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Verfasst: Sa Jan 26, 2008 3:12 pm
von waldorfcave
yes on both channels, exact the same

in the motherforum someone changed the rectifiers , and then his hum disapeares

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Verfasst: Sa Jan 26, 2008 4:03 pm
von waldorfcave
...change the retrifiers still hum

Verfasst: Sa Jan 26, 2008 4:29 pm
von [silent:arts]
all looks good at the photo.
when you disconnect the heaters from one side, is there still hum on the other side ?

Verfasst: Sa Jan 26, 2008 4:34 pm
von [silent:arts]
waldorfcave hat geschrieben:gain ratio have to be tuned like volker said (i use sowters) i do it later....

one problem is, that my unit hums a bit. more then it is ok for me :-(
how much is "a bit" ? (LA2As are not famous for low noise)
is the hum constant or gain depended ?

Verfasst: Sa Jan 26, 2008 4:39 pm
von waldorfcave
hum is to loud, not normal

hum is constant not gain depended.

rewire my ac heaters and check in one minute

Verfasst: Sa Jan 26, 2008 5:04 pm
von Harpo
Did you mount the 7805 isolated from heatsink and caseground? Doppelkühlung :) You may have a ground loop. Is hum frequency pre (mains side) or post rectifier (double mains frequency)?

-Harpo

Verfasst: Sa Jan 26, 2008 5:05 pm
von bernbrue
Lars,
did you use shielded wires for pots, output tranny and XLR? The HT wiring is a real pita. :x :) :) :) :)
Never mind, Volker
regards
Bernd

Verfasst: Sa Jan 26, 2008 5:16 pm
von waldorfcave
7805 is isolated, i checked without heatsink...

shielded wire for pots and xlr, output tranny has no shielded wire...

i disconnected the ac heaters from other side and nothing happens
rewireing of the ac heater not helped

for info: hum is there also when nothing is connected to input

later i will measure the hum

Verfasst: Sa Jan 26, 2008 5:41 pm
von waldorfcave
here u can see and listen to my hum

direct patch the output of one side in my motu hd192 converter

http://www.waldorfcave.com/humla2a.wav

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Verfasst: Sa Jan 26, 2008 6:16 pm
von bernbrue
just a guess, did you connect the output tranny to ground? (black wire) XLR outs to ground as well? hmm, strange? :?

Verfasst: Sa Jan 26, 2008 6:27 pm
von [silent:arts]
at first I thought the soundfile is empty - never try to hear this on the Computer :lol:

but in Pro Tools I can hear it. compared to mine: yes, there must be something wrong :shock:

mhm - have to think about it ...
may be do the Gain pot change first.

Bernd, the black wires from the 8940 are grounded on the PCB.
The HT wiring is a real pita.
:shock: it is just the 3rd layer of the PCB :wink:

Verfasst: Sa Jan 26, 2008 7:17 pm
von waldorfcave
thx...

but can i also use a 25k log pot? where should iconnect the 75kr resistor?

Verfasst: Sa Jan 26, 2008 7:23 pm
von [silent:arts]
25K log is even better. the 75K resistor goes between pin3 of the pot and pin3 of the molex connector. (the circuit stil sees a 100K pot, but you can only move it from 0R to 25K). less gain, but even with this "mod" I never went behind the middle of it.

Verfasst: Sa Jan 26, 2008 7:35 pm
von waldorfcave
cool volker, i order such pots.

tomorrow i will measure some voltages, maybe there is afault

Verfasst: So Jan 27, 2008 12:03 am
von waldorfcave
whats about with this "grid stopper" thing?

i read a lot in prodpro forum. should i try it?

http://www.prodigy-pro.com/forum/viewtopic.php?p=258781