SWR Aural Enhancer Bypass

Discussion in 'Amps and Cabs [BG]' started by duff2, Mar 29, 2010.

  1. duff2

    duff2 Guest

    Dec 16, 2007
    Has anyone modified their older SWR amp to bypass the aural enhancer circuit and achieve a flat frequency repsonse?

    Specifically, I'd like to replace the original aural enhancer knob with a pull pot on my SM-400 so that I can have access to the original and modified preamp voicings.

    Is this possible? If it is, where should the switch be positioned in the circuit below?


  2. Lemoore-on


    May 11, 2008
  3. aBoMoFo


    Jun 16, 2008
    Houston, TX
    Good idea for a neat mod. Taking a quick look at that schematic, I don't see a quick and easy way to do it. One physical limitation to using the push/pull pot MIGHT be the actual space alloted for it within the casing of the 400.

    If you get the mod done though I'd love to see it and hear your thoughts on the results.

    [edit in the line talking about "push/pull pot WOULD be the..." I intended to say "push/pull pot MIGHT be the..."]
  4. duff2

    duff2 Guest

    Dec 16, 2007
  5. Turn it all the way down?
  6. Bongolation


    Nov 9, 2001
    No Bogus Endorsements
    "Flat frequency response" is much more an issue of remedying the tone stack on SWR amps, which are notoriously scooped, not the "Aural Enhancer."
  7. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    With my old SWR GP the closest to flat response was with the aural enhancer off and everything flat apart from the lower mid semi-parametric EQ which had an 8dB boost at 250Hz. I also found the the aural enhancer did was just a simple boost for the first third of its rotation, after which it added yet more cut at 250Hz. If the SM400 is similar I'd try that first.
    Jim Carr likes this.
  8. duff2

    duff2 Guest

    Dec 16, 2007
    Here's how it's done folks (well everything except the push/pull addition that is).


    In text format:
    1. remove the 0.1 uF capacitor
    2. Use it to replace the 0.001 uF capacitor

    This most awesome modification will apparently flatten the preamp voicing and leave the added gain available by turning up the aural enhancer knob intact.

    Thanks to kringle77 and Walter Harley (of Cafe Walter headphone amp fame) for all their help in this matter.

    I am planning on taking my amp to a tech later this week. I'll let you know how the mod turns out. :bassist:

    mjm0, CrocMule, Alex_H and 1 other person like this.
  9. Fuzzbass

    Fuzzbass P5 with overdrive Gold Supporting Member

    Wow. Very cool!
  10. SanDiegoHarry

    SanDiegoHarry Inactive Supporting Member

    Aug 11, 2008
    San Diego, CA

    And if you dial that "Aural Enhancer" all the way down, are you not removing it from the chain?
  11. I have a SWR Grand Prix Pre that has this mod and I highly recommend it,the sound is more usable in a live setting.
    It is good to see how it was done now.
  12. duff2

    duff2 Guest

    Dec 16, 2007
    Not exactly, the mid-scoop is present even with the aural enhancer set to zero (unlike the Enhance control on the Eden and Thunderfunk amps). Up to a 2 O'clock setting the SWR aural enhancer controls how much additional signal from your bass passes through the second half of the 12AX7 tube. This adds gain and extra high end harmonic coloration to your sound but does not change the preset EQ curve present when the control is in the off position. At settings beyond 2 O'clock there's no extra gain, but rather an additional mid scoop added around 200 Hz.

    Graphically, here's what the aural enhancer control does on an SWR amp:


    CrocMule likes this.
  13. mhduffl


    Nov 27, 2008
    Hong Kong
    Hey duff2,

    Do you have any data for aural enhancer at lower setting , such as 8, 10 o'clock???

  14. aBoMoFo


    Jun 16, 2008
    Houston, TX
    Very cool! I'm anxiously awaiting the results and your thoughts. Looking at this mod and my mouth is watering.

    I took a look through a few resources and it looks like the Aural Enhancer pot is a 50K for your SM400 (http://www.musicpartsguru.com/closeup.asp?searchWord=swr&pid=911&offset=10)

    If you wanted one with a pull switch for defeat then this might be the fella for you.

    Now I can't be dead certain that this is the right pot but a quick call to the parts retailer would confirm the fact that the one with the switch is the same as the non switch version. (i.e. Log vs. Linear)

    It also looks like SWR used 50K pots all over the place on their front panel controls. Those of you with "aural enhancers" in the 750X, 550X, Super Redhead, Black Beauty, Workingman Series, Cali Blonde, Strawberry Blonde, Studio 220, Bass 350, Silverado, Redhead, Baby Blue II, Basic Black, SM-900, SM-500, and Grand Prix units might be able to use this 50K w/ switch for the original 50K pot that your units came with.
  15. I am dying to do this mod to my old SM900. But how do I find the physical location of the two caps?
  16. Jim C

    Jim C I believe in the trilogy; Fender, Stingray, + G&L Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Nice work Duff; just assumed that when the knob was turned down it was out of the circuit; await you report back regarding performance
  17. duff2

    duff2 Guest

    Dec 16, 2007
    It would look the same as the black curve but it be located between the black and green curves. Soundwise, the volume would increase slightly and the top end will have more sparkle added to it.

    Direct Box Rox likes this.
  18. mhduffl


    Nov 27, 2008
    Hong Kong

    Thank you so much Duff2....:)

  19. jb63


    Jan 3, 2002
    Cleveland, Ohio

    This completely addresses my only gripe with the SM400, that mid scoop right in the meat and potatoes frequencies. I'm thinking about doing this mod even though my SM has been sidelined in favor of an LMII.
    murphy likes this.
  20. baddarryl


    Oct 26, 2008
    Cape Fear!
    I take it that this is applicable to the WorkingPro 400?
    KeithFTW likes this.