Digital pots v. Vactrols

[cherler]

Digital control is rad, and I wanna do more of it in future builds. I've got a few digital pots I've tested just mirroring a volume knob and there doesn't seem to be any added noise or anything. The nice thing here is I can ground the output from the pot and internally disconnect the input to completely silence the output if needed. I havent used vactrols before really.

What opinions are to be had on this shit?

[BetterOffShred]

My opinion is less science based than maybe you're hoping for, but I really dig the lushness of analog components. I know vactrols and LDR/LED combos are fickle and it takes some doing to dial them in, but the sound is just fantastic.

In the circuit you're describing the shit I mentioned above may be completely unnoticeable, but for control sweeps and envelopes etc, analog bits always sound king to my ears.

[Pladask]

The potentiometer-side of digital pots are analog, but has a finite resolution. They're just controlled digitally. If you look beyond the thru hole options you'll find options with better resolution (steps). E.g. there are some relatively cheap SMD digipots with 1024 steps (good enough for audio) over at digikey.

[cherler]

If you control a vactrol from a uC, isnt the output quantized anyway? PWMs and DACs both have limited resolutions.

I was looking at some of the 1024 tap pots, but a lot of them limit the voltage to the pot side of the device to within your digital power rails. I grabbed some 256 tap ones that let you exceed the rails just to be sure I'd have headroom even with massive gain. Probably completely unnecessary though. I cant really tell the resolution is lower, but it's about 1 deg of a knob per step so it could be a problem at some point.

[controlFreak]

If you control a vactrol from a uC, isnt the output quantized anyway? PWMs and DACs both have limited resolutions.

yes. however, vactrols are slow to react and so have "filtering" built in

[crochambeau]

Vactrols are also quite distant from being a linear system.

As Pladask said, the signal path in these is analog. It's little different than FET switching, I believe level adjustments are accomplished via PWM while switching on & off at a supersonic speed.

Functionally speaking, the biggest difference between digital pots and vactrols is that vactorls are a two leg component, whereas potentiometers are three. So if you're hoping to do anything more complex than a variable resistance/rheostat you'll need to stack them - which will really illustrate that fact that they are not linear in response to control signal (and are slow getting there).

Of course, if all you're after is volume control you can toss VCAs into the mix as potential candidates as well, but if you need a wiper equivalent to your circuit, digital pots will probably prevail over vactrols (presuming you can drive them on the logic side of things).

[Gone Fission]

The voltage limitations are what stops me when I look at digital potentiometers, but I'm usually looking for tube circuits. They're bound to improve as ROHS won't tolerate Vactrols or photoresistors because of the Cadmium.

[cherler]

Vactrols are also quite distant from being a linear system.

As Pladask said, the signal path in these is analog. It's little different than FET switching, I believe level adjustments are accomplished via PWM while switching on & off at a supersonic speed.

Functionally speaking, the biggest difference between digital pots and vactrols is that vactorls are a two leg component, whereas potentiometers are three. So if you're hoping to do anything more complex than a variable resistance/rheostat you'll need to stack them - which will really illustrate that fact that they are not linear in response to control signal (and are slow getting there).

Of course, if all you're after is volume control you can toss VCAs into the mix as potential candidates as well, but if you need a wiper equivalent to your circuit, digital pots will probably prevail over vactrols (presuming you can drive them on the logic side of things).

Yeah not wanting a rheostat is the biggest thing driving me away from vactrols. The volume was just a quick circuit to test I could get the things working and see if there was any noticeable steps or noise. So far I can't tell the difference.

The voltage limitations are what stops me when I look at digital potentiometers, but I'm usually looking for tube circuits. They're bound to improve as ROHS won't tolerate Vactrols or photoresistors because of the Cadmium.

I found some with large voltage tolerance, but the number of taps really drops. https://www.mouser.com/ProductDetail/mi ... cycode=USD