FM-PAK producing a lot of audio noise (hum/buzz)

By hansgoos

Rookie (17)

hansgoos's picture

15-01-2021, 20:13

Hi all,

I recently made a FM PAK cartridge based on the design from Erik Luppes which can be found here: https://hansotten.file-hunter.com/do-it-yourself/sound-mods/

"FM-PAK Circuit diagram, Eagle PCB design"

The cartrigde is working but it's generating a lot of audio 'noise' (hum / buzz). When I move my hand closer to the cartridge the frequency of the 'noise' is changing (grounding issue?)

I tried the cartridge in two different machines (NMS8250 and VG8020) both have the same issue.

I'm wondering if someone else used this schema to build an FM PAK and is experiencing the same problem. Or if there is a simple solution to get rid of this 'noise'.

Thanks in advance,
Hans

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By Alexey

Guardian (3239)

Alexey's picture

15-01-2021, 20:37

We used Eric's schematics as the base for our FMPAC/F4 combo board. The first 2 versions had noise issues because back then I had little experience in designing circuit boards. There are certain rules you have to follow if you design hardware that uses both digital and analog signals.

Paste the schematics and the layout of your board here, maybe someone will see what the problem is.

By st1mpy

Paladin (838)

st1mpy's picture

15-01-2021, 20:39

Build another one with new parts (without removing bits from the first one) ? As it could be one of your components that is faulty. I normally put sockets on the chips so they can be swapped to check. Some chips if bought on ebay, sometimes have 40% faulty batch out of 10.

By AxelF

Champion (389)

AxelF's picture

15-01-2021, 22:32

You can reduce a lot of noice by inserting a ' LM7805' voltage regulator on the 12Volt line and feed the 5 Volt circuit of the cartridge through that regulator with a capacitor for extra filtering.
Now you can cut the noisy 5v line that comes from the MSX.
Its not perfect but noice is a lot less.

By Daemos

Paragon (1955)

Daemos's picture

15-01-2021, 22:58

To get rid of noise there are a few things you can do. You describe shielding issues so make sure that your analogue audio path is properly shielded from the outside world. So add tons of metal and hook them to ground. Then check for noise on the power supplies. If present like more then lets say 10 mV. Its time for some serious filtering. Do the following. Take 12 volts and a bc548 transistor. From b to ground hook up a 100uF capacitor and from c to b a 100 ohms resistor. Then connect e to a lm78l05 or lm7805 (depending on power consumption and size) and make sure to decouple with lets say 10 nF on the output of the voltage regulator. In case of opamps add 10nF @ + and - power supply inputs of the opamp to ground as close as possible to the chip. Use thick wire for ground to prevent ground resistance. Then in all extreme put some small silver mica capacitors (10nF) as close as possible to the digital chips to ground and some very small inductors. There shall never be noise in your life ever again. Wink

By hansgoos

Rookie (17)

hansgoos's picture

16-01-2021, 09:21

Many thanks for your responses

The schematics and PCB board layout can be found here: https://hansotten.file-hunter.com/uploads/files/fmpak.zip

Here are the pictures of the PCB

The build I made also uses sockets for all ICs. I already tried to replace some (but with no effect), but I will try to replace them all to see if that helps. If that doesn't help I will also try to use the 12V line to feed the 5V using a regulator.

By Alexey

Guardian (3239)

Alexey's picture

16-01-2021, 13:17

You uploaded 2 identical images. Please upload the bottom's image as "fmpak-pcb-bottom.png".

I am not an electronics engineer, but I already see a few beginner's errors:

  • YM2413 too far from the amplifier, output goes through the entire board (collects interference on the way)
  • No filter capacitors on -12v and +12v rails (need at least one 22-47uF for each rail)
  • No blocking capacitors on any chip's 5v rail (need at least one 0.1uF for each chip and one 10-47uF on the rail)
  • The amplifier circuit is spread on a large area (should be more compact and ordered)
  • Ground should be routed separately for audio and digital circuits starting from the slot pads
  • The power and ground rails for all ICs are too thin (need to be twice thicker)
  • The output from the amplifier should be close to SNDIN slot pad (slot pin 49)
  • Autorouter is not the best idea for such circuit

There are possible errors on the board, so it would be good to see how the schematics was drawn in the CAD. Also, I've seen fake/broken YM2413 chips from China, so try to replace the chip and see if it solves the problem.

I am not sure that this board can be easily fixed to remove all interference. Better to design a new board IMO.

By st1mpy

Paladin (838)

st1mpy's picture

16-01-2021, 11:07

Ym2413 projects are quite popular (even on other platforms like arduino, raspberry pi, sega master system etc) and you can find many attempts by others on youtube, blog, and there were some on this forum here which can be searched (even talks about noise).

Also, which ym2413 did you get, as there are variants like ym2413b. Some people talk about fake ones (I haven't checked my ones I ordered last year from China...)

I find in general msxs are noisy, and especially bad with after market flash or sd carts (I'm not naming specific ones but a lot of them you can hear interference through the speakers.) So, remove them when testing.

The pcb layout is a possible issue like others have mentioned and missing caps etc. Also the amp choice here is not the best you can get (eventhough I probably have used one similar because its cheap.)

By hansgoos

Rookie (17)

hansgoos's picture

16-01-2021, 20:07

Here is the back of the PCB and the schematic


I didn't made / designed the schematics and PCB, I just found them on the hansotten website. Send the GERBERS to pcbway, ordered the parts and assembled everything. I don't have the knowledge for designing such schematics :)

The YM2413's I have only show 'YM2413 9950 THINK' on the top.

@Alexey: the FMPAC/F4 board (which is this one I suppose: https://github.com/RBSC/F4-FMPAC) does it work in any MSX2 (or MSX1) computer? In my case the NMS8250? Maybe I will have a look and try building it (also saves a cartridge slot)