Hybrid line Level Valve and MosFet

[J.Schmaedecke]

I started with a CCDA amp line, and I did not like the sound.
So, I modified slightly from the CCDA and got the result I expected.
The frequency response at -1dB is from 5Hz to 250kHz.
The valve I used is the E88CC PentaLabs. I used the QA400 for distortion measurements.

 

[Mark Tillotson]

R1 and R2 will worsen the noise performance - there are better ways to reduce the gain.

 

[Bigun]

no gate stopper needed for the FET ?

To lower the gain, if desired, I would increase the un-bypassed cathode resistor values

 

[J.Schmaedecke]

Mark and Bigun thank you for your comments. The high gain in this type of circuit is a problem. But it is necessary to maintain the performance. The increase in noise due to resistors R1 and R2 is not relevant. The signal-to-noise ratio is -74dB only by powering the cathode in AC mode as you can see in the spectrum. I'm going to redo the source in regulated DC mode and I'll show you the result.
Bigun, I will try to use the source of the heater to create a negative source to power the cathode and thus increase the cathode resistance without modifying the polarization. But if this increases the distortion of third and high harmonics, I will not use it. The gate resistor is not required in this circuit. The polarization of the Mosfet is in class A not entering the region susceptible to the oscillations as in class AB or key mode. But if this worries you, a 47 Ohm resistor is enough.

 

[J.Schmaedecke]

The image below shows 10kHz square wave with no tendency to oscillation.
With -7.5V at the cathode it was possible to use the 1.2k ohms resistor reducing the gain to 10dBV. Nice option.
I will post the schematic, and the QA400 images another time.

 

[J.Schmaedecke]

...gain to 10dBV...

gain to 10dBs.

 

[dudaindc]

Nice one - I am definitely following this thread.

 

[J.Schmaedecke]

This is the schematic of the low gain version with increase of the cathode resistor without loss of quality.

 

[J.Schmaedecke]

THD and IMD

 

[Ken Newton]

I started with a CCDA amp line, and I did not like the sound.
So, I modified slightly from the CCDA and got the result I expected.
The frequency response at -1dB is from 5Hz to 250kHz.
The valve I used is the E88CC PentaLabs. I used the QA400 for distortion measurements.

Just curious to know:

1) What was the level of the 2nd harmonic in the CCDA stage?

2) How would you subjectively characterize the difference in sonics between the two stages, what didn't you like about the CCDA that sounds better to you with the hybrid stage?

 

[J.Schmaedecke]

Just curious to know:

1) What was the level of the 2nd harmonic in the CCDA stage?

2) How would you subjectively characterize the difference in sonics between the two stages, what didn't you like about the CCDA that sounds better to you with the hybrid stage?

Ken Newton, thank you for your comments and questions.

First of all, when I think of valve amplifiers, I want to feel the presence of musicians and singers. I owned a tube amp earlier. I currently have a solid state system. This system delivers me high quality but I missed the velvet sound of the tubes mainly in the voices.

When I listened to the CCDA, the sound of the voices was brighter than it should be. I did not feel the presence of the voices. And it did not change by adjusting the bias up or down.
When I used the mosfet and power supply at 130V and set the bias to 1/3 of the source, I heard the sound I was looking for, and a deeper sound stage.

Below, the QA400 image showing the CCDA THD only with tubes.
Bias: 65V (anode)
Power source: 130V

 

[Ken Newton]

Thanks, for such a complete response.