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

[sage]

OK so i had the luxury of being at Henson Recording Studios yesterday and my buddy Gary, the head tech there helped me going thru the unit to make sure it was all good, I did have the pots wired backwards and we are still trying to figure out the LED situation cause I am using some MCI VU meters I had...

Anyhow this is the graph, clean, amazing bench responses... then we brought to the mix room, my friend Miguel was working on some stems for a project project so i asked to hook it up and we put thru some clean gtr stems - AMAZING !!!!

First of all it's DEAD QUIET - secondly it's a WARM, FULL SOUNDING, ROUND, SMOOTH yet not too slow.
We even tried the all stereo mix and yep the sonic boost is phenomenal !

Someone said on this forum everyone should have one of those... well I agree!
Thank you Serpent Audio for this great project! I will post some better pics in a couple of days!

sage

 

[sage]

ah... here is the pic...

still on the bench the knobs are very sensitive - has anyone experienced that?

 

[Biasrocks]

Mission completed.

Fired up and worked perfectly the first time. Absolutely quiet, ton's of gain. Didn't need to fiddle with placement of anything, it all fell into place. Easy build, and they sound great. Waiting on my pair of T4B's from Bill, tested well using my Drip T4b from my LA2A.

Thanks Rukus for this cool project.

Regards,
Mark

 

[juanjovincenzo]

Hi guys!!!

I finish to populate the SA-3A's PSU components  and I want to check the correct performance the PSU with the power transformer before to continue soldering the others components. So please tell me if this wiring beetween Secundaries to J16 PWR XFMR conector is the correct set-up (series-secondaries 2x voltage: e.g.  2x15 VAC in series = approx 34VAC).
thanks and regards
Juanjo

 

[sage]

Finally me and Marson put up in the studio the gear built in the past few months and here is a FB page with pictures, thanks to all on this Forum that have helped with these great projects:

http://www.facebook.com/marsonaudio

sage

 

[Biasrocks]

Strange problem here.

I'm getting oscillations when I engage the HF filter and turn up the peak limit control. This is happening on both channels the exact same way. I initally adjusted the internal HF pot using a 10K tone to get 5db of gain reduction vs normal mode, so nothing excessive there.

Any ideas?

Regards,
Mark

 

[dandeurloo]

Did you remove the coating on the back of the panel around all the pots and switches?  I was just having the same issue on a second one I built and that cleaned it up.  Next time I will do that first!

 

[vineyardgray]

Another finished and fully functional unit.

This thing is the tits.

Might hafta buy some of the close out stock to get a couple more of these in the rack.  Great job Mike!

Only issue with mine is that I can't seem to get the 35V to drop to a useable level for the power LED.  The resistor value is OK (6K7), but still shows nearly the same as the source voltage.  I tried it in-circuit then I pulled the resister and tested it with the 'power link' output out-of-circuit.  No difference - still hardly any drop from 35v.  Then I tried a 47k resistor I had laying around - dropped a little, but not much. Still around 34V or so.

I kinda fried the poor little bridge gizmo by putting the centre tap to the 0V spot on the xfrmr input, I'm guessing that may have something to do with it.  Although all of my voltages test good and the unit works perfectly.

Any ideas WTF?

Thanks,

MR

 

[Harpo]

Only issue with mine is that I can't seem to get the 35V to drop to a useable level for the power LED.  The resistor value is OK (6K7), but still shows nearly the same as the source voltage.  I tried it in-circuit then I pulled the resister and tested it with the 'power link' output out-of-circuit.  No difference - still hardly any drop from 35v.  Then I tried a 47k resistor I had laying around - dropped a little, but not much. Still around 34V or so.

It is an LED, not a lamp. LEDs are polarized and current driven devices. Depending on colour of your LED, this LED might have a forward voltage drop of about 2V. You probably want the LED to operate at 10mA. (35V DC supply - 2V LED forward voltage) / 0.01A current allowed for the LED giving 3300 ohm. Get a 3K3 resistor (from schematic, parts designator of this resistor in question is R51) in series in between +35V, LED and 0V. Double check your LEDs orientation.
(Your 6K7 ? limits LED current to 5mA, that might give dimmed results. Your 47K will limit LED current to 0.7mA, so the LED will never show up)

 

[Biasrocks]

Did you remove the coating on the back of the panel around all the pots and switches?  I was just having the same issue on a second one I built and that cleaned it up.  Next time I will do that first!

Thanks Dan, I'll give it a try.

Regards,
Mark

 

[vineyardgray]

It is an LED, not a lamp. LEDs are polarized and current driven devices.

Aye aye aye!  [slaps forehead with hand]

I think I see the disconnect in my logic:  I'm dropping current with that 6K7 resistor, not Voltage, correct? I should have known that.

I didn't see the LED light up when I first powered on and after checking for voltage, and then seeing how high it was across R51, I assumed I had burned the little bugger out.  I'm guessing now that I've got instead a connection issue of some sort. 

Thanks, Harpo.

Regards,

MR

 

[mulletchuck]

Got mine wired up and powered on tonight!    I bought the kit obgk was selling in the black market back in january.  Onwards to calibration!

 

[mulletchuck]

ok, so after getting thru calibration I had some issues. 

1)  the left channel was noisier(ground hum) than the right, so I disconnected the shield from the input on channel 1 and that cured the ground hum i had when the unit was cranked past 8 on the gain.  However, the side effect was that channel 1 was quieter than channel 2 now.    So, i disconnected the shield on the input of channel 2 and now the two channels' levels were pretty well matched ~0.5db.  no joke, gain on 10, peak reduction on 10, Mod & 50db mode, i get no noise or hum with the input shields disconnected.  as soon as I connect the shields, GNNNNNNNNNNNNNNNNND

2)  There are some erratic moments where i'll have it in link mode and will be pumping material thru it and in order to get them to match output levels, i'll have to keep the gain of one side down by 6 or 7 points on the dial.  For example, both channels will have identical settings for everything except Gain.  I will set the gain identically on both channels, play the material, and then i'll have to increase the left channel's gain all the way up to 7+ for it to match the right channel's gain, when set to 2 or so.    I'll turn the unit off, or into bypass or turn the knobs all the way back to 0 (both PEAK and GAIN knobs) and redial in my settings and the channels will have similar(read: correct) levels after that. 

3)  What does HF do besides occupy space on the faceplate? lol    seriously, this has no audible effect.  In fact, on my right channel, when i turned the trimmer all the way CCW, it dropped the signal level a good 4-5db

4)  my voltages at TP4 were 62 and 67 for the two channels in 50db mode.  sound about right?      The voltages for every other test point were perfect. 

5)  The only differences between the left and right channel was that every cable on the right channel except one used shielded mic cable.  the left channel, if shielding wasn't required, i used regular pairs of wire. 

oh yeah, I should clarify that I didn't stuff the PCBs.  i bought 'em stuffed from obgk.  seriously, all he had to do was wire the thing up.  I don't know why he sold it, but i'm glad he did!  :-D    Sounds great so far, ignoring the weirdness that shows up here and there

 

[mulletchuck]

I'm surprised no one noticed this:

in the schematic, Com goes to Pin1 for the compressor/limiter mode switch
but on the PCB, com is labeled pin2. 

Also, i rewired my whole case with Screw Terminals instead of those sh*tty molex connectors and man, this thing was muuuuuuuch easier to wire. 

For anyone building some of the remaining stock, do yourself a favor and order some of these:

3pin 2.54mm terminal blocks:
https://www.mouser.com/Search/ProductDetail.aspx?R=1725669virtualkey65100000virtualkey651-1725669
2pin 2.54mm terminal blocks:
https://www.mouser.com/Search/ProductDetail.aspx?R=1725656virtualkey65100000virtualkey651-1725656

there's a company called "todaycomponent" and they had better prices but their shipping rate was all F'd up because they had an incorrect weight listed for those things, so google around for those things, they're made by Phoenix Contact

edit:  man, my cellphone camera is terrible smh

 

[rob61]

Should I get continuity between switches on the front panel? I get chassis continuity throughout the case except the front panel. Even the screws holding it in show continuity to chassis. However, the front panel, and all the switches and knobs (casings) don't show it.  Is this going to be a problem?

 

[rainton]

I just received my stereo kit - thanks Mike!

Well now before I start off I have one question:
my enclosure has the recess in the backplate for the IEC on the left side - not as most of you here - on the right side.

So - regarding noise, do you think it'd be better to mount the pcb and the power xfmr the same way most of you did and just let the necessary wires go from the very right side (power xfmr) along
the back to the IEC on the far left?

OR

rotate both of the pcbs - place them closer to the right and mount the power xfmr on the left side right next to the IEC?

I just want to avoid making all the drillings and stuff and then realizing in the end it's not the best solution...

Thanks!!!

 

[mulletchuck]

can't you just flip over the rear panel?  my case's rear panel could be screwed in either way cuz of how it was drilled. 

 

[rainton]

Yes - that's what I did now 
But the rear panel has inputs and outputs engraved which is now
upside down - but I decided it doesn't matter that much...

Better than getting in trouble sound wise and the orientation and placement of the pcbs
all of you have seems to be tested and true so I went for it!

Thanks

 

[rainton]

Alright - I'm finished stuffing the pcbs and I though I would post some close up shots with higher resolution for other people
building a SA3A. Sometimes when I was not sure about something I went through the pics in this thread here and thought
to myself it would be nice to have some more detailed pictures 

 

[rainton]

and another one...