Report - The Path From Passive to Buffered --
My Experiences with Line-Level Preamplifiers - January, 1995
By Steven McCormack
Audiophiles already were talking about running their CD players directly into their power amplifiers when I first began modifying early players back in 1984. Bypassing the preamplifier was possible because, as line-level sources, these players required neither the RIAA post-equalization circuitry, nor the gain circuitry, nor the cost and distortion associated with such circuits.
Going straight into the power amplifier, however, presented two problems. First, this made it inconvenient to switch from one source to another. Second, the variable outputs of most CD players did not sound as good as the fixed outputs. After evaluating all the available information and performing extensive listening tests, I decided that the best option was to design a high quality passive preamplifier.
I chose this path because I felt that the sonic potential offered by this type of component far outweighed the modest limitations. The potential was for achieving true transparency - for having a component that would not sound different from top quality interconnect cable and would have no colorations of its own. Though these goals are unattainable in the absolute sense, I was convinced that I could approach them to the degree that either the shortcomings would be beyond the hearing threshold, or that the audible element would be so minute that "apparent" transparency would be achieved.
Several other designers obviously reached the same conclusion because a number of manufacturers began producing passive line-level preamplifiers within two years of the time I realized my first design.
There are two system limitations imposed by this type of component. First, the source has to have sufficient output to drive the power amplifier with no additional gain supplied by the preamplifier. Fortunately, this level of output is very common among today's musical sources, such as CD players, tape decks, and tuners. In fact, insufficient source component output is the exception.
Second, a passive preamplifier does limit the length of output cable that connects it to the power amplifier when using a high capacitance cable. (There is no corresponding limitation on the input cables.) For my passive preamplifiers, I recommend that the total capacitance of the output cable be held at or below 1,000 picofarads per channel for optimum performance. A higher capacitance level will cause some high frequency rolloff.
Other manufacturers of passive line-level preamplifiers recommend similar limits. Cable manufacturers can help you determine the length which hits the appropriate level. My experience suggests that this limitation becomes important to system design in a small percentage of all cases.
As the design developed, I discovered that what appeared to be a simple, straightforward project was actually a sophisticated set of problems demanding complex solutions. Their highly revealing nature and susceptibility to subtle influences made designing a passive line-level preamplifier as challenging as designing an active one. I feel confident that the paths of other designers must have paralleled my own in this matter.
This experience explained to me why passive preamps suffered from a less than pristine reputation among DIY (do-it- yourself) audiophiles. Some individuals, including both engineers and hobbyists, viewed passive components as nothing more than a few wires, knobs, and a box. They built their units with little attention to parts quality or design, found that it sounded terrible, and concluded, incorrectly, that all passive units must sound terrible. One wonders why they did not conclude that all active preamplifiers sound bad, if they heard one bad sounding active unit.
During the development process, I spent many hours listening to the effects of minute variations in the design. I was continually impressed by the large sonic changes caused by these small adjustments. As a result of this process, I gained great respect for both the sonic potential and the design sensitivity of passive preamplifiers. Conversations with designers of other line- level preamplifiers suggest that my experiences were far from unique.
The sonic performance of the finished product, called the Line Drive, fulfilled all of the original design goals. It was characterized by the exceptional transparency and natural musicality which are now recognized as classic passive features when the preamplifier is properly designed and manufactured, yet rarely heard from other types of components.
With further experimentation, I discovered that the very best performance of my design came from running it with a high input impedance (20 kOhm or higher), high gain power amplifier. This too is characteristic of all passive units. The amplifier's high input impedance prevents loading effects while the high gain assures ample capability to drive loudspeakers to an appropriate volume level. Only extremely low loudspeaker sensitivity combined with low gain amplifiers present a problem for attaining high volume levels. Again, this combination of factors is rarely present in the real world.
While investigating early reports of "insufficient" volume, I found that people simply were not turning the volume control far enough to achieve the levels they wanted. Following the lessons they had learned about not turning up the volume of active preamplifiers too much as a precaution against system damage, these individuals were setting their controls too low. Since passive line-level preamplifiers have unity gain (1X), there is no reason not to turn the volume all the way up if that is required.
Please understand that I have nothing against active circuits, and I assume the same is true of other creators of passive designs. I build active units for applications where they are appropriate, such as phono preamplification. However, I prefer the passive path in some other applications because of the exceptional transparency and natural musicality possible through this approach.
During the latter years of the original Line Drive's seven year tenure, I worked with design refinements in preparation for releasing its successor. These experiments confirmed most of my earlier conclusions, but they also led me to appreciate the advantages of offering a buffered output in addition to the passive output. I had resisted that option for a while because the buffer circuits I knew had a negative effect on sonic performance. It was during the development of the Active Line Drive ALD-1 preamplifier that I designed a special buffer circuit that was based on a simple complimentary FET pair, but which required a very high quality power supply to make it work properly.
This was the first electronic circuit I had worked with that complimented the passive circuit and did not compromise sound quality. It allows the use of any length of cable between the preamplifier and power amplifier. Adding the buffered circuit brings the opportunity to enjoy the sonic benefits of minimalist preamplification to all systems.
The current version of the Line Drive is the TLC-1 (Transparent Line Control) which is completely passive, with a choice of buffered or unbuffered outputs. Its sophistication shows the evolution of our designs from the first product we released in 1984 - Tiptoes (spiked feet for turntables, components, and loudspeakers).
At that time our company was called The Mod Squad because we specialized in custom modifications. As the years went by, we turned our attention more and more to manufacturing, then in 1988 we closed the modification shop. Two years later we changed the name of the product line and corporation to McCormack.
My future plans include designing high performance electronics for home theater systems and, hopefully, returning one day to my first love which is producing live recordings. During the 1970s, I was fortunate to be involved with M&K's Realtime label which produced several of the finest direct-to-disc recordings I have ever heard.
Some of my experiences at M&K were instrmental in starting me along the path to designing and manufacturing my first component: the Line Drive passive line-level preamplifier.
McCormack Audio Corporation
© Copyright 1995, 1996, 1997 Secrets of Home Theater & High Fidelity
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