Oscillation - Page 2

davidallancole · 16th February 2006, 08:27 PM

The waveform of the oscillation looks just like how you describe the sound anatech. I put the scope every where and I see this signal every where. When the op-amp section is seperated from the output devices and the top transistors base is grounded, I don't see any of this happening. The power supply lines are pretty clean. I am going to put the resistor from the op-amp output to the base of the transistor.

Poobah, you were saying that even if the heatsinks are not hot, the devices might still be burning up. I put my finger on the case everytime I put my hand on the heatsink. Is the case a better measure of the temperature?

I am going to take all your peoples suggestions and use some careful planning and layout to build it this weekend.

Thank you again.

anatech · 16th February 2006, 08:36 PM

Hi davidallancole,
Can you describe exactly what you see? HF bursts or not?

I don't think you should run the op amp off the same supply as the output. The negative supply terminal of the op amp is more sensitive to noise than the positive, but you need to pay attention to all of it. Pay close attentoin to the speaker current return path as well.

-Chris

EWorkshop1708 · 16th February 2006, 08:40 PM

Might want to check out your grounds. Make sure all the ground points of the amp are connected and you did not forget a connection.

I built an amp where the NPN and PNP output pairs had their own heatsinks, no insulator, just good thermal grease, and I don't get oscillation. I don't think your non insulated heatsinks are the problem. If you still are uncertain, you could connect a zobel (4.7 ohm resistor with 0.1 or 0.2 uf cap) to each heatsink and to ground.

poobah · 16th February 2006, 09:03 PM

Dave,

All other things being equal... your heatsink will run at the same temperature above ambient, with or without grease. But your devices temperature (not power dissipation) is function how well it is connected thermally to heat sink... the better the connection, the lower the temp of the device. A transistors life goes in half for every 11 deg C rise in temp. Always use grease or a silpad and don't get carried away when you tighten the screw.

anatech · 16th February 2006, 09:12 PM

Hi EWorkshop1708,
You can run the heatsink at a supply voltage and not have any trouble ... as long as the AC impedance to ground is low across the frequency band.

-Chris

davidallancole · 16th February 2006, 09:57 PM

Well, the first time I built it, it worked perfect. I think its just an issue with the physical layout because I tried rebuilding it a bit and thats when all hell broke loose. Like I said a few times before, i will rebuild it this weekend and see what happens.

Thank you again for your suggestions.

davidallancole · 17th February 2006, 04:20 PM

Well, I solved my problem. It looks like I had a case of dumbassness. I have a digital oscilloscope that displays the frequency in the bottom corner. I haven't been wearing my glasses lately (yes, an excuse) so I didn't notice the k in front of Hz.

So I took a .1uF cap from the output to ground and it totaly eliminated my problem.

Thank you all for your kind input.

anatech · 17th February 2006, 05:12 PM

Hi davidallancole,

LOL

It happens to all of us. Shouldn't you stick some resistance in series with your cap?

-Chris

davidallancole · 17th February 2006, 06:54 PM

I could put a resistor in there, and probably will this weekend, but it signifigantly reduces the oscillation all ready.

anatech · 17th February 2006, 06:58 PM

Try it with a value 4R7 to 10 R.

-Chris

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16th February 2006, 08:27 PM	#11
davidallancole diyAudio Member Join Date: Nov 2005	The waveform of the oscillation looks just like how you describe the sound anatech. I put the scope every where and I see this signal every where. When the op-amp section is seperated from the output devices and the top transistors base is grounded, I don't see any of this happening. The power supply lines are pretty clean. I am going to put the resistor from the op-amp output to the base of the transistor. Poobah, you were saying that even if the heatsinks are not hot, the devices might still be burning up. I put my finger on the case everytime I put my hand on the heatsink. Is the case a better measure of the temperature? I am going to take all your peoples suggestions and use some careful planning and layout to build it this weekend. Thank you again.

16th February 2006, 08:36 PM	#12
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi davidallancole, Can you describe exactly what you see? HF bursts or not? I don't think you should run the op amp off the same supply as the output. The negative supply terminal of the op amp is more sensitive to noise than the positive, but you need to pay attention to all of it. Pay close attentoin to the speaker current return path as well. -Chris

16th February 2006, 08:40 PM	#13
EWorkshop1708 diyAudio Member Join Date: Nov 2004 Location: Florida	Might want to check out your grounds. Make sure all the ground points of the amp are connected and you did not forget a connection. I built an amp where the NPN and PNP output pairs had their own heatsinks, no insulator, just good thermal grease, and I don't get oscillation. I don't think your non insulated heatsinks are the problem. If you still are uncertain, you could connect a zobel (4.7 ohm resistor with 0.1 or 0.2 uf cap) to each heatsink and to ground.

16th February 2006, 09:03 PM	#14
poobah diyAudio Member Join Date: Nov 2005	Dave, All other things being equal... your heatsink will run at the same temperature above ambient, with or without grease. But your devices temperature (not power dissipation) is function how well it is connected thermally to heat sink... the better the connection, the lower the temp of the device. A transistors life goes in half for every 11 deg C rise in temp. Always use grease or a silpad and don't get carried away when you tighten the screw.

16th February 2006, 09:12 PM	#15
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi EWorkshop1708, You can run the heatsink at a supply voltage and not have any trouble ... as long as the AC impedance to ground is low across the frequency band. -Chris

16th February 2006, 09:57 PM	#16
davidallancole diyAudio Member Join Date: Nov 2005	Well, the first time I built it, it worked perfect. I think its just an issue with the physical layout because I tried rebuilding it a bit and thats when all hell broke loose. Like I said a few times before, i will rebuild it this weekend and see what happens. Thank you again for your suggestions.

17th February 2006, 04:20 PM	#17
davidallancole diyAudio Member Join Date: Nov 2005	Well, I solved my problem. It looks like I had a case of dumbassness. I have a digital oscilloscope that displays the frequency in the bottom corner. I haven't been wearing my glasses lately (yes, an excuse) so I didn't notice the k in front of Hz. So I took a .1uF cap from the output to ground and it totaly eliminated my problem. Thank you all for your kind input.

17th February 2006, 05:12 PM	#18
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi davidallancole, LOL It happens to all of us. Shouldn't you stick some resistance in series with your cap? -Chris

17th February 2006, 06:54 PM	#19
davidallancole diyAudio Member Join Date: Nov 2005	I could put a resistor in there, and probably will this weekend, but it signifigantly reduces the oscillation all ready.

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17th February 2006, 06:58 PM	#20
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Try it with a value 4R7 to 10 R. -Chris