1. Welcome to TalkBass 2014! If you're new here, we just went through a major site upgrade. Please post all concerns and bugs to the Forum Usage Issues forum. We will be monitoring that forum. Thank you for all of your feedback.

    The TalkBass iphone/android app is NOT WORKING currently. We're working on it. Tapatalk IS working, so if you need to use an app, use Tapatalk. Try using your browser though - TalkBass is now 100% responsive to your phone/tablet screen size ;)

    Please read the TalkBass 2014 FAQ for lots of great info on the new software.

Dual Concentric Volume for G&L L-2000 - Has anyone done this?

Discussion in 'Pickups & Electronics [BG]' started by 4848277, Sep 7, 2011.

  1. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    I recently purchased a G&L Tribute L-2000 and it's great, but I've been contemplating adding a second volume control so I can control the volume of each pickup independently. I have the basic idea of how I want to accomplish this, but I was uncertain of the consequences (sonically) of moving the volume control(s) to the other side of the EQ circuitry. As some of you know, the circuit is backwards compared to other basses. Here's the way it's wired normally with one volume...

    [​IMG]

    And here's a diagram of what I propose to do... (NOTE: People seeking a proper diagram to perform this mod. should use the most recently updated diagram in a later post.)

    [​IMG]

    I'm wondering if I should add a 250K resistor to ground where the original volume pot (electrically) used to be in the circuit.

    I haven't started this yet, I still need to get some parts. I want to get it right the first time though. Any comments from people "in the know"...?

    Cheers!
  2. line6man

    line6man

    Joined:
    Jun 20, 2008
    Likes Received:
    2
    You have your volume pots wired to simply vary the output impedance of the pickups by placing a resistance parallel to them. The volumes probably won't work well that way. Traditionally volume pots are wired as voltage dividers, placing resistance both in series with and parallel to signal.

    What you need to do is wire the volume pots normally. The first terminal is the input from the pickups, the wiper is the output to the pickup selector switch, and the third terminal gets grounded. -Or- If you prefer independent volume controls, wire the wiper as the input and the first terminal as the output, for both pots. The disadvantage to doing it this way is that the pots will place a variable impedance load on the pickups when adjusted.
  3. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    There... I've corrected the schematic in the original post.

    Anyway, I'm still wondering if the tone circuit will function as before given that the volume control was moved to the the other side. Should I put a 250K load resistor in place of where the volume control used to be...?

    Cheers!
  4. line6man

    line6man

    Joined:
    Jun 20, 2008
    Likes Received:
    2
    My first answer would be no, but conversely, this is something easiest to simply try both ways so you can see for yourself what difference it will make.
  5. lowfreqgeek

    lowfreqgeek

    Joined:
    Mar 15, 2010
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: Regenerate Guitar Works
    The bass tone pot will not work correctly as the constant 250k load of the volume pot is part of the bass tone control circuit. You could put a 250k fixed resistor in at that point in the circuit to load the bass control - just as you already suggested.

    I have one of my L2500s wired up with independent volume controls and I love it - mostly. I do use a lot of variations, but in actual use, I either use full front, both, full rear, or full rear with a little front added in. I don't often find myself setting other blends, so a simple selector switch with an extra "rear bias" switch would be just as useful, a la Fender Roscoe Beck basses. You could replace the volume pot with a switch/pot for that purpose. You'd have to figure out what size resistor to put in to drop the front pickup level for the right blend, but that's easy enough.
  6. bassbenj

    bassbenj

    Joined:
    Aug 11, 2009
    Likes Received:
    1
    I agree with all of this. The problem with dual volumes in a passive bass is that they tend to vary the impedance seen by the tone circuits which in the case are more complex than a simple capacitor. And they vary the loading on the pickups. In a passive bass all of these things go together to give you it's tone. So what happens is that tone changes with volume settings. My own experiments along these lines were not very productive and my own conclusion agrees with lowfreqgeek that in practice "mixed" values are used little so that switch pretty much does the job. Plus as you are playing twiddling volume knobs doesn't seem very practical for me, so I've come to prefer pre-set tones you can select with a switch.
  7. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    Thank you. I thought so! :)

    I don't twiddle volume knobs WHILE I'm playing either, but I believe having the option to adjust pickup volumes independently has it's advantages and is more versatile than a pickup balancer. :)

    I also don't want to drill any additional holes in the bass, or modify the current functions of any of the switches. I'm not interested in coil tapping options either.

    Cheers!
  8. SGD Lutherie

    SGD Lutherie Supporting Member

    Joined:
    Aug 21, 2008
    Likes Received:
    0
    Disclosures:
    Owner, SGD Music Products
    Sure it will. It's a bass cut control. When the pot is at zero, the signal is passing through the .0022µF cap, removing the low end. The volume control resistance to ground has nothing to do with that circuit.

    I wired my Ric up like this back in '76 when I saw this circuit in the instructions that came with Carvin pickups. My Ric had two volumes and a pickup selector. Worked just fine.

    If you really wanted the concentric volumes after the switch, just use a regular DPDT on-on-on switch, and wire the switch before the volume pots.
  9. lowfreqgeek

    lowfreqgeek

    Joined:
    Mar 15, 2010
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: Regenerate Guitar Works
    It'll work - sorta - but it won't work correctly or as intended. The 250k load and the .0022uF cap form a 290Hz highpass filter. When the bass control is at max (no cut), the capacitor and bass pot are both bypassed and the signal sees a ~250k load. When the bass control is at min (fully cut), the signal above 290Hz is passed through more or less unaffected, while the signal below 290Hz is divided by a 1M into 250k divider. The 250k is an integral part of that as it determines where the bass-cut set point is with some stiffness (assuming a high impedance input stage such as the 2.2Mohm on the active preamp).

    I actually simulated this whole circuit a year ago or so. I don't have the simulation any more, but I can say that the bass control will not work correctly without some sort of path for low-frequencies to ground, i.e. the volume pot (in this case). I bet if you fully analyzed the complete end-to-end circuit in your Ric, you had a load on the highpass capacitor - somewhere (the amp input impedance, maybe?).
  10. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    Yes, but doesn't that .0022µF cap also form an RC high pass filter in combination with the 250K to ground after it David...? (289.4Hz and above) With that resistance absent, the signal sees the .1µF coupling capacitor (to the preamp) in series, and a 2.2M resistor to ground. The capacitors combine in series (making 2.15nF) and form another RC high pass filter with the 2.2M resistor making a pass frequency of 33.6Hz instead.

    As I'm composing this post, I just noticed lowfreqgeek has just posted a reply very similar to mine! :p

    Cheers!
  11. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    Here's what it should look like... (NOTE: People seeking a proper diagram to perform this mod. should use the most recently updated diagram in a later post.)

    [​IMG]

  12. SGD Lutherie

    SGD Lutherie Supporting Member

    Joined:
    Aug 21, 2008
    Likes Received:
    0
    Disclosures:
    Owner, SGD Music Products
    The passive bass and treble controls work with, or without the preamp. The preamp is not part of that passive circuit. Neither is the volume pot. Passive treble cut tone controls work without volume controls after all.

    The coupling cap is only present when you are using the preamp, and is to prevent DC from the battery from going through the pickups, and is large enough that it doesn't really matter tone wise. The effect of the bass control is the same either way.

    Switch the preamp out, and the control still works. It was used years before G&L did it and without preamps. The 250K control is supposed to be large enough to also not matter much to the pickup. But it can probably be larger in that aspect.

    This is also how the vintage Rickenbacker bridge pickup works, in series with a .0047F cap. It's a low pass filter regardless of any resistance. If you run the pickup right through the cap to the jack it will still remove the low end.

    http://www.rickenbacker.com/pdfs/19507.pdf

    Try it and see. :D

    Here's the 1960s Carvin wiring diagram. You can see the bass and treble controls, same as used with the G&L, in the upper right hand diagram. The call the bass control "treble" and vice versa. This was I got the idea when I bought a Carvin APH-4N neck pickup for my Rick in 1976. I added the bass control to the bass.

    The Carvin Museum - 1962 Pickup Schematics
  13. SGD Lutherie

    SGD Lutherie Supporting Member

    Joined:
    Aug 21, 2008
    Likes Received:
    0
    Disclosures:
    Owner, SGD Music Products
    You can't determine the frequency unless you factor in the pickups. When people try to model these things in something like SPICE, it doesn't really work. I guarantee you that the calculated frequency will not match the real instrument.

    As an example, take a guitar or bass with series parallel switches either for a single humbucker, or two pickups. Select one pickup in series, and turn down the passive tone control. Listen to the tone. Now switch that pickup to parallel. The frequency that the tone cap is working at has now shifted up. Same with "over wound" vs. "under wound" pickups. The tone control will have more of an affect on the higher impedance pickups. As another example, look at an active EMG pickup. The original version had passive tone controls with .1µF caps. The tone controls hardly had any affect because of the fairly low impedance output of the pickup. (not all that low however, more like 10K). The newer X series has a much lower Z output, so they need active tone controls.

    Or the pickup? That's a smaller resistance load than the pot! Craig Anderton designed a preamp for Mighty Might once that had no input impedance setting resistor. He used the pickup for the load, and got much lower noise.

    You don't need a load at the output, since the amp is a load. You won't get any current flowing if you aren't plugged into something! There are a lot of instruments with no volume pots, and everything works just fine. Nothing stops working, right?

    Also if you are running the pickup into a buffer, the load is so high as to be non existent. You can wire this G&L circuit up with the volume at the output of the preamp and it will work the same. But then you lose the volume control in passive mode.

    Just take a pickup, wire it though a small value cap, direct to the jack, and see what you get. Now add a volume pot. The volume pot will alter the resonant peak of the pickup, and maybe even the high pass frequency, but then so will a pickup with a different resistance.

    These circuits were not designed with any specific frequency in mind. It was done by ear. Same is true of a standard low pass passive tone control. You can't determine the frequency unless you account for the impedance of the pickup, and the capacitance of the cable between your pickup and the amp.

    The rest is over analyzing. This is not an RC filter anymore than a passive treble cut tone control is. It's just a cap. There is no resistance used as part of the circuit. They are primitive tone controls from like 50 years ago. They could have done the same thing with an inductor, but they are more expensive than a cap.

    But I can say with all certainty that these simple bass and treble circuits work fine with no volume controls, because I've used them that way over the years.
  14. lowfreqgeek

    lowfreqgeek

    Joined:
    Mar 15, 2010
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: Regenerate Guitar Works
    I understand where you're coming from with your experience David, but I'm also looking at these circuits as a trained analog engineer. I fully understand the effects of the pickup impedance, as well as the load impedance, (and I simulated those, as well), but I also understand at a fundamental level how and why an RC filter works. While the original designers may have not done much analysis, I can guarantee you that these circuits do not break any laws of physics.

    Capacitance and inductance are simply frequency-dependent impedances. When used in RC filter circuits, the capacitor forms a frequency-dependent voltage divider with the resistor. Obviously, the placement of each component determines the type of filter. However, qualitative analysis is pretty straight forward and, if you look hard enough, you'll find that there is indeed some sort of R-component (or L component) whenever a capacitor is used as a filter. For low-pass RC filters, the cap IS the load. For high-pass, there is a resistive load somewhere.

    In the case of the Rick circuit you sent, the treble volume control (330k) forms the high-pass filter with the .0047uF cap; that load is there when the volume is turned up. When it's rolled back, the filter value changes. In the Carvin circuits, the "bass tone control" circuits are all low-pass with the cap to ground. By the very nature of the configuration, they could not sink low-frequencies to ground - the cap would block those low frequencies from sinking to ground and allow them to pass through the circuit.

    As far as Craig Anderton's Mighty-Might preamp, the circuit had an input impedance. He may not have used a resistor, but if he buffered in any way, using any sort of active transistor, the circuit had an input impedance. That's is exactly what transistors do: they provide current gain. Current gain is what happens when you turn a high impedance (low current) into a low impedance (high current).

    I'm sure you know as well as anyone that sometimes in electronics and circuitry things are hidden and less than obvious. Heck, I work with a Sr. EE (my boss) who doesn't understand most of this stuff. He's a computer guy and totally lost when it comes to circuit analysis (seriously). I won't go much further than that... And, just for the record, I don't actually simulate much of anything. I was trained and mentored by several retired analog engineers who could intuitively analyze a circuit so well that they didn't need to simulate anything. I hope to someday be like them...

    Back to the G&L circuit at hand, I would bet *something* that if I take my old G&L bass/treble controls and put them on the bench with a function generator and scope, that I will be able to accurately predict the effects of that 250k volume pot arranged like it is, as well as changes in the value of that resistance. I won't be at work tomorrow, so I don't have access to the function generator, but I'll try to remember next week.
  15. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    I was talking about the modified diagram as I presented it initially (and hypothetically) without the 250K resistor. I'm not sure you realize this, but (even the present, unmodified state of this bass) regardless of the position of that active/passive switch, a portion of the preamp is still going to be present in the circuit. That being the .1µF coupling capacitor and the 2.2M resistor to ground (at the + input of the OP Amp) It's kind of like the old "tone suck" story with non-true bypass effect pedals. You can't tell me part of the signal WON'T go there, even if it is negligible. ;)

    Also, the frequencies I mentioned in my previous post were calculated from the good old fashioned electronics formula which still stands today:

    f = 1 / (2 π R C)

    Where R is the resistance value and C is the capacitance value.

    Anyway, when I get all the parts together, I'll be assembling it with the 250K resistor as shown earlier. :)

    Cheers!

    P.S.: By schooling (college), I'm an Electronic Engineering Technician. Although I'm quite rusty, and my employment doesn't actually include "electronic" work per se, I've still retained quite a bit of what I learned 25 years ago. ;)
  16. lowfreqgeek

    lowfreqgeek

    Joined:
    Mar 15, 2010
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: Regenerate Guitar Works
    One more quick thing...

    I don't contend that a "volume pot" is needed for the bass-control to work in the G&L circuit. I do, however, contend that the 250k load that the volume pot presents in it's stated configuration is integral to how the G&L bass control responds.

    And, having compared my G&L's bass control to a parametric highpass EQ set around the 290Hz point, I can say with some certainty that that number is very close to where the cut-off occurs when the bass control knob is turned all the way back.
  17. lowfreqgeek

    lowfreqgeek

    Joined:
    Mar 15, 2010
    Likes Received:
    0
    Disclosures:
    Endorsing Artist: Regenerate Guitar Works
    Yes, the 2.2Mohm and .1uF are still part of the circuit, regardless of the switch position. Forgot to mention that...

    David is right that the RC filter equation isn't entirely comprehensive because the passive pickups do change things slightly. However, you can look at the source impedance (in this case, a complex impedance) and determine if it's even relevant at the desired frequencies. I am relatively certain than the RC circuit impedance is dominant at the low frequencies.

    Just for grins, how about you try leaving the 250k out of the circuit, check the effects of the bass control, then put it into the circuit and see what changes. Should be a simple mod.
  18. 4848277

    4848277

    Joined:
    Mar 4, 2011
    Likes Received:
    0
    Disclosures:
    Name: Tom
    Yeah, OK... That part of it is simple. More later.

    Good night. -_-
  19. SGD Lutherie

    SGD Lutherie Supporting Member

    Joined:
    Aug 21, 2008
    Likes Received:
    0
    Disclosures:
    Owner, SGD Music Products
    Lets break the schematic apart. The volume control is a volume control. It's the same way it was done for a long time in guitars. The tone controls are just tacked onto that. You can remove the tone controls and the volume works the same, and vice versa.

    You are assuming they designed a standard tone control as an RC filter, but they did not. It just looks and possibly works that way. These are just building block passive circuits stuck together.

    If they REALLY wanted to implement true tone controls or RC filter they could have, but then would have had insertion loss to make up for. RC filters generally have a series resistance. These circuits do not. These kinds of tone controls were quick and dirty "hacks" that had minimal impact on the output of the passive guitars. So while these may form a certain filter circuit, that wasn't the intention.

    Note that the parts chosen are just commonly used pot and cap values. I can guarantee you that no calculations where done!

    Once again the bass blocking cap and the treble tone control are separate entities. You can remove one or the other and they still work. They were never designed as part of a filter that required the other part. it was just a way to contrast to the tone of the two pickups. My Ric used 500K pots. It still worked the same. I also removed the treble pickup tone circuit and bridges the cap with a 1sM pot, as in the G&L circuit. Then I had an additional stereo master volume control. None of those things drastically altered the tone of the bass.

    This is something to remember about treble cut tone controls. When on 10, the cap is doing nothing. You do have some load from the pot however. As you turn down the tone control pot from ten, the major effect is changing the resistive loading on the resonant circuit. This is because the impedance of the capacitor in the range of the resonance is a lot less than the pot value. As the pot gets closer to zero, the capacitor becomes important, but the initial effect is just resistive loading.

    First, it was an op amp circuit. Most audio circuits with op amps have an input impedance setting resistor. Without that they run at very high input impedance. That buffers the pickup, but can produce noise. I don't have the circuit in front of me to look at, but it was pretty novel and allowed the input impedance to be set by the pickup.


    By the way, I also have a background in the electronics industry, and used to work for a large corporation that did defense communications contracts back in the late 70s.

    The function generator is not going to work the same way a pickup does. With a passive system like this, you are forming a tuned resonance circuit. You can use a driver coil over the pickup, but even then it's very difficult to get accurate graphs of what's going on. The pickup has a lot of different things going on.

    If you are interested in discussions on this kind of thing, you should hop on over to the Pickup Makers section of the Music Electronics forum. A few of the guys there are scientist and one works for a major semiconductor firm. At least one is a PhD. You will also find people like Jason Lollar and Rick Turner. We have had long discussions on modeling passive guitar circuits and using function generators and scopes to plot frequency response, etc. Trust me, it ain't that simple, and hasn't really ever been 100% successfully done.

    Pickup Makers

    Heres some good recent threads:

    Why does ^ reluctance ^ RAC?

    Making good measuremnets with imperfect driver coils

    measuring resonant peak



    Just try this, try wiring up the various parts and using your ear to listen to them. Then you will see. I did that back when I was 16 after I opened up various guitars and my Ric bass and saw when they were doing there.
  20. SGD Lutherie

    SGD Lutherie Supporting Member

    Joined:
    Aug 21, 2008
    Likes Received:
    0
    Disclosures:
    Owner, SGD Music Products
    But it's part of the preamp circuit. When the preamp is bypassed the signal going through the .1µF cap isn't going anywhere. That has no effect on the bypassed signal. Same with the 2.2M resistor. It's such a large load that I seriously doubt you wold hear it.

    But that is a sloppy way to do an active passive switch.

    Maybe, but it's still really not a RC circuit. Not intentionally anyway. A first order RC high-pass filter has the capacitor in series with the signal and the resistor across the output. In this case we have the resistor across the input. I seriously doubt anyone would design a RC filter like that, and also not using a 250K resistance. But you need that value for the pickups.

    If you remove the pot, and have the cap in series with the pickup, you will still eventually see a load at the amp end and have your resistance across the output.


    That's what I said. Just do it. I'm not talking abstract conjecture. I've sat and wired these types of circuits up in actual instruments. A popular mod for EMG pickups used to be to wire up a cap in series to make the humbuckers sound like single coils. I've done the same thing with passive pickups, and also used caps to tune the coil resonance.

    The only reason the G&L circuit has that 250k pot in there is to control the output level. You can remove it and the bass will sound the way you would expect, just a little brighter. I bet with the preamp switched in you wouldn't hear much of a difference.

    But you can either sit and think about it all day or try it out. Some things that you think will make a difference hardly do at all.

Share This Page