Reverse engineering Line6 Relay G30 - Page 1
Forum section: electronics / projects

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 26, 2021, 08:11:55 am
content: HiI've been reverse engineering a piece of gear I have, it is a audio wireless system for guitar, I looked at the receiver:It's quite a busy board with an FPGA (Lattice LCMXO256), an ARM cortex (Atmel SAM7S64), a DAC (AKM AK4396) and two 2.4GHz transreceiver (CC2400), so I am far from reverse engineering the whole thing, I looked at a few things, among others the output audio circuit.The wireless audio is sent digital, this is done by the ARM+FPGA+Transreceiver, then sent to the DAC.The DAC has two balanced outputs (4 pins) which are then combined using only one opamp (NE5532) to be sent as an unbalanced signal in the jack plug.My questions:1) I do not really understand the opamp output circuit - it looks like a Sallen Key filter combined with a diffential amplifier?2) Also why combining OutLeft- and OutRight- of the DAC but then only using OutLeft+ on the other opamp input?3) What are C? and C79 at the end for? They are the small gray capacitors on the picture4) R10 does not seem to be connected to anywhere? I see no traces, no via, and it is not connected to ground.Thank you!

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 26, 2021, 08:17:44 am
content: And here are a few pictures

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 26, 2021, 08:18:48 am
content: And a few more

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 26, 2021, 08:59:42 am
content: Alright regarding 1) it seems the opamp circuit is the one they suggest in the datasheet. Not a circuit I have seen before though - does anyone have some documentation on it?

author: mskeete
author_rank: Contributor
timestamp: February 26, 2021, 08:07:31 pm
content: it would make sense if R10 is connected to the junction of R24/R56.Looking at one of your pics, there's a track from R24 to R56. Might be a branch to R10 which isnt visable as its hidden underneath R10.A continuity test with your meter should confirm ir not.

author: Saimoun
author_rank: Frequent Contributor
timestamp: February 27, 2021, 12:52:55 pm
content: Oooh yes of course! Well spotted! That trace goes right under the four 1k resistors, including R10, so it can connecte underneath!Makes much more sense nowThat answers 2) and 4)What about 3) - What are C? and C79 at the end for? They are the small gray capacitors on the picture

author: ConKbot
author_rank: Super Contributor
timestamp: February 28, 2021, 08:56:55 am
content: The 1k resistor on the output is to isolate cable capacitance from the opamp output, to prevent oscillating. The two small caps can help filter stray RF to keep it from wreaking havoc at the opamp inputs (only the feedback cap between the input and output, which won't block RF) IIRC bipolar opamps are decently vulnerable to RFI (input transistor rectifying RF and inducing offset voltages), so in something designed in the last 2-3 decades, especially if it's using RF itself, some RF filtering would be prudent.

author: Saimoun
author_rank: Frequent Contributor
timestamp: March 01, 2021, 08:41:28 am
content: Ok thanks - I did not know about that. The datasheet of the NE5532 does not mention it though, and I have not seen it in other products using similar opamps. But like you say this one is "using RF itself", it also has a plastic case, so is probably more prone to get stray RF than the other audio products I looked at (which had a metal case and were not using RF).And yes about the 1k resistorit is also to set the output impedance of the signal, which is an "instrument" signal, so a 1k impedance is very standard.