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M3 (AO29) FX loop. Simple!

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    M3 (AO29) FX loop. Simple!

    After weeks of trying to find a line level loop cut in point I gave up. Probably why Hammond came out with AO43 for reverb loop. My AO35 reverb was not cuttin it for me, so I have another guitar amp. What I discovered was there is a high impedance point available for use with a guitar level multi FX unit. I'm using my Digitech RP-255. I have left hand bass, looping, reverb etc. This simple mod allows both mannuals to be effected. Bass pedals and vibrato are not. Just as well though cuz I don't usually put verb on bass. The regular vibrato is still available just not with FX. I can have vibrato on the top manual while the lower is effected. If I add the lower vibrato it's out of the loop. It's easy to understand how this works by looking at the M3 later version with PM speaker schematic at Capt. Foldback. What I did is open the larger ground box cover and access connection point 'A'. The cover is the one to the right with all the shielded wire connections. In the box you'll see tabs with wires connected. The one to the right (black) is the vibrato tab (B). Just to the left is tab (A) yellow wire. A few tabs left of that is a double wire connection for bass pedals and vibrato return (D). All I did was unsolder the yellow wire (A) and connect it and it's tab to seperate switched 1/4" jacks. I used heat shrink on the wire connections. I put the send/return jacks in a project box alongside the amp. Just connect the jack switches together. When unplugged the mod is bypassed. Another side benefit of this mod is being able to plug a guitar into the amp. This amp is fantastic for guitar. Just plug into the return jack. Space is tight in the box so I used shielded rca cord. I put a small notch in the upper right hand side of the cover to thread the rca cords in. I joined the shields together and added a small lead to bring the ground outside and connected it to the cover screw. This must be done powered down as the cover is shielding 3 preamp inputs. If you want a taste before unsoldering the yellow wire just alligator a clip on the 'A' tab and play guitar. Until you have your FX unit programs adjusted keep the low volume tab on while comparing the bypass volume. Only quirk I've discovered is a slight volume reduction in the organs vibrato because line A and B share the TWG transformer connection. Not a big deal. Probably won't be using the vibrato anyway as the FX unit offers that and so much more. So far this seems like a perfect marriage. Super fun with a looper. Left hand bass sounds excellent. I'm sure this mod can be taken further by switching or connecting to the other tab 'B'. Connection 'D' is not high impedance as it's the pedal and vibrato return. I'm hoping someone else will enjoy this as much as I am.
    C3, Leslie 720, A100, E100, Have owned L100 and M3, http://soundcloud.com/twiggybush

    #2
    I've got exactly the same loop on my M-102 with a dual 15 band EQ and a pair of 19" processors. I set up effects which vary widely in volume due to compression, distortion, etc. and use adjust knobs on the EQs to boost or cut volume to adjust headroom and final return volume. The adjusts also allow for volume matching with the vibrato channel. A little EQ makes a spinet sing much more. Late at night, I cut the bass way down.

    The broken/for repair category in musical gear on ebay has broken processors on it all the time that just need a re-cap of the power supply. I won't pay over $20. If I get it, I do, if not, c'est la vie. I'm up to 4 rack processors and a pair of rack EQs. The big cost was the rack to put them in which was $1 on ebay with $40 in gas to go get it.
    Hammonds: '52 S, '56 S-6, '61 M-3 (w/'56 JR-20), '66 E-182, '67 M-102, '69 H-112 ; '61 Gulbransen H-1, '61 Mercedes 220B, '83 Bridgestone MB-3, Various cheap guitars, homebrew amps, DSPs, FPGAs, and the magic smoke that makes them run, but no LSI TOGs (yet)

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      #3
      Bravo, Twiggybush and an added Huzzah! I get the "repair" stage done on my M3, I will give your mod a whirl. Sounds just like what I was looking for.
      DrLunk aka Mike Head
      Unofficial Senior Member of the yet to be organized Organizational Committee for "Burning Hamm"
      "Maximum Use of Available Technology"

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        #4
        So, just to be clear on this, the yellow tab out is what you'd call "line" level rather than "instrument" level? As I understand it, most guitar effects pedals are designed to operate at instrument level and as that it what I intend to use for my effects processing, it sounds like I'll need to incorporate a DI box into the loop here. I've never messed with this kind of stuff before so I'm still working on getting things straight in my head. If I bring it down to instrument level for the pedals (primarily a Leslie simulator) I'll need to bring it back up to line level to feed back into the amp?

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          #5
          Assuming that no reduction and reamplification of the line out signal needs to occur, does this drawing look like it would work? A switch on the box would send the signal straight through like stock in one position and enable the loop in the other. And in the stock position it would still allow the addition of a signal through the effects return jack (like an electric guitar.) Do I have this right?

          If so, how does the grounding work? The effects loop send and return jacks are grounded to the box? And then is the box grounded to the amp chassis? I'm like piltdown man when it comes to this stuff.



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            #6
            Originally posted by Hamfeather View Post
            Assuming that no reduction and reamplification of the line out signal needs to occur, does this drawing look like it would work? A switch on the box would send the signal straight through like stock in one position and enable the loop in the other. And in the stock position it would still allow the addition of a signal through the effects return jack (like an electric guitar.) Do I have this right?If so, how does the grounding work? The effects loop send and return jacks are grounded to the box? And then is the box grounded to the amp chassis? I'm like piltdown man when it comes to this stuff.[ATTACH=CONFIG]25321[/ATTACH]
            I'm wondering the same thing...did you ever figure this one out? I'm suspicious that it's actually instrument level as it hasn't entered the preamp yet.

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              #7
              No, I haven't figured it out. I'm stuck at how to wire the damn thing up. I want it to work like this diagram illustrates but I just don't know about what does or doesn't get grounded and where. I think I have determined that for using guitar effects pedals, this source should be okay though, whatever level it is. I think the voltage is low enough and the range of effects pedal voltages varies enough that it's not terribly crucial. For instance, I think my EHX Soul Food voltage output at its max overdrive is 10x what it puts out on the low end.

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                #8
                What we are dealing with in the case of the AO29 insertion of effects, is a "series loop" as opposed to a parallel loop.

                the grounds are really insignificant, other than for shielding from errant signals, because the signal is being added inline...
                and is either on, or off. No mixing with organ signals.

                I am using a parallel loop, because my powered mixer, has the ability to adjust the amount of (effect) signal used with the organ signal. My remote switch, is just to add and remove, the FX signal altogether, as pictured in the {Parallel Loop} photo array in the link

                This may help clear any confusion

                http://www.mackamps.com/articles/wha...they-work.html

                and as they state:

                "Bottom line:

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                  #9
                  Thanks Rotohead. I think you make a lot of sense. "Instrument level" is not defined like Line Level is and thus as long as you don't have a volume problem or you're clipping something hard you're good. I've thrown a couple diagrams together with various options and hopefully some helpful tie in points.

                  The one thing I'm concerned about would be an impedance matching issue. I'm sure the current off the matching transformers on the drawbars is not high. If someone puts this into low impedance gear it's going to sound bad. I'm also not sure if I'd need a resistor on the output jack to limit current pull.

                  Here's the diagrams if anyone wants them Hopefully they make sense. Let me know if you have questions or these are wrong.


                  Comment


                    #10
                    Originally posted by moonmeister View Post
                    Thanks Rotorhead. I think you make a lot of sense. "Instrument level" is not defined like Line Level is and thus as long as you don't have a volume problem or you're clipping something hard you're good.
                    Herr moonmeister,

                    Let me define my interpretation of those differences. Very simply :

                    Instrument level = generated signals, prior to any amplification.

                    Line Level = generated signals that have had some, amplification (i.e. been Pre amped)

                    Speaker level = Generated signals that have completed all amplification stages,
                    and are ready to be fed to the speaker

                    No mention of: amperage, voltage, current, resistance, reactance, inductance,
                    capacitance, or impedance required. If we could all adopt this there'd be a lot less confusion.

                    The one thing I'm concerned about would be an impedance matching issue. I'm sure the current off the matching transformers on the drawbars is not high. If someone puts this into low impedance gear it's going to sound bad. I'm also not sure if I'd need a resistor on the output jack to limit current pull.

                    We're talking here, as you said...about the isolated output from the drawbars.
                    It's a very low voltage. [it's unamplified, therefore by my definition... Instrument level]

                    If one stays, with passive FX units (i.e. [instrument level] Ones that would usually go between the guitar output,
                    and the guitar amp input,) [aka-Stomp boxes] impedance should not be an issue.

                    Keeping to the above, I don't see clipping as an issue.
                    However, adding a 5K or better, potentiometer, in the return line,( RED addition) and bleeding off
                    some of the signal to ground, to limit the input signal, should do the trick if necessary. .

                    Click image for larger version

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                    Once again.... If it sounds good.... use it! If not...
                    RotorHead

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                      #11
                      double posting removed

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                        #12
                        Thanks Rotorhead. You make lots of sense. I've done production for a while but never modified instruments or done a lot of circuit design and such so I know enough to trip myself up. Thanks for the input.

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                          #13
                          In case anyone does not want to mess with the DIY approach, you could always use a Trek II ELB-1A.

                          Their approach is done neatly at the matching transformer, and uses the preamp stage cathode voltage to power its buffer stage opamps.

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                            #14
                            I am new to this group, and I purchased a 1960 M-3 from a church here in Florida a few weeks ago. After cleaning this old girl up and lubricating the tone generator & such I want to find a way to either insert a Leslie Pedal into an FX loop or use the output to and external Leslie or other amplified speaker. After days of scouring the web I came up with a working FX loop on the AO-29 amplifier. The key is where to insert the loop: What I found is there is a small capacitor called C21 that feeds off the output of tube V4 in the preamp portion of the AO29. I clipped the output of C21 and fed the output of C21 into a Switchcraft 13A 1/4" phone connector and from a second Switchcraft 13A back to the other end of the clipped lead of C21. After connecting the correct pins on the 13As to one another. I have a full functioning Series FX loop on an AO29. By using the 13A connectors by simply removing the phone plugs, the amp functions as if it wasn't there. This set-up also allows the use of any external pedals or amplifiers as the internal speaker won't play if you use only the send on the FX loop. My plan is to add a Hammond/Lesle K pedal, as I don't have room for a real Leslie cabinet.

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                              #15
                              Here are a few photos of the finished project Note: I also added a fuse holder and fused the organ for safety.
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