Active vs passive tone control

[rv_bass]

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?

[adriaan]

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.

[hieronymous]

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.

[jazzyvee]

Here is some info on Low Pass filters
https://electronicbase.net/low-pass-filter-calculator/

[rv_bass]

Thanks for the comments and link, very helpful!

[edwin]

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.

[hieronymous]

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.

[keith_h]

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. 

[rv_bass]

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.

[keith_h]

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. 

[cozmik_cowboy]

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

[edwin]

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.

[gtrguy]

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.

[growlypants]

I may be relatively stupid when it comes to understanding how tone controls work, but the helps ALOT!!  Thank you.

[edwin]

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