geoff004

Dual La2a finally finished
« on: December 21, 2008, 01:06:41 PM »
I just finished up my [silent:arts] dual la2a
Just a couple little things to fine tune:  I wired the pots backwards (not a big deal) and I need to adjust the trimmers.  Also I should shorten up some of the power cables from the power transformer.
But the big thing to do is to get a 25k pot for the gain and use it in conjunction with a 75k resistor.  The unit puts out so much gain I can hardly turn up the gain before it's clipping.
Overall it's very quiet.  The right channel shows more noise than the left which might be a result of how I'm running some of the other wires carrying power.  But it only shows noise when it's cranked - in normal operation it's silent.

1 question - what size fuse should I be using?  I've got a 1 amp fast-blow in it now.  If I turn it on with the relays engaged it'll blow the fuse.  Maybe just switch it to a slow-blow?
I always seem to do something stupid in my builds.  If anyone spots anything please let me know.

here's some picts:




Holger

Re: Dual La2a finally finished
« Reply #1 on: December 21, 2008, 01:14:07 PM »
Looks cool...What types of I/O transformers are you using here?

1954U1

Re: Dual La2a finally finished
« Reply #2 on: December 21, 2008, 01:18:20 PM »
Great job!


Overall it's very quiet.  The right channel shows more noise than the left which might be a result of how I'm running some of the other wires carrying power.  But it only shows noise when it's cranked - in normal operation it's silent.

Stupid hint:
did you try to put a steel shield in front of the toroid?
Maybe you have the right ch more noisy because of proximity with transf..

bernbrue

Re: Dual La2a finally finished
« Reply #3 on: December 21, 2008, 01:38:25 PM »
Geoff,
very nice build. In order to reduce noise: the output trannies are quite close to the XLRs, try another position, had the same problem with mine (caused hum & oscillation). Also try to avoid that mains power is crossing wires for gain and peak reduction. Ground the potentiometers (housing, chassis ?)
regards
Bernd

Quote
But it only shows noise when it's cranked - in normal operation it's silent
What else did you expect from silentart. As I know Volker, he would react in the same way  ;D :) ;)
« Last Edit: December 21, 2008, 01:43:01 PM by bernbrue »

[silent:arts]

Re: Dual La2a finally finished
« Reply #4 on: December 21, 2008, 04:03:00 PM »
 :) hey, nice and clean build, congratulations 8)
yes, at least with those Sowter replicas gain is way too much. try the 25K pots first before hunting for hum or noise ...

slow blow fuses are far better with this, since powering up is beefy.
filling all those caps, heating the eight tubes ...
IIRC I used a 500mA slow blow for 230V.

geoff004

Re: Dual La2a finally finished
« Reply #5 on: December 22, 2008, 07:46:18 AM »
Thanks for all the suggestions.
I'll try to get to it all tonight.
I think the big part of my right channel noise is having my relay power wires right up next to my gain wires, but I think I'll try moving the transformers (Sowters in and out BTW) also.
I'll post my results when I have them.

geoff004

Re: Dual La2a finally finished
« Reply #6 on: December 28, 2008, 06:25:43 PM »
1 thing I didn't do that I should have from the start was shield the comp/limit switch.  I was getting a ton of hum as a result.  I also found 1 bad solder joint that made the thing behave a little strangely.  And I changed out the gain pot for a lower value.

So with those things fixed it's totally brilliant!  With gain at full, which is way hotter than I could ever see using it, noise/hum measured in at less than -75db.  I didn't bother moving the transformers as they don't seem to be an issue.

2 things I would like to change about it if I could (maybe someone can point me in the right direction):
First, the output meter is kinda useless.  Is there a little mod to change it to -10 operation?  Otherwise it just gets slammed against the stop.
Second, the GR pot doesn't do anything until you get to 12 o'clock.  I wouldn't mind having it kick in from the start and have more gradual control.

Thank you everyone for your posts.  Every little piece of information has been useful.

Whoops

Re: Dual La2a finally finished
« Reply #7 on: December 28, 2008, 10:18:06 PM »
Amazing!!

Congratulations


[silent:arts]

Re: Dual La2a finally finished
« Reply #8 on: December 29, 2008, 03:35:45 AM »
Quote from: geoff004
First, the output meter is kinda useless.  Is there a little mod to change it to -10 operation?  Otherwise it just gets slammed against the stop.
you did use the 3K9 resistor in series with the meter (it is not on the PCB) ?

Quote from: geoff004
Second, the GR pot doesn't do anything until you get to 12 o'clock.  I wouldn't mind having it kick in from the start and have more gradual control.
strange - you did use a log Pot ?
my normal settings for recording vocals or while mixing never went over 12 o'clock yet.

great you sorted out the hum :)

geoff004

Re: Dual La2a finally finished
« Reply #9 on: December 29, 2008, 07:52:18 AM »
Quote from: geoff004
First, the output meter is kinda useless.  Is there a little mod to change it to -10 operation?  Otherwise it just gets slammed against the stop.
you did use the 3K9 resistor in series with the meter (it is not on the PCB) ?

Quote from: geoff004
Second, the GR pot doesn't do anything until you get to 12 o'clock.  I wouldn't mind having it kick in from the start and have more gradual control.
strange - you did use a log Pot ?
my normal settings for recording vocals or while mixing never went over 12 o'clock yet.

great you sorted out the hum :)

I used the 3k9 for the GR meter setting.  I didn't think I should use if for regular output too.  I'll try it and see.

And just to be sure, the value for the GR pot is 100k log, correct?  Maybe I've got it wired incorrectly.  Is it wired like the gain pot? or should I not use the ground and tie pin 2 to pin 3?


[silent:arts]

Re: Dual La2a finally finished
« Reply #10 on: December 29, 2008, 09:03:49 AM »
use the 3K9 for the output too.
100K log is correct for the GR pot, wired like the gain pot (and ground to pin 1 is important)

geoff004

Re: Dual La2a finally finished
« Reply #11 on: January 01, 2009, 06:47:01 PM »
use the 3K9 for the output too.
100K log is correct for the GR pot, wired like the gain pot (and ground to pin 1 is important)

I got the meter sorted out - it's much better.
But I still don't get the GR pot.  I've got it wired just like the gain pot - coming off the molex connector 1-2-3 to the pot 1-2-3 (100k log).
Is it possible that the T4 sensitivity would be different to that much of a degree?  I'm just searching.

If anyone's got any ideas I'd appreciate it - thanks.


 

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