Quality Lies in the Details Page 2
Bottom line: High-output processors are not an ideal match for high-gain preamps. On the other hand, a high output level is a bonus when used with a passive level control instead of a preamplifier.
Another important measurement to consider when determining if a particular processor or CD player is a good candidate for a passive level control is output impedance. JA wrote an excellent exposition on the subject in the December '92 Stereophile (Vol.15 No.12). Rather than rehash his explanation, the gist of it is reproduced here as a sidebar.
We measure output impedance by driving the processor with a full-scale digital signal, and measuring the output voltage with no load connected (actually, the System One's 100k ohm input impedance) and with a low-impedance load (150 ohms). The voltages are measured with both loads at 20Hz, 1kHz, and 20kHz. To make life easier for us reviewers, the measured voltages are then input to a computer program JA wrote which calculates the device's output impedance using Ohm's Law (footnote 1).
If a passive level control is used between a digital processor and power amplifier, the burden of driving the cables and power amplifier falls on the digital processor's output stage. If the processor has a high output impedance, dynamics can become compressed and the bass may get mushy. In extreme cases, a processor with a very high output impedance can even become current-limited, and flatten the waveform peaks if it's asked to drive a low-input-impedance preamplifier. The Mk.I CAL Sigma, for example, with its very high output impedance of nearly 2k ohms at 20kHz (2000 ohms) and 1.5k ohms at 1kHz, will clip if driving impedances less than about 15k ohms (footnote 2).
Further, the output impedance of the passive level control itself must be added to the equation. The passive control's output impedance (which varies depending on which resistor is switched in, or on the position of the potentiometer) must be added to the driving component's source impedance to find the total source impedance driving the power amplifier.
Digital-processor output impedances vary from less than 1 ohm (the Proceed PDP and PCD), to the original CAL Tempest CD player at 5.6k ohms, and the original Sonic Frontiers SFD-2, which featured 8k ohms at 20Hz. Generally, if the output impedance is less than a few hundred ohms, the processor should have no trouble driving a passive level control.
However, a significant factor in suitability for passive level controls is the interconnect's capacitance. High-capacitance interconnects and high-output-impedance sources are a recipe for soft bass and lack of dynamics. In extreme cases, the treble rolloff can even intrude on the audio bandwidth, robbing the music of air and openness. Cable capacitance is usually specified in picofarads/ft. Less than 100pF/ft is considered low capacitance. Note that 20' of 100pF/ft interconnect has the same capacitance as two feet of 1000pF/ft cable. It isn't the length, but the total capacitance, that's important (see the "Capacitors & Capacitance" sidebar).
It's also useful to know the input impedance of the load. The lower the input impedance, the more difficult it is to drive. Moreover, a low-input-impedance power amplifier will cause high-frequency rolloff at a lower frequency than one with high input impedance. This only becomes a practical matter, however, when driving the power amplifier through very-high-capacitance interconnects. [And, if the processor has a much higher output impedance at low frequencies than in the midrange and above, due to an undersized coupling capacitor, bass frequencies will be rolled off to a degree depending on the input impedance of the preamplifier, resulting in a lean tonal balance. The Mk.I Sonic Frontiers SFD-2 suffered from this problem.—Ed.]
Bottom line: Don't drive a low-input-impedance power amplifier (less than about 20k ohms) with a passive level control through long, high-capacitance interconnect, driven by a high-output-impedance (greater than about 800 ohms) digital processor. And don't connect a processor or CD player with a high output impedance—greater than 1k ohms—to an preamplifier with an input impedance of less than 20k ohms. These aren't set numbers, but guidelines.
Using a voltmeter, we also measure the amount of DC voltage present at the processor's output. DC may cause a thump through the system when the preamplifier is switched to select the processor's output. If the preamplifier and power amplifier are both direct-coupled (no DC blocking capacitors in the signal path) and use no DC servo correction, then the DC from the processor could be amplified and appear at the loudspeaker terminals—not a good condition. Even if it doesn't cause the woofer's voice-coil to burn out, DC offset will displace the cone from its central, rest position, increasing distortion levels. Low DC—less than, say, 3mV—is therefore a desirable condition.
We determine if the processor inverts absolute polarity by driving it with data representing a positive-going impulse and looking at the processor's output on an oscilloscope to see if the pulse is still positive-going. Most processors don't invert polarity, but if one does, that's no reason for concern. Without knowing the absolute polarity of the recording and the rest of your system, it's a 50:50 chance the absolute polarity will be wrong. Knowing that a processor inverts absolute polarity, however, is important when comparing processors: all processors/players under audition should maintain the same polarity for the listening comparisons to be valid.
Footnote 1: Ohm's Law ties together the concepts of resistance, voltage and current flow in a circuit. The DC voltage V across a circuit element is equal to the current I passing through it multiplied by its resistance R. Mathematically, V = I x R.—JA
Footnote 2: See Vol.15 No.10, p.229 for a description of how the Sigma's output becomes current-limited into low impedances.