Why do amplifier onboard tone controls change output levels?

djmay

djmay

Apr 8, 2022
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Switzerland
#1
My signal chain is 5-string fretless with LPF on each pickup > Alembic SF-2 > TC Electronic BQ500 with Bass, Lo-mid, Hi-mid and Treble controls. I set and forget the LPFs and HPF. If I change any of the LPF/HPF filters it does not affect the output level. If I change for example the amplifier Bass control it changes the output level. I typically adjust the amplifier controls for the room environment. Is this typical of all amplifier onboard controls? It seems to me that good design would keep frequency control and output levels separate.

Before my 16-year hiatus from playing bass, I used bass > SF-2 > power amp. Therefore, I never was bothered by level changes. First-world problem.
 
#2
djmay said:
My signal chain is 5-string fretless with LPF on each pickup > Alembic SF-2 > TC Electronic BQ500 with Bass, Lo-mid, Hi-mid and Treble controls. I set and forget the LPFs and HPF. If I change any of the LPF/HPF filters it does not affect the output level. If I change for example the amplifier Bass control it changes the output level. I typically adjust the amplifier controls for the room environment. Is this typical of all amplifier onboard controls? It seems to me that good design would keep frequency control and output levels separate.

Before my 16-year hiatus from playing bass, I used bass > SF-2 > power amp. Therefore, I never was bothered by level changes. First-world problem.
Click to expand...
Yes, apparent volume changes are a thing when adjusting EQ controls. And more so with the regular EQ controls than the high-pass or low-pass (depending on what frequency is set).
With EQ controls, the level of a certain set of frequencies is being changed. If the bandwidth of a frequency is only 1/3 octave wide, (say 500hz to 1000hz), a larger gain difference is noticed than if a smaller bandwidth is used (say, 500hz to 630hz).

It’s always a good idea for players to spend time with their EQ controls on their bass amps, or even better, spend time playing with the EQ controls on a mixer. Learn how frequency selection, gain of boost or cut, and the bandwidth, or “Q” and how it’s affecting thr changes.

I learned all this stuff years ago in my dual musician/pro audio guy life. But I’m absolutely terrible at explaining it. I refer folks to YouTube, where I’ve found great videos that explain and demonstrate this stuff.
 
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#4
This sounds like it might be a case of imperfect perception. Your SF-2 preamp had a Fender-type, cut-only EQ; the BQ-500 has more modern, active EQ that can provide both boost and cut, and much more effectively. Any EQ circuit changes the level of a portion of the spectrum; whether or not this has an audible or real effect on the signal depends on what portion of the signal is being affected and by how much. If your HPF were set to 1000Hz, you would most certainly see a change in the overall output level. And I'm not sure your recollection of your old preamp is entirely accurate. My experience with the Fender EQ circuits, most especially the MF control, is that they can have a significant impact on the overall volume if you crank them down enough -- your BQ's EQ circuits will apply more cut (or gain) with a lot less relative knob travel.
 
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#5
Redbrangus said:
This sounds like it might be a case of imperfect perception. Your SF-2 preamp had a Fender-type, cut-only EQ…
Click to expand...

You’re probably thinking of the F1X or the F2B. The SF2 is the phase aligned, low pass/high pass/band pass filters + mixer thing that Alembic made to allow for Series II type electronics for any instrument.

but I would contend that it’s because of how unique the SF2 is that OP has expectations of no level change when adjusting EQ. That phase aligned aspect of its mixer is very unique, and allows you to do some drastic tone shaping in ways that conventional equalizers simply cannot achieve.
 
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#7
Bob_Ross said:
but I would contend that it’s because of how unique the SF2 is that OP has expectations of no level change when adjusting EQ. That phase aligned aspect of its mixer is very unique, and allows you to do some drastic tone shaping in ways that conventional equalizers simply cannot achieve.
Click to expand...
Yes, the SF-2 is unique and may have raised my expectations to unrealistic levels. However, if I change the LPFs on the instrument the output level change is imperceptible. If I understand correctly, the amp bass and treble are shelf filters and Lo/Hi-mid are somewhat parametric. What is disturbing is that output levels change like a volume control on the tone knobs.
 
#8
Redbrangus said:
This sounds like it might be a case of imperfect perception. Your SF-2 preamp had a Fender-type, cut-only EQ; the BQ-500 has more modern, active EQ that can provide both boost and cut, and much more effectively. Any EQ circuit changes the level of a portion of the spectrum; whether or not this has an audible or real effect on the signal depends on what portion of the signal is being affected and by how much. If your HPF were set to 1000Hz, you would most certainly see a change in the overall output level. And I'm not sure your recollection of your old preamp is entirely accurate. My experience with the Fender EQ circuits, most especially the MF control, is that they can have a significant impact on the overall volume if you crank them down enough -- your BQ's EQ circuits will apply more cut (or gain) with a lot less relative knob travel.
Click to expand...
The SF-2 has two state variable filters plus mixer. Nothing like Fender controls.
 
#9
You should probably try a few options to hone in on what is happening. Plug the bass straight into the bass amp, and try adjusting the EQ. Is it the same effect? Then it's the amp. Plug a different bass straight into the amp. Plug the other bass into the SF-2 going into the amp. Plug the SF-2 into a recording interface. Plug a line-out from the amp into a recording interface.

And yes, the SF-2 can do extreme filtering, so be careful how you use it.
 
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