Effects loop levels

[mjurban]

Hi folks -

At what voltage and impedance levels do guitar pedals typically operate? If I'm adding an effects loop to an amp, what voltage levels do I need to provide at the "send" jack, and what level should I be ready to accept at the "receive" end? My understanding of the typical guitar output is a high impedance signal of about 100 - 500 mV, and up to 2V for a real thrasher. Is that what I need to work with in an effects loop?

Thanks, Mike

 

[jjman]

Those voltage figures look good. You want the impedance of the "send" to be "low" if possible. The normal goal of a loop is to allow for potential preamp overdrive before the loop. The signal coming out the "send" needs to be bought back down to guitar level one way or another.

 

[dotneck335]

Peavey nicely publishes their "nominal" levels as:
Input--- 40 mV rms
Patch--- 200 mV
Pre out-Power In--- 1 volt rms

 

[thoglette]

A.k.a line level(wikipedia) or 0dbV

Normally a 4db peak is allowed for.

600ohm is the nominal line impedance. But 10K load with 100ohm source is also pretty typical (as we're not really driving lines)

This is what NE5532s were designed for. But note that they're still fed with ±18 V supply rails for this application.

 

[mjurban]

Dotneck - thanks, I think the patch level is what I’m looking for.

Thoglette - I’d be surprised to find many pedals (or amps) working at 600 ohms impedance levels. If they are designed to accept a guitar input, I would expect an input impedance on the order of a megohm, or more.

Thanks both,
Mike

 

[thoglette]

Thoglette - I’d be surprised to find many pedals (or amps) working at 600 ohms impedance levels. If they are designed to accept a guitar input, I would expect an input impedance on the order of a megohm, or more.

Agreed, and that's where this thread is a little confused: are we talking about a line level effects loop or are we talking about pedals?

I have seen send/return loops that have a guitar/line level selection switch. Don't ask me where.

 

[mjurban]

Agreed, and that's where this thread is a little confused: are we talking about a line level effects loop or are we talking about pedals?

I think you’ve nailed the source of my confusion. My buddy wants an effects loop, which I intuitively think should run at line level, but he wants use pedals which he is now plugging a guitar into directly.

As I think about it, I’m convincing myself the problem isn’t as hard as I thought. If a guitar pickup can produce 100 - 500 mV, that’s right in the range of consumer audio line level, which I understand to be -10dBV, somewhere near 300mV. If true, I’m home free.

OTOH, if Peavy, via Dotneck, is correct and 40mV is more typical, then I need 15 - 20 dB more gain.

Either way, I think I have my answer. From mV to V is 60 dB dynamic range. The answer looks like lots of headroom, graceful clipping, and perhaps a gain control.

Thanks, Mike

 

[Gnobuddy]

I have seen send/return loops that have a guitar/line level selection switch. Don't ask me where.

My little Mackie mixer has a switch that lets you set the output level to either "instrument level" or line level. The little ART Tube MP/C preamp/compressors I use have a similar switch.

My general impression is that, at least with the equipment I use, "instrument level" is typically around 100 mV, while "line level" on pro-audio equipment is usually around a volt, give or take a couple of dB. That may be nominally 775 mV (0 dBu), nominally 1 volt (0 dBv), or nominally 1.23 volts (+4 dBu). All three are close enough to 1 volt for all practical purposes.

All the line levels may be equal for all intents and purposes, but there is roughly a 20 dB gap between instrument level and line level, and that is sufficiently large that, in my experience, they are not interchangeable. I find I need to use an external preamp if I want to run an instrument-level signal into one of the line-level inputs on my mixer.

As for the (-10 dBu) consumer line level, I've never encountered that in any electronics designed to work with guitar, microphones, keyboards, bass, etc. Usually I only find -10 dBu levels on RCA connectors - the sort you find on the back of your 1985 Hi-Fi stereo receiver, the one collecting dust in the attic because you only listen to music on earbuds now.

No self-respecting instrumentalist or vocalist has any music performance gear that uses RCA plugs. If your buddy does, tell him he's sentenced to sit in the corner with a bowed head, and must watch the inimitable Vinh Bui perform "Imagine" five times in a row: YouTube

Back to the FX loop, if you really want to be able to operate at both line level and "instrument level", my suggestion would be to include a 20 dB switchable pad on both send and return jacks. The send level would be nominally a volt, switched down to 100 mV by the 20 dB pad. The receive level would be nominally 100 mV, switchable to 1 V by its 20 dB pad.

Even with switchable 20 dB pads, I suggest you still include a level pot on the send, and a gain pot on the receive, to allow the circuit to cope gracefully with unruly signals that are neither proper instrument level, nor proper line level, signals.

(I lied about the earbuds, by the way. Personally, I loathe earbuds, and still use an old stereo receiver and a pair of decent studio monitor loudspeakers for all home music and TV listening. But that makes me a member of a vanishingly small minority these days.)

-Gnobuddy