G&L Bass Preamp 1.3 frequency plots n fun

Seeing as how we are fortunate to count Paul Gagon as a member here, it seemed like a good time 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 kind of windings are on the S500 pickup and why?" 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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Re: G&L Bass Preamp 1.3 frequency plots n fun

Postby Paul Gagon » Mon Oct 08, 2018 11:32 am

Ken Baker wrote:
jcburn wrote:Thread resurrection time - Paul can you remember when you created these graphs - for the preamp bypassed reading were the tone controls completely dimed, completely rolled off etc? I find it interesting that the pickups have a completely flat response!


The plots are dated 7/1-8/2016, as noted at the top of each one. All were posted here 7/14/2016.

These plots are only for the preamp and do not show the output of the pickups.

Ken...

Ken is correct. These plots are for the preamp only. To get a good sense of the output response of the L series MFD pickup, check out my post on “The Taming of the Climax/L-1500 Bass.”

Thanks,
Paul
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Re: G&L Bass Preamp 1.3 frequency plots n fun

Postby jcburn » Tue Oct 09, 2018 1:21 am

Paul Gagon wrote:
Ken Baker wrote:
jcburn wrote:Thread resurrection time - Paul can you remember when you created these graphs - for the preamp bypassed reading were the tone controls completely dimed, completely rolled off etc? I find it interesting that the pickups have a completely flat response!


The plots are dated 7/1-8/2016, as noted at the top of each one. All were posted here 7/14/2016.

These plots are only for the preamp and do not show the output of the pickups.

Ken...

Ken is correct. These plots are for the preamp only. To get a good sense of the output response of the L series MFD pickup, check out my post on “The Taming of the Climax/L-1500 Bass.”

Thanks,
Paul


Sorry guys, my initial post may not have made sense in the way I wanted it to. I find it interesting that the frequency response of the preamp.in passive mode is completely flat.
I'm not sure how the frequency responses were generated, but I assumed it us by passing some sort of noise through the preamp. I was also then interested what would happen to the graph should the tone controls be included in the test set at different levels.
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Re: G&L Bass Preamp 1.3 frequency plots n fun

Postby jcburn » Tue Oct 09, 2018 1:22 am

And if I'm still making no sense I'll stop trying! I kinda know what I want to say but don't understand technically how to put it in words!
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Re: G&L Bass Preamp 1.3 frequency plots n fun

Postby Ken Baker » Tue Oct 09, 2018 8:52 am

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...
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