Salas hotrodded blue DCB1 build - Page 207

BigE · 1st August 2012, 06:44 PM

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.

boywonder · 2nd August 2012, 03:30 PM

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

Salas · 2nd August 2012, 03:48 PM

Never seen that before. At what level did you set your generator?

boywonder · 03:55 PM

Quote:

Originally Posted by Salas

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.

Salas · 2nd August 2012, 03:58 PM

If you will measure the output as it is with the 150K dummy on it, is it really 150K or something parallels?

Salas · 2nd August 2012, 04:00 PM

Is there a 50 Ohm T BNC terminator on the scope maybe?

boywonder · 2nd August 2012, 04:04 PM

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!

Salas · 2nd August 2012, 04:06 PM

No worries, I do worse all the time.

boywonder · 2nd August 2012, 04:10 PM

Dc offsets are -1.8mv and -3.1mv..........should I do anything to reduce these, or are they acceptable?

Salas · 2nd August 2012, 04:19 PM

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.

2nd August 2012, 04:04 PM	#2067
boywonder diyAudio Member Join Date: Jan 2008 Location: So.Cal.	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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1st August 2012, 06:44 PM	#2061
BigE diyAudio Member Join Date: Aug 2008 Location: Toronto	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.

2nd August 2012, 03:48 PM	#2063
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Never seen that before. At what level did you set your generator?

2nd August 2012, 03:58 PM	#2065
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	If you will measure the output as it is with the 150K dummy on it, is it really 150K or something parallels?

2nd August 2012, 04:00 PM	#2066
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Is there a 50 Ohm T BNC terminator on the scope maybe?

2nd August 2012, 04:06 PM	#2068
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	No worries, I do worse all the time.

2nd August 2012, 04:10 PM	#2069
boywonder diyAudio Member Join Date: Jan 2008 Location: So.Cal.	Dc offsets are -1.8mv and -3.1mv..........should I do anything to reduce these, or are they acceptable?

2nd August 2012, 04:19 PM	#2070
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	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.