Salas hotrodded blue DCB1 build

[BigE]

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]

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

 

[Salas]

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

 

[boywonder]

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]

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

 

[Salas]

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

 

[boywonder]

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!

 

[Salas]

No worries, I do worse all the time.

 

[boywonder]

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

 

[Salas]

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.

 

[boywonder]

got it, thanks......assuming you mean swap the inner/outer positions for each pair.

 

[Salas]

Yep.

 

[mjock3]

heat sink

Would a heat sink rated at 1.3 C/W 11mmx61mmx35mm be enough on each side for a 600ma hot rod?

Thanks

 

[pinnocchio]

Would a heat sink rated at 1.3 C/W 11mmx61mmx35mm be enough on each side for a 600ma hot rod?

Thanks

I think you will see fire and explosions of the mosfets 😱

 

[Salas]

He probably meant cm.

 

[mjock3]

How do I go about figuring out what I need. I don't understand what to look for.

 

[mjock3]

I meant 116mmx61x35

 

[Salas]

W/C is the key. 1.3W/C when receiving 12W heat per rail assuming 10V across each Mosfet @ 0.6A they gonna rise 15.6C above box's ambient.

 

[Salas]

If your box is roomy, ventilated, even with 40C inside you are safe at under 60C on the fins.

 

[mjock3]

Thanks Salas🙂