I have gone though 2 upgrades since the release of the CT-11H preamp. However, I finally decided to create a new model based on the CH-11H with all the upgrades implemented and some changes in the actual circuit design. It's amazing because a very low level of distortion is achieved by LTspice simulation.
In the new circuit, the cathode follower now has a cathode resistor connected directly to ground and feedback is taken from the grid of the cathode follower with an added capacitor in the loop. Also made change is the output from the second stage which is now taken from the cathode of the top tube , making the second stage a typical mu-follower. The top and bottom tubes for the mu-follower are now separated in two triodes by the new PCB design. An ECC88 is employed for the cathode follower instead of 6BQ7A, running at around 8ma current. In the chats, THD is 0.000061%, and second, third harmonic distortion are all below -120db.
Although simulation can hardly reflect exact the same thing in reality, but simulation results for the original circuit were never so good. Listening experience also tells me that it's absolutely worth the change. The sound is much more refined and I can feel more timber/body like with piano. Bass is also fuller and richer.
The main PCB has been re-designed in order to reflect all the changes and improvement.
UPDATE:
Some reality measurements using ARTA software and Focusrite Scarlett 2i2 USB sound card.
Frequency Response
FFT Chart
2nd/3rd Harmonic Distortion
At -3db output, second harmonic distortion is -80db, the third is about -100db. then the rest of are all less than -100db. This could explain the preamp sounds very natural, no harsh across entire audible range.
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Tube used
3X JJ802S new production
1X Beijing 6n11 NOS
1X 6X4 Sylvania NOS
Filament Arrangement:
12.6V regulated for 2 X JJ802S
6.3V regulated for 6n11 and JJ802S
Output Noise level
0.1mv~0.12mv when input shorted to ground
Gain:
21.5GB
Power Input:
AC: 230V~240V
Connectivity:
3 set of RCA for input signals.
1 set of RCA for output (output signal can be muted by a mute switch in the front panel and automatic delayed for appox. 100 seconds on start-up)
For more info please contact peter@consoletronics.com
