Noise is enemy one in a phono preamp; it threatens to overwhelm the tiny signals (only fractions of a millivolt) from the cartridge and climb up to annoyingly audible levels on the high gain multiples (up to 10,000 times) essential in such amps. So it makes sense to begin our tour through phono amp design with a close look at what goes into silencing the noise.
There is no one device or technique that will deliver the very low noise figures needed. A nearly silent phono stage is the result of intense focus on the details, lots of them. The power supply is critical. The AC power grid acts as a giant antenna that picks up all sorts of electromagnetic interference and delivers it to the audiophile’s wall socket. Power conditioners are helpful, but the phono amp itself ought to have an effective filter to strip away high frequency “junk” that gets past the conditioner. Inside the phono unit, the power transformer is a source of 60 Hz hum and must therefore be well shielded or, better, kept a few feet away from the sensitive gain circuitry. Rectifier diodes are a source of high frequency noise; the thoughtful designer chooses ones that minimize this effect and then filters remaining diode noise.
Even after being rectified, the resulting DC power carries a strong 60 Hz ripple, which has to be filtered. Several stages of filtering and voltage regulation are typically necessary to produce power rails that are clean and smooth enough for use in phono amplification. Once the power rails are properly filtered, it is essential that the power distribution system be designed so as to prevent the power from being re-contaminated by stray electromagnetic radiation within the amplifier enclosure. Long runs of wire are susceptible to such contamination, so distribution through a dedicated power plane within the printed circuit board is preferable. All potential noise sources in the power supply circuitry, such as voltage references and transistors, should be bypassed to ground with capacitors.
Clean, smooth power is essential, but it is just the beginning. Virtually every component in the gain circuitry is a noise source, and each will contribute noise to the output. The designer’s task is to minimize the contribution of each. The most common component is the resistor. There can be a few hundred in a given phono amp. Each one has two noise components that, added together, determine its noise contribution. The first is proportional to the resistor’s value in ohms and is independent of the type of resistor. The second is determined by material and construction, e.g., wire wound and metal film resistors are quieter than carbon composition ones. Thus, the designer must not only select low noise types of resistors, but also design the circuit to use resistors with the lowest feasible values, especially in critical locations. But, tradeoffs can be necessary in choosing resistor values. Lower resistances mean higher currents that can, in their turn, increase noise in other components such as transistors. Small value resistors can also require the use of relatively large value capacitors in some parts of the circuit, particularly the RIAA network. Big capacitors can add distortion at high frequencies if current is limited. In these situations a balance must be struck.
Each active device (e.g., transistors, jfets, op-amps) also adds noise. The amount varies widely among devices depending on their type and construction. So the designer seeking low noise will naturally choose low noise types, but those choices must be informed by the other effects the device has on the sound. Some low noise devices just sound better than others. Moreover, each active device should be used within the circuit in a way that optimizes its low noise performance. For example, the impedance of the source signal fed into a transistor, the current running through it and the voltage across can all influence its noise contribution. The designer needs to provide an environment for each device which will allow its low noise characteristics to come to the fore. Also, using more than one transistor in parallel can reduce noise.
Even some seemingly innocent parts can be noise culprits. Switches create noise because their contacts are necessarily not soldered solidly together. There is always a tiny bit of space between the two surfaces of switch contacts. When current runs through the switch, the electrons have to jump over that space and, in doing so, cause a bit of noise. A designer will choose switches carefully, and avoid using them in particularly noise-sensitive parts of the circuit, such as the input of a phono amp. Wires and circuit board traces can act as antennae that pick up stray radiation and convert it into noise. They need to be kept as short as possible and, in the case of wires, shielded or twisted together.
Noise from all of these sources must be addressed if the phono stage is to be really quiet. But low noise should not be considered in isolation, or pursued to the exclusion of good performance in other important areas, such as distortion, dynamics, tonal accuracy, gain and good sound. Successful designers understand they are building a system in which the parts interact with each other, and strike the appropriate balance. Combining thoughtful engineering with careful listening to accommodate these competing factors is where design merges into art. 66dB
Applying these principles to a phono amp design can produce some remarkable results. I recently took two already quiet phono stages and, by rigorously applying the techniques described above, reduce the output noise voltage of each of them by over 50%. These noise reductions have resulted in increased retrieval of the low level detail and better soundstage and imaging.