Decware Zen Taboo mk4 technical measurements Standard Prologue If you are unfamiliar with audio measurements please use a search engine with the query: "audio measurements" or "audio measurement handbook" Look for publications by Richard C. Cabot and also by Bob Metzler, both from Audio Precision. There are other useful publications as well. These will provide basic knowledge. Interpretation of the following measurements is beyond the scope of technical measurements posts. The data presented were collected as follows: 1. PrismSound dScope III, picoscope 5243B, Keysight 34465A, Cal Test CT2593-2 balanced probe 2. Balanced XLR cables Belden 1800F with Neutrik 110R AES connectors (if used) 3. Single Ended cables Audioblast HQ-1 3 ft with Rean RCA connectors 4. 32 and 300 ohm loads used for measurements 5. dScope analyzer sample rate 48 KHz unless otherwise noted 6. 0dBu level used for testing unless otherwise noted 7. Amplifier input to output gain set for 0 dB unless otherwise noted 8. Audioquest Forest and Schiit Pyst USB cables used with measurement equipment 9. Vaunix Lab Brick USB hub 10. Shielded 14AWG and 16AWG power cables 11. ESD, EMI/RFI controlled lab bench and workspace 12. Tung-Sol (TS) EL84 tubes Measurements are made in accordance with AES17:2015 Sensitivity data for two headphones to keep in mind while viewing these measurements: HD 650 impedance 300R, sensitivity 98 dB/mW HE-500 impedance 38R, sensitivity 89 dB/mW SPL levels for above headphones for reference: 0 dBu 300R 2.00 mW - 101 dBSPL @ 98dB/mW 0 dBu 30R 20.00 mW - 102 dBSPL @ 89dB/mW All testing performed at 0 dBu unless otherwise noted. This level is consistent with listening to headphones (referenced above) at 90 dBSPL average with peaks to 100 dBSPL, if the music has 10 dB Peak to Avg ratio. That is LOUD for long listening sessions. Zen Taboo mk4 Measurements commenced after 1 hour of warmup. Measurements were performed over a period of several months. Index Post 1 – Measurement setup description, highlights Post 2 – Bal input 300 ohm load, High gain setting Post 3 – Bal input 32 ohm load, High gain setting Post 4 – SE input 300 ohm load, High gain setting Post 5 – SE input 32 ohm load, High gain setting Post 6 – Bal input 300 ohm load, Low gain setting Post 6 – Bal input 32 ohm load, Low gain setting Post 7 – Square wave response Post 8 – Frequency Response Post 9 – reserved for corrections and / or additional data Measurement setup picture: Standard setup with Decware load for 300R and 32R loads Listening evaluation picture: Zen Taboo mk4 inputs switched between Holo Spring 2 KTE Bal & SE outputs. Most listening evaluation with gain set to High, crossfeed bypassed, amp volume set for 0 dB gain with Goldpoint SA2X and SA2 external stepped attenuators employed to set level. There are plenty of impressions threads on the forum where more detailed and nuanced thoughts about the various components are discussed. For my preferences, either I like how a system renders music or I don’t. Notable highlights: Due to some interesting attributes of this amp I will provide additional commentary and interpretation from which I would normally refrain. Decware Zen Taboo tonality has always impressed me during listening sessions at various meets. Typically with HD800 headphones and yggdrasil DAC. Meet conditions are often burdened with higher ambient room noise than my dedicated lab listening environment. With the opportunity to hear this wonderful amp in my own setting, the tonality continued to impress. I acquired HE-500 planars when they were introduced over a decade ago. They have never provided sufficient auditory satisfaction to justify the head and neck load from their extra weight. Even after replacing the stock stiff silver wire with a high quality supple woven cable from Forza Audio Works. Taboo mk4 must have been designed with planars in mind. HE-500 spent more time on my head over several weeks than all the past listening session attempts over a decade combined. Many hours at a time. Previously I would only listen for ten minutes before returning to dynamic headphones. Taboo + HE-500 renders music with some seriously good synergy. However, AC mains noise was on the threshold of perception with the various HD800 family and other dynamic headphones in my lab. It must have been just below my threshold of perception with the HE-500. Keep in mind my listening lab is typically 10 to 20 dB lower than most home listening spaces. When I moved the Taboo to our family room, residual ambient noise masked my perception of Taboo AC mains noise with HD800. Making the assumption that Balanced input would be the best way to experience this amplifier I spent the first few weeks with that configuration. Imagine my surprise when I switched to SE, RCA inputs and found I preferred the music rendering of SE input over Balanced. Transient sounds were a touch more natural for example. Take note of SE measurements in both the Square Wave response and Frequency Response sections. I did try the crossfeed feature for a short evaluation. It does provide some spatial presentation but I still prefer Waves NX or Redline Monitor VST processing. Let's get this out of the way. Generator level set to 0 dBu, Amp set for 0 dB gain with 300R load, High gain selected, Balanced input, Tung-Sol EL84 tubes: THD+N: 0.03166% Left channel (SINAD 70.0 dB) THD+N: 0.03619% Right channel (SINAD 68.8 dB) Those who worship THD+N / SINAD should stop at this point and dismiss this amp. The remainder will be a waste of time. However if you consider there is more to an amp than SINAD / THD+N please continue. This special sounding amp offers another opportunity to demonstrate how chasing low distortion numbers out of context is the very definition of foolishness. Decware Zen Taboo mk4 TS EL84 Distortion 1KHz 0dBu 300R Hi gain The greatest components contributing to Taboo’s THD+N are AC mains noise. 60 Hz and 180 Hz AC mains noise components are at the threshold of perception for many common headphones. For Sennheiser and other headphones with similar sensitivities that threshold is approximately -75 dBu. Most environments have sufficient ambient noise to mask the hum for most headphones. IEM users may want to seriously consider their personal sensitivity to AC mains noise. Observe 60 Hz hum at -71 dBu and 180 Hz 3rd harmonic at -80 dBu. The following two charts aid in calculating headphone SPL at these levels, ignoring headphone frequency response. Taboo mk4 TS EL84 60 Hz headphone SPL estimate: Taboo mk4 TS EL84 180 Hz headphone SPL estimate: Using data from the above estimates levels for several common headphones and IEMs are plotted on the iso226-2003 Equal Loudness Contours chart: From the chart above note that most headphones won’t produce sufficient SPL to allow audible detection of Taboo AC mains noise for typical listening environments. If a particular space approaches the level of quietness found in my lab, the hum might be perceptible. Insanely sensitive Shure SE215 and Westone UM2 (and IEMs with similar sensitivities) are likely to reveal the Taboo’s AC mains noise in quiet ambient conditions. Note about threshold of AC Mains noise perception In my acoustic lab when listening between 65 and 75 dB SPL on headphones with sensitivity of 95 to 100 dB/mw, mains noise of -75 dBu is at the threshold of perception. At this level I perceive the blackground compromised and a certain sourness to the listening experience that would mostly disappear when listening level was increased to 75 to 85 dB SPL. Mains hum between tracks is still evident. In normal home environments ambient noise masking is likely sufficient to hide this issue if AC mains noise is at or below -75 dBu. If AC mains noise is at or below -85 dBu then I no longer perceive it with the headphones of the sensitivity mentioned in my very quiet acoustic lab. Most listeners will be undisturbed by this issue to which I am so sensitive. Taboo mk4 TS EL84 Distortion vs Amplitude 300R load - hi gain Trace ID: Yel: THD+N Grn: 2nd harmonic distortion (D2) Blu: 3rd harmonic distortion (D3) Red: 4+HD+N (crap factor including noise above 400Hz, AC mains noise removed) As previously shown AC mains noise dominates THD+N measurement below approximately +6 dBu. Above D2 dominates THD+N. With AC mains noise removed from 4+HD+N Taboo demonstrates a much lower distortion profile than the THD+N sweep would indicate. This probably hints at why the Taboo has such great tonality achieving this distortion profile with low Negative FeedBack (NFB.) Again, THD+N obscures a clear picture of the distortion profile for a given component. Think of THD+N as a lossy data compression technique. The danger with such data reduction is some distortions are of more concern than others. Distortion at some frequencies are more audible than others thanks to the non-linear behavior of the Human Auditory System as demonstrated with ISO Contours of Equal Loudness (Fletcher-Munson curves.) Zen Taboo mk4 gain linearity 300R Hi Gain L ch Consider what happens when a pair of headphones produces 100 dB SPL at 0 dBu output from Taboo. This implies 20 dB SPL when Taboo output is -80 dBu. Over that range Taboo has excellent gain linearity. This measurement was made with the Continuous Time Detector with bandwidth from 20 Hz to 20 KHz. AC mains noise is contributing to the measurement deviation below -80 dBu. A narrowband FFT centered around 1 KHz would have eliminated the measurement deviation. Zen Taboo mk4 Output Impedance High gain output impedance is low at 1.2 Ω Changing to low gain increases the OI to approximately 50 Ω. While this was purportedly to reduce AC mains noise perception, I did not care for the change in tonality it produced in the amp with any headphone I tried. Minimal time was spent with listening with low gain. Likewise I made only a few measurements with low gain. Zen Taboo mk4 Output Level change vs gain switch position It should also be noted output levels change considerably with low impedance headphones when the amp is switched to low gain.