The Bachelor technical measurements

The Bachelor 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. 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 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.

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 - 32 ohm load SE input LoZ SE output part B
Post 4 - distortion at various output levels part C
Post 5 - IMD sweeps part D
Post 6 - distortion vs amplitude (power) part E
Post 7 - reserved for corrections or additional data

Listening evaluation picture:


Measurement setup picture:


Output Impedance:
HiZ: 5.7 ohm (5.7R)
LoZ: 2.2 ohm (2.2R)

Notable highlights:
Very low output impedance for a transformer output tube amplifier this size.
Bachelor proves smaller size and lower cost doesn't necessarily correlate to lower performance.

Greatest differences between Bachelor and SW51+:
Bachelor output impedance is roughly 10x lower than SW51+.
Distortion sweet spot is approximately 8 dBu higher than SW51+.
Overall size and footprint are smaller than SW51+.
Bachelor has smaller transformers.
Bachelor uses 6z1p whereas SW51+ uses 6z51p. Both are triode strapped pentode topologies.
Bachelor is lower cost.

Otherwise this report is substantially similar to SW51+ both measured data and comments.

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 the Bachelor. 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.

Bachelor 6z1p A04 THD+N THD nth-HD FFT 300R HiZ - 4+HD+N with 60Hz 0dBu

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 narrative will break the complexity into smaller increments and explain what is represented. Specific numbers referenced are for the LEFT CHANNEL.

Beginning with THD+N we note what appears to be a rather high number of 0.14% 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 one.

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.

4+ HD+N is a measurement I developed 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. Bachelor has 0.003% crap factor which is approximately -90 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, horizontal Blue line on the Right channel we note mains noise and other residual noise are all at or below -90 dBu, relatively inaudible. Consider the following:
-90 dBu 300R 98 dB/mW HD800 will produce 11 dBSPL
-90 dBu 55R 104 dB/mW Focal Clear will produce 24 dBSPL
-90 dBu 27R 106 dB/mW ER4P-T will produce 29.5 dBSPL

Gain Linearity

Exceptional gain linearity down to -95 dBu, remarkable for a SET amp, especially considering the price point.

Frequency Response

-3 dB at 5Hz and 75 KHz for 300 ohm load on HiZ output!

Bachelor Polarity - input Red, output Blue

Output polarity is inverted from input. For those sensitive to this should take appropriate notice.

Bottom line
a minimalist, small footprint, low cost SET amplifier with very special sound, a 2nd harmonic heaven for SET magic.

Well done again @Zampotech!

 

300 ohm load SE input HiZ SE output part A

Bachelor A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output


Bachelor 50 + 7000 Hz 300 ohm load SE input HiZ SE output - Left Channel


Bachelor Gain Linearity 300 ohm load SE input HiZ SE output - Left Channel


Bachelor THD+N vs Frequency 300 ohm load SE input HiZ SE output - Left Channel


Bachelor Residual Noise 300 ohm load SE input HiZ SE output - Left Channel


Bachelor Frequency Response 300 ohm load SE input HiZ SE output - Left Channel


Bachelor Square Wave 2000mV 5uS/div 300 ohm load HiZ SE output - Left Channel
Inset displays overall square wave response at 10mS/div

Bandwidth estimation: BW (MHz) = 0.35 / RT (mS)
Where RT = 10 to 90% Rise Time
0.35 / 5.189uS = 67.45 KHz

Complete 300 ohm load SE input HiZ SE output analysis report pdf attached

 

32 ohm load SE input LoZ SE output part B

Bachelor A04 THD+N THD nth-HD FFT 32 ohm load SE input LoZ SE output

Bachelor 50 + 7000 Hz 32 ohm load SE input LoZ SE output - Left Channel


Bachelor Gain Linearity 32 ohm load SE input LoZ SE output - Left Channel


Bachelor THD+N vs Frequency 32 ohm load SE input LoZ SE output - Left Channel


Bachelor Residual Noise 32 ohm load SE input LoZ SE output - Left Channel


Bachelor Frequency Response 32 ohm load SE input LoZ SE output - Left Channel

-3 dB at 5 Hz and 45 KHz

Bachelor Square Wave 2000mV 5uS/div 32 ohm load LoZ SE output - Left Channel
Inset displays overall square wave response at 10mS/div

Bandwidth estimation: BW (MHz) = 0.35 / RT (mS)
Where RT = 10 to 90% Rise Time
0.35 / 6.374uS = 54.91 KHz

Complete 32 ohm load SE input LoZ SE output analysis report pdf attached

 

distortion at various output levels part C

300R load HiZ output

Bachelor A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output at 0 dBu


Bachelor A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output at -10 dBu


Bachelor A04 THD+N THD nth-HD FFT 300 ohm load SE input HiZ SE output at -20 dBu


32R load LoZ output

Bachelor A04 THD+N THD nth-HD FFT 32 ohm load SE input LoZ SE output at 0 dBu


Bachelor A04 THD+N THD nth-HD FFT 32 ohm load SE input LoZ SE output at -10 dBu


Bachelor A04 THD+N THD nth-HD FFT 32 ohm load SE input LoZ SE output at -20 dBu


Distortion lowers with lower operational levels, common to all three measurement sets. The sweet spot to me is -10 dBu and -20 dBu with 300 ohm headphones.

 

IMD sweeps part D

Bachelor 300 ohm HiZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to 0 dB gain


Bachelor 32 ohm LoZ IMD sweep 19+20 KHz to 5+6 KHz - amp set to 0 dB gain


Bachelor 300 ohm HiZ 19+20 KHz IMD vs amplitude - amp gain set to 0 dB


Bachelor 32 ohm LoZ 19+20 KHz IMD vs amplitude - amp gain set to 0 dB

 

distortion vs amplitude (power) part E

Note: some measurements made at 40 Hz, 440 Hz and 1000 Hz. Watch graph annotation.

300R load measurements

Bachelor 6z1p THD+N vs amplitude at 440Hz - 300R load HiZ: with annotations:

Red and Blue lines are gain linearity. Where THD+N crosses over Gain Linearity falling off is a good indication for quality maximum power.

Power levels and approximate SPL levels for HD6x0 and HD800. Listening to music at 90 dB SPL with peak to average ratio of 10 dB will have peaks hitting 100 dB SPL. That is LOUD. In that range the Bachelor is running between 0.06 and 0.13 % THD+N. But remember in the posts above Bachelor THD+N is 2nd harmonic dominant. So really the amp is presenting between 0.06 and 0.13 % 2nd harmonic distortion. Sweet! Even better for those listeners that prefer 80 dB SPL which is still pretty loud.

Bachelor 6z1p THD+N and 4+HD+N vs amplitude at 1000Hz - 300R load HiZ: annotated 1:

This sweet spot occurs between approximately -5 and +5 dBu, consistent with listening level peaks of 95 to 105 dB SPL
THD+N is 2nd and 3rd harmonic dominant while 4+HD+N distortion is at a minimum below 0.02 %
Bachelor sweep spot is 8 dBu higher than SW51+

Bachelor 6z1p THD+N and 4+HD+N vs amplitude at 1000Hz - 300R load HiZ: annotated 2:

Now if the music has a peak to average ratio of 10 dB, then the amplifier preferred operating area is approximately bounded by the green rectangle above.

40 Hz 300R load measurements
Bachelor 6z1p THD+N vs amplitude at 40Hz - 300R load HiZ:

For completeness 40 Hz performance should be considered, especially for a transformer coupled amplifier.


32R load measurements

Bachelor 6z1p THD+N vs amplitude at 440Hz - 32R load LoZ with annotations:

Again, approximating for a low impedance headphone. The HE-500 is actually 38R instead of 32R used for testing. But the concept remains close enough.

Bachelor 6z1p THD+N and 4+HD+N vs amplitude at 1000Hz - 32R load LoZ: annotated 1:

This sweet spot occurs between approximately -10 and 0 dBu, consistent with listening level peaks of 90 to 100 dB SPL

Bachelor 6z1p THD+N and 4+HD+N vs amplitude at 1000Hz - 32R load LoZ: annotated 2:

Now if the music has a peak to average ratio of 10 dB, then the amplifier preferred operating area is approximately bounded by the green rectangle above.

40 Hz 32R load measurements
Bachelor 6z1p THD+N vs amplitude at 40Hz - 32R load LoZ:


So how much power does one need?
Depends on the listening level, distortion level tolerated, type of distortion produced by amp, etc, but for many headphones and listeners 20mW at 300R and 100 mW at 32R is more than enough; Keep in mind the Bachelor is 2nd harmonic dominant at those levels.

 

reserved for corrections or additional data