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
. 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.
August 02, 2020, 08:15:57 AM