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My hybrid hedhpone amplifier for digital audio players and Moodeaudio.

Another my unusual project. Hybrid headphone amplifier. Powered by a USB power bank. So portable as a booster for a portable player. No volume control because this is already the case in the sound source. 

The tube are Soviet-made low-voltage triodes 6S63N with a metal-ceramic structure known as a nuvistor. Power amplification is provided by LME49600 buffers (audio version BUF634A, diamond topology). There is a power-on delay in switching on the outputs. The whole is closed by a DC/DC converter that provides voltages: + 12.6V for tube heating and output buffers, -11V for DC-servo and + 36V anode voltage for tubes. DC/DC converter works with a clock speed of 1.6MHz.

The first approach is to have no voltage gain for the entire amplifier. I achieved this through the voltage divider between the tube and the output current buffer. At the same time I have a reduction of the signal level and I have a voltage reduction of about 18V at the anode to the level of about 6.3V polarizing the input of the buffer. Tube-buffer DC coupling requires a DC-servo system. A series resistor at the output provides short-circuit resistance and offers an almost constant output power for various impedances ranging from 16 to 40 ohms.

Unfortunately, there were no readable characteristics of the 6S63N tubes available. Since I have a curve-tracer for electron tubes (uTracer 6), I took the characteristics off myself and adjusted the spice model of this lamp to them.

The problem of the constructed unit turned out to be the stability of the converter. The voltage adjustment loop compensation capacitor (C32) had to be removed. The good working point of the tubes offered the dominance of the second harmonic in nonlinear distortions, a much lower level of the third harmonic and a very low and negligible level of higher harmonics. The prototipe made it confirmed with the sound of, and it was confirmed by the measurement results. The sound is a bit brighter and the bass has more energy. 

However, I immediately made a decision that this amplifier should offer a slight voltage gain. This will allow the sound source to be set quieter, which will favorably affect the distortions that arise in it. I did it through a simple modification consisting in adding 1.5uF capacitors in parallel to the R13 and R14 resistors. I obtained an additional 3.3x voltage gain (~ 10dB) for the 60 ohm impedance of my Koss Porta Pro headphones. The modified amplifier was measured.

The next version, keep this voltage gain, there will be a slightly different operating point of the tubes. There will be no DC tube-buffer coupling, which also means no DC-servo system and -11V power supply. All other functional features will remain unchanged.

Finally, I will add that there is a potential in this system to drive high-impedance (400 and 600 ohms) headphones with good efficiency. This will require a higher signal level and work with a THD level of around 3%, but here the second harmonic will dominate, which is not perceived by our sense of hearing as a distortion.

Simplified internal structure of LME49600 buffers:

Matching the model to the characteristics of the 6S63N tube obtained from uTracer 6:

Schematic diagram of the first version, assembled as a prototype:

PCB project:

Measurement results for the input signal level 1Vpeak (0.707Vrms)

Intermodulation distortions as defined by SMPTE, DIN and CCIF:

Crosstalk at 1kHz, 100Hz and 10kHz for each channel:
Channel L: -67.9dB, -67.5dB, -66.2dB 
Channel R: -67.7dB, -67.3dB -65.8dB

Noise floor:

THD without weight filter:

THD with A filter:

Spectrum of THD with specification of harmonics for different levels of the input signal (0dB = 1Vp):

THD as a function of frequency, detailing the first harmonics. Two measurements for each channel:

Visualization of THD spectrum differences for both channels resulting from the dispersion of lamp parameters (input lever differs from 1Vp):
My little brick in this device - a tube buffer similar to the diamond topology (double cross coupled balanced path).
Design of the second version of the amplifier. 

The SPICE model is a tube with a slightly different adjustment to the characteristics than for the first version of the amplifier assembled as a prototype.

**** 6S63N_LV ******************************************
* Created on 07/09/2022 10:30 using paint_kit.jar 3.1
* Plate Curves image file: 6s63n1a2.bmp
* Data source link:
.SUBCKT TRIODE_6S63N_LV 1 2 3 ; Plate Grid Cathode
+ PARAMS: CCG=3.5P  CGP=2.4P CCP=1.3P RGI=2000
+ MU=17.57 KG1=180.95 KP=47.04 KVB=148.5 VCT=0.1465 EX=1.289
* Vp_MAX=50 Ip_MAX=25 Vg_step=0.5 Vg_start=0 Vg_count=9
* Rp=8200 Vg_ac=0.5 P_max=40 Vg_qui=-1.5 Vp_qui=19
* X_MIN=190 Y_MIN=134 X_SIZE=948 Y_SIZE=662 FSZ_X=1938 FSZ_Y=1038 XYGrid=false
* showLoadLine=n showIp=y isDHT=n isPP=n isAsymPP=n showDissipLimit=y
* showIg1=n gridLevel2=n isInputSnapped=n 
* XYProjections=n harmonicPlot=n dissipPlot=n
E1 7 0 VALUE={V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(VCT+V(2,3))/SQRT(KVB+V(1,3)*V(1,3)))))}
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}

Simulations of second version: diagram, oscillogram, frequency response and enlarged diagram of the frequency response to show the limits at a decrease of 0.5dB and the THD coefficient:

Schematic diagram:

PCB project and visualisation:

Bll of materials:
My little brick in this device - a tube buffer similar to the diamond topology (double cross coupled balanced path).

OMG! I haven't seen a nuvister since I earned pocket money during college helping a friend repair RCA New Vista TVs (among others) back in the '60s!

I tip my hat to you, sir, for your hardware design and implementation efforts. 

Many types of nuvistors were produced in the Soviet Union until the  begin 90's. Unlike the American types, the Russians produced them with long, flexible leads, because they did not use plugs for these tubes. In addition to triodes, tetrodes also produced pentodes in this form, even power pentodes.

Some nuvistors from Soviet Union:
Standard triodes  6C51H (7586), 6C52H (7895), 6C62H
Low voltage triode 6C63H

UHF triode 6C53H (european EC1010):

Tetrodes  6Э12H (7587), 6Э13H, 6Э14H
Small signal pentode 6Ж54Н  (and triode 6C65H has identical form, differs from standard classical nuvistors)

Power pentode (Pa = 15W! grater than EL84 tube) 6П37Н

Suffix -B means military version of tube (6C51H - standard version, 6C51H-B - military version). 

Some of them were like 6C51H, 6C52H, 6Э12H produced in huge numbers, still easy to buy, cheap. The rest are considered rare (small amount of production). Relatively easy to acquire is this 6П37Н power pentode.

Another very interesting tubes are the rod tube, invented in the Soviet Union and only produced there.
My little brick in this device - a tube buffer similar to the diamond topology (double cross coupled balanced path).

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