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Old 14th February 2006, 11:10 PM   #1
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Default Oscillation

Anybody ever build an amp that oscillated between 750 to 1.2 kHz?

The first time I built this amp, 2 weeks ago, it worked perfect.

This time I did a little reworking and now it oscillates when its just sitting there with the input grounded. The only difference is that I have no heatsink compound between the transistors and the heatsink. Each transistor is on there own heatsink so there electrically isolated. I did this only because I didn't want to fiddle with the goop and the amp last time barely got warmer than room temperature and never really warmed up much this time.
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Old 14th February 2006, 11:21 PM   #2
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Heres the amp:

http://www.diyaudio.com/forums/attac...amp=1139676950
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Old 15th February 2006, 12:40 AM   #3
anatech is offline anatech  Canada
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Hi davidallancole,
Sounds like feedback for sure. This could be through your power or ground routing.

-Chris
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Old 15th February 2006, 03:36 AM   #4
jaycee is offline jaycee  United Kingdom
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Sounds like your amp is picking up RF interference through the heatsink which acts as a big antenna, and this is causing it to oscillate. Isolate the transistors, and ground the heatsink, and it'll probably stop.

This could mean your design is close to instability though.
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Old 15th February 2006, 07:49 AM   #5
Eva is offline Eva  Spain
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It makes little sense to see that simple a circuit oscillating at such a low frequency. Where does such a big time constant come from? In case of instability, it should oscillate at several hundred kilohertz.
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Old 15th February 2006, 01:56 PM   #6
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Thanks for all the responses.

Yes, I figured it would oscillate at a lot higher frequencys as well. When I had it hooked up the first time, it oscillated around 3 MHz, so I put a cap across the output, which took that away.

I am going to come in and build it again this weekend, making sure I take my time at each step.

I think I will put the proper heatsink goop on to isolate the transistors and try grounding the heatsinks.

Thanks again.
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Old 16th February 2006, 02:31 AM   #7
anatech is offline anatech  Canada
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Hi Eva,
It's just a higher frequency motorboating. I've seen it a few times.

-Chris
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Old 16th February 2006, 01:55 PM   #8
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Anatech, are you refering to the 3 MHz oscillation? If you are, then what do you mean by motorboating?
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Old 16th February 2006, 02:44 PM   #9
anatech is offline anatech  Canada
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Hi davidallancole,
Motorboating is normally a "plop .. plop .. plop" sound at a low frequency. Depending on wiring this can be higher in frequency. There are times that high frequency oscillation can be modulated by a lower frequency (blocking). This may make a similar sound.

So use you 'scope. Look at the power supply lines (B+ and B-) for similar waveforms to what's coming out the speaker terminal. Also check for a high frequency or bursts of high frequency on the supplies and output. Once you know what's going on you can track it down.

Jaycee has already pointed out that ungrounded heatsinks can cause these problems. It's happened to me before. That was easy to fix (thank goodness!). You will need to use insulators and grease on your outputs before grounding the heatsinks (sorry if I'm pointing out the obvious).

-Chris
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Old 16th February 2006, 04:41 PM   #10
poobah is offline poobah  United States
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Could be a few things here. For openers use heat grease... the current gain of those darlingtons go up with temp... that doesn't help. Just because the sinks aren't hot doesn't the device isn't melting.

You should have some resistance between the op-amp and Q2... maybe a hundred ohms or so.
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