jcburn wrote:Sorry guys, my initial post may not have made sense in the way I wanted it to.
I think I get what you're after.
I find it interesting that the frequency response of the preamp in passive mode is completely flat.
The frequency response of the preamp will be the same, passive or active, unless component values are changed to affect the response. For example, the setting Active With Treble Boost adds a resistor's value to part of the preamp's circuit which causes the preamp to increase high(er) frequency output. Under this scenario, the curve would no longer be flat but would show a gain increase at higher frequencies.
I'm not sure how the frequency responses were generated, but I assumed it us by passing some sort of noise through the preamp.
Passing noise, say white noise, through the preamp would likely mess up the curves because most "noise" of this type carries lots of frequencies. More likely is that Paul used an audio generator to create a nice, clean, constant amplitude, signal to feed to the preamp; then swept that signal from low to high frequency. On the bench, if you feed a nice flat frequency spectrum to the input and you get a nice flat frequency spectrum at the output, you can say that the frequency response of the preamp under test is flat. Remember this.
I was also then interested what would happen to the graph should the tone controls be included in the test set at different levels.
That could be a trick question.
Open this link in a new tab and give it a read, then come back here. The preamp in a G&L L Series bass is "downstream" of the pickups and Tri-Tone Passive-Treble-Bass (PTB) part of the circuit and is essentially (except for Active/Treble Boost) non-adjustable. Because of its simple design and location in the overall circuit, its output will be directly proportional to its input. If we reduce the amplitude of part of the input signal relative to the rest of the input signal, such as with a treble cut, the preamp's output will reflect that reduction and you'd see it in a plot. So long as the treble cut at the input is proportional to the treble cut at the output, the preamp's (just the preamp) frequency response is still flat. The
overall circuit's frequency response is not flat because we cut some treble, but the preamp handling of the various frequencies fed to it is equal across the total spectrum.
Ken...