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Converting to/from line level signal for guitar effects? Answered

So I've got some effects pedals that don't get along with my Egnater Tweaker's effects loop, namely the HBE Frost Bite Flanger, but my Carbon Copy doesn't do too well either.  I've determined the problem to be the effects loop (which is at line level), and the previously mentioned pedals won't accept line level signals without a significant volume drop and high-end attenuation.

So my idea is to convert the line level signal being sent from the effects loop to instrument level, run it though my pedals, and then back up to line level and to the loop's return.  This would do the trick to get it down to the right level, but how do I get it back up to line level?  It's passive so I wouldn't think you could go from -10db to +4db, right? I'm also concerned about introducing noise and tone sucking.  Additionally, the send and return both have a different impedance, would I need to compensate for that?

Any ideas on a way to achieve this, or a better method that hasn't occurred to me?  Any help is appreciated.



Best Answer 7 years ago

Nice amp. But the specs for the send/return aren't exactly "line level."

Line-- in: 10K, out: 100
Tweaker-- in 220K, out: 600

guitar-- out: 3K to 10K
tube amp-- in: 100K to 1M+

The Tweaker is much closer to guitar-level signals than line levels. And those send/return values are within the ranges usually supplied by FX themselves (I.E., once the signal enters an "active" chain, it no longer has the high output impedance of a dry guitar signal. Generally--some old FX have high output Z).

The largest difference is the impedance of the Send loop: 600 ohms vs. 5K or so for pickups. However, a lower output impedance feeding the F/X should ADD high end, not remove it.

However 220K of input impedance on the Return is a little low, and might load the signal--somewhat.

Try adding a buffer after your FX (to lower the output impedance of your stompboxes) and see if the tone-sucking disappears. Something like a simple JFET or opamp buffer.

Perhaps I worded my question poorly - the problem stems not from a difference in impedance but rather from the pedals inability to handle the +4db signal from the amp. I believe instrument level is -10db, and so exceeding the input level that the pedal is designed to handle has some strange results.

I'm concerned about matching impedances only if I succeed in converting the signal from +4db to -10db and back, since I imagine most commercial devices are designed for applications with a mixing board or the like, and not for guitar related things.

Also, the effects loop is listed as buffered in the manual, although I'm not entirely sure how that affects things...

I'm kind of doing things the hard way since I could just get a distortion pedal and not use the effects loop... I just really like how the distortion from the amp itself sounds.

So I guess I'm wondering what the best way would be to drop the signal down to -10db and then back up to +4db with minimal signal degradation. Thanks for commenting!

"I believe instrument level is -10db "
-10dB, compared to what? -10dB is a relative, not an absolute measurement. Plenty of dB difference between a single coil pickup and an active humbucker.

"Also, the effects loop is listed as buffered in the manual, although I'm not entirely sure how that affects things..."
Yes, clearly those impedance levels are buffered (600 ohm Z for the send), but by nature impedances interact--that's why some pedals like buffers and some don't. And some pedals (like a fuzz face) sound best with the input being inductive (pickup) and not so great unless they are first in the FX chain.

You can get killer overdrive and tone with a clean boost up front and a decent tube amp--it doesn't have to go through the FX loop. Pushing the amp up front vs. via the return will be different of course...but one is not necessarily better, it depends on taste.  It depends on preamp vs. power amp distortion--there's no "right" way here, just preference.

The Tweaker manual outlines that the send/return levels should be controlled by the FX--so it's best to use a box that has input and output levels. That sounds more like a rack-mounted FX to me (like my old Digitech GSP5). Some distortion/boost pedals will have a gain adjust, of course.

So not all FX are appropriate for inserting into a send/return. Just because you have the loop doesn't mean it will replace the old setup completely. Some sound better up front.

If you still want to use your old stompboxes on the loop, the easiest way to drop signal is to insert a voltage divider--I.E., a POT to drop the signal strength. That's a very simple circuit, you certainly don't have to pay $80 for a fancy line level box--which is probably an impedance-converting transformer. You don't need that to simply drop the signal level, a POT or two resistors will do the trick.

If you need recovery and the pedal itself doesn't have enough gain, use a boost afterwards--a pedal with over-unity gain.

-10 dBu (should have gotten my units right in the first place), but I'm pretty sure that that's wrong. Instrument level should be much lower than that now that I think about it... Fairly sure that the effects loop is a +4 dBV though. So it's probably not easily feasible to go from line level to instrument level and back without introducing considerable amounts of noise.

I'm thinking at this point it's probably best to just buy a good quality distortion pedal and run everything up front.  Any pedal(s) in particular that you'd recommend?  And thanks for the input, you may have noticed I'm a little green on the technical side of audio.

I dunno--what's your taste for distortion?

I tend to like pedals like the "Bad Monkey", which is more of an overdrive. Or the "Dirty Little Secret" from Catalinbread.

I'm looking for more of a classic tube distortion sound, as tonally neutral as possible so that the EQ options on the Tweaker can still shine through (it does a pretty good job of emulating British, American, and Vox AC tones). Or will a distortion pedal just totally take over no matter that the amp's EQ is set to? Anyways I was looking at the Tonebone Classic, any opinion?

Nice thing about playing through a good tube amp--you've already got "classic tube distortion", even somewhat when the amp is clean!

Both the pedals I mentioned are more overdrives than fuzzes; they each are designed to push tube amps hard enough to clip themselves (rather than the pedals themselves having much tonality). The Bad Monkey especially is similar to a Tube Screamer.

Egnator makes great-sounding amps--I'm sure you can emulate a Fender or a Vox sound pretty easily with the on-board EQ. As far as other sounds (like Mesa/boogie), I don't really care for that, but with the right stompbox it can be done.

Unless you've got unlimited $$$, you might demo pedals at the music store you find what you want...

Sadly, the music shops in my area don't have a very good selection, I've tried that in the past. I think I've found what I'm looking for though, the Seymour Duncan SFX-03 has got some pretty delicious distortion, not too much in the EQ department but I can always use the amp for that. Thanks for you help.