05-02-2019, 05:37 PM
In general, if you want a flat frequency response when the tone controls are in the centre, and also wish to have symmetric lift and cut, then yes, the "passive Baxandall" requires logarithmic pots.
Confirmation: http://www.duncanamps.com/technical/tonestack.html - or: http://sound.whsites.net/articles/eq.htm
Interestingly, it's not correct to call the passive arrangement "passive Baxandall" - we should be calling it "James" - but unfortunately for EJ James, "passive Baxandall" is the common usage. Searching for "James tone control" does bring up a decent number of results, though. I will make an effort to use the right term in future.
Anyway, if you had bought linear pots, then the circuit would have provided considerable bass and treble boost when the controls were set to the centre, and there would have been much more cut available than boost. Buying log pots was the right thing to do here.
The active Baxandall circuit uses linear pots. These are often easier/cheaper to source, and for stereo applications the matching between the two sections is usually better with linear pots, so that's just a couple of reasons why manufacturers like the active scheme. No insertion loss either.
The input impedance will depend on the setting of the volume control and the tone controls. At very low volume settings, it will be close to 100k. It will fall to perhaps less than 10k as you turn up the volume to maximum. I haven't done any calculations, and it's probably not a huge problem, but just be aware of it. If a high input impedance is definitely required, then a buffer after the volume pot is a good idea. Even just an emitter follower might well do the job. We can cross that bridge if needed once you've used it for a while (all the best projects are never finished - they always evolve in light of experience).
Without knowing the TS parameters of that drive unit, it's hard to give a precise answer about the back panel. My best guess would be along the lines of "it was probably originally intended to fit in a valve radio with a well-ventilated rear panel, so if you put it in a sealed box, the Q would be too high, and it will "honk" at the bass end". I'd try a sealed box (with some stuffing) first of all, and compare it with and without the back, and perhaps slide it around to give different openings. In terms of LF, the original rear panel has such large openings that they would note have been "tuned" in any way - anything over perhaps 2 or 3 square inches probably falls into the same category, so there's plenty of latitude here.
Confirmation: http://www.duncanamps.com/technical/tonestack.html - or: http://sound.whsites.net/articles/eq.htm
Interestingly, it's not correct to call the passive arrangement "passive Baxandall" - we should be calling it "James" - but unfortunately for EJ James, "passive Baxandall" is the common usage. Searching for "James tone control" does bring up a decent number of results, though. I will make an effort to use the right term in future.
Anyway, if you had bought linear pots, then the circuit would have provided considerable bass and treble boost when the controls were set to the centre, and there would have been much more cut available than boost. Buying log pots was the right thing to do here.
The active Baxandall circuit uses linear pots. These are often easier/cheaper to source, and for stereo applications the matching between the two sections is usually better with linear pots, so that's just a couple of reasons why manufacturers like the active scheme. No insertion loss either.
The input impedance will depend on the setting of the volume control and the tone controls. At very low volume settings, it will be close to 100k. It will fall to perhaps less than 10k as you turn up the volume to maximum. I haven't done any calculations, and it's probably not a huge problem, but just be aware of it. If a high input impedance is definitely required, then a buffer after the volume pot is a good idea. Even just an emitter follower might well do the job. We can cross that bridge if needed once you've used it for a while (all the best projects are never finished - they always evolve in light of experience).
Without knowing the TS parameters of that drive unit, it's hard to give a precise answer about the back panel. My best guess would be along the lines of "it was probably originally intended to fit in a valve radio with a well-ventilated rear panel, so if you put it in a sealed box, the Q would be too high, and it will "honk" at the bass end". I'd try a sealed box (with some stuffing) first of all, and compare it with and without the back, and perhaps slide it around to give different openings. In terms of LF, the original rear panel has such large openings that they would note have been "tuned" in any way - anything over perhaps 2 or 3 square inches probably falls into the same category, so there's plenty of latitude here.
