Good Day,
My power supply has a "pi" filter network, along with 22,000 uF filtering per rail and ceramic disc capacitors from each rail to ground. The power suply can still oscillate with all this around? I will check the power supply, but other than that, where else a fault in the amplifier can cause oscillation? Thank you for your time.
My power supply has a "pi" filter network, along with 22,000 uF filtering per rail and ceramic disc capacitors from each rail to ground. The power suply can still oscillate with all this around? I will check the power supply, but other than that, where else a fault in the amplifier can cause oscillation? Thank you for your time.
Good Day
The weird thing is, the oscillation problem is only in the left channel amplifier. The right channel is perfectly fine and both amplifiers share a common +/- 44V power supply (60 W RMS per channel). The two things i did that i think may be the cause:
1) To avoid corrosion of the hand drawn copper PCB, I sprayed the entire bottom of both amplifier PCBs with laquer, several times. This is the common wood type that is available at hardware stores. Could the oscillation be cause by the parasitic capacitance of the laquer?
2) Between the bases of the output Bipolar Junction Transistors, there is a 1 uF capacitor that is paralleled with a 150 ohm resistor. The capacitor is supposed to speed up the "turn off" of the output pair. I forgot to buy the capacitor when I bought the components for the amplifier, so I used a second hand one I salvaged from somewhere. The second hand capacitor had too short legs to be directly soldered to the PCB, so I ran about 1 cm of single uninsulated strand of wire to connect the capacitor to the PCB. Could the oscillation be caused by the inductance of the single strand wire together with the capacitance of the capacitor?
Please tell me if any of the two conditions above could contribute to the oscillation. Thank you.
The weird thing is, the oscillation problem is only in the left channel amplifier. The right channel is perfectly fine and both amplifiers share a common +/- 44V power supply (60 W RMS per channel). The two things i did that i think may be the cause:
1) To avoid corrosion of the hand drawn copper PCB, I sprayed the entire bottom of both amplifier PCBs with laquer, several times. This is the common wood type that is available at hardware stores. Could the oscillation be cause by the parasitic capacitance of the laquer?
2) Between the bases of the output Bipolar Junction Transistors, there is a 1 uF capacitor that is paralleled with a 150 ohm resistor. The capacitor is supposed to speed up the "turn off" of the output pair. I forgot to buy the capacitor when I bought the components for the amplifier, so I used a second hand one I salvaged from somewhere. The second hand capacitor had too short legs to be directly soldered to the PCB, so I ran about 1 cm of single uninsulated strand of wire to connect the capacitor to the PCB. Could the oscillation be caused by the inductance of the single strand wire together with the capacitance of the capacitor?
Please tell me if any of the two conditions above could contribute to the oscillation. Thank you.
HI raveenvijendren,
I don't think the laquer is the problem because you did this on both PCB's, and only one has the oscillation. Using laquer is a good trick to keep your PCB's very nice throught the years, I also do this with every PCB on the copper side.
I don't know whether the capacitor of 1µ can be the problem. I would probably remove it or change it with another one, and do a test. Schematics would be very usefull here.
Good luck
Ben
I don't think the laquer is the problem because you did this on both PCB's, and only one has the oscillation. Using laquer is a good trick to keep your PCB's very nice throught the years, I also do this with every PCB on the copper side.
I don't know whether the capacitor of 1µ can be the problem. I would probably remove it or change it with another one, and do a test. Schematics would be very usefull here.
Good luck
Ben
Good Day,
Here are the schematics of my amplifier. It is an ordinary class AB amplifier capable of 60 W rms per pair of output BJTs at +/- 42V. Looking at the attached schematic, the capacitor which I was talking about was C9.
Also, I am using a 10 ohm ground lift resistor between the signal grounds of the amplifiers and star ground for both amplifiers. I did this to remedy a +40V DC offset which came on both channels. After adding that resistor, offset came to about 6.2mV for the right channel and 15mV for the left channel.
The left channel output transistor also blows for no reason. This could be due to oscillation and the weird thing is, only the NPN output blows but the PNP is fine. It is always the case.
If this does not help, I will take some pictures of the amplifier and paste it here for you to see it. Thank you.
Here are the schematics of my amplifier. It is an ordinary class AB amplifier capable of 60 W rms per pair of output BJTs at +/- 42V. Looking at the attached schematic, the capacitor which I was talking about was C9.
Also, I am using a 10 ohm ground lift resistor between the signal grounds of the amplifiers and star ground for both amplifiers. I did this to remedy a +40V DC offset which came on both channels. After adding that resistor, offset came to about 6.2mV for the right channel and 15mV for the left channel.
The left channel output transistor also blows for no reason. This could be due to oscillation and the weird thing is, only the NPN output blows but the PNP is fine. It is always the case.
If this does not help, I will take some pictures of the amplifier and paste it here for you to see it. Thank you.
Hi raveenvijendren,
Is R24 a wirewound? Is it inductive? Try replacing it with a same value 1W or 2W carbon film or metal film.
If still a problem with slight oscillation , try paralleling another with it, totalling 4 ohms. It may be due to component spreads one channel stability is a little less benign than the other.
Cheers,
Greg
Is R24 a wirewound? Is it inductive? Try replacing it with a same value 1W or 2W carbon film or metal film.
If still a problem with slight oscillation , try paralleling another with it, totalling 4 ohms. It may be due to component spreads one channel stability is a little less benign than the other.
Cheers,
Greg
Good Day,
All the resistors I used in this amplifier is the cheap carbon resistors with 5% tolerance(not exactly high fidelity). No wire wound or metal film(too expensive and I'm 16). What about Mr. Thanh's idea of:
Q12 Q13 ,protection tst
a 40n cap from B to C
a 47p cap is parallel with R10
Anyway, I am not an expert, so I will try everything I get from you all, and hopefully be able to null the oscillations. Thank you very much for the help.
All the resistors I used in this amplifier is the cheap carbon resistors with 5% tolerance(not exactly high fidelity). No wire wound or metal film(too expensive and I'm 16). What about Mr. Thanh's idea of:
Q12 Q13 ,protection tst
a 40n cap from B to C
a 47p cap is parallel with R10
Anyway, I am not an expert, so I will try everything I get from you all, and hopefully be able to null the oscillations. Thank you very much for the help.
I weren't using those cap in my amp before . My amp became a occsilator after I had added the protection circuit . I went to www.diyaudio.com 
, and a guru recomend me that thing
, and a guru recomend me that thing
A real BIG sorry.
Good Day,
I would like to apologise to everybody in this thread about my oscillation problem. I tested the amplifier PROPERLY this time, and I only found about 0.8 mV of noise and about 4 mV of hum present(on both channels). No 20 MHz Oscillation. The reason I saw the oscillation was because I connected the amplifier to a Philips CD ROM headphone output. What a cheap junk that thing was. I read the scope properly this time and found that the CD ROM outputs a 1.25 MHz sinewave in the headphone out. My silly mistake of connecting the amplifier there. Very sorry for troubling everyone in this thread.
On the other hand, everyone here has given me very useful information regarding amplifier oscillations. Thank you all so much and again, I am very sorry for the inconvenience caused by my sloppy troubleshooting. Thank you.
PS: Please be careful where you connect you amplifiers.
Good Day,
I would like to apologise to everybody in this thread about my oscillation problem. I tested the amplifier PROPERLY this time, and I only found about 0.8 mV of noise and about 4 mV of hum present(on both channels). No 20 MHz Oscillation. The reason I saw the oscillation was because I connected the amplifier to a Philips CD ROM headphone output. What a cheap junk that thing was. I read the scope properly this time and found that the CD ROM outputs a 1.25 MHz sinewave in the headphone out. My silly mistake of connecting the amplifier there. Very sorry for troubling everyone in this thread.
On the other hand, everyone here has given me very useful information regarding amplifier oscillations. Thank you all so much and again, I am very sorry for the inconvenience caused by my sloppy troubleshooting. Thank you.
PS: Please be careful where you connect you amplifiers.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.