SIT Startup Problems - What did I miss?

[ilimzn]

Without a load the coupling cap cannot charge or discharge and since it has self-discharge, the voltage on it will tend to drop towards zero, making the output voltage with no load equal to the voltage on the drain of the VFET.

[Michael Rothacher]

And that, as they say, is Numberwang!

[dantwomey]

Originally Posted by Michael Rothacher

Now, musn't blame Nelson for my omission.

For the sake of others:
If one tests with a dummy load (as one should) the coupling cap discharges rather quickly through the load.

Since speakers don't always like this sort of thing, we use a 100 Ohm or so resistor connected from the output to ground to bleed off the cap when there's no load connected. This can potentially save you or your speker from a shock.

The reason I came across this is I'm in the habit of checking for DC offset on the outputs of new amplifiers. With one slight deviation in the grounding scheme I know my wiring is perfect and an exact duplicate of the original schematic. That said, when my new VFets arrive I'm going to install a 100 ohm resistor across the output terminals of my amp. Hopefully this will prevent my speakers (or myself) from seeing any DC.

Regards,
Dan

[Michael Rothacher]

ilizmn is trying to clue you in on why you're measurement is wonky. Your cap is hanging in space.

The 100 ohm resistor will improve your measurement situation as well as bleed the cap, but I'd like to emphasize that you should always test with a load resistor. There's one in the schematic, and if you had you'd probably be listening to an amp right now.

[dantwomey]

Originally Posted by Michael Rothacher

ilizmn is trying to clue you in on why you're measurement is wonky. Your cap is hanging in space.

The 100 ohm resistor will improve your measurement situation as well as bleed the cap, but I'd like to emphasize that you should always test with a load resistor. There's one in the schematic, and if you had you'd probably be listening to an amp right now.

Regards,
Dan

[dantwomey]

Originally Posted by Michael Rothacher

ilizmn is trying to clue you in on why you're measurement is wonky. Your cap is hanging in space.

The 100 ohm resistor will improve your measurement situation as well as bleed the cap, but I'd like to emphasize that you should always test with a load resistor. There's one in the schematic, and if you had you'd probably be listening to an amp right now.

I probably mistook the load resistor as representing a nominal speaker load. That aside, I'll do all my next startup tests with a resistor load in place. Thanks for the heads up.

Regards,
Dan

[generg]

Hi Dan,

we are mostly used to the capacitor free outputs and check the offset.....

I was in the last days also in the situation to be in panic with the F3 clone and my newly implemented 15 x 1000uF Silmic ... it is easy to forget what ilimzn wrote...

especially when building a new project.....!


but what was the reason your SIT was blown?

[Zen Mod]

Originally Posted by Zen Mod

besides bad cap , there is missing bleeder resistor on speaker side

that's the price when Pa is in a hurry to please everyone

it can be a problem when connecting speaker to still excited cap

[Michael Rothacher]

Have we beaten ths one to death yet, because I've had enough.

Unsubscribed.

[Zen Mod]

not first and not last subject , beaten to death

there is no need to take it from wrong side .........

you can find mistakes in Pa's manuals , left there most probably because life isn't serious thing , so there is no need that electronic must be ........ and what Greenhorns can catch , if not own mistakes



(sez Mighty Moi , who scorched more than few mosfets and just later found that Pa was laughing all the time )