The LKV Approach to Phono Preamp Design: Science and Art in the Service of Good Sound.

A phono pre-amplifier has two essential functions. It must amplify by as much as 1000 times the tiny signals that the cartridge generates as it traces the groves in a vinyl LP record. It must also compensate for the RIAA curve that is used to attenuate the low frequencies and boost the highs when vinyl records are made.

At LKV our approach to creating an excellent preamp to perform these functions involves (1) identifying the necessary characteristics of such an amplifier, (2) applying sound engineering principles and techniques to design an audio component that will achieve those characteristics, and (3) most importantly, listening to a variety of music played through the preamp over an extended period of time. Based on the results of this listening, we select among various parts and circuit topologies and modify the design to achieve an optimum balance among the key characteristics. The first two steps are the science, the third is the art.

Here we identify the key characteristics and explains why careful listening is essential to selecting the best components and getting the balance right. For those who are interested in more technical information about the process of designing to optimize the key characteristics, we have posted a separate White Paper that discusses those technical issues.

Key Characteristics.

1. Gain is the amount of amplification a device provides. The typical 5 mV output of a moving magnet cartridge must be amplified by a factor of roughly 100 (40 dB) before the signal is fed to the rest of the audio system. The much smaller 500 uV signal from a moving coil cartridge needs to be multiplied approximately 1000 times (60 dB). Many cartridges have outputs falling between these extremes. A good phono stage needs to have gain that is adjustable in steps from 40 dB to 60 dB.

2. Low noise. The amplification factors discussed above are large and the signals from the cartridges are small. Any noise entering the input of the phono amp will be amplified along with the signals, as will any noise generated by the amp itself. If the amp does not block incoming noise and minimize its own noise contribution, it will bury the musical signal. Good signal to noise ratios exceed 80 dB for the typical input (5 mV) from moving magnet cartridges and 60dB for that from moving coil devices.

3. The RIAA equalization used in making LPs attenuates the lowest frequencies (20 Hz) about 20 dB and boosts the highest (20,000 Hz) by about the same amount. The effect of the RIAA curve must be reversed to restore the original frequency balance of the music. This task is performed by a network of capacitors and resistors. Such a network should produce as close to a flat frequency response as possible; a variation of less than +/- 0.2 decibels is good.

4. Headroom (Also referred to as Overload Margin) refers to the amount (typically expressed in decibels) by which a signal can be amplified before clipping occurs. Even though the input signals from the cartridge are very small, phono amps need output headroom of 20dB or more because, among other reasons, their amplification factors are great.

5. Low Distortion. In discussions of amplifier specifications, distortion is the bending or deformation that an amplifier’s circuitry causes to the shape of the incoming electrical signal, be it a simple sine wave or the very complex waves that carry music. It is not, of course, desirable to deform the signal being amplified; doing so will surely take us further from the sound of the original performance. But distortion is a tricky (and controversial) subject. Below some minimum levels, it is inaudible. Techniques used to lower distortion can compromise other aspects of amplifier performance. Most distortion measurements, including the commonly reported ones for total harmonic distortion (THD) and inter modulation distortion (IMD), are made using sine waves as the test signal. The problem is that musical signals are far more complex than these sine waves and present a much greater challenge to the amplifier than do the test signals. An amplifier that shows high distortion in a sine-wave test will certainly also distort the more complex musical signal badly. But, the opposite is not necessarily true. An amplifier that has good results on sine wave based tests may still distort the more challenging musical signal. All other things being equal, less distortion is better than more. But, because all other things never are equal, measurements must ultimately give way to careful listening in judging an amplifier’s performance.

6. Cartridge loading refers to the amount of resistance which the signal from the cartridge “sees” at the input of the phono preamplifier. For moving magnet cartridges, 47 K Ohms (47,000 ohms) has become the standard and works well. But moving coil cartridges present a different situation. For them, the amount of load resistance affects the amplitude of the musical signals at the high frequency end of the audio spectrum. In effect, it tunes the signal from the cartridge. The optimum resistance varies from cartridge to cartridge, system to system and listener to listener. To be suitable for use with a variety of cartridges and systems, a phono preamp should have a 47 K ohm load for moving magnet cartridges and be configured so the load resistance for moving coil cartridges can be to be adjusted in several steps from 100 to 1000 ohms.

7. Relatively Simple Circuitry. No physical device or process is perfect. Each time we manipulate an electrical signal (e.g., amplify, filter or attenuate it) we do some damage to it. Thus, it is desirable to use simpler rather than more complex circuits in order to minimize the number of devices through which our vulnerable signal must travel and the number of operations to which we subject it. This criterion favors the use of circuits using few gain stages, discreet parts, and no loop feedback. However, simplicity of circuit may come at a cost in poorer performance in other important characteristics, for example, gain, distortion, headroom, etc. Thus we arrive at the need for listening to balance a phono amplifier’s characteristics and to help in selecting the best sounding parts and circuit topologies¹.

Listening.
The effort to maximize an amplifier’s performance in one of the characteristics described above can impair the performance in others. Once reasonably good performance is achieved in all seven, the real work begins. Only through repeatedly listening to music played through the amplifier and tweaking it based on what is heard, can intelligent choices among the necessary trade-offs be made.

Most audiophiles are familiar with the ongoing controversy regarding use of negative loop feedback to reduce distortion. Well designed circuits can achieve low measured distortion without feedback, but the very lowest levels of such distortion are achieved with high gain circuits using substantial loop, negative feedback. Some say such very low levels of distortion improve the sound, others that the added complexity damages it. At LKV, we believe that good results can be achieved with each approach. But our listening experience indicates that, so long as proper design techniques are used, the very best sounding amplifiers are made without loop feedback. But the real point is simply that a choice like this can only be made by listening. There is no formula or calculation that will tell whether one design or another sounds better to the human ear.

A similar, but much less well known, issue arises in the design of the resistor-capacitor network that performs the RIAA equalization in a phono amp. The resistors in the network add noise to the circuit in proportion to their value: the larger the resistors, the more the noise. The capacitors contribute to slewing distortion in direct proportion their value, the larger the capacitance, the higher the slewing distortion². We would like to be able to use both small value resistors and low value caps. But we cannot, because the ratio between the resistors and capacitors must be kept constant if the network is to function properly. So, the designer must decide how to balance slewing distortion against noise³. What balance will produce the best sound? Again, only careful listening will provide an answer.

As much as a designer might wish to be able to measure and calculate precisely all aspects of an amplifier’s performance, experience listening to different designs using different parts compels the conclusion that there are factors at work that we have not yet devised a reliable means of measuring. For example, two circuits using different transistors may have virtually identical measurements (noise, distortion, etc.) but sound quite different when heard through revealing speakers. The same can be true of two different circuit topologies. Only by listening over an extended period can one make judgments regarding which part or circuit is to be preferred. This method is admittedly subjective and subject to the biases of different listeners. But, until our measuring technology improves, it is the best we have.

Summing Up.
LKV uses the best engineering principles, techniques and measurements to design circuits that achieve good performance in all of the areas described. Having done so, we listen and modify and listen some more to get the best sounding balance in each of our products.

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