POT Fissle.  Scratchy tracks.  Can they be illiminated? - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: April 13, 2021, 04:06:09 pm
content: I have found the downside of running a high gain headphone amp with the input attenuated by a stereo pot is that all the pot track scratches are amplified 11 times too.There has to be an easy way to take the filtered output of a volume pot and use it to electronically set the volume.Something like a Baxendal config where an op amp uses a heavily filtered output of the pot in it's feedback loop used in the primary amp feedback loop.I can't see why I can't decouple the unfiltered pot from the audio path and still use it to control the audio path... but I don't know how.

author: mvs
author_rank: Frequent Contributor
timestamp: April 13, 2021, 04:33:00 pm
content: How about LDR volume control? It was quite popular solution to exclude noisy pots from audio signal path.

author: Wolfram
author_rank: Frequent Contributor
timestamp: April 13, 2021, 05:10:27 pm
content: Do you have DC current running through the wiper? Even small amounts can significantly increase scratching noise. A simple fix is to use a DC block capacitor in series with the wiper, while making sure you have an alternative biasing path for the following stage.Edit: It's possible to make a voltage controlled attenuator, but making one with low distortion and good tracking between channels can be pretty expensive and involved, so it depends on your requirements. LDRs are a disaster in this respect, I once tried to make a two-pole state variable filter with fancy NOS Vactec vactrols, and the two devices I used from the same batch had a 3:1 variation in resistance at the same current between each other. This would obviously not be an issue with mono sound, but they also change with temperature and light history. Some amplifier ICs have built-in VCAs for volume control, and you get dedicated volume control ICs from THAT, Alfa RPAR, Sound Semiconductor and a few others.

author: Bassman59
author_rank: Super Contributor
timestamp: April 15, 2021, 03:27:08 am
content: Quote from: Wolfram on April 13, 2021, 05:10:27 pmI built (and use daily) a studio monitor controller based on the TI PGA2320 stereo volume-control chips. One of the outputs is for headphones. The PGA outputs drive the fixed-gain headphone amplifier directly. Gain steps are silent. Of course, you need a microcontroller to manage the gain control.

author: paulca
author_rank: Super Contributor
timestamp: April 15, 2021, 09:49:24 am
content: Quote from: Bassman59 on April 15, 2021, 03:27:08 amI did ponder the DSP route. Does anyone know just how complicated it gets if you wanted to drop a DSP into the signal path and presumably have an MCU setting parameters for volume, EQ etc.I gather there is a route requiring a full ADC stage, a DSP chip and a DAC to the fixed gain amp.The question is.. does anyone make a DSP which has build in analogue IO?

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2021, 12:17:35 pm
content: How about a digital potentiometer?

author: ElizatronicWarfare
author_rank: Contributor
timestamp: April 15, 2021, 12:43:30 pm
content: Surely it's not that complicated (unless I've missed something)? Why not just use the trimpot as part of the feedback resistance for an op amp? If the resulting amplitude range is too wide, you can simply attenuate it back down into an useful range afterwards with a voltage divider.

author: paulca
author_rank: Super Contributor
timestamp: April 15, 2021, 06:28:29 pm
content: Quote from: Zero999 on April 15, 2021, 12:17:35 pmI never knew they existed. Thanks.An MCU can sample, filter, and smooth the real pot's input and control the digipot. That or just use a rotary encoder.

author: paulca
author_rank: Super Contributor
timestamp: April 15, 2021, 06:34:50 pm
content: Quote from: ElizatronicWarfare on April 15, 2021, 12:43:30 pmNot sure how this would stop scratchy tracks/wipers being amplified. Putting the pot in the feedback loop is possible, but needs 2 op amps to be stable. That would give me variable gain though. It was just simply to use the basic stereo logrhithmic pot directly on the input before the AC coupling cap. The opamps I'm using are single channel and cost nearly £10 each. Throwing another pair in to create a baxandal config, still wouldn't solve the problem as the pot scratches will just offset the feedback causing it to be amplified anyway. Noise is noise, input or feedback.The amplifier signal path is gained 11 times, so ANY noise from the POT wipers is amplified 11 times. Even if the input is attenuated right down near 0. Result is quiet music, loud scratches.

author: Zero999
author_rank: Super Contributor
timestamp: April 16, 2021, 08:24:59 am
content: Quote from: paulca on April 15, 2021, 06:28:29 pmIt sounds complicated though.I think there's a problem with DC bias in your potentiometers, they're really bad quality, or are worn out and need replacing. Put capacitors, preferably film, in series with the potentiometer and wiper, to block any DC, coming from the tiny bias currents from the op-amp and previous gain stage. They need to be a high enough value to pass 20Hz, but not so big it takes ages for the voltage to stabilise.EDIT:If you're already using AC coupling capacitors. What sort are they? Tantalum, aluminium, or film, ceramic? The latter two are better. If they're tantalum or aluminium, the tiny leakage current won't help and it's possible they're old, or bad quality, which is increasing the leakage. Replace them with film or ceramic, preferably the former, as ceramic capacitors can be piezoelectric.

author: paulca
author_rank: Super Contributor
timestamp: April 17, 2021, 09:00:09 am
content: I have AC coupling caps on the input. Kennet 22uF aluminium polymer caps and 220uF on the output.You could be right about the pot, but it was sourced from farnell, I think it's Alpine brand.

author: Bassman59
author_rank: Super Contributor
timestamp: April 19, 2021, 02:23:17 am
content: Quote from: Zero999 on April 15, 2021, 12:17:35 pmThat's the PGA2320. A digital pot in front of a buffer amplifier. (actually two of each.) The advantage is that the digital pot is trimmed so each step is 0.5 dB and the two sides are matched better than you could do with a mechanical pot.