Pre-amp output/Power amp input-style Effects Loop question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by hipfan, Jul 7, 2003.


  1. hipfan

    hipfan Member

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    I have a Mojotone Tone Machine amp with a pre-amp out/power amp in type arrangement instead of a feature labeled "effects loop". I use some modulation effects (Deluxe Memory Man, occasional phaser and trem) in my rig and prefer to put them in an effects loop, after the pre-amp, whenever possible. However, they sound overloaded and strange in this amp's "loop". I have a couple of questions about this.

    First, is this arrangement just a series effects loop without the official name? It kind of seems that way, with all of the signal exiting the pre-amp, going into the effects chain, and then returning via the power-amp input.

    Second, is there any way to make this kind of loop work better with stompbox-style effects like I use? I've read a little about the Nobels SPX-31, which claims to convert a series effects loop to an adjustable parallel one. This sounds like just what I need, but I don't have access to try one out right now. Are there any other units that perform this function? Thanks for any help!!!
     
  2. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Pretty much, though there might not be buffering the way there is in many FX loops.

    Your problem sounds like an issue with levels. It's probably operating somewhere around line levels instead of the instrument levels that your stompboxes are used to seeing. The series/parallel thing isn't gonna help unless your pedals can deliver a 100% wet output. IMHO, your best bet is to try an Ebtech line level shifter or equivalent. If that doesn't work, you'll prolly have to use rack FX in the loop.
     
  3. hipfan

    hipfan Member

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    aleclee - Thanks for the reply. Yep, the Line Level Shifter looks like it might be my best bet. I sent an e-mail to EbTech inquiring. I'll post anything I find out from them.
     
  4. hipfan

    hipfan Member

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    I'm still waiting for a reply from Ebtech about the LLS, but, while I'm waiting, I decided to download the user manual for the Nobels SPX-31. Here's a noteworthy excerpt:

    "The SPX-31 can be mechanically combined with other NOBELS units. 4 units together are not bigger than a 19" unit (1U). Also it can be screwed. Best place is e.g.: Inside your combo amplifier."

    This sounds like one hell of a piece of kit! However, I'm not sure if I can manage to fit myself inside my amp.:p



    On a more serious note, from the product descriptions, I think I might be able to accomplish what I need with a combination of the Nobels and the Ebtech products. I could run it as follows: pre-amp out of amplifier -> input of Ebtech LLS (knocking the signal down to instrument level, as my effects want to see) -> input of SPX-31. Then, I would loop the effects through the SPX-31. Next, on the return, I would go as follows: output of SPX-31 -> return of Ebtech LLS (knocking the signal back up to line level, as the amp wants to see) -> power amp in of amplifier.

    My theory is that this would accomplish the buffering that aleclee speaks of, while also giving me an adjustable, parallel effects loop instead of straight series. Thoughts?
     
  5. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    I don't know that the FX you use would necessarily give good results in a parallel loop, particularly the phaser and trem.
     
  6. hipfan

    hipfan Member

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    Ah, good point. I ordered one of the Ebtech units from Zzounds today, and I'll try it by itself first. From my understanding, this will just create a series loop. I'll see how this sounds, and then maybe try out the Nobels box if the sound is not right and creating a parallel loop might help.
     
  7. hipfan

    hipfan Member

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    I received the Ebtech unit from Zzounds, and, while I'm sure it's a fine device, it is not intended for this application. Ebtech confirmed this in an e-mail exchange.

    Back to the drawing board on this one, I guess. Any other thoughts for turning this into a "true" effects loop in the modern sense?
     
  8. Scrutinizer

    Scrutinizer Member

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    Mojo has a schematic of the amp on their site, but unfortunately does not show the pre-out/pwr-in jacks.

    http://www.mojotone.com/tone/schematic.pdf

    V4B is a cathode follower stage that mixes the signal from the two channels. C14 couples the output of this stage into the PI circuit. I would assume that the preamp out is after C14. I’d also expect to see another coupling/dc-blocking cap after the poweramp-in jack.

    If my assumptions are correct, signal level matching in your ‘effects loop’ is going to require two circuits. One before the FX unit would attenuate the signal to prevent overdriving the FX. This attenuator could be a simple potentiometer in a box (similar to a volume pedal).

    The second circuit would be inserted between your FX output and the poweramp input. This would boost the signal back to the desired level - in fact, it could be used as a master volume control. Not sure if there is anything off the shelf that has a hot enough clean output to work in this application. Without knowing the expected signal levels at the PI input node on that amp, I can't suggest anything in particular.

    In any case, it will important to remember that the signal level at the preamp output will increase as you adjust the channel volumes (and tone controls). In other words, suppose you set your amp volume level at soundcheck, then nudge the volume knobs up gradually throughout the night (everyone does this, right?). Eventually, you will be overdriving the input of your FX devices unless you allowed sufficient headroom to begin with.

    Seems that this is perfect application for a Dumbleator. I wonder - does anyone makes Dumbleator clones?
     
  9. Fuchsaudio

    Fuchsaudio Member

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    There's a guy (In Germany I think), who's selling on on E-bay. Just put Robben Ford in your search box in musical instruments. Looks pretty well made.
     
  10. aleclee

    aleclee TGP Tech Wrangler Staff Member

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  11. hipfan

    hipfan Member

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    Wow, thanks for the input guys! It looks like the Dumbleator is exactly what I would need for this amp to have an actual effects loop. I'll send an e-mail to Graydon and do an E-bay search for that guy in Germany. I'll keep this thread posted with any progress.
     
  12. GDS Racing

    GDS Racing Member

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  13. hipfan

    hipfan Member

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    I contacted the German fella and received a prompt and promising response. I think I'm going to go ahead and order one of the Dumblelator clones from him. Should be less than $200, including shipping from Germany to Chicago.

    Thanks to all for the guidance. If I get the thing, I'll post a follow-up to let everyone know how it works out.
     
  14. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    Cool. Let us know how it works out. I'd be interested in one if it does the trick for ya.
     
  15. hipfan

    hipfan Member

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    I just placed my order for one of the German Loop Interfaces. Here is the Loop Interface Info Page.

    It should ship on Monday, 8/18. I'll let everyone know how it does after I've had a chance to put it to the test. :)
     
  16. hipfan

    hipfan Member

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    OK, just received the Loop Interface. It sure looks like a nice piece of kit, but I can't fire it up just yet. The power supply came without a plug, so I'll have to wire it up this weekend. I'll follow up here when I've had a chance to test it out.
     
  17. LIONEL69

    LIONEL69 Member

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    how did it work out ???
     

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