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Old 14th July 2006, 01:05 PM   #1
DACMan1 is offline DACMan1  South Africa
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Default Aleph-X with common-mode oscillation

I built Grey Rollins' version of the amp and now I get 13.3V common mode oscillation of about 500Hz. Anyone familiar with this problem? I havent had any time to investigate further. I will statr with powersupply decoupling first, as I don't have a clue where else this type of problem could originate.

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Old 14th July 2006, 05:02 PM   #2
Netlist is offline Netlist  Belgium
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What boards did you use?
Try adding a 1nF cap between Collector and Base of Q3 and Q8 or a 10nF between Collector and Emitter of the same transistors. As described in the Wiki under 7: More unsorted info from The Big Thread.

/Hugo
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Old 14th July 2006, 05:31 PM   #3
DACMan1 is offline DACMan1  South Africa
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I believe those caps are for high frequency oscillations? The boards are ones I did myself.
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Old 14th July 2006, 05:34 PM   #4
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Yes, HF. As for the 500Hz, no idea. I'm sure someone with more experience will help you out.

/Hugo
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Old 14th July 2006, 07:34 PM   #5
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Before I do anything else, I would carefully check the connections
of the PSU capacitors whether they are tight.
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Old 15th July 2006, 09:53 PM   #6
DACMan1 is offline DACMan1  South Africa
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Default Solved

Seems that grounding the inputs solved the problem (even though I thought I grounded them previously). Now the only problem is heatsinking for the diodes on the powersupply which is inadequate but there is not enough space for bigger heatsinking
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Old 16th July 2006, 04:30 AM   #7
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Default Re: Solved

Quote:
Originally posted by DACMan1

Now the only problem is heatsinking for the diodes on the powersupply which is inadequate but there is not enough space for bigger heatsinking

For your info. I often fix the bridge diodes on the main heatsinks.
In this case, we need to be generous with somewhat lengthy wire arrangement.
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Old 16th July 2006, 06:57 AM   #8
DACMan1 is offline DACMan1  South Africa
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That won't work with my layout and the fact that I am using 4 BYV diodes. Also, I want to minimse the wires to and from the diodes, as they carry large current spikes to top-up the reservoir caps. I have other heatsinks that are suitable, just don't know how much better they will be. I will look into it tomorrow.
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Old 16th July 2006, 07:09 AM   #9
EUVL is offline EUVL  Europe
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You can estimate the heat dissipation by multiplying you total bias in Ampere by 3V (2 diode drops). Then choose the heatsink to give you max. 30 degC temperature rise.

Patrick
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Old 16th July 2006, 07:20 AM   #10
DACMan1 is offline DACMan1  South Africa
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In other words, if I have individual diodes, Multiplying half the current by 1.5V per heatsink.

Which gives 1.5*2.25 = 3.375W

30degC/3.375W = 8.88degC/W
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