digital pot instead of analog pot

[brownt]

Hi,

I would like to use a digital pot in place of the analog pot in this circuit. However, it does not work properly. There are some very tiny sound fluctuations but that is all. I wonder what needs to be done, or why its not working as expected, how I can work out what the problem is.

thanks

[mikerj]

It would help if you told us which digital pot you used, and ideally a schematic showing how you wired it up.

[brownt]

Hi, its a microchip digital pot. like mcp4361

Wired just like the analog pot is wired. two ends and a wiper. Perhaps I am missing something there, as the questions seems pointless.
 

[mikerj]

Perhaps I am missing something there, as the questions seems pointless.

Did you bother looking at the datasheet?  Hint: What is the min and max voltage permitted on the wiper pins?

[brownt]

The voltage on the pins must stay within the power supply range of the digital pot, so in this instance 0 - 5 volts.

There is no DC on the pot as there are caps on either side. So, do I need to measure the amplitude of the AC, and have a positive & negative supply for the dig pot within that amplitude?

[Wolfram]

An easier method would be to bias the grounded end of the pot to + 2.5 V. As both the other terminals of the pot are AC-coupled, this DC bias will not affect the rest of the circuit.

[alanb]

Is the digital pot linear whereas the analogue pot was logarithmic?

[Gibson486]

So, you move and change the digipot, the bias changes as well. This is fine with a regular pot, bit with a digipots, you need to mindful of where the 0 signal is referenced or you risk going beyond the rails.  For your application, there are better digipots available, but it will cost you, however, for the point of learning, this is a good first step.

NVM...it is biased at 0V.  So yeah, you may be clipping on the lower end.

[Zero999]

If the digital potentiometer is biased at 2.5V, then another capacitor is required on the output, to block the 2.5V DC.

Here's an example of how it can be done. The 0V node of the potentiometer is biased at 2.5V, via a potential divider and AC coupled to 0V. As shown the bias is taken from the 9V supply, but it might be better to change the 39k and 15k resistors to both the same value, say 10k, and take it from the 5V rail.

[Alex_MGM]

I used this very simple schematic. It works very well but it's operation depends on transistor used.
I tried several ones and it turned out that the best (for this application) are very cheap Russian (and Soviet, olad stock) KT315. Similar is for example BC847B.

[Zero999]

I used this very simple schematic. It works very well but it's operation depends on transistor used.
I tried several ones and it turned out that the best (for this application) are very cheap Russian (and Soviet, olad stock) KT315. Similar is for example BC847B.

That won't be very linear and will cause some distortion. It's also not a digital potentiometer, unless it's connected to the output of a DAC, but a crude voltage controlled resistor. J-FETs make better voltage controlled resistors, than BJTs, because they're more linear.

[ahbushnell]

Hi,

I would like to use a digital pot in place of the analog pot in this circuit. However, it does not work properly. There are some very tiny sound fluctuations but that is all. I wonder what needs to be done, or why its not working as expected, how I can work out what the problem is.

thanks

The pot needs to be controlled by a serial data link.  I don't see that in the schematic?

[Zero999]

Hi,

I would like to use a digital pot in place of the analog pot in this circuit. However, it does not work properly. There are some very tiny sound fluctuations but that is all. I wonder what needs to be done, or why its not working as expected, how I can work out what the problem is.

thanks

The pot needs to be controlled by a serial data link.  I don't see that in the schematic?

That's probably because only the analogue part of the circuit is shown. There will be a microcontroller, but it isn't relevant to the question which was about using a digital potentiometer, in an analogue circuit.

[Alex_MGM]

In my post I only offered regulating element for electronir resistor, even method of regulations.
I tested it and it works rather good.
It is possible to use FET transistor instead og bipolar, there are different ways to apply  regulating voltage to base of the transistor. You may use mechanical potentiometer of DAC capability of microcontroller. But method of regulation seems very elegant to me.   

[ahbushnell]

Depends on how he is doing ot if it is an issue.

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