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
The thing in #307 is what I call overkill. We don't need protection for BJT input. The PN junction in the BJT acts like a zener junction. Thus, it is protected by its nature.
I like the one in the OP.
I like the one in the OP.
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I released a v1.0 of this and built two units that perform well in my house. I’m currently working on a 200W version of it. So it’s still alive.
The latest version definitely sounds better than the OP. The OP has some issues with large orchestras sounding a bit muddled. Very slightly so, but noticeable.
And after that it's noisy like hell. Driving a transistor into emitter-base avalanching is the way to cause damage to it that results in increased non-ideal base current and increased 1/f noise current.
I did some reading on BJT breakdown, and those 7 cent diodes are definitely staying. While breakdown might not fry the transistor, they DO degrade.
Most any BJT diff pair is susceptible to this unless protected. Even a singleton input sufficiently overloaded. I might be inclined to use two diodes in series to ensure that the normal differential
voltage doesn’t make them conduct, and to halve the capacitance.
Avalanching the emitter base is also THE way to make a nice white noise source, or to ensure the fastest possible fall time in switching applications.
voltage doesn’t make them conduct, and to halve the capacitance.
Avalanching the emitter base is also THE way to make a nice white noise source, or to ensure the fastest possible fall time in switching applications.
The overload will never happen in normal operations. We are talking about audio amps.
It only happens when something goes wrong badly, such as one side of output transistor or fuse blows up and the output is pegging at rail voltage without DC offset protection kicking in. In that case, you would blow more stuffs up for sure.
That’s why I said it is overkill.
There are enough threads here about amplifiers that got noisy due to damaged input transistors. You only need to connect a preamplifier and accidentally turn up the volume too high (which can easily happen when you forgot to turn on the main amplifier). Besides, there are electrostatic discharges, especially when you connect or disconnect stuff.