This Causing the hum?

MIDIbox SID
1

Hi all.

I’ve gotten my 4xSIDs working again, but am getting a huge amount of hum. I can hear the sids working but the noise has a higher decibel level than the sids. :frowning:

I know it’s not the SIDs and the Sid boards since I’ve tested each individually and get the same 50-60hz hum.

I need opinions and suggestions on how to correct this.

Here’s a layout of the circuit I’m using to distribute the power from a 12v wall adapter.

And the board…

The wall adapter is connected to J1.

The adapter is rated as 12v@1a. the 5v connection on the layout is used to power J2 on the Core, and power the Vd Vs connection for j2 on the Sid. The 12v connection on the layout is connected to Vd on J1.

Here’s what I’ve tested. when I have only the SID module connected (J2 to the 5v and J1 to the 12v (don’t know if this matters or helps), there is no noise. Once I connect the core, then I hear the noise.

I have a slight fear of building anything that deals with creating a DIY PSU (electrocution). :stuck_out_tongue:

I want to use a wall adapter, but I’m open to suggestions.

Here’s an image of it stuffed into a case for your enjoyment.  :slight_smile:

Cheers!

t_xen

2

start by fitting another 2200uf cap between th 12v+ and the common 0v (gnd)

are you sure the plugpack is 12Vdc?

3

That was the thought I was having. I Just wasn’t sure so I kinda left that on the back burner and out of the post before trying.  :wink:

And yes, the plugback is 12v DC. The voltage on my multimeter shows 14.6v DC supplied by the adapter. I do have the SID modules stuffed with the 7812 since I’m using four 6581 SIDs.

I’ll give the 2200uF a shot and see if this reduces the hum and post my findings.  :slight_smile:

4

To keep costs down, some of the low cost plugpacks simply rectify the 12vac from

the tranny with no filtering at all. I have seen some which dont even use a bridge

recitifier (full wave rectification) and simply use a single diode (half wave rectification)

which produces a large amount of noise (if you look at the DC from a half wave

rectifier you will see that the DC is actually “switching” on and off 50/60 times a

second.

You may need to play around with the value of the filter cap to get rid of all the

noise. Especially if it is only a halfwave recifier. Just make sure that the voltage

rating is no less than 16v, preferably 25v.

It is sometimes worthwhile to pay a bit extra for the fully regulated plugpacks.

5

Hey t_xen, that is a nice housing. I can imagine someone walking around with it. ;D

6

Hey t_xen, that is a nice housing. I can imagine someone walking around with it. ;D

I can imagine the Boston police department going into caniption fits over it.

7

Yeah, let’s *not* carry a box that has lots of electronics in it and a reddish display on it around.

“OMG! Which wire to cut - there are too many!” :smiley:

8

lol!

Yea, I’ve gotten quite a few bomb comments.  :wink:

The case was made from an old poker chip case. I saw a guy walking around with it and asked where he bought it and told him what I wanted to use it for. he thought the idea was neat so he sold it to me for 10 bucks.  :smiley:

Warland, I added the 2200uF cap from the 12v line to the common ground. still got some noise, although there is just the slightest difference. I think your right about the half wave rectification. I checked and I get about 0.9mVAC coming through it.

Well, it was a cheapie Radioshack adapter. Are adapters labeled if they have full wave rectification?

o0O"not a bomb, but it can sound “tha bomb!”"O0o

:stuck_out_tongue:

9

Looks like fixing the hum was a lot simpler than that. the ground line was looping through the audio ground via the case.

A wire for the power jack broke. I removed the jack from the case, resoldered and tested it by plugging it into the adapter. lo and behold, there’s little hum on the thing now. I could do with less, but hey, it’s sounding great!

10

good to hear :slight_smile:

I haven’t seen any that state the rectification method but I guess it’s a

case of price will probably dictate the quality :wink:

11

the ground line was looping through the audio ground via the case

Yes, the case could be a problem that you dont look at when searching. I have some problems because of encoders touching the front-panel. I searched everywhere but couldnt see the problem.

12

lol!

Yea, I’ve gotten quite a few bomb comments.  :wink:

The case was made from an old poker chip case. I saw a guy walking around with it and asked where he bought it and told him what I wanted to use it for. he thought the idea was neat so he sold it to me for 10 bucks.  :smiley:

Hahah what a great story! (and a real nice guy)

Try carrying electronics parts and a martial arts training weapon case (which I have learned appears to be a rifle case to the uninitiated) at the same time. I did it twice, got quizzed by police one time and security the next, and gave up :smiley: