Comments on my first PCB layout please

mind me askin what they set you back and where you got them made? Some of my more recent layouts are not so self-etch friendly.

I had 3 made, and it was $64.31 for tooling (got them from PCBCart though so i won't have to pay again if i reorder) and then $17.96 per board, plus some shipping.

Cheers

I made an adaptor today and it all worked, managed to do it with just one layer of veroboard mounted diagonally, but i was worried about the durability and i didn't want to damage the board while trying to remove the one test transformer i put on the bottom. So in the end i decided to just put them all on the bottom side, it isn't an issue as i wouldn't have wanted to mount the board too low anyway, else the input connectors would be in a wierd place. The 4 pin molex is for output and the 8 pin is to go to a bypass switch.

Cheers

OK so i have been doing some testing with the scope. The audio passes ok the only thing is that the levels dont seem to be what they should from the theory.
With a 50Ω source, from the schematic (attached at the bottom) i got that there would be 12.04dB loss from summing 4 channels, and then a further ~18.5dB loss due to the 680Ω shunt, and then 13.97dB gain from the transformer (1:5) :

25Ω source + 10kΩ summing (each leg) x4 in parralel = 2506.25Ω
2506.25Ω x 2 = 5012.5Ω (2 legs)

Potential divider loss from 680Ω shunt = 20log(680/5012.5+680) = -18.45dB

So the total loss should be -12.04 + -18.45 = -30.49dB

Add in the transformer gain and the overall level should be -30.49 + 13.97 = -16.52dB

Yep?

However, here are the scope readings i have got, obviously not extremely accurate as its an analog scope and human reading error. The top line is voltage, all taken differentially until it is unbalanced at the xfmr. The next line is loss from the previous stage, and the bottom line is total loss from input. w/o 680Ω is where i lifted the shunt resistor so i could measure just the summing loss.

Input w/o 680R Post 680R Output
1.6v     0.55v         0.16v         0.85v
Stage -9.275dB   -10.724dB   14.505dB
Overall  -9.275dB     -20dB         -5.494dB

So where the reading is -9.275dB it should be -12.04 from summing loss. And where it is -10.724dB it should be -18.45 from the 680Ω shunt. The transformer seems ok, only 0.5dB out from theory.

I did some calcs to try and identify the problem, and found that the summing loss from 3 inputs is -9.54dB, close to my reading so i thought maybe an input isnt connected properly, but nope, they all check out ok. Further to this, if there was only 3 inputs connected, then the loss from the 680Ω shunt would have increased, but the readings actually show it as decreasing.

Thats about as far as i got, any help appreciated.

Thanks

It's more than a little bit late but:
The XLR connectors pin #1 (and G) are not part of the audio circuit and should not be connected to the audio ground.
They should be connected directly to the chassis with as short a wire as is practicable.

Yeh at the minute there isn't really a chassis to connect it to though :/ I've heard this before though and always got a bit confused, because if you're following star grounding then the audio ground will be connected to the chassis at the same point as the XLR ground pins, so really all the grounds are common surely?

Anyway that doesn't explain the unexpected levels does it?

Cheers

If you have no chassis, then the XLR's pins #1 should not be connected to anything.

The XLR connector does not have a ground pin, it has a shield pin!
So the entire length of the cable's shield acts as a noise antenna.
The XLR shields should connect to the chassis near the XLR connectors.
The RCA grounds should connect to the chassis near the RCA connectors.
Try cutting the boards ground plane around the XLR connectors and then  add a jumper to the chassis.

"EJ" I'm a little confused.
The schematic in post #63 is different than the board in posts #42 & 52.
One has XLR connectors and the other has 1/4inch TSR connectors.
Also the board seems to have more parts than the schematic.

Speedskater said:

"EJ" I'm a little confused.
The schematic in post #63 is different than the board in posts #42 & 52.
One has XLR connectors and the other has 1/4inch TSR connectors.
Also the board seems to have more parts than the schematic.

Click to expand...

The actual board is shown a couple of posts up, with dual stacked TRS connectors. That schematic above is just a quick one i made to post on here, and i just put XLRs as it was quicker to find those than TRS symbols. Regarding more components, thats because basically the board is pretty much just the same circuit 4 times, with the exception of a molex connector on one to send to a switch, I'll attach the full schematic that the board is built from.

I'll try what you've said about cutting the grounds on monday when i'm back at work, but i've listened to the outputs and if it was extra noise picked up by the shield acting as an antenna then surely A) I would see it on the scope B) I would possibly be able to hear it (obviously depends if its in the audible range)??

Thanks for the help
Elliott BTW

Well you will only be able to see or hear the noise when it's present and of course that will be at the worst possible time.