digi Pot causing click noise

Help Support GroupDIY:

oran.outan

Member
Joined
May 12, 2021
Messages
20
Location
France
Hi all

Not really a studio project, more about an sound installation project and an experimental instrumental/electronic design...
I made an assembly with a microcontroller and a digital potentiometer MCP42xxx.
I can send messages to the microcontroller for control the resistors levels of the digit Pot connect to a custom 'amplifier board', basically a circuit based around a 10w audio amplifier TDA2003... which changes the level of a magnetic microphone (guitar pick-up) send to speakers.
The pick-up level change (the sound on speakers moving up/down)… but I can ear a 'small audio click noise' when I change the level/Value of the pot, at each step of the pot. If I don’t change/send a value to the pot, sound is clear and I haven’t any noise, but while I change the pot level ‘click noise’ appear. So, it’s clearly coming from there !

I understand that the digi Pot resistance is 'jumping', step by step and is not continuous/linear (which is sad) like a rotary pot, causing 'cuts' in the signal.
Could a small capacitor for smooth the resistor jump can help !?

My ability on audio design are pretty poor, any help is welcome.

here is the TDA circuit, I just replace the 50k rotary pot by the digi Pot...

thanks and best
0-0
 

Attachments

  • Capture d’écran 2021-06-24 à 15.14.16.png
    Capture d’écran 2021-06-24 à 15.14.16.png
    634.6 KB · Views: 18

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,036
Location
Marcelland
There may be more than one explanation to your problem.
The most likely is charge injection. That happens when the logic circuit drives the gate of an individual CMOS switch; there is a small fraction of the voltage that couples electrostatically to the rest of the circuit. This is generally documented in the datasheet.
There may be some residual DC across the digipot, so any chnge in volume results in a small DC variation at the output.
The TDA200x produces an input noise current. When the volume changes, the noise also changes; that is called noise modulation, which also can be perceived as a click, because it varies suddenly.
 

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
2,632
Location
New Jersey, USA
I'm not surprised. Digital pots are obviously nothing like regular potentiometers. They have voltage limitations, wiper resistance, the level is stepped, care has to be taken to keep digital noise out of your analog signal, ...

I have never used a digipot (mostly for reasons above) but if you design the circuit around the part, I think it could be made to work just fine and noise free.

If I were trying to fix this, I would study the situation and try to figure out exactly why you're getting a click. For example, there is no input capacitor. So if there is even a few 10's of mV DC across that pot, the near instantaneous change in the digipot will cause a jump in level that would certainly sound like a click.

Or you can just throw parts at the problem and see what happens. Specifically, I would add a capacitor to the input. Depending on the desired fidelity, I would actually make this cap a little small to filter out hum. Even 100n would be a little large. Use one of those RC calculators to figure out what part in your bin would do what you want. Then I would also add a cap from the wiper of the digipot to ground. Again you might choose a part to deliberately cut the highs a little. A conventional driver speaker isn't going to do well over 10kHz anyway. So maybe 1nF.

However, I do see another issue worth mentioning that is somewhat unrelated. I'm not familiar with that TDA amp but generally you want to feed a power amp with low impedance. Otherwise, it could actually be hissy. So technically there should probably be a buffer after your pot. And that's another opportunity to bandpass.
 

oran.outan

Member
Joined
May 12, 2021
Messages
20
Location
France
Hi

Thanks for your reply... I will have now to figure out what your are talking about and how the parts work... read/learn/understand all of this...

other question :
When I connect an output just after the input (for get the raw pick-up sound, like the 'thru' of a DI) I also have the 'click noise', And cannot make use of the pick-up. Could it be possible to 'isolate' or make two road ? one going to a direct output without any action, the other one passing by the Digi Pot and the amplifier !?
Like this I could have the pick up (going to a pre) and the other one passing the amplifier circuit...

Thanks again
0-0
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,036
Location
Marcelland
Hello! Let me also insert my five kopecks.
Since 2019, I have been implementing my project in the construction of a studio console. For maximum simplicity, I do not use any digital technologies, keys on FETs, TTL and relays. But without compromising the functionality and sound quality. So far, I am building three modules: a master section, a section of 12 subgroups and a section of 12 input channels.
Master: 3 stereo monitor lines+ 1 mono monitor line, 2 headphone outputs, 10 aux lines. Next, there will be blocks for expanding the functionality.
Input channels: separate microphone and linear preamps, adjustable edged LPF and HPF, 4-band 100% parametric equalizer (LF and HF - shelf\bell), 10 aux sends (pre \ post fader), sends to 24 subgroups (now I'm building the first 12 subgroups), sends to master (L\R) and mono (C).
Subgroups: receive a signal via the internal bus from the input channels, receive an external signal from the FX section (they can work as linear input channels), from FX sends to 8 aux (pre \ post fader) and master, from internal buses sends to 8 aux (pre \ post fader) and master. Each subgroup has the same equalizer as the input channels, which is switched between the FX and the internal bus. There are INSERTs on the input channels, subgroups, auxes, and L\R master.

P.S. I apologize for possible mistakes in English - I use an online translator.
I apologize for the quality of the images - I took pictures on a mobile phone with not very good lighting last year.

Hi

Thanks for your reply... I will have now to figure out what your are talking about and how the parts work... read/learn/understand all of this...

other question :
When I connect an output just after the input (for get the raw pick-up sound, like the 'thru' of a DI) I also have the 'click noise', And cannot make use of the pick-up. Could it be possible to 'isolate' or make two road ? one going to a direct output without any action, the other one passing by the Digi Pot and the amplifier !?
Like this I could have the pick up (going to a pre) and the other one passing the amplifier circuit...
You would need to buffer the pick-up with a unity gain...buffer.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,019
Location
Hickory, MS
I have done a lot of work with DPOTs but not that specific microchip family... I worked with higher voltage/higher bit parts.

That microchip part is low voltage CMOS single supply so presumably your AC should be biased at 1/2 that supply voltage.

There should be some clicks for switching large wiper changes in the presence of audio, just like switching conventional audio circuits. With no audio, it should switch silently.

====

This is probably too much information but DPOTs can switch pretty much silently if wiper changes are coordinated with signal zero crossings, but that is probably not your issue.

JR

PS: I saw that microchip published app notes have you read those?
 

oran.outan

Member
Joined
May 12, 2021
Messages
20
Location
France
Hi

I actually have click at every step of the Dpot, small change or big... when Audio. that work good when driving a led... but not the project...
In a naive way I just replace the rotary pot by a Dpot hoping that will work... but digital world is not analog one's°°°

I imagine I have to do something like this !
where the MCP606 is a Micropower CMOS Op Amp.

Best
F
 

Attachments

  • Capture d’écran 2021-06-25 à 16.03.41.png
    Capture d’écran 2021-06-25 à 16.03.41.png
    36.3 KB · Views: 10

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,019
Location
Hickory, MS
that topology is probably even more sensitive to switching noise, injecting stray charge directly into the op amp - input.

sadly I checked out their first ap note (an1316) and that circuit is right there, so it better work. :unsure:

I see little discussion about power supply but presumably the the DPOT connected to the op amp's output shouldn't be driven beyond the DPOTs max rail voltage. I would be apprehensive about connecting the op amp + input to ground unless using split supplies like +/- 2.5V.

JR
 

FIX

Paul Wolff
Joined
May 5, 2021
Messages
42
Location
Nashville
Keep in mind that with audio, you will get clicks as the transition looks like a square wave when it drops or increases. With any circuit that uses FETs, you get control bleed through, the data sheet shows as much as a 50 m/v pulse. You want to insure that the input has no DC offset, and you might want to try the dual version and have the wiper feed both + and - inputs of an opamp, with only the signal going through one of them. It might null it further.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,019
Location
Hickory, MS
Keep in mind that with audio, you will get clicks as the transition looks like a square wave when it drops or increases. With any circuit that uses FETs, you get control bleed through, the data sheet shows as much as a 50 m/v pulse. You want to insure that the input has no DC offset, and you might want to try the dual version and have the wiper feed both + and - inputs of an opamp, with only the signal going through one of them. It might null it further.
Yo Paul that you? Hi
===
The audio from a DPOT will look like instantaneous steps between large gain changes that will sound like clicks. In theory the signal is multiplied by a step function.

This 5V CMOS DPOT can probably be made to work statically, but I wouldn't expect click less gain switching without much work (I worked with much higher voltage parts with more bits (small step size).

JR
 

Bo Deadly

Well-known member
Joined
Dec 22, 2015
Messages
2,632
Location
New Jersey, USA
you might want to try the dual version and have the wiper feed both + and - inputs of an opamp, with only the signal going through one of them. It might null it further.

That's a interesting idea. I'll have to file that away in my brain somewhere if I ever have an occasion to try a dpot.
 

oran.outan

Member
Joined
May 12, 2021
Messages
20
Location
France
that topology is probably even more sensitive to switching noise, injecting stray charge directly into the op amp - input.

sadly I checked out their first ap note (an1316) and that circuit is right there, so it better work. :unsure:

I see little discussion about power supply but presumably the the DPOT connected to the op amp's output shouldn't be driven beyond the DPOTs max rail voltage. I would be apprehensive about connecting the op amp + input to ground unless using split supplies like +/- 2.5V.

JR
"sadly I checked out their first ap note (an1316) and that circuit is right there, so it better work. "

Yes, I take the screenshot from the datasheet... In a next order I will take one and make a try, but it not be soon as I not need much for now...
of course, I will let you know...
 

FIX

Paul Wolff
Joined
May 5, 2021
Messages
42
Location
Nashville
Yo Paul that you? Hi
===
The audio from a DPOT will look like instantaneous steps between large gain changes that will sound like clicks. In theory the signal is multiplied by a step function.

This 5V CMOS DPOT can probably be made to work statically, but I wouldn't expect click less gain switching without much work (I worked with much higher voltage parts with more bits (small step size).

JR
Yes that is I... Still doing the do. Making immersive analog consoles now. "I KNOW". LOL
 
Top