Must touch power cable to stop noise?!

[Slater]

Hello.
I built a small mono audio amp with an 8 pin TDA2822. The schematic is
in this document (http://www.utc-ic.com/spec/TDA2822.pdf) on page 6,
labelled "bridge".
This is the problem: it starts working normally, but at a certain
point, the speaker starts buzzing and rustling, at first very quietly,
then louder and louder. A few seconds later, the noise gets quieter
and quieter and then vanishes.
It does this thing with and without the input connected.
Today I noticed that I can stop the noise, or worsen it, by touching
and twisting the power cable. The power cable is just an extension I
made to bring power from a PC power supply to the board (+12V, +5V and
GND, as my board also contains other circuits). If I connect the PSU
to the board directly, it gets worse (more noise).

What do I do?

 

[hamilton]

Hello.
I built a small mono audio amp with an 8 pin TDA2822. The schematic is
in this document (http://www.utc-ic.com/spec/TDA2822.pdf) on page 6,
labelled "bridge".
This is the problem: it starts working normally, but at a certain
point, the speaker starts buzzing and rustling, at first very quietly,
then louder and louder. A few seconds later, the noise gets quieter
and quieter and then vanishes.
It does this thing with and without the input connected.
Today I noticed that I can stop the noise, or worsen it, by touching
and twisting the power cable. The power cable is just an extension I
made to bring power from a PC power supply to the board (+12V, +5V and
GND, as my board also contains other circuits). If I connect the PSU
to the board directly, it gets worse (more noise).

What do I do?

OK, you said "I built", how did you built it ??

PCB board, peg board, point to point wiring ??

How did you build it ??

Please post a pic somewhere.

hamilton

 

[Slater]

OK, you said "I built", how did you built it ??
PCB board, peg board, point to point wiring ??
How did you build it ??
Please post a pic somewhere.

Here you go:
http://oi44.tinypic.com/ilaumh.jpg

The other circuits on the board are:
- Hsync & Vsync to Composite Sync converter
(http://members.optusnet.com.au/eviltim/scarte.png)
- V-USB MAME panel (http://vusb.wikidot.com/project:mamepanel)
- NE556 based circuit for high frequency Hsync rejection

Thanks for your help.

 

[hamilton]

Here you go:
http://oi44.tinypic.com/ilaumh.jpg

The other circuits on the board are:
- Hsync& Vsync to Composite Sync converter
(http://members.optusnet.com.au/eviltim/scarte.png)
- V-USB MAME panel (http://vusb.wikidot.com/project:mamepanel)
- NE556 based circuit for high frequency Hsync rejection

Thanks for your help.

WOW, what a mess !!

1. have you done this type of project before ?
2. do you plan on rebuilding this in a cleaner package ?
3. will you make a PCB from the schematics ?

When you say "touch the power cable" where is this power cable located
in the pic ?

Your "power cable" is transmitting noise everywhere, it needs to be
stopped there, where ever there is.

Your circuit the way it is built, may not be fixable.

Your audio is picking up noise for the rest of the circuit.

Touching anything is inducing noise everywhere, I bet they can "see"
your circuit next door.

Good luck

hamilton

 

[Slater]

WOW, what a mess !!


1. have you done this type of project before ? No.

2. do you plan on rebuilding this in a cleaner package ?

I would rebuild the audio amp. If I had to rebuild everything, I'd
rather buy some commercial product (there are a couple around).

3. will you make a PCB from the schematics ? No.

When you say "touch the power cable" where is this power cable located
in the pic ?

It's the one at top right, starting with the white plug. It goes into
the PC.
The power reaches the audio amp through the yellow and black wires
that start from there. The audio amp is the circuit at lower right.

 

[Johann Klammer]

I noticed you placed it on top a CRT...
Is the CRT on when it happens? They _do_ generate a little noise.

 

[Slater]

I noticed you placed it on top a CRT...
Is the CRT on when it happens? They _do_ generate a little noise.

Sometimes it's on, sometimes it's off. It doesn't make a difference.
It also does that with a totally different PC in another room.

 

[gregz]

Hello.
I built a small mono audio amp with an 8 pin TDA2822. The schematic is
in this document (http://www.utc-ic.com/spec/TDA2822.pdf) on page 6,
labelled "bridge".
This is the problem: it starts working normally, but at a certain
point, the speaker starts buzzing and rustling, at first very quietly,
then louder and louder. A few seconds later, the noise gets quieter
and quieter and then vanishes.
It does this thing with and without the input connected.
Today I noticed that I can stop the noise, or worsen it, by touching
and twisting the power cable. The power cable is just an extension I
made to bring power from a PC power supply to the board (+12V, +5V and
GND, as my board also contains other circuits). If I connect the PSU
to the board directly, it gets worse (more noise).

What do I do?

Is there local bypassing of power lead?

I always use flea clips when building boards like this. Make putting
components on and off easier.

Greg

 

[HectorZeroni]

Is there local bypassing of power lead?

I always use flea clips when building boards like this. Make putting
components on and off easier.

Greg

Could also assemble a battery and power it with pure DC to see if the
problem is being injected externally or not.

 

[Sjouke Burry]

Sometimes it's on, sometimes it's off. It doesn't make a difference.
It also does that with a totally different PC in another room.

Time to check, it sounds like a classic case of oscilating
electronics.
A scope, to check, or if that is not possible, touching
different parts of the electronics, while connecting a
volt or amp meter here and there, will show fluctuations,
when you touch part of the oscillating circuit.

 

[hamilton]

I would rebuild the audio amp. If I had to rebuild everything, I'd
rather buy some commercial product (there are a couple around).

It's the one at top right, starting with the white plug. It goes into
the PC.
The power reaches the audio amp through the yellow and black wires
that start from there. The audio amp is the circuit at lower right.

Do as Fred stated, use a separate regulator and one of these:

http://www.ebay.com/itm/Active-TDA2822-amplifier-DIY-electronic-kit-PCB-/180713725250

 

[Slater]

It seems I made the problem disappear by cutting the red wire (+5V) in
the power cable, thus shutting down the sync converter and the NE556.
I'm trying to add better decoupling and bypass and will made further
tests afterwards.

 

[Slater]

I'm trying to add better decoupling and bypass and will made further
tests afterwards.

Actually, I forgot to put any decoupling in those!

 

[Slater]

2) Put PI CLC or CRC filter(s) at the amp power input point(s) AND
add a good 0.1uF or 1.0uF cap across chip supply pins.

Thank you. I'll put a PI CLC filter. Would you please suggest the
values for the components? The amplifier chip is powered from a PC
PSU's 12V line, while the other two chips get 5V from the same source
and they process signals of 15 - 16 KHz.

Fortunately, the problem disappears when the whole device is used: I
was making tests with the input to the sync processing chips
disconnected. If I connect everything, no problem.

 

[mook johnson]

Thank you. I'll put a PI CLC filter. Would you please suggest the
values for the components? The amplifier chip is powered from a PC
PSU's 12V line, while the other two chips get 5V from the same source
and they process signals of 15 - 16 KHz.

Fortunately, the problem disappears when the whole device is used: I
was making tests with the input to the sync processing chips
disconnected. If I connect everything, no problem.

You have to be really careful with any type of amplifier.

Good rules of thumb.

1) Filter the PC power supply: buy a ferrite torroid or bead and put around
the cable coming from the PC power supply. Those are switching power
supplied that generate noise in both the common mode and differential mode
Just snake your cable through the torroid for a few turns and that will help
knock down the power supplies radiated noise. Next, right were the power
enters your board add some bypass caps (100 - 1000uf electrolytic with .1uF
ceramic in parallel. The reason is that the electrolytics are not very good
at filtering very high frequencies (MHz range). The .1uF ceramics are
effective at those frequencies and will take over where the electrolytics
peter out.

2) Amplifier power leads: Bypass the power supply leads on the amplifier
directly across the Vcc (V+) and Vss (GND) leads with an electrolytic and a
..1uF ceramic capacitor in parallel. The leads must be very short between
the caps and the power leads of the chip or radiated noise can enter the
loop and get on our power rails. Also any excessive lead length can
introduce inductance when inhibits the filtering action of the capacitors.
Run the supply (V+) and return (gnd) wires next to each other (twisted is
best) back to the power plug bypass caps. You want to distribute power to
the high power parts in a "star" arrangement where each chip has in
independent pair of power wires to the power supply entry to the board. Do
NOT daisy chain ground or power or that will allow feedback to occur. When
they talk of "ground loop noise" daisychaining is a big cause if that.

3) limit the bandwidth of the frequencies going into your inputs. You can
put a small RC filter with a cutoff frequency on the 40K - 100KHz range
something like a 1k ohm in series and a 3300pF from the input pin to ground.
This will prevent any RF noise that gets picked up on the wires leading to
the input from getting into your amplifier. You might have to play with the
values for the 1K resistor since your amp has in input impedance spec of
your amp says is has a 100K input impedance, the 1K in series with that you
reduce the gain by ~ 1%. The 10K call out should be before the 1K so the
circuit should be signal in -> 10K to ground -> 1K in series -> C to
ground -> then chip input pin.


4) Tighten up your wiring: You don't want output signals crossing input
signals. You also don't want input signals close to power supply rails. If
you cross them do so a as close to 90 degree angles as possible. Also
eliminate any excess wire loops, make the wiring as close to point to point
as possible.

5) Does the back of your perf board have a ground plane on it? if so make
sure it is grounded. Leaving unconnected copper is a good way to couple
unwanted signals.

 

[Slater]

Thank you Mook for the precious advice!

 

[josephkk]

Here you go:
http://oi44.tinypic.com/ilaumh.jpg

The other circuits on the board are:
- Hsync & Vsync to Composite Sync converter
(http://members.optusnet.com.au/eviltim/scarte.png)
- V-USB MAME panel (http://vusb.wikidot.com/project:mamepanel)
- NE556 based circuit for high frequency Hsync rejection

Thanks for your help.

Geez, put some power supply bypass caps on the boards, preferably near the
ICs you are using.

?-)