Power Supply Help - How Do I get rid of the hum??

[mydementia]

Hi Guys.
I finally got all the parts I needed to put together my power supply.  I've been experimenting with 12V tube amps (like the stuff over at sopht.ca) and needed more current capacity. 
I built this circuit yesterday:


I plugged in my last build (that didn't work with my 1A supply before) and it worked... only problem is the hum...  I turned off my flourescent work light - no effect.
I've read several PS tutorials for star grounding in tube amps and twisting leads to cancel hum, but I'm curious if my basic design is causing the problem... Specifically, I have AC and DC in the same switch... just realized I should have clipped out the LED just to see if that's the problem, or if I have AC wires in close proximity to DC output...

Any words of wisdom or good references on this specific problem?  Isn't the rectifier the noisiest part of the circuit? 

Thanks for looking.
Mike

[frankclarke]

Are the heaters DC?

[mydementia]

Yep - The heaters want 12VDC (approx 0.5A each).
For reference, I'm working to a variation of this schematic:
http://www.sophtamps.ca/schematics/sopht_12k5-v6.jpg

[mydementia]

UPDATE
I got took the LED out of the DPDT switch and updated the schematic (below) - still hums.


I decided to bite the bullet and rewire the box (only a few wires...)... before and after below - Still hums...
Before:


After:


Any ideas?

Thanks for looking.
Mike

[scaesic]

i don't see any ground connection to the chassis? if not you could be pickuing up noise.

edit: im not sure i can read your schem, are the 1r, 0.1uf cap conencted to the output smoothing caps? is this supposed to be extra filtering? if so, i think you should be taking your V+ from the other side of the 1r: this means you are using an unfiltered voltage source, as you have taken the voltage from the wrong place, which would also cause noise. The schem could however be drawn wrong...

on the plus side the wiring looks a lot better.

[mydementia]

Hi scaesic - thanks for the reply.
I did secure the safety ground under one of the bolts on the tranny (sanded the enclosure down to get good metal-to-metal contact), but I did not ground the chassis... guess I should do that (usually taken care of by my I/O jacks - just didn't think about it).  Good thing to try tomorrow...

I used one 'leg' of Brian's power supply schematic as my reference (page 4 on: http://www.chipamp.com/docs/lm3886-manual.pdf) - I know it worked great on my LM3886 amp... did I honk something up in my conversion?  I haven't figured out how to get 'nodes' using Visio yet... still learning...

Thanks.
Mike

[ambulancevoice]

add some 12v regulators too, help reduce the noise even more, and make the supply cleaner

also, i think scaesic meant was if you chassic grounded the ac wall plug
the ground from the power supply can go to the dc output jack

[the_random_hero]

add some 12v regulators too, help reduce the noise even more, and make the supply cleaner

also, i think scaesic meant was if you chassic grounded the ac wall plug
the ground from the power supply can go to the dc output jack

Good luck getting 2.2A out of a standard 12V regulator without some trickery. I'd be shortening all the wires coming from the power transformer, or possibly housing it out of the box. For my power supply, I used a 12VAC plugpack which then runs straight into the onboard wiring.

[mydementia]

Regulators don't pass enough current for my application (typically max 1A) so I'm trying it this way...  In a previous thread, Minion pointed out a datasheet schematic for a 'high current voltage regulator' that uses a 'pass transistor' (http://www.diystompboxes.com/smfforum/index.php?topic=61543.msg484242#msg484242) - looks interesting, but I didn't have the parts...

So do or do not run the DC ground to the chassis?

EDIT:
I updated the schem in Visio when I got to work - looks like I did something funky with the filename... here it is for reference:

[scaesic]

all your grounds should be connected together, but you have to make sure you don't ground something twice, even by accident, this would cause a ground loop.You just have to think about it really carefully, make sure you havnt run a ground wire to uninsulated jacks/pots with an electrcal connection to the chasis, as this could potentially cause a ground loop if you have grounded the chasis. (a metal chasis should be grounded btw)

also to reiterate - someone else shoudl verify this because i'm not sure - although it is the same as the pdf,  by the look of y're schematic you're taking V+ from the wrong side of the 1r resistor, that is presuming it is there as a low pass filter, it could be there for some other reason?

*edit* ok apparently the 1r rc network is to rid the cap of inductance, as far as i know the 0.1uF caps should NOT be electrolytic (iv just had a very similar thread about this...), maybe that is the problem

[mydementia]

The PS schem I'm plagiarizing here is from chipamp.com. 
After a bit more reading, I learned that the 1R/2W resistor and last 0.1uF cap are the 'snubber' resistor and cap.  From diyaudio.com, it looks like this resistor/cap combo does something with shifting the output impedance of the power supply (if I'm reading right... here's a post for someone who understands this better than I do: http://www.diyaudio.com/forums/showthread.php?postid=546197#post546197. 

Aha - just saw your 'edit':
The 0.1uF caps are not electrolytic
symbol= |(
I'm using poly film box caps
symbol= ||

Maybe I'll try cutting out the snubber section...

[scaesic]

i'd double and triple check all your grounding - ground loops are the culprit of noise 90% of the time.

[BrianGT]

Mike,

I sent you a reply to your mail.

You are right on the "snubber" schematic, and the best explanation is the thread you linked to.  From my attempt at analysis, it seemed useless.

Post more details about your wiring, as the hum could be related to a ground loop.

--
Brian Bell

[mydementia]

Here's what I'm going to try when I get home today (should look more familiar to everyone here - I removed the snubber cap and resistor and added the grounding scheme).


I need to take the board out to remove the snubber section, so I'll go back through and re-verify that I only have the grounds as shown. 

My next question is how to get the output voltage down from 15.8VDC (loaded) to the ~12VDC my circuit 'wants'.  I'm considering a series resistor (or pot for variability) but I'm not sure if there's a better way...

Thoughts/references appreciated.
Mike

[jrem]

You might want to try a 'star' grounding scheme, where each ground is 'home-runned' back to one common ground point, versus running the PS ground to tube 1, then tube 1 to tube 2, then tube 2 to input jack, etc.  The latter can cause grounding loops. 

[Gus]

Take the 1 ohm 2 watt from the first schematic and install it after the + out of the bridge and before the 10,000uf to add some series resistance going into the cap.

Lets us know what happens.

Anyone think they know why I suggested this?  There are two reasons.

However don't let it run to long just long enough to do a quick test,  because of the  power rating of the resistor  P = E x I  =  about 2.2V x 2.2amps =4.8 watts,    5 watts should work but I would use a 10watt

[the_random_hero]

Take the 1 ohm 2 watt from the first schematic and install it after the + out of the bridge and before the 10,000uf to add some series resistance going into the cap.

Lets us know what happens.

Anyone think they know why I suggested this?  There are two reasons.

However don't let it run to long just long enough to do a quick test,  because of the  power rating of the resistor  P = E x I  =  about 2.2V x 2.2amps =4.8 watts,    5 watts should work but I would use a 10watt

Low pass filter and lowering +V voltages?

[jrem]

Take the 1 ohm 2 watt from the first schematic and install it after the + out of the bridge and before the 10,000uf to add some series resistance going into the cap.

Lets us know what happens.

Anyone think they know why I suggested this?  There are two reasons.

However don't let it run to long just long enough to do a quick test,  because of the  power rating of the resistor  P = E x I  =  about 2.2V x 2.2amps =4.8 watts,    5 watts should work but I would use a 10watt

Low pass filter and lowering +V voltages?

1 ohm ain't gunna lower the voltage any . . .    a) it will keep the cap from trying to back charge the rectifier 120 times a second reducing ripple (1 ohm seems a little low for that, though, IMO), b) put an ohm meter across the 1 ohm resistor, set it to volts, and the reading is the current draw through the power supply.

I would lose the 2.2k resistor, too.  It's probably only there to drain the cap when the power is off . . .

[Gus]

At a 2.2 amp current draw there will be a 2.2VDC drop across a 1 ohm resistor

A 10,000uf cap has low ESR so you can get high current surges.  The 1 ohm or even bigger will limit this.  A larger value to drop the voltage even more.

[jrem]

At a 2.2 amp current draw there will be a 2.2VDC drop across a 1 ohm resistor

A 10,000uf cap has low ESR so you can get high current surges.  The 1 ohm or even bigger will limit this.  A larger value to drop the voltage even more.

2 for 2,  I win!  paypal me the prize, thanks!