LA3A 500 BUILD

[peter purpose]

Can't remember... what does it sound like?

 

[BrodieSound]

Can't remember... what does it sound like?

Pretty good actually!!
The calibration I've done is pretty basic though; RV28 fully ccw, RV7 voltage for amp board output, and zero the GR meter. The rest didn't make a great deal of sense to me. I've no idea if the meter is accurate! Perhaps it's not that important if it sounds ok, but having got this far I'd like to think I'd finished it properly!

Thanks,

S.

 

[peter purpose]

Just found an extra kit..... £155. PM me if interested.

 

[haima]

Sorry typo! I meant to say when there is no signal present! Is that normal?

I'm just trying to put the finishing touches on my build (finally!!) and I'm getting the same issue here...

With no input signal, when I turn the squash pot all the way up I get some oscillation visible on the output pin of the sidechain amp - this is when the squash pot is in it's last 15degrees or so of rotation. The oscillation is above the audible range - shows up as about 1 division @ 20uS on my scope (what is that 40kHz?).

I guess I need to play with maybe putting a small cap across some resistor in the sidechain amps? or maybe shielding the cable run to the meter (which is very short right now, but it DOES sit right up agains the sidechain amp).

My pot cables are shielded.

I also had trouble getting the meter to calibrate - it seemed impossible to get it to both zero and show roughly the correct amount of compression. Basically the needle doesn't swing far enough - i.e. -20db of actual compression shows up as around -10 db on the meter.

I tried different values of resistors for pretty much every resistor in the meter driving section, no luck.

I ended up removing the stock meter face and hand drawing a new one... but it's not great, the meter only wiggles about 4mm for 10db of compression - not much resolution. It's a shame more of the meter face can't be used. Perhaps my LDRs are really mis-matched... Or perhaps I really need a active meter driver to get the proper swing.

BTW - apart from these minor issues it SOUNDS amazing! Thanks Pete!

 

[mulletchuck]

Just curious, Is the metering in this unit similar to in the 1176, where it's strictly visual, and has no effect on the audio, right?

 

[peter purpose]

The meter accuracy will always be off until someone with theoretical skill fronts up with a solution.

 

[mulletchuck]

But, the meter has no effect on the audio, correct?

 

[peter purpose]

Correct in my glimpse of the schematic, but don't take my word for it.

 

[mulletchuck]

Question about the meter.  On another thread posted today, someone was talking about biasing their LA3A that they just recapped.  they included a link to the UREI schematic and the meter portion is different than yours, peter.    Perhaps this is how the meter circuit is supposed to be set up?  it seems much simpler than how your schematic shows the meter being connected.

 

[peter purpose]

You can try it like that, but as it's a cheap Chinese meter the extra stuff was nailed on.
I'm sure this is covered in the thread somewhere.

 

[mulletchuck]

I've read thru this thread a couple times but couldn't find any information on what voltage rating to use for the caps that don't have a voltage listed in the BOM.  is 63V ok? 
Also, in the schematic, C5 is 400uF/25V but in the BOM it's listed as 470uF/16V.  is this an error?

Is transistor matching if building a stereo pair necessary?    I'm currently assembling a mouser Cart, and am unsure if I need to order 2 of each transistor in the BOM, or do I need like 20 of the 2N508X and should match them between units?

The links in this thread for the OA91's were all discontinued.  will this work instead?:  http://www.mouser.com/ds/0/NTE/nte109-15221.pdf
it says here that it is a replacement for the OA91:  http://www.weisd.com/test/GenericParts_WEISD_view.php?editid1=OA-91

 

[dmp]

caps at 63v will be fine since that is higher than any voltage in the unit.
I don't think you would need to put effort into matching anything except the t4b - there is a stereo adjust trimpot for the overall sidechain.
I have some OA91s left over I think if you need them.

 

[mulletchuck]

awesome.  I ordered all the parts today, now I gotta text Drip (i'm glad he posted his cell # on his website) and find out how he's doin' with those T4B's designed for this kit.

 

[adrian]

hello,
i'm building 2 LA3a, when i searched on the web i bought some cv7130 on ebay in order of the oa91 for GD1-GD2-GD3-GD4, is that really the same thing?

 

[mulletchuck]

According to Kato:

OA91's do not need to be germanium. They're just for rectifying the fakie VU meter. You could use Bat85 diodes, like so: 771-BAT85133. (mouser p/n)

Also, depending on what pins and pin sockets you get, you might want to mount all that stuff on the underside of the PCB instead of on the top of the PCB, inside where the opamps fit.

 

[adrian]

ok thank you!

 

[haima]

Sorry typo! I meant to say when there is no signal present! Is that normal?

With no input signal, when I turn the squash pot all the way up I get some oscillation visible on the output pin of the sidechain amp - this is when the squash pot is in it's last 15degrees or so of rotation. The oscillation is above the audible range - shows up as about 1 division @ 20uS on my scope (what is that 40kHz?).

Just a little update - I experimented with trying to cure the small amount of oscillation in the sidechain (which only occurred when the squash pot was near maximum). I tried small caps in various places, I was able to kill the oscillation, but not without effecting the frequency response.

I then realised I could use the "stereo adjust" trimmer on the front panel to attenuate the total amount of gain the sidechain - this had the effect of preventing the oscillation by effectively limiting the range of the squash pot slightly.

The oscillation also seemed to be more common when the HF/de-ess trimmer on the small plugin board was maxed out - which makes sense as this boosts top end in the sidechain... so try playing with this too.

Just thought I'd mention my findings for those who might have encountered this minor issue in their build.

Now that the oscillation is gone - I've finally mounted my unit in my lunchbox - next to my pair of EQNs - thanks Pete!

 

[Pliplo]

Is there a kit for me??

 

[peter purpose]

Is there a kit for me??

Not from me, but someone else might sell you one.

 

[adrian]

I just finished 1 unit,
i'm trying to calibrate RV7 but I can't really change the output voltage! it stay betwenn 4,8V and 4,9v;
i checked and the audio pass threw, no problem, but there is very few compression..
i tried to change the amp card but it's the same...

i saw  nielsk had the same problem with very few compression but it was a solder bridge, i double checked i don't find...