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When using a gutar amps FX loop to route the preamp through a pedal, is it any different from the internal circuit when the FX loop is bypassed?

Anything you put in the loop will affect the signal, even if it's just a cable. What I want to know (for some unknown reason) is if the output from the FX loop is any different to the internal output from the preamp to the power amp? Worded differently, does the power amp get the exact same signal as the fx loop send?

I've tried looking at a few schematics but i don't know the symbols n stuff Traynor mono bloc-b schematic page1 Traynor mono bloc-b schematic page2 Marshall ModeFour MF350 block diagram

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    I've always thought that send/return interrupts the signal path between pre and power amp. But, like you, I find it hard to read these schematics. Looking forward to an educated answer!
    – Tim
    Feb 9 at 11:37

2 Answers 2

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In the diagram Mono Block II there are no additional components between preamp and the power amp (besides a 100Ω resistor), while in the other one (what is it for?) there is a 14 dB attenuation on the FX send, compensated by 14 dB amplification on return. That should be transparent, but yeah, that's some extra components.

Whether the impact of these extra components is audible, you must check on your own. But if you overdrive something you plug into the loop with too high signal – that's likely audible!

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  • so the preamp needs a higher signal than your standard fx loop (actually i think this one is switchable between -10dBV and +4dBV). oh wait, is that the 14dB your talking about?
    – yarns
    Feb 10 at 9:00
  • @yarns Yes, I think so. I'm not sure how much "standards" there are though, I saw guitar effects clipping already with signal going from guitar with passive (but hot) pickups. So, YMMV Feb 11 at 4:22
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    To me the second diagram looks like the connection between preamp and power amp would be at +4dBV and there would be an option to attenuate the effects loop level by 14dB down to -10dBV, then boost it back.
    – ojs
    Feb 11 at 16:30
  • @ojs yeah the +14dB joined by the dotted line labelled 'level' is controlled by one switch. it allows you to switch between levels depending on what gear you plug in can handle.
    – yarns
    Feb 14 at 13:22
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does the power amp get the exact same signal as the fx loop send?

Depends on the exact amp but in general: yes, that's the design goal. In practice, there might be slight electrical differences: any external connector will have some extra protection. In the case of your mono-blocks that's a 100 Ohm resistor that protects the FX send against short circuit. Given that the expected input impedance of an effects processor would at least by 20,000 Ohm for a line input and much higher for an instrument input, that's negligible.

... even if it's just a cable.

Not really. Unless there is something seriously wrong with the cable or the connectors, this will not make an audible difference.

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