Techtalk

Charles,
I had some problems with hum, too. When I mounted the pots to the front panel, hum was no problem any longer. I´ve still got some high frequency noise when gain pots are cranked up to maximum, shurely because of one or two crappy tubes, have to find out which one it is. I used shielded wires for all connections. The Edcors have no extra connection for ground. My sowter input is grounded. Pictures??
regards
Bernd

Ok I tried a few things all of which made no difference:

1. Connected the center tap secondary of the edcor output to ground. This should have better balanced the output signals in the hope that the two channels would cancel each other out on noise. But it did not work.

2. Shielded the power lines ... No change but I don't think this is magnetic noise.

3. lifted the output XLR grounds. No Change

4. Tried some old Mesa boogie 12AX7s I had here. No difference seen.


I still think it might be a ground loop. ?? it's pretty close to 60Hz.


Also with gain off no noise .... but as I turn it up the noise turns up as well.

If I set my Scope to trigger on Line and crank down the time I can see the 65Hz waveform. Also IF I just put the probe next to the any of the Caps I can see it clearly as well. So there is a waveform lurking in there.


Any ideas on what this problem might be? Could bad components cause this such as old caps ? Wrong cap values? What are some possible causes to this kind of thing?

Thanks
-Chuck

ChuckD hat geschrieben:Also with gain off no noise .... but as I turn it up the noise turns up as well.

this lets assume the hum develops before the gain potentiometer - but looking at the schematic there is not too much.
try it without the T4B.
disconnect the input transformer.
any changes ?

Ok I think I found a big problem Bernd gave me a hint when he said his hum went away when he screwed the pot to the front panel.

One problem was the cables need really really good shielding. I used some nice shielded cable but still if I just move a cable bend an end the noise jumps from -60dBu to -40dBu

tomorrow I will have to try different cables to see how that can help

-Chuck

I couldn't ask everyone I big favor? If everyone could please explain how they each grounded one side of their shields for each line (gain, Peak Reduction, Limit/Comp) <= these seem to be the most import ones.

1. If you can include photos that would be very very helpful.
2. Explain where you took the ground for each shield for each channel.
3. Did you ground only one side or both of the shield
4. Did you not shield ?


I am certain now that this is one source of problem that I am seeing.

This would be very helpful and also since the PP guys will be making these soon it might be nice to get this info now. Then I will try to put together a PDF application note for everything in this thread. Volker you can give it to the guys over there.

-Chuck

Chuck, (is it Chuck ? )

for the gain and peak reduction I just used normal, symmetrical, shielded wire.
if you look at the schematic, pin 1 of the potentiometers goes to ground - I used the shield for this connection.
I read the Limit / Comp should be shielded, my first plan was to use shielded wire and connect the shield to the frontpannel - but since I'm lazy sometimes I just used twisted wire at the end, with no problems.
just another thing is: I never used this switch yet, and I did not see too much changes while measuring.
for my next one I will leave it out

Yup it's me !

Ok so Volker used :

Gain / Peak : Standard two conductor wire with shield connected at both ends (board and pot) as pin 1. Since Pin 1 is ground on board.

Limit/Comp : twisted pair with no shield


Anyone else?

- Chuck

Hi Charles,
I sent you an email with some pics.
regards
Bernd

Great! thanks! those photos will be a big help!


Any others?

-Chuck

He Bernd,

in one photo you have listed a Transformer Ground . Is that the power transformer? in the other photo there is no real trasformer earth ground connection.

Is there any real need to have our star ground connected to earth ground?

Someone here at work mentioned that is where the 60Hz "nasty" stuff is coming from. So why bother connecting it?

Ground_Lift?

-Chuck

Hi Charles,
my power torroid transformer has got a screen winding that should be connected to the star ground. Don´t nail me on that, but I think you have to connect star ground to earth ground. A ground lift switch disconnects only XLRs input pin 1 from earth ground.

Yes you are correct

I just spoke with a friend and very popular old school amp builder (founder of SUNN amplifiers). Conrad Sundholm.

He told me that for his stuff he always star grounds all of his ground planes to Pin 1 of the Input XLR then he connects the pin one to chassis. Now he also connects earth ground to chassis as well but somewhere different as he put it "as far away from the XLR connection as possible". He says that will reduce the 60Hz ground noise from getting in there.

I ground lift would only disconnect the pin1 from the star and not the star chassis. He rarely would do this he said.

I will try this out tonight to see.

-Chuck

OK Got it down to -75dBu at max Gain !! At 70 it is non-existent!!!
Yeah!

Quote .... "My Photo-love Story"








I am going to make another very soon and I plan on trying to put together some of the tips on reducing the hum on this unit.

Thanks Volker ! Bernd!

Sounds great! Very quick with my Vactrol T4b clone

-Chuck

Wow, nice looking unit as always
and good you sorted out the hum

Sweet