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I have a Vox AC4 amplifier which lack of FX loop.

Hence, it is possible to add the effects only in front of the amplifier. The chain would be: Guitar --> looper --> Reverb --> Amp

Because the looper is in front, it is important to match the impedance for the rest pedals:

From the looper I will have 330Ω of the output

HolyGrain which is stomp box has 680.000 Ω on input and 360Ω on the output RV 300 which is modulatory pedal has 250.000Ω on input and 1.000 Ω on output

What is the better option assuming that I would like to send to Ac4 a very undisturbed in terms of frequencies range signal and ii) not lose the amplitude of the signal because AC4 lack of the head-room ?

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    As a general rule of thumb, connecting low output impedance to high input impedance doesn't cause any problems. In any case the "mismatches" between 330Ω 680Ω and 1KΩ are not likely to be significant. Factors of 2 are important for speaker impedances, because the output impedance of a good transistor amp is 0.01 ohms or less, not 4 to 16 ohms, so "turning the amp up to 11" can easily overload the amp if the speaker impedance is too low. But "overloading" a line-level output with an impedance of a few hundred ohms won't "damage" anything, whatever you do - the power levels are too low. – user19146 Sep 1 '17 at 9:33
  • What about replacing speaker with more efficient one? – teodozjan Sep 1 '17 at 10:34
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    "Is it possible to connect high-impedance headphones into the 16 Ohm speaker output" That is not a good idea. On an "old-school" tube amp design, it may damage the amp simply because the output impedance is wrong (note, tube amps are different from transistor amps and effects boxes here), and if that doesn't happen, the amp can generate enough output to fry the headphones, or permanently damage your hearing. With phones the sound is generated right in your ear with close to 100% efficiency. A speaker cabinet is often less than 10% efficient, and several feet from your ears. – user19146 Sep 1 '17 at 11:19
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    If you're asking if you can plug the output of the reverb and looper into the guitar input on the amp, then yes that is ok, there is nothing wrong with that. You don't even have to think about the impedances, they are correct, they shouldn't match. Matched impedances is bad for pedals, good for amp to speaker connections. If you're asking if you can connect the amp speaker output to the input of the reverb or looper, you can't because that will damage the amp, pedal, or both. Both the impedances and the power levels are wildly wrong for that kind of hookup. – Todd Wilcox Sep 1 '17 at 14:56
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    @user3470313: you are getting confused here (or I am). YOur question is pretty straightforward, but then in a comment you mention hooking something up between the speaker and the amp. That is the Bad Thing Todd is speaking about. There is a lot of power there and things can break and even burn. Don't confuse speaker impedance matching with lowZ-hiZ input impedance matching. – Yorik Sep 1 '17 at 14:56
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Don't over-think this. Just hook them up and see what happens. Low output impedence to high input impedence is good. Inputs are about detecting voltage, not about transferring power. If there's a problem or it doesn't sound good, come back and ask again. But I don't think there will be. These boxes are INTENDED to be used in-line with guitar inputs.

  • This answer doesn't seem to clarify why the askers concerns are unfounded. It also could be misinterpreted to mean it's ok to connect the pedals in any way at all, such as to the speaker output of the amp. – Todd Wilcox Sep 1 '17 at 13:31
  • I repeat - "These boxes are INTENDED to be used in-line with guitar inputs." The concerns are unfounded because (a) LoZ out to HiZ in is good and (b) these boxes are designed to be used in-line with guitar inputs. – Laurence Payne Sep 1 '17 at 13:59

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