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Overall build quality on both stock and mod units was exceptional. Unless you know what to look for with the mod, I doubt most people would notice the swapped parts on the circuit board. I experienced no operational issues with either piece. Both performed flawlessly. I flipped back and forth from 96 to 192kHz upsampling and slightly preferred the lower setting for most discs. I didn't think there was too much difference between the two but 192 seemed a little too bright. Therefore all my notes reflect the 96Hz setting.


Music playback over the stock version was clean, crisp and detailed, with well defined bass and good pitch definition. While the DL III presented a warmish tonal balance, highs were extended and airy. There was also a good sense of space and soundstage layering on large-scale classical recordings. However, I did note a hint of sizzle and a sense of electronic artifice up high. Textures were a tad on the dry side but not quite what I'd consider thin or threadbare. Overall I thought the stock DL III was excellent value and would be a decent upgrade for someone with a DVD player or a mid-priced CDP.
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Now let's have a look at Cullen Circuits' Stage Three mod: Since Rick Cullen has been associated with PS Audio 35 years, who'd better to offer mods than the bloke who designed and built the gear in the first place? Plus, there's all that long-standing experience with digital gear referred to above. Remember, Rick helped develop PS Audio's very first DAC, the iconic Digital Link.

This particular mod, unlike most others I am aware of, actually improves the component's circuitry. While upgraded caps, resistors and connectors are included, Cullen goes a step further. Instead of feeding the PPL (Phase Locked Loop) with an add-on low jitter clock as other mod firms do, Cullen instead tears out the PPL chip completely and adds two low jitter clocks to support the SRC (Sample Rate Converter). This apparently results in considerably more effective jitter reduction.
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My review liaison at Cullen Circuits, EJ Sarmento, sent me the following email about their mods and I also grabbed a quote from Rick Cullen regarding improved measured performance from PS Audio's forum: "I decided to write this message to fill you in a little more about our mod. Let me first say you would not usually see this type of modification done by anyone else. Our mods are true and detailed and actually improve the electrical circuit. Most other mod companies change caps, crystals and resistors and add ERS and chassis damping material which do improve sonics but not to the extend as an improved circuit does. Below is a quote from Rick."


"In the stock design, a 27MHz crystal is fed into a phase lock loop to synthesize these frequencies. The majority of the jitter is generated in the PLL. Granted, the 27MHz crystal has some influence but simply feeding a low jitter clock into the PLL (as others do) can only have a minimal effect on the clock jitter that supports the sample rate converter. I would expect the jitter to be roughly 200ps given the specs on the PLL. If you're looking for low jitter, the other modders' approach simply does not cut it. The same applies to the clock replacement for the USB chip. In my tests, replacing this clock yielded minimal improvement, not enough to justify the cost. I opted to put the effort into upgrading the analog components in the audio circuit.
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"My approach is to provide two different low jitter clocks each having less than 1ps jitter to support the sample rate converter. My dual clock board contains two low jitter oscillators, the switch necessary to select the appropriate clock for the selected sample rate along with an extremely low-noise voltage regulator and support circuitry. This approach feeds the sample rate converter with a <1ps jitter clock directly with no PLL chip in the path. I remove the PLL chip entirely.


"My upgrade places the next focus on upgrading as many components in the signal path as possible. 28 resistors and 11 capacitors are replaced along with 8 FETs and the output jacks with WBT NextGen 210Cu RCAs. ERS cloth is also carefully applied near the digital circuitry." From EJ again: "Our resistors are non-magnetic Dale RN55D with copper leads (unlike stock). Where we replace caps, we use Nichicon Muse audio-grade capacitors. Definite measurement improvements are noticeable on the Audio Precision post mod, which most modifications won't show."


And from Rick again: "The more significant changes I have measured on my Audio Precision are reduced THD across the board, with the most significant improvements between 10kHz and 20kHz. The unmodified DLIII typically measures 0.009% at 10kHz, 0.0084% at 14kHz and 0.008% at 20kHz. The modified DLIII measures 0.005% at 10kHz, 0.0038% at 14kHz and 0.002% at 20kHz. Noise is typically reduced by 10uv and DC offset by 10mv."