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Are most onboard preamps Baxandall type EQ ?

Discussion in 'Pickups & Electronics [BG]' started by grillman, Feb 19, 2016.


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  1. grillman

    grillman

    Dec 15, 2014
    So if you cut the bass and treble on a 2 band EQ onboard preamp does it boost the mids ?
    Or vice-versa if you boost the bass and treble does it cut the mid ?

    I am especially wondering about the musicman 2 band EQ preamp (cut/boost) and the Aguilar OBP-1 (only boost)
     
  2. ex-tension

    ex-tension

    Jun 11, 2009
    grillman likes this.
  3. khutch

    khutch Praise Harp

    Aug 20, 2011
    suburban Chicago
    Based on the relatively few makers who publish curves it appears that most onboard bass preamps are Baxandall filters. Boosting both bass and treble does have the effect of cutting the mids and vice versa. There are a few people who make variable LPF preamps and some have variable damping in addition to variable cutoff frequencies. You can make this type of filter for a passive bass too by using a rotary pot in place of the tone pot to select multiple tone caps, one at a time. It is more complex but you can also include damping resistors to get two levels of damping on the filter.
     
    grillman likes this.
  4. Technically, the answer is no, because the mids usually stay put, depending on how close the treble and bass centre frequencies are. You'd really need to adjust the gain as well as the treble and bass to boost or cut the mids. But I think I know what you mean, and yes, with a baxandal filter you can get a smiley face or frowny face frequency response.

    Also, technically speaking the original james-baxandall filter is actually passive (complete with log taper pots). With the controls in the middle, you have a flat response but with an overall cut of around 24dB. With gain recovery, you get back to zero, then the treble and bass controls give you +/- 12dB or thereabouts.

    Around the late 70's, with cheap and small ICs, the baxandall was adapted to be wrapped around an opamp, and (IME), this acounts for many treble and bass preamps since then - onboard, rack-mount, amp, hifi, mixing desks etc etc etc.

    Around the late 80's a third control was often added as part of the same filter. A mid control done this way has a low Q and therefore often only has half to two-thirds the cut and boost of the treble and bass controls. This is what's going on with the 3-band stingray pre from the early 90's. The 3-band fender bass preamps I've seen, and lots of 90's acoustic guitar preamps have also been like this.

    These are both notable preamps that aren't baxandall filters.

    The MM 2-band is quite unique. If you google, I'm sure you'll find some response curves online. (If I dig around I'm pretty sure I modelled this myself years ago.) This is a preamp that has been talked about at great length. It's not flat with the controls in the middle, but it is cut and boost, and yeah to a certain extent you can get a smiley or frowny face. The preamp has only one opamp, so it's input Z is quite limited. (virtually all more recent preamps have a 2nd opamp that serves as an input buffer.) That's why it works so well with just one pickup that is wired in parallel mode (IOW, low Z), and directly coupled (IOW, no vol or blend pot between the pickup and the pre).

    AFAIK, the treble and bass filter in the obp1 (as well as the sadowsky pre, and probably the old bart tct) is simply a scaled/tweaked fender tone stack. (I'm happy to be corrected here.) This is a filter that is in virtually all fender valve amps in one form or another. It's a passive filter (complete with audio taper pots), and like the original baxandall, needs around 20-24dB of gain recovery. With the controls in the middle, you get a big smiley face, and with both controls all the way down, you get pretty close to a flat response. A frowny face is usually not an option. Be aware that these controls don't act in a symmetrical way, and also interact quite a lot. Plus their centre frequencies shift. The treble is kind of a shelf, and the bass is kind of a peak type response. Kind of...

    If you aren't into spice (electronics circuit modelling software), there used to be a freeware tonestack modelling programme made by someone-or-other. If memory serves, this covers 4 or 5 common tone controls, and you can tweak the pot and cap values and see the freq response curves. Pretty cool really for the hobbyist, and worth tracking down if it's still out there.

    FWIW, it's important to not just look at the one element of filtering in its own right. It's very common to have anything but a flat response in one part of a system to account for the shortcommings of another part of the system. Cassettes and records have a pretty radical EQ applied at the mastering stage, then your boombox or turntable applies the opposite (RIAA) eq to improve the noise response. Dolby and Bose are more examples of a similar thing (...well sort of). It's common for guys to desire a flat response from a bass preamp when their pickups and speakers especially, often have huge peaks, troughs and roll-offs in frequency response. Not to mention the fact that our instrument is a piece of wire tensioned on a plank of wood. Have a listen to your bass unplugged in a quiet room. Sounds pretty honky, right? OK, I'm getting OT now... :p
     
    Last edited: Feb 20, 2016
    NickRich, Growlmonkee, Jim C and 4 others like this.
  5. grillman

    grillman

    Dec 15, 2014
    Thank you guys ! I never expected such a detailed answer.

    Sorry if my question wasn't perfectly clear. I am starting to understand how EQ work. A very interesting topic indeed.

    I learned a lot from your posts. Thanks again.
     
  6. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    FWIW, the Baxandall active EQ design was published back in 1952: 'http://www.learnabout-electronics.org/Downloads/NegativeFeedbackTone.pdf'

    which I found here: Amplifier Tone Control

    And here is the original E.J. James article on the passive design, from 1949: http://www.americanradiohistory.com/Archive-Wireless-World/40s/Wireless-World-1949-02.pdf

    And here is Duncan Munro's Tone Stack Calculator: TSC
     
    Last edited: Feb 22, 2016
  7. grillman

    grillman

    Dec 15, 2014
    Is there a computer (or mac) program showing me the signal wave if I plug my bass into it ?
     
  8. Passinwind

    Passinwind I know nothing. Commercial User

    Dec 3, 2003
    Columbia River Gorge, WA.
    Owner/Designer &Toaster Tech Passinwind Electronics
    You can run a .wav file recording of your bass through LTspice and both see and hear the modeled results.
     
    grillman likes this.
  9. grillman

    grillman

    Dec 15, 2014
    Thank you good sir !
     
  10. ofajen

    ofajen

    Apr 12, 2007
    92.4W 38.9N
    One minor detail, Fender style tone stacks will typically cut at least 10 dB in the mids, while the bass and treble will cut less when turned up.

    So if you want a flatter response, the mid control should be set high and the treble and bass turned down to also approximate that 10 dB cut.

    Otto
     
  11. FourBanger

    FourBanger

    Sep 2, 2012
    SE Como
    10 o'clock - 2 o'clock - 10 o'clock
     
  12. ofajen

    ofajen

    Apr 12, 2007
    92.4W 38.9N
    Possibly. Of course it depends on all the specific component values.

    Otto
     
    FourBanger likes this.
  13. The obp1 (and the sadowsky, tct etc) doesn't have a mid control. It's a fixed resistor in these preamps. I'm not sure why guys are so keen on having a flat response (see the last paragraph of my last post), but if you really want one with one of these preamps you pretty much have to turn both bass and treble down all the way. And FWIW, if there was a mid control, then with both bass and treble down so low, the mid control simply acts as a vol/gain control across all frequencies.

    I don't want this to sound like I don't like the fender tonestack as an onboard filter. I love it and have done for many years. The current preamp in my main bass has a (very tweaked/evolved) version of it. And both my amp rigs have this filter as well.
     
  14. ofajen

    ofajen

    Apr 12, 2007
    92.4W 38.9N
    I'm very fond of the tone stack in my YBA-1, which is much like a '59 Bassman. I don't ever actually adjust for a flat response. Passive tone stacks are sorta organic and the controls interact, so you basically just adjust til it sounds good.

    I typically have the controls toward the middle of their range, which will reduce the mids some with a bit of bass boost and a hint of treble boost. The overall sound from the amp and cab is great.

    Otto
     
  15. Yeah I agree. Actually I have a silver face bassman 100 as well, and it of course, also has a tonestack. There's just something nice about that filter isn't there. Perhaps it has to do with the phase response...

    Ha Ha! so do I. It's funny isn't it. I don't think I have a particularly scooped tone, but the (eden) 210 I use for my big rig is quite middy, so it evens out I guess.

    When I bough this 112 aguilar last year to use with my class D amp for smaller gigs, I tweaked my bass's onboard pre again for more mids. I'm also thinking of adding a mid-boost circuit of some sort to the amp's preamp. The aguilar 112 is very nice, but is quite scooped sounding already.
     

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