[IDEA] Hybrid amplifier: tube + class D

[STUDI]

My hybtrid tube preamp + class D power stage prototype amplifier.
First step: volume potentiometer with switch, switch power by relay, DIP switch to configure oscillator frequency of power stage. 
Major changes in next version: no DIP switches, oscillator set to 1.2MHz, dealy muting and no power switch because capacitors with low ESR can stick the contacts together.
Minor changes: increased load on the anode voltage converter to reduce the noise level.

The appearance of the first (prototype) version:
 

Final schematic (for the next versions):


Inductors:
MSS1210 - 10uH: L1-L4, 1.5mH: L5
MSS1278 - 330uH: L6
MSS7348 - 680uH: L7, 1mH: L8
All inductors manufactured by Coilcraft

Capacitors types:
MKS2: C3, C4, C9-C12
0805 size, X7R: C31, C32, C41-C43, C44, C46, C48, C49, C54, C55
1206 size, X7R: C13-C16, C52, C57, C58
1812 size, X7R: C21-C24 
0805 size, C0G: C1, C2, C5, C6, C17, C29, C30, C39, C40
1206 size, C0G: C17-C20, C25-C28
hybrid (OSCON): C7, C8, C45, C47, C51, C53
standard aluminium: C33-C38, C50, C56, C59

Capacitors voltages:
C5, C6, C56, C57, C58: 100V
C7, C8, C9, C10, C51: 63V
C3, C4, C11, C12, C20-C32, C39-C43, C46, C48-C50, C54-C56: 50V
C33, C34, C35, C36, C37, C38, C59: 35V
C47: 25V
C44: 16V
C45: 10V

Resistors:
Mini-MELF 0204: R1-R16
MELF 0207: R17-R20
1206 0.5W: R30
0805 0.125W - all others

Resistors tollerance:
1% all Mini-MELF, MELF and R31, R32, R34, R35
5% all others

Electron tubes:
V1, V2, metal-ceramic (nuvistor) triodes, mu=32, gm=11.5mA/V - 6S51N or 6S51N-V (russian products).
V3, subminiature magic eye DM70


Layout of PCB

top side:


bottom side:



PCB dimensions: 147mm * 98mm


Preamplifier stages, typical voltage stages on triodes. Cathode resistors unblocked. The achieved gain of the degree is approximately 14 times.

Magic eye controlled by two voltage doublers with a common output condenser. Both rectifiers are supplied directly from the positive output terminals. No regulation, for full control of the 22.5 - 24 V voltage generator, the "exclamation mark" is completely extinguished.


IC2 - a buck converter that powers lamp glow. Set the VR1 potentiometer to 6.3V at TP1 point.
IC3 - a boost converter for 47V anode voltage for tube preamplifiers and 70V voltage supplying the magic eye. The R33 resistor adds an additional load to the converter, reducing the noise level.


T1 - automatic mute after failure detected by TPA3116 system.
T2, RL1 - mute after turning on the power, eliminating two crackles in the loudspeakers, the first after switching on the voltage and the second when the preamplifier tubes are already emission.

[STUDI]

Design assumptions:
Sensitivity is sufficient for commercial level (-10dBV / 316mVrms).
Output power: 2 * 32W/8Ω at 24V supply
Bandwith -0.5dB: better than 20Hz-20kHz
Bandwith -3dB: better than 10Hz-45kHz 


Measurement:
Software: RMAA 6.4.5 on Windows 10 x64
Audio interface: Focusrite Scarlett 2i2 2gen.
Load: real speaker sets
Supply: industrial power supply 24V/100W, manufacruter: Delta


Results for full undistorted power

Noise RMS:
L: -85.6dB
R: -84.2dB

Noise RMS (A):
L: -86.9dB
R: -85.5dB

Noise peak valuse:
L: -71.8dBFS
R: -70.1dBFS

DC offset:
L: 0.00%
R: 0.00%

THD:
L: 0.366%
R: 0.387%

THD+noise:
L: 0.367%
R: 0.387%

THD+noise (A):
L: 0.483%
R: 0.509%

IMD+N:
L: 0.407%, R: 0.356%

IMD+N (A):
L: 0.251%, R: 0.271%

IMD+N (swept freq):
5kHz: L: 0.422%, R: 0.450%
10kHz: L: 0.226%, R: 0.240%
15kHz: L: 0.175%, R: 0.184%

Crosstalk
100Hz: L->R: -58.6dB, R->L: -65.1dB
1kHz:  L->R: -66.1dB, R->L: -67.3dB
10kHz: L->R: -62.6dB, R->L: -51.8dB

Noise level:


THD:


IMD:




Results for ~-12dB level

Noise RMS:
L: -73.0dB
R: -72.6dB

Noise RMS (A):
L: -74.5dB
R: -73.8dB

Noise peak valuse:
L: -58..4dBFS
R: -48.8dBFS

DC offset:
L: 0.00%
R: 0.00%

THD:
L: 0.103%
R: 0.111%

THD+noise:
L: 0.112%
R: 0.120%

THD+noise (A):
L: 0.138%
R: 0.149%

IMD+N:
L: 0.123%, R: 0.114%

IMD+N (A):
L: 0.079%, R: 0.089%

IMD+N (swept freq):
5kHz: L: 0.120%, R: 0.128%
10kHz: L: 0.083%, R: 0.126%
15kHz: L: 0.080%, R: 0.083%

Crosstalk
100Hz: L->R: -62.9dB, R->L: -67.7dB
1kHz:  L->R: -68.7dB, R->L: -66.1dB
10kHz: L->R: -65.6dB, R->L: -51.4dB

Noise level:


THD:



IMD:



Note:

The distance from the right speaker system to the tubes during measurements is less than 50 cm, from the left one about 80 cm. Please note that the microphone of the tubes affects the crosstalk.

The DC voltages of the SPICE simulation match better than 10% with the measurements of the assembled prototype.

*$

.SUBCKT 6S51N  1 2 3 ; P G C (Triode) 21 May 2004 by GaLeX
+ PARAMS: MU=54.72  EX=1.248  KG1=123.5  KP=99.63 
+ KVB=300.0  VCT=0.00 RGI=1500
+ CCG=4.2P  CGP=1.8P CCP=2.5P
E1 7 0 VALUE=
+{V(1,3)/KP*LOG(1+EXP(KP*(1/MU+(V(2,3)+VCT)/SQRT(KVB+V(1,3)*V(1,3)))))}
RE1 7 0 1G
G1 1 3 VALUE={(PWR(V(7),EX)+PWRS(V(7),EX))/KG1}
RCP 1 3 1G    ;
C1 2 3 {CCG}  ;
C2 2 1 {CGP}  ;
C3 1 3 {CCP}  ;
D3 5 3 DX     ;
R1 2 5 {RGI}  ;     
.MODEL DX D(IS=1N RS=1 CJO=10PF TT=1N)
.ENDS

[STUDI]

In action: https://files.fm/down.php?i=znh8ub82e