Salas hotrodded blue DCB1 build - Page 207 - diyAudio
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Old 1st August 2012, 06:44 PM   #2061
BigE is offline BigE  Canada
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Thanks... it will be a couple weeks before I start... there are a couple of other things in the way.

I will certainly post photos on completion.
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Old 2nd August 2012, 03:30 PM   #2062
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Default I need a little help.......

I just completed a build of the DCB1 using the new black PCB and it appears that I have an oscillation problem (at least I think I do.....)

I'm running 10R/6.5W (200ma config), mosfets bolted to the floor with kapton insulators, and my DC voltages are +10.22/-10.25VDC. I've got a 10K stepped attenuator before the input. The DC voltages out of the power supply section are rock solid and look very clean.

I see the attached scope trace (same behavior) on both channels. This is with the output disconnected, and with a 150K R across the outputs, any suggestions?

Thanks
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File Type: jpg DCB1.JPG (120.8 KB, 208 views)
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Old 2nd August 2012, 03:48 PM   #2063
Salas is offline Salas  Greece
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Never seen that before. At what level did you set your generator?
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Old 2nd August 2012, 03:53 PM   #2064
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Quote:
Originally Posted by Salas View Post
Never seen that before. At what level did you set your generator?
It's set at 50 ohm Z out, and from very low output from the generator all the way up to clipping of the DCB1 (by cranking up the output of the generator) the wave looks like the picture (at different amplitudes of course). I tried two different scope probes as well, as a sanity check.

Last edited by boywonder; 2nd August 2012 at 03:55 PM.
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Old 2nd August 2012, 03:58 PM   #2065
Salas is offline Salas  Greece
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If you will measure the output as it is with the 150K dummy on it, is it really 150K or something parallels?
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Old 2nd August 2012, 04:00 PM   #2066
Salas is offline Salas  Greece
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Is there a 50 Ohm T BNC terminator on the scope maybe?
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Old 2nd August 2012, 04:04 PM   #2067
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Default I figured it out.....

I recently got my func generator back from loaning it to someone and the sweep function was turned on.

Everything looks fine now.......off to check the DC offset.

Sorry for the newbie diversion!
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Old 2nd August 2012, 04:06 PM   #2068
Salas is offline Salas  Greece
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No worries, I do worse all the time.
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Old 2nd August 2012, 04:10 PM   #2069
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Dc offsets are -1.8mv and -3.1mv..........should I do anything to reduce these, or are they acceptable?
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Old 2nd August 2012, 04:19 PM   #2070
Salas is offline Salas  Greece
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There have been some better reports, depends on your amp's gain & if its input is DC coupled. In general under 5mV its largely innocent though not perfectionist. Some just swap positions between the audio JFETs pair for each channel to see if they can strike it less when they already had used their best matches. It can even end up more that way though. Lucky shot if you got the curiosity.
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