Universal Audio 2192 Master Audio Interface - Manual (Page 36)

The Technical Stuff
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Model 2192 Circuitry
The 2192 Master Audio Interface was designed with two primary goals: the highest-quality audio fidelity
and ease of use.
Specifications can be useful but are sometimes misleading because they don’t always tell the whole story.
Something can look great on paper, but still sound bland. Phase and frequency response, distortion,
dynamic range and linearity are important indicators, but they’re based on sine wave signals that no one
listens to. Transient response in a complex, dynamic music signal is very difficult to measure because the
simplistic measurement tools we have are very primitive compared to the human hearing system. Also, the
behavior of a complex musical signal in a real circuit is very different from the theoretical behavior of an
idealized model. This is something that Univeral Audio’s engineers have been studying extensively while
designing both analog circuits and DSP systems that model them.
One of the biggest criticisms of many high-sampling rate converters is the “spectrum shift” that occurs as
the sampling rate is increased. One important question to ask is, how is the low end when sampling at 192
kHz? With some converters, it sounds like you’re viewing your signal through a fixed window that slides up
and down the spectrum: Either you get to hear tight bass, or good highs, but not both at once. This appears
to be caused by the phase distortion in the analog high-pass and digital filters used by converters. The
solution adopted by the 2192 designers was to eliminate all DC blocking capacitors and all DC-servo
circuits (which are a type of high-pass filter), and to use a digital offset calibration scheme that maintains
maximum headroom without capacitors or sample rate-dependent digital high-pass filters.
A very significant aspect of coupling capacitors, otherwise known as DC-blocking caps, is their low-
frequency phase response. All coupling capacitors are high-pass filters, which means they introduce low-
end phase distortion. Unfortunately, phase distortion is most apparent at low frequencies, and it severely
affects transient response and imaging. When the beater of the kick drum hits the drumhead, the signal
spikes rapidly because there is more information in that signal than most musical signals. Any smearing of
that transient is heard as a lack of presence and detail, and a general “it’s not quite there” effect.
Since a 24-bit converter has a theoretical dynamic range of about -145dB, circuit noise becomes a
significant issue. With a maximum output level of +22dBu, this means the circuit noise cannot exceed -
123dBu. At these low levels, thermal noise becomes a dominant factor, so circuit impedances must be
reduced as much as possible. This causes a problem with capacitor coupled circuits because as the
resistance is lowered, the capacitors must get bigger. Capacitors have a critical influence on sound
quality, and big capacitors that sound good are very expensive. The 2192 designers decided to completely
eliminate all coupling capacitors in order to realize the lowest possible circuit impedances and thus the
lowest noise floor.
Most UA products use custom components, and the 2192 is no exception. During the course of our testing,
we realized that all the venerated high-quality IC op-amps available were simply not good enough. Even
after playing some tricks to make them sound as good as possible (biasing them Class-A to eliminate
crossover distortion, etc.), we couldn’t get them to sound the way we wanted, so we decided to design our
own op-amps that met our sonic requirements without compromise.
Although many respected designs utilize IC op-amps, one of the biggest problems with using them in audio
applications is their high open loop gain. This means you have to wrap a lot of negative feedback around
them to use them at the low gains required by an analog converter. The problem with high loop gain is that