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dBu, DBm, power amps...What's going on?

Started by GLUGSTER, January 21, 2009, 07:27:30 AM

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GLUGSTER

I am at a loss. What is the deal with Zoom? All the other processors' output are rated at dBu. Zoom's: DBm. What is the difference? I tried to research the relationship on Wiki and I found it really difficult to understand. I did get this though:




I just don't understand, what does it mean?....Why am I asking this question? what does this have to do with MFX? Simple. DBm doesn't interact with a power amp properly. The experiment I did was to plug both sides of a a QSC into stereo sides of a 4-12. So here is how I did it:

guitar 1=>G7.1ut=>BBE 482 ch. 1=>QSC left side=>left side of a Carvin cab
guitar 2=>RP350=>BBE 482 Ch. 2=>QSC Right side=>right side same cab

I played through the Zoom, my buddy played though the RP350. I was peaking out the BBE and the tone was really bad, plus the output was pretty low. My buddy playing though the RP350 however WAS NOT peaking the BBE, and he was VERY LOUD and clear. So does the Zoom's DBm leveled outputs have the wrong impedance or is something else going on?

J M Fahey

Excuse me Glugster but your explanation is somewhat confusing, at least to me (sorry).
I guess what you say, but I shouldn´t be guessing, just concentrating in your problem.
You speak about a Zoom , but you don´t name it in your signal chain. (A bit like starting to tell a story about "Joe" and others, later talking about "my brother" and later saying something about "the skinny one") are rthey the same or not?, confusing.
As far as I understand, you have two multi-effects, an RP350 and a "Zoom", although you talk about a G7.1ut
You also talk about a BBE 482 ¿Why?
And you are two different people, presumably with different guitars.
The basic point to get a valid comparison is to reduce the "extra variables" to a minimum.
For your comparison to be valid, you should plug your guitar, with an Y cable or a splitter box (very easy to improvise) into the Zoom and the RP350, and rig a switch (footswitch or friend/buddy/girlfriend operated) to be able to send either of your signals, switching quickly, to the same amp/cab or even a combo.
The human ear has *very* short term memory, so switching quickly back and forth, even within the same chord, ensures the best comparison.
Please try something like that, you´ll be surprised at the incredibly small nuances you'll be able to detect.
Cheers.

GLUGSTER

Quote from: J M Fahey on February 11, 2009, 12:32:14 PM
Excuse me Glugster but your explanation is somewhat confusing, at least to me (sorry).
I guess what you say, but I shouldn´t be guessing, just concentrating in your problem.
You speak about a Zoom , but you don´t name it in your signal chain. (A bit like starting to tell a story about "Joe" and others, later talking about "my brother" and later saying something about "the skinny one") are rthey the same or not?, confusing.
As far as I understand, you have two multi-effects, an RP350 and a "Zoom", although you talk about a G7.1ut
You also talk about a BBE 482 ¿Why?
And you are two different people, presumably with different guitars.
The basic point to get a valid comparison is to reduce the "extra variables" to a minimum.
For your comparison to be valid, you should plug your guitar, with an Y cable or a splitter box (very easy to improvise) into the Zoom and the RP350, and rig a switch (footswitch or friend/buddy/girlfriend operated) to be able to send either of your signals, switching quickly, to the same amp/cab or even a combo.
The human ear has *very* short term memory, so switching quickly back and forth, even within the same chord, ensures the best comparison.
Please try something like that, you´ll be surprised at the incredibly small nuances you'll be able to detect.
Cheers.

See, I think that my explanation is VERY clear. To answer another one of your questions, the BBE was used for level monitoring as I explained. What I tried to do with my experiment was to show that the output of the Zoom G7.1ut was different or incompatible with the other pieces of gear. As far as all the other scientific testing (swapping cords, guitars etc.) from one side of the chain to the other, I did all of that as well. We aren't talking about some *subtle* little problem here. The Zoom has an impedance mismatch that make the other pieces incompatible. I figured this problem out though, by using a Behringer G100 DI box. The DI buffers the signal, and changes the Zoom G7.1ut's output making it useful again. I hope that clears things up for you and thanks for responding.