My 6SN7 Based Preamp

Always wanted to hear how the big bottles sound. Recently I build my version of 6SN7 preamp. It actually took many months in tuning.  Finally a CCS was added to the cathode follower to replace the cathode resistor. Although no significant improvement in measurement but great improvement in listening. It results much deeper and richer sound in mid and lower frequencies range. Here is the circuit topology 

Here is the LTspice FFT chart: 

THD and THD+N:

Proof of concept, components are point to point soldered  

As the cathode voltage of the cathode follower has reached around 170V, there is a need to lift the filament voltage to around 150V,  therefore two separate filament supplies are required. As in picture, 2 sets of filament supply circuits are around the transformers. They are not regulated but heavily filtered to supply the required DC for the filaments of three tubes. Both AC supplies are snubbed before bridge rectifier. 

HV from transformer is also snubbed before full ware rectifier and then feed to a R-C-R-C filter , then to the shunt regulator board. +300V of regulated DC output with 45ma current is adjusted to power the preamp. Actual measurement of residual noise is about 0.25mv.  That ends the proof of concept build. 

PCB design is in progress. 

Update: 

It has been quite a while since. Well,  I did not stop this project at all. Finally the sample of production version has been made. There are some minor updates in the final PCB layout. 

PCB with 2oz copper and 2mm thickness

Components populated

Back side

Finished 

Tubes Used: 

1X 6SN7 GTB  Sylvania  coin base  NOS

2X JJ 6SN7 new production 

Finally, some real time measurements done by REW and Focusrite solo 2i2

                                                                              

As can see, the frequency response is about 35hz - 24khz @-3db

THD= 0.061%,  THD+N=0.062%  @ 0.775V output ,  second harmonic is dominating.

(LTspice result was THD=0.042% and THD+N=0.052% at the same output level. ) 

Also, as can see,  the 50hz component is about -106db which equals to around 10uv. Also measured the output residual noise using an analog millivoltmeter, the reading is 0.04mv RMS 

Have to say, the PCB version measures much better than the "point to point "soldered version. 

Listening test setup : 

Source:  Singxer  SU6 USB interface + Rockna Wavedream R2R DAC

Power Amplifier:  Audio-GD C1 

Speakers: Sonus Faber Amati Homage 

This speaker has 92db sensitivity, but when stick my ears very close to the speakers, it's very very quiet ! :)