Hi! I salute you from Argentina (so, pardon for my english). First advice: I'm not an electronics connaiseur. Second advice: I already googled & read the unit's manual. So, here we go. I recently bought an old Yamaha SPX90 rack unit. I never owned a rack unit before. I play a Stratocaster guitar with some distortion pedals & i want to "properly" add the rack to my ""gear"". The SPX90 has a -20dB / +4dB level switch for the input jack and another one for the output, as you can see in the unit's manual (http://www.yamaha.com/yamahavgn/Documents/ProAudio/SPX90.pdf ). (I thought that the normal thing in audio gear was -10dB / +4dB for unbalanced/balanced line level & that -20dB was like an unbalanced instrument level, but i might be wrong.) My amp has no loop effects send/return on it -it's a normal practice crappy amp (but i'm looking forward to buy another amp with loop fx anytime soon). So, i have three simple and specific questions:

1. if I select -20dB (which is a normal guitar/normal guitar pedal output level?) in the input level switch of the SPX and -20dB (which is a normal amp/a normal guitar pedal input level?) in the output one's, could I use my rack unit "as a pedal" and connect it from or to any normal guitar/pedal output or normal pedal/amp input without having impedance/level issues? Or it would still be better if I use the unit in an effects loop amp's chain?
2. If I use the SPX in the effect loop chain of an amp, which level should i choose for the input and for the output? -20dB or +4dB? Doesn't an fx loop send/return of an amp work at -10dB line level?
3. In which case could i need to use a diferent switch level for the input than for the output? (Has the Yamaha some kind of DI box incorporated?)

Thank you all so much for your collaboration!! As you can see I'm kind of confused
Last edited by jimbomonjeau at May 14, 2014,
1. that switches are the input and output gains.
It's as simple as that.
The impedance has nothing to do with it.
Start from -20db both in the input and output, try the thing and see how it sounds.
Then adjust both of the switches according to your personal necessity/taste.
You'll not gonna break anything anyway, don't worry.

2. same thing as before.

3. same thing as before.

As for the DI box...
A DI box is a device that brings down the impedance of the signal coming from your guitar from the unreasonably high level of around 500kOhm (that's a generalization, passive pickups can go from lower to way higher output impedances) to line level - around 10kOhm.

Your reverb processor wasn't desined to be used with guitars but since you have one just plug the last of your pedals into it and flip the switches to make it sound good.
I reckon +4db for the input will be the best setting.
Name's Luca.

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