Dual La2a - [silent:arts] version

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Postby waldorfcave » Sat Jan 26, 2008 2:12 pm

yes on both channels, exact the same

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

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Postby waldorfcave » Sat Jan 26, 2008 3:03 pm

...change the retrifiers still hum
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Postby [silent:arts] » Sat Jan 26, 2008 3:29 pm

all looks good at the photo.
when you disconnect the heaters from one side, is there still hum on the other side ?
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Postby [silent:arts] » Sat Jan 26, 2008 3:34 pm

waldorfcave wrote: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 ?
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Postby waldorfcave » Sat Jan 26, 2008 3:39 pm

hum is to loud, not normal

hum is constant not gain depended.

rewire my ac heaters and check in one minute
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Postby Harpo » Sat Jan 26, 2008 4:04 pm

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)?

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Postby bernbrue » Sat Jan 26, 2008 4:05 pm

Lars,
did you use shielded wires for pots, output tranny and XLR? The HT wiring is a real pita. :x :) :) :) :)
Never mind, Volker
regards
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Postby waldorfcave » Sat Jan 26, 2008 4:16 pm

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
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Postby waldorfcave » Sat Jan 26, 2008 4:41 pm

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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Postby bernbrue » Sat Jan 26, 2008 5:16 pm

just a guess, did you connect the output tranny to ground? (black wire) XLR outs to ground as well? hmm, strange? :?
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Postby [silent:arts] » Sat Jan 26, 2008 5:27 pm

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:
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Postby waldorfcave » Sat Jan 26, 2008 6:17 pm

thx...

but can i also use a 25k log pot? where should iconnect the 75kr resistor?
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Postby [silent:arts] » Sat Jan 26, 2008 6:23 pm

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.
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Postby waldorfcave » Sat Jan 26, 2008 6:35 pm

cool volker, i order such pots.

tomorrow i will measure some voltages, maybe there is afault
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Postby waldorfcave » Sat Jan 26, 2008 11:03 pm

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
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