ELEKIT TU-8200R

Here are some of my attempts at measuring noise and distortion of my TU-8200R, using REW with a Focusrite Scarlett Solo as DAC and ADC.
Solo loopback at 1 kHz:
Focusrite Scarlett Solo loopback 3.37 V out min input gain (balanced).jpg

Signal to noise and SINAD are at about 93 to 94 dB, not great, but sufficient for these purposes.

With the TU-8200R in the loop, first a typical distortion plot at 1 kHz, this one for 100 mW output into 8 Ohm. 100 mW is a bit higher than my typical listening volume:
TU L 100 mW 0 dB.jpg


There is quite some mains noise; I can't exclude that some of it is due to my measurement setup. The Solo DAC-to-ADC loopback does not show that noise (see plot above).

Below is a summary table showing signal-to-noise ratio SNR, signal to noise and distortion SINAD, and total harmonics THD and the 2nd and 3rd contribution to it, as function of amplifier output power. I believe REW quotes SNR unweighted, whereas SINAD is A-weighted, which means the elevated noise we see at low frequencies in the plot above is de-weighted in the SINAD computation relative to SNR.

Power per

channel

(8 Ohm) [W]
SNR

[dB]
SINAD

[dB]
THD

[%]
2nd

[%]
3rd

[%]
1 mW
48
52
0.03
0.018
0.019
10 mW
58
59
0.07
0.060
0.027
100 mW
68
54
0.18
0.17
0.031
1 W
77
45
0.49
0.43
0.23
2 W
81
43
0.61
0.48
0.38
4 W
78
39
1.03
0.37
0.73
5 W
82
27
3.88
1.49
2.66
I think the THD numbers are generally in line with Ken Rockwell's measurement of the TU-8200 (without the Lundahl output transformers) up to about 1 W. Above 1 W my amplifier seems to have relatively lower (but still high, of course) distortion.

If we can believe the above numbers, 10 to 100 mW seems to be the sweet spot of this amplifier. Noise and distortion are at the 1-2 permille level, and the second harmonic dominates. At 1 to 2 W distortion approaches the percent level, and the third harmonic creeps up on the second. Beyond that the third harmonic dominates.
 
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I'm curious too, Gotan. First I have to gain some more experience with REW and my measurement setup. I also have a Digilent AD2 on order, which supposedly allows similar measurements, but with completely different hardware (with only 14 bits, but still). That should give me some more confidence in what I'm doing.

I'm also not giving up hope that Victor - or somebody else near Seattle - might drop off an amplifier or two with Amir of ASR, who could do much better measurements.
 
Here are some measurement results from the 8200 as well:
https://www.diyaudio.com/community/threads/tu-8200-measurements.389413/

Before I bought my own TU-8200R, I also read almost every source for reviews. Also Ken Rockwell's review illustrated with measurements. These reports made me totally curious about the amplifier, and I am completely satisfied with it. I mentioned that I sometimes listen to a Moth 30, which is a real little "bastard". I've never had an amplifier that gave me so many goosebumps. It is literally chilling how it interprets the emotions in the right music.

And in addition to this special effect, I listen to the TU-8200R much more because of its calm musicality and spatialization of the event. Mainly 807 in triode mode + ECC40 drive tubes enchant me. The music is simply that much calmer, and the atmosphere it creates is more than exciting.
I'm writing this just to show you whatever THD and distortion measurements, this is a damn good amplifier.
 
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The power cord from the TU-8200R kit came in handy today, when I replaced my external monitor's power cable with it (the monitor does have a plastic enclosure, unlike the TU), and got rid of a noise source. I couldn't do any measurements with this monitor connected, but it is entirely possible that there was some inaudible remaining contribution.
 
I think my power transformer it's toasted, the amp shut down itself a couple days ago and now it won't turn on, no led or heating power. In fact, After disassembling the whole unit PCB by PCB I couldn't find any bad soldering or any other faulty components (at least visually).

So I took some measurements at the power transformer output connector, blue and yellow cables are normal (about 6 and 24v respectively according to the manual) but red ones are dead, zero, nada.

Is there anything else I should check?. Thanks in advance!
 
I think my power transformer it's toasted, the amp shut down itself a couple days ago and now it won't turn on, no led or heating power. In fact, After disassembling the whole unit PCB by PCB I couldn't find any bad soldering or any other faulty components (at least visually).

So I took some measurements at the power transformer output connector, blue and yellow cables are normal (about 6 and 24v respectively according to the manual) but red ones are dead, zero, nada.

Is there anything else I should check?. Thanks in advance!
Stupid question but the fuse is OK?
 
I tried to compare distortion at the speaker output and at the 6L6GC grid of the TU-8200R. I posted the same odd (to me) finding also at ASR, but I think the DIYaudio forum might be a better place to get feedback.

Victor, moderators, if you think this content is better homed in another subforum, please move it there. It is about the TU-8200R though.

So, it seems the 2nd harmonic is higher at the grid than out the output, or, saying it differently, there seems to be 2nd harmonic compensation between the driver and the output tube. I hooked up my Analog Discovery 2 USB scope & signal generator to my Elekit TU-8200R, and I find the harmonics at the speaker output to be more or less in line with what I had seen earlier using REW and the Focusrite Scarlett Solo as DAC and ADC, namely that from a few Watt onwards the third harmonic overtakes the second. But to my surprise the situation at the grid of the output/power tube (the 6L6GC) is different: the second harmonic is higher than the third at 6.25 W output.
Here is the FFT derived from the AD2 scope observation; ch.1 is the grid, ch.2 the speaker output:
1688020961602.png

Settings:
  • 1 kHz 100 mV amplitude in
  • 10 V amplitude out at speaker terminals, with TU-8200R volume turned all the way up
  • 4 V amplitude at grid
  • Gain = 20 dB
  • Amp set to ultralinear mode
  • Output impedance switch to low (at high setting, gain is 0.5 dB higher)
  • Speaker output terminated into 8 Ohm 50 W resistor
  • Output power is 6.25 W (10 V ^2 / 2 / 8 Ohm)
So, is the TU-8200R designed to compensate (second) harmonics between the driver and the output stage, or is there another explanation for this observation?
 
I promptly confused the two scope channels (but not the FFT channels). The amplitudes and the gain number in the post above are wrong.
The grid amplitude was 10 V, and the output amplitude 4.0 V. So the gain is 32 dB, and the output power is 1 W in ultralinear mode for 100 mV input amplitude, and maximum gain.

Also, because I'm now switching the amplifier mode setting quite regularly for my measurements, I measured the decay time for the anode/screen bias voltage: ten seconds after turning the amplifier off it is already below 40 V. so it becomes save pretty quickly.
 
Here are some graphs and tables comparing triode, pentode and ultralinear mode at 1 Watt output power.
If I'm not mistaken, the conclusion is that you want to use this amplifier in triode mode at this power level for minimum distortion.

Also, while using the high impedance setting for the output transformer buys a bit (0.5 dB) of power into 8 Ohm,
you get a few dB lower distortion by keeping the switch set to low impedance.

In the graphs below, the speaker output is in blue, on channel 2. Channel 1 shows the grid of the 6L6GC output vacuum tube.
While the tables are a bit uneven in that output levels are not exactly the same (which doesn't change the relative distortion levels and the conclusion), to produce the graphs I adjusted the gain so that the fundamental at the speaker output was at 9.03+/- 0.03 dBV for each mode.

Pentode mode at 1 W; 2nd and 3rd are equal, and you can see some higher harmonics already creeping up above the noise on the grid:
TU-8200R Pentode mode grid + output R 1 kHz 1 W FFT.jpeg


Ultralinear mode at 1 W; third harmonic is higher than second (on the speaker output, not on the grid):
TU-8200R UL mode grid + output R 1 kHz 1 W FFT.jpeg


Triode mode at 1 W:
TU-8200R Triode mode grid + output R 1 kHz 1 W FFT.jpeg


I'm a bit surprised, but that's what it is.

Pentode:
Distortions1 kHz
fundamental
2nd
Delta dB
3rd
Delta dB
Max Grid R 9.0 dBV
3.5 V
-46.7-40.7
Max Out R7.6 dBV
4.1 V
1.05 W
-45.8-46.5

Ultralinear:
Distortion1 kHz
fundamental
2nd
dB relative
3rd
dB relative
Grid L16.94 dBV-37.4-58.6
Out L9.0 dBV-49.7-52.6

Triode:

Low impedance setting
Distortions

1 kHz
fundamental

2nd
Delta dB

3rd
Delta dB

Grid L

20.7 dBV
15.5 V

-33.5

-74.5

Out L

8.5 dBV
3.9 V
0.95 W

-48.8

-70.7

High impedance setting
Distortions

1 kHz
fundamental

2nd
Delta dB

3rd
Delta dB

Grid R

20.2 dBV
15.7 V

-32.4

-63.2

Out R

9.08 dBV
4.3 V
1 W

-41.4

-56.8
 

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Nice job Gruesome, thank you very much!
Several times I switch to pentode or UL mode while I am listening music, but the favourite mode for me is Triode.
Triode mode lacks a bit of punch of pentode but it has a very uniqe presentation.
So I am sorry that some of my 807 tubes don't run in Triode mode. They have in triode some weird noise (like a ground hog effect), but they working perfect in UL or pentode. Strange.
Some of 807s and esp. all of my ATS25 and 5933 working perfect in triode mode too.
 
I will have to repeat the measurement with the REW + Focusrite DAC/ADC setup. Maybe also with the KEF R3's and the microphone.
I'm just a bit baffled that pentode and UL mode at the same low power show more distortion than triode mode.
I expected these modes to surpass triode mode in useable power. That does not seem to be the case.

It's true that I can squeeze another two Watt out of both UL and pentode mode, but the higher modes looks truly awful at 3.3 W in pentode mode:
TU-8200R Pentode mode grid + output R 1 kHz 3.3 W FFT.jpeg



Ditto for UL mode at 3W:
TU-8200R UL mode grid + output R 1 kHz 13.8 dBV FFT.jpeg


Both these power levels were picked as the maximum before the higher harmonics jump up to a constant level independent of order.
With the REW & Focusrite Solo setup I was measuring a few percent of THD up to 5 W power in UL mode, so these USB scope results need cross checking.
 
I think it's easier to compare the 1 kHz distortion measurements in graphical form. First the data taken with the low impedance output setting (into an 8 Ohm resistor):
Triode mode is blue, Ultralinear orange and Pentode mode yellow. The 3rd harmonics have the same colors with dashed lines.
The lowest power setting is 0.1 Watt.
1688362302321.png

Up to 1.5 Watt, Triode mode has lower 3rd harmonic than both Pentode and Ultralinear mode.

The distortions measured with the output impedance switch set to high (again, into the same 8 Ohm resistor):
1688362480897.png

In the high impedance setting as well, Triode mode has lower 3rd harmonic than Pentode mode and Ultralinear mode up to 1.5 Watt. At 1.5 Watt, Triode mode in the high impedance setting has lower harmonic distortions than in the low impedance setting. At 1 W and below, the distortions are lower at the low impedance setting.

Ultralinear mode has lower 3rd harmonics relative to 2nd in the high impedance setting.

For Pentode mode at both output impedance settings, and for Ultralinear mode at the low impedance setting, the 3rd harmonic is disappointingly high compared to the 2nd harmonic.

So far I have cross checked only one data point with REW, Triode high impedance setting at 1 W output power. I am getting the same numbers for 2nd and 3rd harmonic, -41 dB and -60 dB respectively. THD is -41 dB (same as 2nd).
 
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The 2nd harmonics data point at 1.5 W in the low impedance plot is wrong; I thought I had a typo in my data table where it said '-67 dB', and corrected it to -37 dB, but upon cross checking my results with REW and the Focusrite Solo DAC/ADC, there is indeed an odd structure in the harmonic distortions around 1.5 W output power. I thought it might be related to an audible output transformer resonance near 7 kHz, but that seems to be a different issue.

Here is a plot of THD, 2nd and 3rd harmonics levels at 800 Hz, 1000 Hz and 1200 Hz. The 2nd harmonics at 1,000 Hz and 1,200 Hz are almost on top of each other. All three frequencies show the dip in 2nd harmonic at the same power of 1.5 W:
1688517699266.png


What is not shown, but contributes significantly to the THD, are the strong excitations of the 7.2 kHz transformer resonance by the 9th harmonic at 800 Hz and 6th harmonic at 1,200 Hz.

Here is the structure in the (right channel) TU-8200R noise near 7 kHz; there are actually four peaks, at 7,138 Hz, 7,200 Hz, 7,272 Hz and 7,348 Hz, and all of them can be excited, either directly or as a harmonic, and lead to audible output transformer noise:
1688517175192.png


Is there a way to get rid of these electromechanical transformer resonances? I tried pushing on the coils and also on the core laminations with a (plastic) pen, but did not observe any change in sound. Would one have to embed the whole transformer in resin? Or (re)paint the coils? I do not really want to take the transformers apart.
 

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