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SWR Aural Enhancer Bypass Mod revisited and updated

Discussion in 'Amps and Cabs [BG]' started by SNCpre, Jan 6, 2019.

  1. SNCpre


    Jan 20, 2016
    8 years ago, under my old Talkbass username Duff2, I started a thread to share an SWR amp mod that originated from Walter Harley (@cafewalter) of Cafe Walter Audio.

    SWR Aural Enhancer Bypass

    As shown in the schematic below, the mod involved removing the 100nF capacitor (C10) from the low pass filter and then installing it across the 1 nF capacitor (C12) of the high pass filter in the fixed value Fender tone stack. This effectively defeated the low mid-scoop and treble boost that define the SWR sound, and resulted in a flatter frequency response. I originally referred to this mod as the 'SWR Aural Enhancer Bypass', however since the Aural Enhancer knob still functioned as a gain control/pad for the signal entering the second half of the 12AX7 tube, a better descriptor would have been the "Harley SWR Preshape Bypass Mod'.

    SWR Harley Mod (original).

    A subsequent contribution to the original thread by Talkbass user @the V, detailed how to incorporate the mod onto a DPDT switch so that one could toggle back and forth between the stock voicing and the bypass option. I had this version of the mod installed on my SWR SM-500 soon after. I was quite pleased with the result, particularly if I used the active input (which bypassed the first half of the 12AX7) and then dialed in the amount of 12AX7 coloration I desired using the Aural Enhancer knob.


    This whole endeavor jump-started a serious electronics hobby for me. In the subsequent years I tackled a variety of projects as I got better with the soldering iron and audio circuit design theory. My most recent project was the restoration of an SWR SM-900 that had been almost trashed to the point of no return. After rebuilding the blown (and burnt!) power amps, replacing the power supply capacitors and swapping the preamp power supply cement resistors for chassis-mounted, aluminum-cased wirewounds, I turned my attention to incorporating the Harley Bypass Mod into the preamp.

    Later in the original SWR AE bypass thread, @KeithFTW had suggested changing the values of the low and high pass capacitors to achieve more of an Ampeg 'ultra low switch engaged' mid-scoop vibe. Since I never used the stock preamp voicing, I decided to follow KeithFTW's advice and change the cap values such that the mid-scoop dip moved from 200Hz to 500Hz while filling in the 100-200Hz gap and attenuating the boost in the 1-3kHz range of the original preshape EQ curve. After working out the details (C10=47nF, C12=330pF), I realized that this mod could be taken one step further. Since the new cap values were smaller than the original values, it would also be possible to add a second switch to toggle between the original AE200 voicing (black curve) and the new AE500 option (yellow curve). More importantly, the Harley Bypass mod (blue curve) would still function for both voicings!

    AE bypass v2.

    If you are interested in trying this out for yourself, here are the wiring diagrams:

    Important: use capacitors with maximum DC voltage ratings that match the specs of the original SWR parts you are replacing.

    Two switch version (SW1 = preshape in/out (bypass), SW2 = AE200/AE500):

    SWR preshape bypass with AE200_AE500 shift switch.

    Single switch versions:

    SWR preshape bypass wiring original.

    Last edited: Jan 10, 2019
  2. This is cool stuff. No longer have an SWR, but good to keep this info out there. Thanks for sharing.
    SNCpre likes this.
  3. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    Hey Sheldon, thanks for this new design.
    For those that don't know, Sheldon produced balnk boards to build your own 2-band Stingray pre.
    If that wasn't enough, he also built a Spector HAZ preamp to support those of us who have a Korean Spector with a bad pre.

    I only just started using my old SM400 again after getting a F'deck HPF.
    I think I will do the original mod with the switch.
    Many of the early amps have no HPF.
    Is there an easy way to add a HPF while I'm inside?
  4. SNCpre


    Jan 20, 2016
    Hi Jim,

    @UncleFluffy was actually responsible for the 2-band Stingray boards.

    With regards to the HPF, as I had mentioned in one of your SWR related threads (see post #33)...

    And to think I nearly tossed my SWR SM400 in the trash; why didn't anyone tell me that

    ...there is (what should be) an adequate HPF filter positioned just before the power amp stage in all post-1992 SWR amps. The reason why the stock HPF fails to control the sub-low frequency speaker-excursion issue is still beyond my level of understanding at this point. I personally have never experienced this issue with the Baby Blue or SM-500 I previously owned or the SM-900 that I currently possess. If this were something that were an issue for me, I could study it and try to help you find a solution. Unfortunately, I cannot offer any sage advice on this subject at the present time. Sorry.

    If anyone can explain why the stock SWR passive HPF is inadequate for some users, I'd love to hear from you.

    Last edited: Jan 7, 2019
    wave rider likes this.
  5. Roxbororob

    Roxbororob Supporting Member

    Jun 8, 2015
    I finally did it (after himming and hawing for the longest time). AE mod last night, amp running fine afterwards, have to wait until band practice to stretch its legs and really hear whats up ( it was Sunday night at home and all).

    EDIT: Ancient SM400, with the dual foot print for the caps. The swap was a breeze.

    Jim C and SNCpre like this.
  6. SNCpre


    Jan 20, 2016
    In order to compensate for the extra 12 dB of preamp gain in the low midrange after the mod, I tried swapping in a 12AT7 (gain = 60) and a 12AY7 (gain = 40) for the stock 12AX7 (gain = 100). Both tubes sounded great as 12AX7 substitutes in my SM-500 and SM-900. The 12AY7 sounded pretty much like the 12AX7 in this circuit, but it offered a greater range of input sensitivity for the gain and limiter controls - akin to how the amp reacted pre-modification. The 12AT7 behaved similiarly to the 12AY7, although it was a touch darker sounding. When pushed into distortion, I much preferred the tone of the 12AT7 over the 12AY7. If you have performed the preshape bypass mod and you have a 12AT7 or 12AY7 laying around, I would recommend that you try them as substitutes for the stock 12AX7 tube.
    wave rider and Roxbororob like this.
  7. moogieotter

    moogieotter Custom Title

    Jun 16, 2009
    Duluth, GA

    I have an SM-500 as my main amp. Would you do this mod for me? I would pay.

    I also have a first gen Redhead that really moves cones so much I suspect no HPF. I also have a dead Grand Prix to fix up.

    PM me as needed. Thanks.

  8. Callused Finger

    Callused Finger

    Feb 22, 2007
    New York
    Thank you for this! I have a 1993 SM-400 that I’ve owned since new. I did the aural enhancer mod last year and while I like the sound I’ve since felt guilty about it and Have been thinking about adding a switch.
    Appreciate the effort and sharing the information.
    wave rider and SNCpre like this.
  9. MarkA

    MarkA In the doghouse. Supporting Member

    Sep 26, 2008
    I don't have an SWR amp (though I was aware of them as a young 'un), but I think that threads like this and the original are super-cool. I like the DIY/revise/repurpose/rejuvenate old gear ethic and the sharing of it with others.
    Red Planet, Jim C, LeonD and 2 others like this.
  10. SNCpre


    Jan 20, 2016
    Update: I thought I'd add these additional mod variations and their associated graphs here for completeness sake.

    If you use a DPDT on-off-on switch for the bypass function, you can access the Muthafunk voicing by placing the switch in the middle 'off ' position. The Muthafunk mode (pink curve in the first graph) offers a little more note clarity and definition over the Harley full-bypass mode (blue curve) IMO. But hey, don't forget about that powerful eq section if you need more top end. ;)

    SWR AE Voicing Mod - Dual Switch Version.

    For those of you who aren't comfortable with a soldering iron or disassembling your amp, you can still enjoy the benefits of this mod by simply popping the top cover and cutting the 100nf (100nF = 0.1uF) cap down the middle, pulling away both halves from the leads with a gentle twisting back-and-forth motion and trimming the remaining leads flush with the PCB. You can achieve a close to flat response curve by boosting the bass control to 2 o'clock and filling in the midrange around 500Hz, but this really isn't necessary to achieve a superior tone relative to the stock voicing. Boosting the AE control after the mod still tightens up the bottom end sufficiently without having to touch the bass control. Be patient and extra careful while cutting and pulling out that capacitor! :)

    SWR AE Voicing Mod - no soldering iron required.JPG
    Last edited: Jul 9, 2019
    miljoneir and Roxbororob like this.
  11. Red Planet

    Red Planet Banned

    May 29, 2005
    I'm cranky in my old age.
    Very interesting thread.
    Cool for these old amps to still be in use!
    Had my share of SWR gear.
    SNCpre likes this.
  12. Roxbororob

    Roxbororob Supporting Member

    Jun 8, 2015
    Who you calling old?!
    SNCpre and Red Planet like this.
  13. Heavy Blue

    Heavy Blue

    Nov 11, 2017
    Prairie Canada
    Great thread!

    So as I have a Headlite which apparently is different than other SWR's so what parts of this mod can I do?
  14. SNCpre


    Jan 20, 2016
    You don't need to do anything to your Headlite. The AE200/AE500 and AE bypass functions accomplish the same thing as this mod.
    Last edited: Jul 10, 2019
    wave rider likes this.
  15. Thanks for this information. I just tried this on my old Hartke 3500 mosfet amp. The tube preamp section of this amp has the same circuit of that filter design as the SWR.

    This mod seems less useful in this amp since it has a solid state preamp channel that is flat.
    However, I prefer to get some tube coloring from the amp. The way the preamp design functions, I means that I always have to mess with the eq to get rid of the resulting mid scoop, which couldn't be fully achieved since the eq points on the graphic eq don't match up.
    After doing this mod, I have an amp that has a tube preamp channel that is almost flat in response, but allows me to get some tube-coloring.
  16. agat38


    Aug 31, 2014
    Very intersting all that.
    I also did the aural enhancer mod few years ago on my 92 SM400 with push pull AE pot so i can use the amp in both mode, last week i make the hpf mod replacing 1µf with 0.22 µf, now i want to make the AE500 mod so i have a question about the influance off the 330 pf and 47 nf values, for exemple if i put 220 pf and 33 nf what append ?
  17. SNCpre


    Jan 20, 2016
    This will maintain the shape of the the mid scoop and move it to the right by ~200Hz, effectively giving you an AE700 version of the mod.
    agat38 likes this.
  18. agat38


    Aug 31, 2014
    Ok thanks, i will begin by the AE500 mod, may be later i'll play with different cap values ...

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