Alembic Guitars Club
Connecting => Miscellaneous => Topic started by: rv_bass on March 10, 2020, 06:30:41 PM
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I was reading online trying to determine the difference between active and passive tone controls on a bass guitar. What I’ve found so far is that low pass filters as a tone control are passive using resistors to determine the cut frequency limit, providing only cut, no additive function. And active tone controls typically involve parametric eq (bass, treble, mid) that can boost and cut. Is that correct? If so, are the low pass filters on an Alembic passive?
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If a circuit needs to be powered to function, it's an active circuit. However, not all constituent parts in an active circuit need to be.
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I believe the Alembic low-pass filter is active - that's what the Q is about, it adds a bump at the cutoff frequency. So with zero Q it may act more like a passive tone control, but you can also emphasize that frequency which you cannot do with a passive tone control.
I don't think passive tone controls are technically low-pass filters but I know very little about electronics so I can't tell you what they actually are.
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Here is some info on Low Pass filters
https://electronicbase.net/low-pass-filter-calculator/
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Thanks for the comments and link, very helpful!
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Various different types of filters can be active or passive depending on design. All active means is that there is a circuit that requires power and provides amplification. You can do complex filters passively, but you end up paying a price with a serious drop in output.
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Edwin (or anyone), would you consider a passive tone control like on a Fender Strat or P-Bass to be a low-pass filter? I have only ever heard of low-pass filters in the context of Moog (and other) synths and Alembics, but occasionally you see the classic passive tone controls referred to in that way.
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Edwin (or anyone), would you consider a passive tone control like on a Fender Strat or P-Bass to be a low-pass filter? I have only ever heard of low-pass filters in the context of Moog (and other) synths and Alembics, but occasionally you see the classic passive tone controls referred to in that way.
Yes the tone control on a passive Fender instrument would be a passive low pass filter as it attenuates the high frequencies allowing the lower ones to pass.
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I think what Keith says gets to my question. I was only familiar with low pass filters on my active Alembic Basses and SF-2. I then read a comment on line referring to a passive low pass filter, so I was a bit confused as to whether low pass filters needed power (active) or if they could be passive (no power needed). Sounds like they can be active or passive. Thanks everyone.
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There are multiple advantages to using an active tone control over passive ones. One that comes to mind concerns the Fender Jazz Bass and that is the tone control becomes less efficient as the pickup volume controls are turned down. With a well designed active circuit the tone controls will see a set signal level so the control is responds consistently regardless of the volume setting. With additional circuitry you can change the characteristics of the tone control such as with the CVQ and Q switches and change the rolloff or slope of the frequencies above/below the filter setting.
As an aside, it was the problem with the way the passive tone control interacted with the volume controls in my Jazz Bass that prompted me in the mid 70's to build and install an active tone circuit/preamp for the bass.
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If I understand it correctly (not a given by any means), the difference is that a passive tone control is low pass filter that operated vertically, shelving with more or less slope at a fixed frequency, going (in very rough form) from --------- to --------\ , while Alembic's active filter operates horizontally, shelving at fixed slope, and moving the frequency, thusly: --\ to -----------\. Someone more knowledgeable, please correct me if I'm wrong.
As to the volume/tone interaction on a J-Bass (and P, and Strat, tele, LP - pretty much every passive bass or guitar) can be remedied by the overwhelming job of moving the wire from the tone pot over one lug on the volume pot; known as "'50s Gibson wiring", it removes the problem completely (and makes one wonder why in the world Gibson abandoned it in the '60s.......). I have wired both my Telecaster & my Sheraton this way; it works.
Peter
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If I understand it correctly (not a given by any means), the difference is that a passive tone control is low pass filter that operated vertically, shelving with more or less slope at a fixed frequency, going (in very rough form) from --------- to --------\ , while Alembic's active filter operates horizontally, shelving at fixed slope, and moving the frequency, thusly: --\ to -----------\. Someone more knowledgeable, please correct me if I'm wrong.
Peter
Both are low pass filters and either one could be built to adjust frequency or have a fixed frequency and adjust amount (or slope). Really, the only difference is that a passive filter doesn't utilize a circuit that requires amplification and therefore a power source. That's why an active circuit is called active.
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I am going to try, with my non-expert knowledge, to explain what I know (or think I know) about a passive tone circuit in a bass:
A passive tone control, like on a Fender Jazz bass, has a tone potentiometer (or control) that bleeds the pickup signal to ground. In between the pot and the ground is a capacitor that selectively bleeds more trebles to ground than bass frequencies, as the pot is engaged. This gives the bass less highs and a bassier tone due to that. It does not add bass, but just removes the trebles. This is because the trebles pass more easily through the cap than the lower frequencies. If there were no cap in the tone circuit, it would be a simple volume control. There is also some overall loss of signal inherent in the tone and volume controls. Some guitar techs will put in a circuit bypass to drop these controls out of the circuit. This gets a louder signal out of the guitar.
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I may be relatively stupid when it comes to understanding how tone controls work, but the helps ALOT!! Thank you.
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I am going to try, with my non-expert knowledge, to explain what I know (or think I know) about a passive tone circuit in a bass:
A passive tone control, like on a Fender Jazz bass, has a tone potentiometer (or control) that bleeds the pickup signal to ground. In between the pot and the ground is a capacitor that selectively bleeds more trebles to ground than bass frequencies, as the pot is engaged. This gives the bass less highs and a bassier tone due to that. It does not add bass, but just removes the trebles. This is because the trebles pass more easily through the cap than the lower frequencies. If there were no cap in the tone circuit, it would be a simple volume control. There is also some overall loss of signal inherent in the tone and volume controls. Some guitar techs will put in a circuit bypass to drop these controls out of the circuit. This gets a louder signal out of the guitar.
That is one type of passive tone control. There are many. You can also have a low cut. Or a mid boost. Or a mid cut. You can wire the resistance (potentiometer) and capacitance (capacitor) in many ways and also add other passive components, such as chokes (transformers without iron) to play with the inductance of the circuit and transformers (such as some versions of the infamous suck switches of Gibson EB-2s and early Starfires). Jack Casady's Starfire had some interesting tone controls that were all passive.
So, it's important to remember that all passive means is that there's no amplification in the circuit and has no bearing on what the circuit does. The passive high cut tone control is ubiquitous because it's simple and it's useful, but the options are really wide open.
Here's a site that provides a calculator for passive filters using capacitance, resistance, and inductance to calculate frequency and Q factor.
https://www.omnicalculator.com/physics/rlc-circuit
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OK, here's where some of the confusion lies for me. I never heard of "low pass filters" until I got into Alembic and Moog. I had never heard of the passive tone controls of a Fender bass (for instance) being referred to as low pass filters - until recently.
In the manual for the SF-2 (here's the recent post linking to the manual (http://club.alembic.com/index.php?topic=25342.0)), it states the following:
Why We Choose to Filter
Natural acoustic instruments behave in large part as filters. All that statement means is that these filters tend to resonate in response to some signal - and acoustic instruments certainly do resonate. Low-pass filters are the closest electronic equivalent to these natural acoustic sounds.
This is precisely the reason we chose to use low-pass filters in our Alembic instruments. As a result, our instruments preserve the natural sound of the string's vibration and a variety of pleasing sounds are achieved. They were, in fact, the first active electronics ever to be used in guitars and basses.
Now I understand that this passage isn't written in a technical manner. But to me it implies that Alembic's use of low-pass filters is a significant, new advance over the traditional method. It doesn't say what that traditional method is, but I would assume the standard tone control. So to me, this implies that the traditional tone control is not a low-pass filter, it is a different electronic method of sound manipulation that could be seen as similar to a low-pass filter, but different. Then again, in the final sentence they point out that this system is "the first active electronics" so you could read it as the active filter is the advancement.
There is also the issue of "resonance" which is emphasized in the manual. Resonance isn't adjustable in the standard passive tone control - maybe that's the key?
I wish I could blame this on it being the morning and my brain not working yet but if my brain's not working then it's been this way for 20 years! It could just be semantics, but I personally see the recent (?) use of "low pass filter" to describe the standard passive tone control as misleading, if not incorrect. Not sure that I'm correct in that though.
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David's simple description of how the cap works in a passive tone control is really great. Thanks for that!
Hieronymous, low-pass = treble roll off = high-cut, yes? That much is semantics. The use of the word "resonance" here is a bit more ... interpretive? There are a few meanings in the dictionary but I'm pretty sure they mean like an acoustic instrument's resonant cavity, as in an acoustic guitar's hollow body. And as sound travels through air the dampening effects first happens on higher frequencies. The further away you are from a sound source the more natural "low-pass" filtering happens. Does that make any sense?
So passive low-pass and active low-pass generally do the same exact thing to the sound. But the passive version is more reactive to resistive and inductive loads, pickup volume levels, etc, whereas the active version has built in "buffering" which isolates the circuit so that it always works as expected.
Alembic being the "first active..." could certainly be seen as an advancement because of what I just described. But it also doesn't claim to be the "only" way to do it. It's certainly MY preference though. :D
Jimmy J
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Hieronymous, low-pass = treble roll off = high-cut, yes? That much is semantics. The use of the word "resonance" here is a bit more ... interpretive? There are a few meanings in the dictionary but I'm pretty sure they mean like an acoustic instrument's resonant cavity, as in an acoustic guitar's hollow body. And as sound travels through air the dampening effects first happens on higher frequencies. The further away you are from a sound source the more natural "low-pass" filtering happens. Does that make any sense?
So passive low-pass and active low-pass generally do the same exact thing to the sound. But the passive version is more reactive to resistive and inductive loads, pickup volume levels, etc, whereas the active version has built in "buffering" which isolates the circuit so that it always works as expected.
Alembic being the "first active..." could certainly be seen as an advancement because of what I just described. But it also doesn't claim to be the "only" way to do it. It's certainly MY preference though. :D
Jimmy J
Thank you Jimmy san! I appreciate the response. I guess the other word is "filter" and a passive tone control I grudgingly admit could be considered a "low pass filter" - maybe "resonant filter" is the important part?
I guess part of my just doesn't want to grant "low pass filter" status to the lowly passive tone control! In a way I feel like it takes away from the achievement of Alembic electronics, but then again, most people won't get it anyway, and those that know, know.
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Resonance is usually created by feedback at a certain frequency. While the hollow cavities in an instrument do this to a degree, a filter's resonance is much simpler than that and is centered around one frequency.
Formants, multiple resonances in speech, are an interesting example. Each formant is created by the resonance of various parts of the head, throat, etc. It defines the vowel characteristics of a voice When you move formants around, you are moving multiple resonances at the same time and that's really the way to make voices sound like different voices. It would be very interesting, and probably more accurate in making a bass sound like different instruments, to have a formant filter onboard, but that's a massive increase in complexity.
Alvin Lucier did an interesting piece which was originally inspired by his trying to smooth out his stutter by playing a recording of his voice in a room and recording the results over and over until the room and the formants of his voice and the room interact and create a very interesting and haunting rhythmic sound.
https://en.wikipedia.org/wiki/I_Am_Sitting_in_a_Room
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Inserts mind blown emoji.