Understanding the pots effect on treble and bass

We are fortunate to count Paul Gagon as a member here, so it seemed natural to give him a home to stretch out in and relax a little. Maybe exercise the grey matter and present a little history of the guitar and bass (amps too!) from his perspective as one of the eminent designers of our time.

Got questions? Great! Good questions might be, "What are the EQ mappings for the M Series preamps?" Or, "What was the thought process behind the MJ-4?" Troubleshooting questions should remain in the regular forums (he reads those too). Finally, please be mindful of how I feel about tech questions via PM or email.

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Understanding the pots effect on treble and bass

Postby Simon C » Sun Mar 07, 2021 8:16 am

Hi,

I have the GL Tribute 2000 and 2500 on my list of basses to try out when the shops re-open post covid lockdown in the UK. I'm trying to do my research in advance so that when I get the basses in my hands, I know what the controls are trying to do.

The frequency response curve for the standard preamp v1.3 in the link below seems pretty clear (I tried pasting a screenshot but couldn't). If the preamp has changed significantly on recent basses and it affects the answer please says so:

viewtopic.php?f=28&t=2294

So if the preamp is on it boosts all frequencies, with the frequencies from say 100Hz to 10kHz getting the most boost. If the preamp with treble boost is engaged then it provides approximately the same boost on frequencies below 350Hz and more boost to the higher frequencies.

I can't work out from the wiring diagram in the thread below which signal the bass and treble pots act on (probably because I don't understand what the white, black and yellow wires do).

viewtopic.php?f=4&t=1684

From what I've read the tone pots act before the preamp so can only attenuate (i.e. cut) frequencies. That being the case I assume they are a low pass filter (for the treble) and a high pass filter (for the bass). Is this correct?

If so I assume this means that if the bass tone pot is rolled off to cut frequencies below 30Hz then the preamp has nothing below 30Hz to work on so it just boosts frequencies above 30Hz. Likewise if the treble pot is rolled off to cut frequencies above 8kHz then the preamp has nothing above 8kHz to work on so it just boosts frequencies below 8kHz. Is this correct?

I assume this also means that only frequencies between 30Hz and 8kHz get passed to the output jack, regardless of whether the preamp is on or not. Is this correct?

If the pots act post preamp can you please explain what they do, ideally making reference to the frequencies in a similar way to how I have done above.

Hopefully it is clear what I'm asking.

Thanks in anticipation,

Simon

P.S. If you want to educate me on the colours of the wires please do.

P.P.S This looks like a great forum to be a member of
Simon C
 
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Re: Understanding the pots effect on treble and bass

Postby Ken Baker » Sun Mar 07, 2021 1:49 pm

Simon C wrote:The frequency response curve for the standard preamp v1.3 in the link below seems pretty clear (I tried pasting a screenshot but couldn't). If the preamp has changed significantly on recent basses and it affects the answer please says so:


To my knowledge, the current preamp in both the Tribute and USA basses is version 1.3. Of course this is G&L we're talking about, so anything can and will change.

So if the preamp is on it boosts all frequencies, with the frequencies from say 100Hz to 10kHz getting the most boost. If the preamp with treble boost is engaged then it provides approximately the same boost on frequencies below 350Hz and more boost to the higher frequencies.


The preamp doesn't provide much broad-spectrum boost. There is a tiny bit of treble boost that is on all the time and the "active with treble boost" setting accentuates this.

I can't work out from the wiring diagram in the thread below which signal the bass and treble pots act on (probably because I don't understand what the white, black and yellow wires do).


The color coding in the linked drawing is from 2000, so it may have changed. It might be better to just use this one and color in what your bass actually has. For the color diagram, yellow, black, and white leads carry signal. How it all hooks together is a matter of following the diagram, or working from memory.

Having said that, The L Series basses are kind of a hybrid. They are, at their core, passive instruments with the V-T-B controls upstream of the preamp. The preamp's input is always in-circuit (even in passive mode), so it always sees signal. The preamp's output and the passive output both are connected to the active/passive switch, and it is this switch's setting that determines what is connected to the jack - passive out or active out.

From what I've read the tone pots act before the preamp so can only attenuate (i.e. cut) frequencies. That being the case I assume they are a low pass filter (for the treble) and a high pass filter (for the bass). Is this correct?


All the controls, volume, treble, & bass, are cut only. This isn't because they're upstream from the preamp, it's because the circuit was designed this way. You could apply these same controls downstream of the preamp and they'd still be cut only, though their actuation would likely sound a bit different. EQ controls with cut/boost (think M Series) are active parts of the preamp and work very differently.

If so I assume this means that if the bass tone pot is rolled off to cut frequencies below 30Hz then the preamp has nothing below 30Hz to work on so it just boosts frequencies above 30Hz. Likewise if the treble pot is rolled off to cut frequencies above 8kHz then the preamp has nothing above 8kHz to work on so it just boosts frequencies below 8kHz. Is this correct?


Your ideas here are correct. Just remember that the preamp doesn't boost much at all other than the switchable treble boost. It's main purpose is to present a low impedance to your amplifier, which helps to keep your tone in good shape. Note that in passive mode, or with any passive instrument, cable length should be held to 10-12 feet to avoid tone suck from flaky capacitance.

I assume this also means that only frequencies between 30Hz and 8kHz get passed to the output jack, regardless of whether the preamp is on or not. Is this correct?


ALL frequencies that current settings provide will be passed to the jack, active or passive.

Hopefully it is clear what I'm asking.


Actually, it's not real clear what you're after. If you're just trying to get your head wrapped around how the bass' electronics work, what I've described above should head you in the right direction.

P.S. If you want to educate me on the colours of the wires please do.


The colors have no particular meaning. For example, signal from the pickup select switch on is carried by black, white, and yellow leads in the color drawing. But that drawing is nearly 20 years old and colors other than the pickup leads may have changed.

P.P.S This looks like a great forum to be a member of


Thanks!

Ken...
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Re: Understanding the pots effect on treble and bass

Postby Simon C » Sun Mar 07, 2021 3:38 pm

Thanks Ken.

I am just trying to get my head round how the electronics work. Your response helps that. Thank you.
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Re: Understanding the pots effect on treble and bass

Postby Simon C » Mon Apr 12, 2021 1:32 pm

Hi Ken and anyone else reading this thread.

I bought an L2K Tribute today. Thanks for the answer to my question at the start of this thread - it helped un my purchasing decision. Other posts on the forum also helped me to get a feel for what to expect and what to try out when I played it in the shop.
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Re: Understanding the pots effect on treble and bass

Postby Ken Baker » Mon Apr 12, 2021 5:40 pm

Simon C wrote:I bought an L2K Tribute today. Thanks for the answer to my question at the start of this thread - it helped un my purchasing decision. Other posts on the forum also helped me to get a feel for what to expect and what to try out when I played it in the shop.


Congratulations, and welcome to the addiction.

Glad we were able to help.

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