Dual La2a - [silent:arts] version

[waldorfcave]

yes on both channels, exact the same

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

[waldorfcave]

...change the retrifiers still hum

[[silent:arts]]

all looks good at the photo.
when you disconnect the heaters from one side, is there still hum on the other side ?

[[silent:arts]]

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 ?

[waldorfcave]

hum is to loud, not normal

hum is constant not gain depended.

rewire my ac heaters and check in one minute

[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

[bernbrue]

Lars,
did you use shielded wires for pots, output tranny and XLR? The HT wiring is a real pita.
Never mind, Volker
regards
Bernd

[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

[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

[bernbrue]

just a guess, did you connect the output tranny to ground? (black wire) XLR outs to ground as well? hmm, strange?

[[silent:arts]]

at first I thought the soundfile is empty - never try to hear this on the Computer

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

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.

it is just the 3rd layer of the PCB

[waldorfcave]

thx...

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

[[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.

[waldorfcave]

cool volker, i order such pots.

tomorrow i will measure some voltages, maybe there is afault

[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