SA-3A (LA-3A Clone) Support Thread

[ruckus328]

This is the offical support thread for the SA-3A Project.  Below you will find the BOM and other documents for the build.

Ordering:

www.serpentaudio.com

SCHEMATIC (REV 2):

https://www.dropbox.com/preview/SA-3A_REV2_SCHEMATIC.pdf?role=personal

BOM (For Stereo Build) - UPDATED JUNE 18th, 2014:

https://www.dropbox.com/preview/SA-3A_REV2_BOM_Stereo_2020.pdf?role=personal

BOM (For Mono Build):

PENDING

PCB Dimension Drawing:

https://www.dropbox.com/preview/SA-3A_REV2_PCBDIM.pdf?role=personal

 

[ruckus328]

OK, some basic info to get you guys started (warning, it's late and I'm a little out of it right now):

I'll try to get some pics up tomorrow to help explain a little better.

Please download the schematic above and get a little aquainted with the circuit.  It's very easy to follow (I hope anyways) and shows very clearly how how to wire the switches and pots.

You're going to need some shielded wire as she's a sensitive gal otherwise, I recommend getting some two conductor (22 gauge) shielded wire.  7 or 8 feet should be plenty.  The schematic shows what needs to have shielded wire and how to terminate it.  The Bypass, GR/Output, and 50db/30db switches do not require shielded cable, however there is no harm in using shielded there if you want to.

WATCH YOUR AC RUN TO THE FRONT PANEL POWER SWITCH (unless you like noise of course  ;D ).  I'd recommend using a piece of two conductor (18 gauge) shielded wire for this, 2 feet long will be plenty.

Enclosure - 10" or 11" Deep is recommended to keep your toroid as far away as possible.  An 11" deep enclosure would allow you to turn the boards sideways for maximum toroid distance if you choose.  You can cram it into a 8" or 9" Enclosure like I did on the prototype, but be fair warned might have to place some kind of mumetal shield over your toroid.

Board mounting holes are for a 6-32 or M4 screw.  A T4B in the socket is a tight fit in a 2U enclosure, so be careful how much you stand your boards off, you'll have to use fairly short standoffs or you won't be able to put the lid on.

If you're using one of my front panels, they have pilot holes for parmetal or modushop enclosures on the back side.  If you're using on of the collective cases (which are in different locations) you'll have drill your own holes from the front (I'll be posting a drill template).

As far as I know, collective cases are the only enclosures out there that do not have a front lip (which is extremely nice for this build as the toggles and meters run very close to the top/bottom of the enclosure.  If you're using a modushop or parmetal enclosure you'll have to notch out the bottom front lip where the toggles are as well as on the top front lip in the meter areas using whatever means are at your disposal (keep in mind it doesn't have to look nice, I just started hacking away with my dremel).

Try to stay with me folks  ;D

PCB Stuffing:

Power Supply Section:

For a stereo build, both boards are identical however the power supply section should only be populated on the board going closest to your toroid transformer (we'll refer to this board as the primary board).

The other board (we'll refer to this one as the secondary board) only gets the 3 caps indicated installed in the power supply section (C10, C12, & C24).

Power is brought from the Primary board to the secondary board via the "power branch" connector (J15).

CRITICAL***: The input transformer (T3) and metal can transistors (Q5, Q6, Q10, Q11) MUST be spaced off of the board some.  Because it is a double sided board, if you don't space them off of the board when you solder the pins, solder is going to flow through the plated holes to the underside of the parts and short to the metal cans.  get my drift?

Be sure the input transformer (T3) is oriented correctly, pin 1 marking is on the underside.

My only screw up, somehow values for Q5 & Q6 didn't print in silkscreen.  They are:

Q5 - 2N3053
Q6 - 2N4037

Front panel power LED connector (J14) and dropping resistor (R51) should only be installed on only one of the boards.  It doesn't matter which one, take your pick, although it'll probably be a shorter wire run from the front panel to your secondary board.

Interstage & Output transformers should be hard wired right to the board.  If you got a component kit, be careful you don't accidentally stick a connector in these xfmr wiring spots or you're gonna find yourself short on connectors.  Try to keep the wires relatively short but be sure to give yourself enough slack to get in there to solder the wires to the board before you mount them.

Interstage Transformer wiring:

Black goes to 5
Red goes to 2.5
Blue goes to .31

That's all there is to stuffing the boards, rest should be self explanatory.

I'll get some more info regarding wiring and pics tomorrow.

 

[ruckus328]

That should get you going on stuffing the boards guys.  More to come.

 

[Neil]

Thanks for the info. When you get a chance how about a pic or diagram of your case layout? I'd like to mirror it to avoid problems, like with the AC run to the front panel power switch.

 

[dtrax]

I hit a snag, and need some clarification. I've stuffed all the 1/4W resistors, but at VR5 and VR6 it's unclear where to solder them to. Below is a pic to illustrate how I have them at the moment. If I lay the resistors flat, they cover up holes.

EDIT: the resisters I'm refering to are 2K61 at VR6 and 3K24 at VR5. Ruckus, I'm looking at your pic here for general guidance. (post #56) http://www.groupdiy.com/index.php?topic=41264.40

Honestly, it's late as hell so I should probably give the project a rest and get some sleep! Looking forward to your reply...

EDIT2: Reading the schematic and looking at the underside of the PCB, it seems there's no problem. I think. lmao... good night!

 

[Purup]

Hi dtrax,
I don't know if you have already solved this problem. The two holes you are using for the VR5 and VR6 are to be used only if a trimmer resistor is used. You are using a normal resistors for the VR5 and VR6, so you should use the two holes right next to the ones you are using on your photo. Meaning the holes a little further towards C24, C22, and C26. There is a direct connection from the holes you are using and the ones you are supposed to use, so everything is ok if you have already soldered these resistors to the PCB. Hope you understand my description. If you need it, I can send you a photo of how to place it on the PCB.

 

[dtrax]

Purup, thank for confirming that. Yea, when I actually looked at the schematic it made sense. I jumped the gun on posting, being excited to get things progressing. I'm actually done populating the boards, save for the interstage and output trannies (just waiting on a parts order from mouser for hardware, wire and such). So far a very painless build!

 

[ruckus328]

dtrax, sounds like you're good?  Post some board pics!


I'll get some unit pics up tomorrow of the build I'm currently doing, I only have 1 channel populated (had hoped to have it done by now ut too busy), but should be enough for you guys to get the idea.

of course you can always bring her by if you have any issues dtrax, you're only 30 minutes away!

 

[Neil]

I'm making good progress on mine. I have both boards stuffed except for trannies and am working on the case. Tonight I tapped and drilled the front panel. I advise everyone to check and double-check their hole placement before drilling! I'm using a Par-metal case I had lying around and the pilot hole markings didn't quite line up with my holes like they should. I'm not sure why, but I'm sure glad I checked before just drilling them out. Even so, I was nervous since it was a one time, get it right or else shot!! As mentioned it's been a very easy build so far.

 

[dtrax]

Ask and ye shall receive. Although my iPhone doesn't do well with indoor lighting.

dtrax, sounds like you're good?  Post some board pics!


I'll get some unit pics up tomorrow of the build I'm currently doing, I only have 1 channel populated (had hoped to have it done by now ut too busy), but should be enough for you guys to get the idea.

of course you can always bring her by if you have any issues dtrax, you're only 30 minutes away!

Thanks for the offer bro!

 

[dtrax]

Am I a ratard? I can't figure out how to attach multiple pics.... and for some reason they're both upside down....

 

[Neil]

Do you recommend we install spacers under the pcb where the output and interstage transformers are and use longer screws to go through the case when we mount them? I'm thinking it might save something if the unit was ever accidentally dropped or otherwise took a shock since they're kind of heavy.

 

[Davo]

No real need for spacers (I wouldn't recommend dropping your unit nonetheless ;D) 

The interstage mounts just fine... the output xformers don't quite flush up though so I put a small washer under each side before
ratcheting it down

cheers,
dave

 

[Purup]

No real need for spacers (I wouldn't recommend dropping your unit nonetheless ;D) 

The interstage mounts just fine... the output xformers don't quite flush up though so I put a small washer under each side before
ratcheting it down

cheers,
dave

Yes, that is a good point. I had to place one washer between the PCB and interstage and two washers between pcb and output transformer. Before I did this, the PCB started bending when I tightened the screws to the transformers.
Cheers,

Mikkel.

 

[ruckus328]

Yes, that is a good point. I had to place one washer between the PCB and interstage and two washers between pcb and output transformer. Before I did this, the PCB started bending when I tightened the screws to the transformers.
Cheers,

Mikkel.

Crap, knew I was forgetting something......yes this is important, I had the same issue as the bracket feet aren't necessarily perfectly 90 degrees to the body.  I recommend placing a washer under each foot on both transformers to give it a little space between it and the board.  If not, as some of you have noticed, there is the potential for the board to flex significantly (not a good thing).  I actually was going to go with .090" thick boards because of the weight of the transformers, however due to the already tight clearance of a T4B in a 2U enclosure, I kept it at .063" thick.

As far as spacers underneath the transformers between the board and enclosure - yes, definately not a bad idea.  For me, I have the boards mounted with screws running from the bottom of the enclosure and then a #6 nut acting as a spacer, then the board goes on top of that and another nut on top of the board to screw it all together (same way I recommend to mount the sb4000 boards in the build manual.

The transformers I have mounted with #8 screws running up from the bottom of the board.  The heads of these #8 screws ended up being the exact same neight as my #6 nut board spacers, so the heads of the xfmr mounting screws are touching the bottom of the enclosure and in turn acting as support for the transformers, just lucked out with all that really.  Whether you do it like that or like you said neil, I would reccommend to somehow put some support underneath them, just to play it safe.

 

[Davo]

Just finished her up this morning.

Nice project, thanks for putting this together Mike!!

It was a bit tricky getting her quiet... really susceptible to AC, but with a bunch of minor tweaks was able to get it down to a just audible 60hz on
the highest gain/highest peak reduction settings of the right channel. 

One observation for builders... don't skimp on the ground wire... I started with a 20 gauge wire and it just wasn't adequate

cheers,
Dave

 

[jandoste]

Hi ,
I got a question,
I have not received 604R( R46). Instead of 604R I received 604k  ???
What value can I use instead of 604R?
Thanks,

 

[ruckus328]

Hi ,
I got a question,
I have not received 604R( R46). Instead of 604R I received 604k  ???
What value can I use instead of 604R?
Thanks,

Umit, please recheck with your meter, there is no 604K - are you sure your meter didn't say .604K?

They should be blue 1/2W resistors.

 

[ruckus328]

Davo, beauftiful job! You beat me to the pics haha.

AC noise - yes, this is what I was talking about, toroid placement/orientation and mains AC routing will be the culprits.  If you look at an original you'll see the pwr transformer is actually mounted on the exterior of the unit.  She's got a fair amount of gain, so if it's only audible at max gain it's probably at the acceptable level, but few more things you can try to see if it gets it down to nothing....ALthough don't think the original is known for being an especially quiet compressor either.

Ground/power connections - yes, I'd really recommend 18 gauge or larger.

Your board placement/enclosure size is the recommended arrangement to keep things as far from the toroid as possible, both of my builds have been in very small enclosures and the toroid is very close to the right board.  On my first prototype I had alot of initial noise, shielding the AC line and rotating the toroid was able to mostly eliminate it.  I temporarily stuck the toroid on the outside of the enclsore and that fixed it, she's still sitting like that and being used for tracking  ;D.  Going to try and put it back inside and try a mumetal shield over the toroid one of these days.

On my second build, with same placement, same size enclosure - the unit is dead silent.....wierd, but I'm not complaining.

If you're still getting more noise on the right channel than the left though I'd recommend to try rotating your toroid (if you haven't already done so) and maybe putting a piece mumetal or steel between it and the boards.  Quick test too is to try and move it to the outside of the enclosure and see if it helps.

Also, something I didn't mention and isn't indicated on the schematic - make sure the shield for the "link" wire is only connected to one of the boards, don't connect it at both ends or you'll cause a ground loop.

Lastly - original unit's manual says this in regards to Pin 1 on input & output xlr's:

"Connect the ground or “shield braid” (usually pin 1 of XLR cable) of the LA-3A output only. However, you may
not want to connect this wire if it causes hum noise in the audio signal. You may need to connect the input
cable ground wire as well if it eliminates hum problems."

I had no difference in noise levels between grounding them and floating them, so both my inputs and outputs are grounded.  YMMV, you may want to try floating them and see if anything changes.

For stereo link calibration:

Stereo Setup / Calibration
1. Set the STEREO ADJ control to full Clockwise.
2. Set the PEAK REDUCTION knob to 0.
3. Set the GR / OUTPUT switch to OUTPUT.
4. Connect a signal generator to the inputs of both LA-3A units. 400 to 1000 Hz recommended.
Set the signal generator output level to produce a typical level for your application.
5. Adjust the GAIN control on each unit so the outputs are equal at O dB on each unit. External
meters can be used to view output level as well.
6. Set the GR / OUTPUT switch to GR (Gain Reduction Metering) on both units. Turn the PEAK
REDUCTION knob on just one of the two units until the meter reads -5 dB; both units should
respond.
7. Note which LA-3A is reading the most gain reduction. On this unit, turn the rear panel STEREO
ADJ control until both LA-3A meters read the same value. The unit is now in calibration.
Bring up the PEAK REDUCTION of the remaining unit to the same numerical value to match
gain reduction.
Stereo Use
You are now ready to use the two units as a stereo pair. Typically the users adjusts both front panel knobs in
tandem to achieve matched compression and output levels. The stereo image should remain solid and both
units should compress the stereo signal equally, regardless of which side (left or right) is triggering the gain
reduction.

 

[jandoste]

Hi ,
I got a question,
I have not received 604R( R46). Instead of 604R I received 604k  ???
What value can I use instead of 604R?
Thanks,

Umit, please recheck with your meter, there is no 604K - are you sure your meter didn't say .604K?

They should be blue 1/2W resistors.

Hi Mike,
You are right! I was a bit tired
But good news : I finished it
Great project and very easy to build it!