And why would it cause crackling? When you turn the pot, you will just shift the balance of current pulled through the pot and R2. Isn't that a smooth transition? I admit it feels weird to feed idle current through the volume control, but is it really bad enough to warrant another large cap in the signal path? It needs to be at least 4.7uF if I want to preserve the excellent bass range I currently have.
That was my thinking too. It doesn’t seem like a big enough problem to warrant another capacitor.
I'm no expert on the physical and chemical processes that cause long-term degradation of potmeters, but I do know that the contact between the wiper and the resistive track tends to get worse with age (albeit not as quickly as when the 5 uA or whatever would have flown out of the wiper of a carbon track potmeter).
As a worst-case assumption, assume that the wiper may temporarily lose contact altogether when you turn it. In the present circuit, 5 uA of base current can then cause a voltage jump of up to 110 mV, this being IBR2. After 23 times amplification, that's a 2.53 V voltage jump at the amplifier output. That's quite a crackling sound that could have been avoided with a capacitor.
If you don't want any extra capacitors, you might as well put the potmeter before C1. The cut-off frequency then changes with volume, but why would that matter as long as it remains high enough?
As a worst-case assumption, assume that the wiper may temporarily lose contact altogether when you turn it. In the present circuit, 5 uA of base current can then cause a voltage jump of up to 110 mV, this being IBR2. After 23 times amplification, that's a 2.53 V voltage jump at the amplifier output. That's quite a crackling sound that could have been avoided with a capacitor.
If you don't want any extra capacitors, you might as well put the potmeter before C1. The cut-off frequency then changes with volume, but why would that matter as long as it remains high enough?
But if you have any DC offset on the input, that will run through the pot to ground. Here's the latest version. I moved the DC blocker and added R36 to make the Bode plot for full volume look a little bit more like the rest of the plots. It eats about 1dB of gain, but I can live with that. Not that the shape of the graph in the 1MHz range makes much of a difference, other than esthetics.
The input LP filter has cutoff at about 750kHz, which happens to be the same as the whole amplifier. Does that make sense? Looking at other designs, it looks pretty common to set the cutoff around 1MHz.
The input LP filter has cutoff at about 750kHz, which happens to be the same as the whole amplifier. Does that make sense? Looking at other designs, it looks pretty common to set the cutoff around 1MHz.
Uploaded the changes we discussed over the last few days to Github: https://github.com/prydin/the-bog-standard