alvinyow

Hi to all the gurus here,
I'm new to this forum. I hv been using TDA8424 Volume chip in my project. I manage to control the level using a pic controller.
However the problem i'm facing are... lots of noise. When the music are paused. Turn the device to a maximum level. The noise level is unacceptable. Beside when i sendind the data to the chip, while it is ramping the vol up or down, I can here some "ramping noise". I dunno how it called. It should be ramp smoothly without any additional noise.
I built this unit using 4 pcs of regulator 7812/7912 for the balance-unbalance input and unbalance-balance output. The ground of this circuit is analog ground and is supply from 1 transformer 909Vac. The other similar transformer 09Vac will supply 5V and 12V from 7805/7812 regulator respectively. The +5V/+12V will supply to the PIC and TDA8424. So here will be a digital ground. I had split the digital and analog ground before they join in certain spot of the board. Is there any problem with the power supply section? I'm quite happy with this chip. The only problem is i wans to get rid of those noise.
Even when i sending the data to increse the bass or treble.... the output will somehow mix with "some unwanted digital sound".
Please help....
Many thanks

Torben

Hi alvin,

Welcome to the forum. Just trying to help, but please post a schematic. It will make it easier to see what's going on with your circuit.


Torben

alvinyow

here is the drawing.
Let me know if i had did something wrong somewhere.
I manage to control the level.
But too many noise to handle.....

Attached Images

Volume Control.jpg (108.5 KB, 24 views)

crutschow

What is the nature of the noise? Can you describe it?

__________________
Carl

mneary

AG and DG should probably be hooked together at one point only, usually near the chip.

alvinyow

I dunno how to describe. The noise is quite unbearable. Okay..... when the volume level is at low level. The noise is barely noticeable (i dun think it is a hum). but when the signal ramps up.. the noise level escalate. Though in a smaller proportion to the signal level. But it is quite noticeable. Besides, when the ramping is in progress....the "ramping sound" is heard (quite noticeable too) it seems like a digital sound...every level it reach u will hear a noise. When the ramping stop, "ramping sound" will stop too.

alvinyow

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AG and DG should probably be hooked together at one point only, usually near the chip.

I did tied both AG and DG together but to no avail. The noise still there.

My last resort will change to other Chip for a new trial. I may try out LM4832. Almost give up on this chip. I had two design on my PCB.
One is all share the same ground plane. Which is the shortest path back to the ground point.
Second is split both DG and AG and finally fixed at a point where both are jointed together

ronsimpson

Try adding 200 ohm resistors between uC and TDA8424 on clk and data lines. Could use 1k resistors if you send data slow.

I know of an amplifier that has the analog audio parts on one PCB and the micro on a second board. The data lines have an opto-isolator on the analog board. There is no direct connection between boards that way.

There are several application notes on digital pots that talk about a “zipper” sound when you change the volume control. Some of the digital pots only change the settings on zero crossing of the audio signal to reduce this noise. I did not look up your data sheet to see if the TDA8424 does that.

alvinyow

Ohh....in my drawing i miss out the 10k pull up resistor for the I2C bus. You mean the 200 ohm place in a series connection? or Bridging the clk and data line?
The TDA8424 did not have any zero crossing detector. Would this be the reason behind for these "zipper" sound?

ronsimpson

Adding a resistor to the clock and a resistor to the data line reduces the current flow. I know the inputs are high impedance so there should not be current flow. I am talking about current (AC) on the edges of data and clock.

Take a look at PGA2500. I know of one high end mixer that uses them on the mic/line inputs as cores adjustment. You set it to get the level close and use a different control for most level control after that. There are several parts like the PGA2500. Look at the application notes, there may be help there.

Mikebits

Not shown in your schema, but you are using bypass caps on your Vcc pins of your IC's are you not?
Other thing you might try is running your TDA part on 12Va, since is has analog paths.

alvinyow

Will try to add the resistor. Hope to get rid of the zipper sound first. Before troubleshoot on the other noise

alvinyow

I did had a bypass cap of 0.1uf on all the supply pins and locate as close to the chip as possible.
My initial design was run TDA on the 12Va. Which i feel that might not balance the load on the +V and -V. During those time the problem remain. Hence I add another 7812 to produce 12Vb. However all the problem still persist.

Mikebits

Hmm, Sounds like our doing the right stuff. Are you using point to point wiring, or pwb?
If point to point, the wiring is critical. Make sure all your analog signals are routed away from the digits. You have a good solid ground plane I assume. Do you have anyway to make sure your power signals are clean? I love my scope

ronsimpson

The sound you hear may be the nature of the beast. Several digital audio volume control chips claim ‘ours don’t make a zipper sound’. Your ear should be able to hear a series of 1db steps.