xDuoo TA-66 technical measurements

XDuoo TA-66 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, Tektronix ADA400A 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. 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. Stock tubes provided by xDuoo used

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 approximately 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 each session.
Measurements were performed over a period of several days.

Index
Post 1 – Measurement setup description, highlights
Post 1 – Distortion measurements and Output Impedance
Post 2 – Other measurements
Post 3 – Distortion surface measurements
Post 4 – reserved for corrections and / or additional data

Measurement setup picture:

Standard setup for TA-66 with 300R load

Listening evaluation picture:

TA-66 evaluated with Holo Audio Cyan 2 and KTE Spring 3 SE outputs. TA-66 amp volume set for approximately 0 dB gain and Goldpoint SA3+2 external stepped attenuator employed to adjust sound levels. HD800-JAR and HD6x0 headphones most used for listening evaluation.

There are plenty of impressions threads on the forum where more detailed and nuanced thoughts about the various components are discussed with an example
found here

For my preferences, either I like how a system renders music or I don’t. I found a lot to enjoy with this amp.

Notable highlights:
Very well balanced design. Intelligent design tradeoffs to achieve a really great sounding amp at such a low price point which are supported by both listening evaluation and measurements.

Quite notable observations for a 6AS7 type cathode follower OTL amplifier with no negative feedback:
± 1 dB gain linearity over 100 dB range
SNR greater than 90 dB
Very good square wave response
Frequency Response: 15 Hz to greater than 88 KHz (-3dB)
Bandwidth: 125 KHz (transient response)
Clarity in the highs, lovely midrange without bloated bass

Due to some very interesting attributes of this amp I will provide additional commentary and interpretation from which I would normally refrain.

Let's get this out of the way.
Generator level set to 0 dBu, Amp set for 0 dB gain with 300R load
THD+N: 0.09308% Left channel (SINAD 60.6 dB)
THD+N: 0.08941% Right channel (SINAD 61.0 dB)

Those who worship THD+N / SINAD should stop at this point and move on, nothing to see here. 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 delightful amp offers another opportunity to demonstrate how chasing low distortion numbers out of context is the very definition of foolishness.

XDuoo TA-66 Distortion at 1 KHz, ~0 dBu, 300R load

In this measurement suite the THD+N measures as expected. Observe that 2rd harmonic is the major contributor to THD+N at 0 dBu. Harmonic distortion follows a very desirable descending pattern. Also note how 60 Hz AC Hum is observed to be below -80 dBu and aggregate noise is approximately -80 dBu in both channels yielding background AC mains and residual noise below audible threshold with typical high impedance dynamic headphones.

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.

The following series of distortion measurements provide a sense of harmonic distortion with respect to AC mains noise as listening levels are lowered.

XDuoo TA-66 Distortion at 1 KHz, 0 dBu, 300R load


XDuoo TA-66 Distortion at 1 KHz, -10 dBu, 300R load


XDuoo TA-66 Distortion at 1 KHz, -20 dBu, 300R load


XDuoo TA-66 Distortion at 1 KHz, -30 dBu, 300R load

Observe how AC mains noise remains constant as the primary signal and related harmonic distortion drop with lowered levels.


Previous four graphs animated for enhanced comparison visualization.

TA-66 Distortion vs amplitude 1KHz 300R -30 to +20 dBu

Trace ID:
Yel: THD+N
Grn: 2nd harmonic distortion (D2)
Brn: 3rd harmonic distortion (D3)
Red: 4+HD+N (crap factor 400Hz to 22KHz)

In the graph above 2rd harmonic distortion, D2 increases with increasing level and is the dominant contributor to THD+N above approximately -9 dBu. Below that level PS Noise becomes the dominant contributor to the THD+N metric. Above approximately 16 dBu D3 becomes greater than D2 and is the dominant contributor to THD+N.

4+HD+N remains considerably below THD+N over the entire range suggesting that crap above 400Hz is likely masked by everything else rendering a pleasing background.

It should be noted that 0 dBu drives many headphones to approximately 100 dB SPL which is too loud for long listening sessions and long term hearing health. A distortion sweet spot exists between -20 and 0 dBu where THD+N is lowest. Target listening average levels of 70 to 80 dB SPL with 10 dB peaks hitting this zone is a good compromise between distortion and healthy listening levels.

TA-66 Output Impedance:

Output impedance is approximately 125 Ω
Clearly high nominal impedance headphones are likely to achieve good results while those with lower impedance may or may not render desirable sound quality.

 

TA-66 frequency response 300R load

Impressive for this OTL amp topology. Note L and R channels are closely matched.

TA-66 square wave response 100 Hz 2Vpp 300R load

Also impressive for this OTL amp topology.

TA-66 Multitone 0 dBu 300R load

Most of the grass between tones is 45 dB or more below tone levels. Very reasonable.

TA-66 stepped volume control behavior

The intersection of the knob positions (1 to 24) and the curve shown are the only levels available. Interpolation was drawn to see the audio contour but those in-between levels are not available. Further, the actual gain achieved will be dependent on the gm of both 6n2 and 6n5pj tubes. Observe again L and R channels are closely matched.

TA-66 residual noise

60 and 120 Hz are below detection threshold for headphones with sensitivities of 98 ~ 100 dB/mW at 100 to 300 ohms.
Having the power switch on the chassis rear confining AC mains to a small area may be contributing to this fine result for this amplifier topology.

Complete 300 ohm load measurement report pdf attached.

 

Distortion surfaces


While interesting, THD+N is a lossy information compression. Decomposing into D2, D3 and 4+HD+N below will provide additional insight into the distortion behavior of this amp.



D2 increases as level increases adding a perception of greater loudness at higher levels. D2 also increases at lower frequencies where perception may consider this to add punch to the bass.



D3 rises with level but remains below D2 until approximately +16 dBu (seen in earlier graphs.) Below -15 dBu and above 1000 Hz it vanishes. This could be perceived as reducing edginess or it might be perceived as diminished excitement depending on personal preferences.



4+HD+N is a smooth surface remaining considerably below D2 until drops under -20 dBu and below D3 until level drops under -10 dBu. Below -20 dBu AC mains noise and other residual noise below 400 Hz will dominate thus masking the crap.

 

reserved for corrections and / or additional data

 

Added TA-66 residual noise measurement to post 2.

 

Thanks @atomicbob for the measurements.

I also perceived some noise from my unit, but that was acceptable in my use. For those who may be bothered by this “feature” it might be difficult to unhear what is there. It’s not the inkiest of amps.

Update: I believe the noise in mine was actually coming from the Yggdrasil. If there is any mains noise, I can’t perceive it…or at least I can’t hear with my tin ears. (I do run a cheap mains filter strip though.) And now, for me to check on the DAC output….