Musical instrument amplifiers often have an "FX-loop" with a pair of jacks which can be used to interrupt the default connection between preamp & power amp, to put the preamplified signal ("send") through an external effect (-chain), and feed its output signal into the power amplifier ("return"). Is there a (perhaps "unofficial") standard for the voltage levels one has to expect at the "send" output / is expected to put into the "return" input, and the impedances, or are there some ball park ranges that a majority of manufacturers are adhering to?

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    \$\begingroup\$ Despite the question I linked as a duplicate having been closed, please see the answers section as I believe that should help answer your question. Also see electronics.stackexchange.com/a/594177/2028 \$\endgroup\$
    – JYelton
    May 27, 2022 at 16:02

2 Answers 2


FX-loop ... Is there a (perhaps "unofficial") standard for the voltage levels one has to expect at the "send" output / is expected to put into the "return" input

You should check your equipment manuals but the general expectation is that these connections would be at line level. JYelton has unearthed a useful answer with a table listing a glorious selection of line levels. However for reasonable quality mixing/PA equipment I'd hope to see the nominal line level at the pro level of +4 dBu/1.23V.

and the impedances

In days of yore when we actually sent audio signals down telephone lines (hence the name "line level") the source impedance, line impedance and load impedance were all 600 Ω. These days 600 Ω is just a nominal (indeed, very nominal) impedance figure.

More modern equipment uses impedance or voltage bridging. The source impedance will be less than 600 Ω, and usually no more than 200 Ω. The load impedance will be greater than 600 Ω, typically 10 kΩ or more. We still call this a 600 Ω connection though, because of the history.

The above answer relates to FX outputs and inputs on a mixer, or the connection between a preamp and power amp, as requested. However your title mentions electric and bass guitars. Note that the voltage and impedance levels of guitar effects pedals is a different can of worms.


FX loops are usually unity gain but they buffer the voltage to low impedance, which helps in some cases where stray noise is high.

But For 60 dB SNR considered adequate for most except studio recordings the signal level to line level must be 60dB higher than the noise at each and every stage if you consider 1mV as the smallest signal then noise must be 1uV by having good balance and low impedance <=600 ohms over the audio range. the load impedance only needs to be 100x or more for 1% loss unless matched impedance for 50% loss with a balun for better CM rejection of E-field hum pickup or humbucker.

  • shielded twisted pairs and a differential preamp will improve the noise reduction caused by SMPS leaking common mode hum into the pre-amp.
  • Ground loops can induce noise as well as the power supply when floating via leakage effects so proximity to ground and power lines can have a good or bad effect depending on ground noise or line noise E-fields nearby.

Remember ground is supposed to mean 0V and even 0uV so a human touch the strings acting as antenna can affect the leakage currents and common mode and differential mode hum according to the preamps ability to reject CM noise. This is why an INA or instrument amp and good cable for balanced wire impedance helps greatly to get noise down to the uV levels at source if the signal is only 1mV. Yet many instruments can play loudly at 50mV or more so they have attenuators to regulate the levels.

OVerall gains from source to line levels vary from 100 to 1k If there is an adjustment, otherwise they may be fixed somewhere inbetween.


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