• WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required as is 20 years of participation in forums (not all true). There are daily reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!
OP
H

horias2000

Member
Forum Donor
Joined
Apr 7, 2021
Messages
44
Likes
15
Location
Cluj-Napoca, Romania
I managed to perform a quick measurement with the feedback closed. Below you can see the frequency response (log frequency scale).

1647861714695.png


I'm also inserting the openloop response with log frequency scale, just for reference.

1647861762039.png



And I'm attaching the square wave response for 1kHz, 500Hz and 100Hz. The frequency response does look better than open loop but I still get a HF rolloff way too early. I'll look into that when I have more time.
 

Attachments

  • rc_zobel_150nF_13R_15nF+1k_v1_plates_100Hz_square_closedloop.png
    rc_zobel_150nF_13R_15nF+1k_v1_plates_100Hz_square_closedloop.png
    43.8 KB · Views: 11
  • rc_zobel_150nF_13R_15nF+1k_v1_plates_500Hz_square_closedloop.png
    rc_zobel_150nF_13R_15nF+1k_v1_plates_500Hz_square_closedloop.png
    49.9 KB · Views: 13
  • rc_zobel_150nF_13R_15nF+1k_v1_plates_square_closedloop.png
    rc_zobel_150nF_13R_15nF+1k_v1_plates_square_closedloop.png
    50.1 KB · Views: 13
OP
H

horias2000

Member
Forum Donor
Joined
Apr 7, 2021
Messages
44
Likes
15
Location
Cluj-Napoca, Romania
I removed C7 and the frequency response got a lot better (as expected).

1647875723503.png



So I now have -0.7dB at 20kHz and -3dB above 40kHz. I will add a smaller capacitor so that I will get -3dB at around 20kHz. I'm not sure what is the best practice here. Is it ok to have -3dB at 20kHz or I should go a bit higher than 20kHz?
As expected, the square wave response improved significantly (as seen below).

1kHz

rc_zobel_150nF_13R_1nF+1k_v1_plates_1kHz_square_closedloop_removed_22nF_detail.png



and 10kHz

rc_zobel_150nF_13R_1nF+1k_v1_plates_10kHz_square_closedloop_removed_22nF.png
 
OP
H

horias2000

Member
Forum Donor
Joined
Apr 7, 2021
Messages
44
Likes
15
Location
Cluj-Napoca, Romania
I did a quick performance evaluation of the amplifier and the results are shown below. I'm happy with this amount of distortion (0.035%) but the 50Hz noise (or oscillation) needs to be dealt with. So I will focus on that in the coming days/weeks. If I can manage to get rid of that (or at least reduce it significantly) I will be very happy with the performance. The measurement was done with 60mA quiescent current through the KT88 output tubes. This can be increased a bit (65 - 70 mA). So the THD will most probably get better. The measured frequency response looks really good as well (apart from the 50Hz issues). I plan to run the other module in ultra linear and compare the performance between the two modes (pentode vs ultra linear).

Tube_1kHz_THD_high_bias_200mV.png


tube_FR.png
 
OP
H

horias2000

Member
Forum Donor
Joined
Apr 7, 2021
Messages
44
Likes
15
Location
Cluj-Napoca, Romania
Just a quick update. I thought that the 50Hz could be coming from the ac filament supply/wires. I tried routing the wires as far away as possible from any sensitive parts and I also twisted the two wires together. Nothing, the 50Hz was still there. I tried using another 6.3V winding but with no improvement. I know that sometimes the 50Hz filament ac voltage can leak inside the small signal tubes. Therefore I used a regulated lab power supply for the two input tubes and BINGO! The 50Hz noise went down almost completely. I then looked at the spectrum and the frequency response again and this time it looks much better. I will most probably use DC for the low signal filaments or I will look online for tubes that do not exhibit this issue so much.

I must say that the performance of the amplifier looks to be very good in my opinion. Frequency response is really flat in the audio band and THD and THD+N look respectable in my opinion. With the 360Vdc supply I can get around 20W before clipping. I know this is not a lot but for my needs it's more than enough. More power can be achieved with a higher supply voltage.

This is for 1W output on 8OHM load

Tube_1kHz_THD_DC_filament_input_driver_optimal_bias_180mV_1W.png


This is for 5W output
Tube_1kHz_THD_DC_filament_input_driver_optimal_bias_180mV.png



tube_FR_DC_fialment.png
 

Remk

Member
Joined
Feb 9, 2022
Messages
7
Likes
11
Location
Netherlands
That looks good. On a pcb you almost have the need to feed the heaters with DC.
Make shure they are not flooting; use 2 resistors or a pot betweed heaters and ground if not already.
 
OP
H

horias2000

Member
Forum Donor
Joined
Apr 7, 2021
Messages
44
Likes
15
Location
Cluj-Napoca, Romania
As I said earlier, the second power amplifier PCB will be setup to work in UL mode so that I can assess witch mode has better performance (UL or pentode). I managed to do this today and in order for it to work in UL mode I had to perform a few modifications.
  1. the grid voltage of the KT88 valves had to be lowered from around -18Vdc to around -40Vdc.
  2. the R-C compensation network that I placed on the anodes of V1 was modified to 1k + 330pF. This gave a -3dB frequency of around 40kHz
  3. the 100nF capacitor placed on the feedback input of V1 was increased to 200nF. This improved the LF response.

The closed loop frequency response is shown below. It looks good but in pentode mode it was a bit better. Not a big difference though.

1648567033674.png


The THD was the big surprise. It's clear that the 2nd harmonic is a lot bigger than it was in pentode mode. In pentode mode the THD was 0.017% (1W) and in ultra linear it is 0.096% at 1W. This is a big difference in my opinion. It gets even bigger at 5W, 0.038% vs 0.21%. Output power was about the same, 20W in UL mode and around 18W in pentode mode. Is this THD difference something normal? I was really expecting the THD in UL to be very similar to the THD in pentode mode but this is clearly not the case.

The conclusion is that I will modify the second module to work in pentode mode and I will use pentode mode for the final build. I will keep this thread updated with construction photos once I start.

1W output power
Tube_1kHz_THD__UL_1W.png


5W output power
Tube_1kHz_THD_UL.png
 
Top Bottom