed active pre-amplifier -- something with a bit of gain and some proper engineering consideration for the impedance interfaces between the audio source components and basic amplifiers will (almost always) make your hi-fi system feel more jumpy tuneful and alive sounding.

The advent of the CD player (many of which have their own integral volume control) has created a consumer climate where the average audiophile now thinks that an active pre-amplifier may be unnecessary and most probably a sonic liability. This notion is a sort of mass marketing perversion of the "less-is-more" zeitgeist. Because... every single component in the audio chain, whether it is big or small active or passive, operates as a filter and, as such, has what is known in electronic engineering parlay, as a "time constant". In audio, every time two disparate things are touching each other either a low-pass, a high-pass or a band-pass filter is created. And every time this happens either the high and or low frequencies are diminished. (A filter alone is slightly unpredictable -- when a bunch of them start touching each other -- look out! Anything can happen.) The damn thing about filters is this: they never extend bandwidth, they only reduce it. They can (almost) never improve the sound of an audio system. A passive pre-amplifier is never better than say, no pre-amplifier at all.

The question a prospective buyer must ask him or her self is: Should I employ an active or passive pre-amplifier? When that question is answered with some assurance, then, questions like which and what type can be answered.

Q: Who should use a passive pre-amplifier?

A: People who can say for sure the following are true about their systems.

1. "I do not need phono equalization." If you have a turntable and you play black vinyl discs, then you need some sort of RIAA equalization (filtering) to restore your source signal to its original form. This usually will take the form of some sort of active "phono" stage with the proper RIAA time constants inserted between a pair of "buffer" input and output stages -- with or without gain. Usually this stage has anywhere from 24 - 78 dB of gain, depending on the needs of the associated MM or MC phono cartridge. And, very often this phono stage is associated with an active "line-level" stage which may have a gain of less than 0dB or it may have as much as 10 or even 20 dB of gain. (Remember, every 3dB of gain represents approximately a doubling of the music's apparent loudness!)

2. "All of my line-level sources (my tuner, my CD player, my cassette, etc.) have enough gain to drive my amplifier directly." This means every source should have an output of (at least) .75 volts. A 2 volt output is industry standard.

3. "The impedance of my sources is low enough and the load impedance of my basic amplifier is high enough that their direct connection will not seriously roll off my system's high frequencies or compromise its transient response." This means that every source component should have an output impedance of no more than 1000 ohms (less than 300 ohms is best) and that your amplifier should have an input impedance of at least 10,000 ohms. 20,000 ohms (or more) is better.

4. "I do not plan on using interconnect wires more than two-meters long." Longer lengths of wire connecting pre-amplifiers to amplifiers present greater capacitance and therefore create low-pass filters with lower turnover frequencies thereby rolling off more of the music's higher frequencies and reducing the illusions of luster, brightness or transparency as well as transient attack. In other words longer lengths of interconnect can cause the reproduced music to appear dull.

The perspective passive pre-amplifier user should always do two things: First consult manuals and/or manufacturers to determine if the above conditions are easily met. And second, you must try the passive preamp in your own system to make sure it does not screw up your music. These types of audio component interfaces are (in reality) quite complicated and typically possesses too many variables to be predictable -- therefore a home audition is essential.

For the above reasons, I will not go into great detail about the sonic wonders-detriments of the three passive pre/control units I auditioned. If an audio system likes a passive pre-amplifier music will sound more clean, open, free and alive than it did with the active pre-amplifier. If the passive setup is good it will always make words in songs more clear and easy to understand. If the inactive control unit is a good match, your system will play loud and clear and be fun to listen to.

If a passive pre-amplifier is wrong for your system, your system will sound dull and rolled off and slow. When passive pre-amplifiers do not work they kill all the fun and boogie factors. They roll of the high frequencies and make your hi-fi play less than loudly enough for satisfying listening, dancing and grooving. When the passive units do not work -- there is no hope for rocking out. Bad, bad, bad.

I inserted all three passive pre-amplifiers between my Audio Note, Roksan, and Flatfish CD players and my Kamuro 845 and 47 Lab amplifiers. None of the component combinations I created rolled off the highs or killed the music in any really noticeable way. All of the setups I invented worked and sounded fine. None were boring, dull or uninvolving. However, all of them were less fun than the 47 labs "Gaincard" amplifier alone. (The Gaincard can be used alone because it is an "integrated" amplifier with its own volume controls.)

What I am using as my sole criteria in evaluating these units is directness. The unit that makes me feel the closest to what (I imagine) is on the disc wins. The unit with the least fingerprint and the softest grip on the music is the one I like the best. The passive pre-amplifier that gives me the rawest, most unexpurgated, unedited version of the recording is the one that delivers the fun factor. Right? With that in mind, let us see how each of these units worked in my system.

Audio Synthesis, "Passion Ultimate"

Audio Synthesis, "Passion Ultimate" This unit is the most expensive and the most expensive looking. It is long, narrow and made from heavy, beautifully finished aluminum. It has no knobs. There is nothing you can adjust from the unit itself. All functions are controlled from the heavy aluminum remote control. So brothers and sisters -- do not loose that remote! And you better be better than me at programming your VCR because this unit requires some learning and instruction reading just to be able to turn up the volume.

The most obvious thing about this unit is: it has a line cord. The unit needs power to drive the volume, balance and selector functions as well as for the illuminated display which practically fills the modest faceplate.

The Passion Ultimate is the unit to buy if you are a deluxe, luxury-oriented person. This is the 1947 port and the 300-thread cotton of passive pre-amplifiers. It feels very expensive to use and it has a very light touch on the music. In audiophile terms, this is probably the best of the three units I examined. Fast and transparent are terms that come to mind when I think of the Passion Ultimate. The only real negative is that the Ultimate has the worst channel-to-channel and source-to-source separation I have ever encountered.

When switching from one source to another with the remote (which is exactly what the remote is intended to do) I would find myself listening to two sources at the same time. I could actually understand the lyrics to two different songs from two different recordings -- at once! The separation was so bad that the source I chose to play was only a few dB above the source that was "unselected". So, unless you have a remote for all your source components, you will have to get up and switch off your previous source before you can enjoy the new one.

The Passion Ultimate's instruction manual describes the unit thusly: "An all aluminum enclosed passive preamp with three line level inputs plus a hardwired direct input that bypasses the input selector and which may be used as a tape output feed for recording purposes." Also mentioned: "Inputs and outputs are made with PCB mounted WBT connectors to eliminate any internal wiring from the minimalist signal path..." Hummm? Am I missing something here?

"...minimalist signal path" -- with PCB traces? Please tell me, when, where or how a PCB trace can be an advantage in a signal path? Personally, I would take a few hundred feet of six-nine silver wire any day over even a few millimeters of lead and tin globs scattered about on the surface some fiberglass board. Sounds like the admen wrote this manual. Wouldn't you say?

Either way this is a very nice, clean-sounding luxury unit that should make some high-priced solid-state amp user very happy. Me? I "pre"fer either of the other two units. (It should also be noted that this unit has only one set of output jacks which means wiring up two separate outputs like a main amplifier and an active subwoofer will be impossible without using yet one more part in the signal path.)

DH Audio Labs PSS Model 500 Silver
(Made By David Hum)

Unlike the Passion Ultimate, this unit had no luxury feel at all. Zero. Nadia. It felt cheep and junky. But! It made the music sound very live and jumpy and fun to be with! This unit was so close to my own in the lightness of its being that the only time I remembered that it was not was when I went to change the volume.

On mine, I spent about $150 each on two 32-position, custom mono attenuators which in turn feed into a custom, impedance matched, stereo volume control which cost almost $400. This extravagant setup allows me to adjust balance with a fair amount of subtlety and repeatability while still offering the convenience of a stereo volume control. The best of both worlds -- dual mono balance (and separation) with stereo gain. (The selector switching is accomplished by two (mono) rows of large-area silver-contact switches which allow the source-amp interface to see only one source-switch combination at a time.

I mention all this about my design because this, along with Audio Note silver wiring and solder and Kimber's best RCA females are very close to as good as you can get with this kind of device. (The next step up goes to input-output transformers and XLR connectors.)

But! When one can not throw crazy money at the solution one must rely on ol' fashion smarts, good ears and creativity. This is what David Hafler always did... and this is what it appears David Hum is now doing. I know the parts Hum selected and I know that many engineers would agree with me that these particular parts will wreck only minimal damage on the music signal. In short, the Model 500 is cheep looking but the designer's experience, good sense and good taste has made it into a very, very enjoyable preamp to use. It is easily the best of the three in purely musical terms. All the music I played with the 500 in the system seemed raw and present and noticeably lively. Fun. Music through the 500 had this beguiling, direct-from-the-source feel that quickly won me over. I would still be using this pre-amplifier today if it were not for the fact that it is dual-mono (I prefer one volume control) and it has only one set of output jacks (I use an active subwoofer). It is also too lightweight for me. The whole chassis jumps around when I am turning the knobs. Bad.

Antique Sound Lab "Passive TIDT"

Antique Sound Lab "Passive TIDT" The TIDT probably had the biggest negative impact on the music of all three. It slowed down and thickened the viscosity of reproduced music -- noticeably more than the others, but still only a little. And, do not laugh, but, I love this pre-amplifier. It is my favorite. In fact I am gonna buy it.

Why would I prefer the TIDT if it made my music sound worse than either of the other two? And why would I buy it if I already own a better one? Because it also does something good to the music that I can not quite put my finger on. Something that not even mine does.

I have had all three of these units for a long time now and for some reason I keep going back to the Antique Sound Labs unit. I like the way my system feels with the TIDT installed. I like the way it looks. I like to turn the knobs. (While it is dual-mono, the chassis is heavy and solid.) I can not even say for absolute sure that it makes my music worse than the others. (Remember all of these units are quite similar in sonic signature.) The TIDT just makes every disc a teeny-tiny bit slower, thicker and less lively, but at the same time, it makes the music seem just enough more smooth and "whole" to make me feel good.

Wish I could say for sure what is going on and wish I could better describe what it is that the TIDT does that makes the music so enjoyable. (The fact that it has two sets of output jacks which allows me to use my sub may be some small part of my happiness.) Something tells me it might be the effect of the auto-transformers. Something tells me it is the lack of resistors and too many questionable mechanical contacts. I have always been partial to autoformers. (Autotransformers are transformers with only one coil instead of two. In this case a single coil with a series of separate tapings that allow for a variety of sequential attenuations. They present the signal with a complex, mostly inductive, low impedance that can be very kind to small-scale energy crossings.) I have used them in loudspeaker crossovers. I used this pre-amplifier with both the Gaincard and the Kamuro 845s and in both cases, it just felt like it belonged there. Some unspoken rightness. With the Antique unit I find myself noticing the texture of the music more. There is a "touchable" thing that happens to the music with the TIDT in the system that I really like. So what can I say? There is also some mysterious "X-factor" thing going on here.

With or without the subwoofer turned on I like the "feel" of music with the Antique Sound Lab preamp. I am wrestling with my impressions, but the best I can figure is the other preamps are more disconnected or disembodied sounding. Compared to the TIDT the other preamps seem to play down or generalize textures.

All of these units are capable of doing a nearly perfect job at selecting your musical program and then controlling the volume of said program. All of these units are quality high end products. Any of them could, quite possibly make you personally very happy. But you must listen for your own self in your own system. And remember -- if it ain't broke don't fix it. The first important question any prospective passive pre-amplifier purchaser must ask is this: Do I really need one? If you answer yes to this question then you must go back and answer the questions at the beginning of this article. Then, if its is yes, yes, yes, to everything. Then you can consider the three units reviewed here and, I would suggest poking around some. Talk to the guy who designed your amplifier. Ask a few smart guys some dumb questions. Then try a couple passive control units that look and feel right and see what you think. And remember, the passive preamp must make the music more enjoyable. What did Johnny Cochran say? "If it don't fly -- you mustn't buy!"

Specifications

Antique Sound Labs

Outputs: Two single-ended and one "tape"

Inputs: Four line-level and one "tape"

Balance Control: No (dual-mono volume controls)
*employs auto-transformers to accomplish gain variation

Dimensions: 3" x 15" x 5.5" (HxWxD)

Price: $399

Audio Synthesis Passion Ultimate

Outputs: One single-ended + record-direct

Inputs: Three line-level

Balance control: Yes

Dimensions: 3.75" x 4.25" x 12.25" (HxWxD)

Price: $1,795

P.S.S. Model 500 Dual Mono

Outputs: One single-ended

Inputs: Four with mute switching

Balance Control: Dual-mono volume control

Dimensions: 3" x 8" x 5" (HxWxD)

Price: $330