G&L Bass Preamp 1.3 frequency plots n fun

[Paul Gagon]

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

[jcburn]

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.

[jcburn]

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!

[Ken Baker]

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...

[Karl Kaminski]

I also have a description of the various revisions that have been made to the G&L Bass Preamp through the years. I have posted this in two other threads but I think it might also belong here as well. The original preamp that G&L was using, before BBE took over in 1991, used an LM4250 chip and very large resistor values around the circuit. These resistors were of the carbon film type and were typically 5% or 10% tolerance. I was told that there were some complaints about the preamps being noisy (circuit noise, not 60Hz hum. Think hiss vs hum). This was primarily caused by the large resistance values coupled with the type of resistors used.

Rev 1.0 was the first preamp to address the noise issue. It was done in September of 1992 and used the LF441 chip and value scaled down the resistors. These resistors were made from metal film and had a tolerance of 1%. The frequency response was the same as the original but the circuit noise was quite a bit less. I took great care in evaluating the sound of this preamp against the original with a ton of bass players. No way was I going to mess with the sound of the bass. I just wanted to reduce the circuit hiss.

Rev 1.1 was done to go back to the LM4250 chip because it was being so difficult to get the LF441. Besides, with the proper value of the bias resistor that connects to pin 8 of the 4250 you can achieve the same noise spec as the 441. This version was done in October 1997.

Rev 1.2 was done in February 2011. This revision switched the position of R8 (1K) to the output side of R9 (100K) to reduce the voltage divider effect at the output (a very minimal change) and to bring the external 10K gain controlling resistor on to the PCB under the name of R11. Also, the on board high frequency boost resistor (R7) was changed from a 21.5K resistor to a 27.4K resistor to reduce the treble gain......a very small amount

Rev 1.3 was done in October 2011. This revision was to address complaints of picking up radio interference in the preamp. An RF input filter was added to the front end by using R12 (1K) and C7 (220p). This blocks any radio frequency signals from entering the input circuitry of the preamp.

Like Ken has said in many posts and on numerous occasions, there really isn't any big sonic difference between the various versions. I was just trying to address various issues as they came up. I hope this helps get these revision numbers cleared up.

Im little outta my depth here, but thought I'd ask all the same...
I recently did a single coil (outer) mod https://www.bassesbyleo.com/forum/viewtopic.php?f=9&t=305&start=20#p20068 on my 2008 USA L2500 (rev1.1 Pre). Even with copper shielding Im getting some RF bleed in 'active', and w/treble boost.

Could I simply add the rev1.3 R12 (inline with input) and add C7 (across the input and ground on the rev1.1Pre) to block the RF on the rev1.1?

Screen Shot 2022-06-17 at 1.04.54 PM.jpg (21.17 KiB) Viewed 1747 times

Thanks for any guidance

Edit: I added the R12 and C7 as illustrated .. and it seems to work. There is the smallest about of RF still but very minimal in treble boost now. I'll post pics and comments in the above linked thread since this Freq Plot thread. Now that I have the presumed a bit, next up I'll try swapping out the various R7 and R10 values shown in Paul's Plots to fine tune.

[Paul Gagon]

Hi Karl,

Sorry for the late reply. Yes!, what you’ve done (adding the 1K resistor and 220pf cap) will work nicely. If you have a situation where you still get some RF because you’re close to a broadcast station/tower, you can increase the value of the 1K resistor to 2.2K. That should help with RF issue.

Paul

[Karl Kaminski]

Wow, thanks Paul for confirmation—and a ton of useful information! No, worries—This has been really great. I'll definitely try the 2.2K.
Just one more (well, two) 'shallow swimmer' questions, curious if I didn't have a 2.2K on hand, could I put two 1KΩ in series? (resistors in series are the sum of their values)

and does increasing the R12 to 2.2K alter/affect the tone/sound of the Pre? (Im guessing the RF that's being blocked is higher up in the spectrum)

Thanks again!
Karl