My MOSFET amplifier designed for music

good evening

i changed the value af R27 because of my lab supply just have 32V. so i use a 10k parallel to R27 =5k
everything looks fine at the DC values. the L channel is out of the amp and still on the heatsink (3U 300mm)
same strange sine wave at lower frequencies without load!
schematic_32Vrail_DC values.jpg
changing of opamp doesnt help .DC offset is nearly 0 / zero volt.
i did a schematic with al measured values.
Z-diode at the opamp shows 11,67Volt

kr
chris
 
The DC voltage conditions look fine (R17 and R22 currents calculated incorrectly 🙂)

Use the scope to check both - and + supplies are clean when the problem is happening. Make sure the grounds are configured correctly, again use the scope on the grounds to check they are clean.
 
  • Thank You
Reactions: chermann
i did the scope measurements
ch1 yellow input signal is via T-connector to channel 1 and input via BNC-kroko
ch2 is signal output - or it is checking the GND
ch3 pink negativ rail
ch4 dark blue positive rail

i checked al situations the rails and the GND looks clean in every conditions. the signal is still distorted
 

Attachments

  • IMG_20250128_142437.jpg
    IMG_20250128_142437.jpg
    488.6 KB · Views: 16
  • IMG_20250128_142424.jpg
    IMG_20250128_142424.jpg
    472.3 KB · Views: 16
  • IMG_20250128_142340.jpg
    IMG_20250128_142340.jpg
    492.7 KB · Views: 16
  • IMG_20250128_142326.jpg
    IMG_20250128_142326.jpg
    501.2 KB · Views: 18
because the only frequency dependent component at the beginning is the input cap -in my situation 2x 10µF parallel i jumpered the caps.
hurra!! the signal is fine

strange that both caps 10µF muse and the UFG nichicon 10µF are defect.....but okay...get out!
 

Attachments

  • caps shorted its okay.jpg
    caps shorted its okay.jpg
    509 KB · Views: 9
i changed to a WIMA 4µ7F and soldered this cap in....put i got the same situation. !!!!!!???? 🤔 😒 🙄 :smash:
input cap not okay...140Hz---if i jumper the cap then signal okay?!???
WHAT?
the PCB is clean and no wrong solder is possible and my solder joints are not bad after 40 years soldering.

maybe an imported information:
if i but out this yellow jumper cable the the singal is okay and all 2 seconds you can see that its getting more and more distorted.

kr
chris
 

Attachments

  • cahnged input cap to WIMA 4u7uF.jpg
    cahnged input cap to WIMA 4u7uF.jpg
    463.1 KB · Views: 46
  • input cap shorted singal okay1.jpg
    input cap shorted singal okay1.jpg
    420.8 KB · Views: 41
  • input cap shorted singal okay2.jpg
    input cap shorted singal okay2.jpg
    460.7 KB · Views: 35
  • IMG_20250128_145749.jpg
    IMG_20250128_145749.jpg
    480.2 KB · Views: 39
It won't be the opamp. Look at the output of the opamp and make sure it is just DC and no AC signal (apart from a very very small value that will rise as frequency falls).

More obscure reasons:
Make sure that if you are using an output coil (inductor) that it is air spaced and not one with a magnetic core.
Don't use and common mode chokes in the DC rails to the amp... I'm sure you are not but such things can do this.
 
It won't be the opamp. Look at the output of the opamp and make sure it is just DC and no AC signal (apart from a very very small value that will rise as frequency falls).

More obscure reasons:
Make sure that if you are using an output coil (inductor) that it is air spaced and not one with a magnetic core.
Don't use and common mode chokes in the DC rails to the amp... I'm sure you are not but such things can do this.
i measure without coil/before the coil. no my coil is an air coil.


but you are right
a have a lot of AC signal on the output pin 6 of the OPV
look at the pics - made 10minutes ago!

yellow input
light blue output (no coil)
dark blue PIN 6 OPV

kr
chris
 

Attachments

  • 1khz_yellow input lblue output dblue pin6 opv.png
    1khz_yellow input lblue output dblue pin6 opv.png
    50.6 KB · Views: 12
  • 10hz_yellow input lblue output dblue pin6 opv.png
    10hz_yellow input lblue output dblue pin6 opv.png
    46.6 KB · Views: 11
  • 10khz_yellow input lblue output dblue pin6 opv.png
    10khz_yellow input lblue output dblue pin6 opv.png
    51.5 KB · Views: 14
  • 50hz_yellow input lblue output dblue pin6 opv.png
    50hz_yellow input lblue output dblue pin6 opv.png
    42.9 KB · Views: 8
  • 90hz_yellow input lblue output dblue pin6 opv.png
    90hz_yellow input lblue output dblue pin6 opv.png
    50.1 KB · Views: 8
  • 300hz_yellow input lblue output dblue pin6 opv.png
    300hz_yellow input lblue output dblue pin6 opv.png
    53.1 KB · Views: 9
This is 200Hz at the equivalent of 1 watt output into 8 ohms (4 volts peak output)

Screenshot 2025-01-29 061741.png



And this is the output of the opamp under those conditions Look at the scale:

Screenshot 2025-01-29 061814.png


And this is 20kHz under same output conditions. Look at the scales:

Screenshot 2025-01-29 061921.png