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set
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Bypass caps yes/no
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Driver load res value
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Driver bias res val
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Results
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Spectrum
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1
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No bypass
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47k load
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1.6k bias
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We see highish THD, "Classic SET" spectrum with 2nd harmonic dominating. Intermediate position of the bias pot did not offer much differences in picture.
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2
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No bypass
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47k load
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1.6k bias
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Classic SET spectrum with 2nd harmonic dominating, high THD.
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3
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No bypass
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47k load
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1.6...3.8k load
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Intermediate positions of the bias pot did not offer much differences in picture.
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n/a
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4
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No bypass
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77k load
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1.6k bias
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Low distortion, very good spectrum, at -3db, same across the volume range. At very low volume, interestingly, 2nd drops very low: at -20 db - 2nd only a bit higher than 3rd, at -40db they are equal.
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5
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No bypass
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77k load
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3.8k bias
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SET character, higher 2nd harmonic level, hence more THD.
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6
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No bypass
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77k load
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1.6 - 3.8k bias
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with deeper driver bias, distortion goes up. The very minimum of the 3rd (and higher odd) harmonic is at 1.6k position, hence it is the best dial for this config
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n/a |
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7
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Bypass
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77k load
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3.8k bias
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SET character, evenly diminishing products, fairly low THD.
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8
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Bypass
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77k load
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1.6k bias
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SET character. notice *huge* distance between 2nd and 34rd. while it is big at -3db, it is even more appealing at under -10db - high products drop to > -80db wrt the base.
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9
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Bypass
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77k load
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2.8k bias
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the lowest 3rd across the board - for ~ 2.8 - 3 k bias.
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10
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Bypass
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47k load
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1.6k bias
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good space between 2nd and 3rd, 3rd lower than 4th, which is very good
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11
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Bypass
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47k load
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3.8k bias
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about the same level of 2nd and THD, but 3rd is comparatively much higher, and it stays that way across volume range, so 1.6 is better here
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12
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Bypass
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47k load
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3k bias
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Shift towards Push Pull character.This dial offers the lowest THD for the bypass, 47k combination, but alas at the expense of the 3rd. Compare -73dB (1.6k) with -60db here. This is harmonic cancellation at work, with its pluses (low THD) and minuses (spectrum drifting from SET to PP, with higher percentage of odd harmonics).
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For comparison purposes, the plots lined up:
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1
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2
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4
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5
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7
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8
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9
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10
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11
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12
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These patters are mirrored by the other channel as well.
The frequency response plot, into 8 ohm resistive load, with the bypass cap, at full power:
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Now, with bypass cap disconnected.
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Note: The frequency response plots are taken using white-noise source. Since the signal distribution is 20hz..30khz, amplitude level is low but the power is full. Also, the plots are not accurate in LF region due to the measurement method. The plots appear showing persistent LF rolloff across the who;e range, but this effect is due to the setup. Here is the signal directly into the soundcard, showing just how much rolloff is due to the setup:
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