Shortest Way 51+ (sw51+) 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 (if used) 2. Balanced XLR cables Belden 1800F with Neutrik 110R AES connectors (if used) 3. Single Ended cables Audioblast HQ-1 1.5 ft with Rean RCA connectors 4. 30 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 to achieve 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 Measurements 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. Shortest Way 51+ (sw51+) Measurements commenced after 1 hour of warmup. Measurements were performed over a period of several days. Index Post 1 - measurement setup description, highlights Post 2 - 300 ohm load SE input HiZ SE output part A Post 3 - 30 ohm load SE input LoZ SE output part B Post 4 - 300 ohm load SE input LoZ SE output part C Post 5 - distortion at various output levels part D Post 6 - IMD sweeps part E Post 7 - possibly more sweeps Post 8 - reserved for corrections or additional data Setup picture: Output Impedance: HiZ: 54 ohm (54R) LoZ: 27 ohm (27R) Notable highlights: Due to some interesting attributes of this amp I will provide additional commentary and interpretation from which I would normally refrain. Those frightened by high distortion measurements will have the bejesus scared out of them by sw51+. This very special sounding amp offers an opportunity to demonstrate how chasing low distortion numbers out of context is the very definition of foolishness. It is noted that both THD+N and IMD measure rather high on this amp. Those bothered by such high levels and regard measurements as some sort of diety should bail out here and not waste time with what follows. The above complex display contains considerable information. The most informative are the FFT spectrums but many want to see numbers, of which there are plenty on this display. The following series will break the complexity into smaller increments and explain what is represented. Beginning with THD+N we note what appears to be a rather high number of 0.5% THD+N which is an oversimplification. THD+N was one of the first audio measurements and historically easy to measure. Think back 60 years and imagine how to make a measurement without computers. THD+N is simply the insertion of a signal to a component, removing that stimulus with a notch filter and measuring what remains. The remainder comprises everything including mains hum and harmonics, harmonic distortion, residual noise, inharmonic distortion, etc. So let's break the THD+N down into components. Expanding scope a bit we see THD+N, THD and 2nd harmonic distortion are almost the same value. This is the first clue that THD+N and THD are 2nd harmonic dominant. I believe 2nd harmonic distortion is part of the magic that provides an enriched presentation of the music. Make no mistake, this is a coloration, but in my opinion a very pleasant euphonic distortion. Now look at 3rd harmonic distortion. Note this value is a factor of 10 lower than 2nd harmonic distortion. Again, my opinion, I believe 2nd and 3rd harmonic distortion ratio comprise the voicing of a component. So this smaller amount of 3rd harmonic distortion adds just a hint of sparkle to the 2nd harmonic enriched sound. 4th and 5th harmonic distortion are a factor of 100 lower than the 2nd harmonic, an insignificant level. Hum and residual noise likewise are a factor of 100 lower than the second harmonic, also insignificant. This is a measurement I developed, 4+ HD+N, with stimulus, 2nd and 3rd harmonic removed. What remains is a simplification of 4th, 5th and higher harmonics, mains noise, residual noise, in-harmonic noise. This is what I consider the crap factor. A low number here is desirable. sw51+ has 0.005% crap factor which is approximately -86 dBu, well below audibility. Putting the foregoing into perspective the harmonic distortion depicted in the FFT is downward sloping as shown by the Blue line on the Right channel, highly desirable for those who appreciate what 2nd harmonic distortion offers, of which I include myself for many recreational listening situations; not all though. Finally, adding another horizontal Blue line on the Right channel we note mains noise and other residual noise are all at or below -85 dBu, relatively inaudible. Consider the following: -85 dBu 300R 98 dB/mW HD800 will produce 16 dBSPL -85 dBu 55R 104 dB/mW Focal Clear will produce 29 dBSPL -85 dBu 27R 106 dB/mW ER4P-T will produce 34.5 dBSPL Exceptional gain linearity down to -100 dBu, remarkable for a SET amp, especially considering the price point. Bottom line, a minimalist SET amplifier with very special sound, a 2nd harmonic heaven for SET magic and excellent gain linearity. Well done @Zampotech !