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Topic: Line Level vs. Pre-amp Level Signals? |
Johnny Thomasson
From:
Texas, USA
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Posted 6 Nov 2011 2:51 pm |
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I know there is a voltage difference between the two; one is intended for the input of a guitar amp and the other for the input of a mixing console. But I'm not sure which is which. Here's what happened. This morning in church I used my Digitek Studio 5000 to go direct to the PA for the first time. When I was gigging I always used it in front of my amp with no problems. But this morning when I used it to go straight to the PA, I got a really annoying hum. I switched back to my LR Baggs Studio Pre with an RV-5 looped in, which of course eliminated the problem. The mixing console did not like my Studio 5000! (I'm using this for fiddle, not steel.)
Now the question. The Studio 5000 produces some really nice, creamy tone shaping capabilities, not to mention tons of available effects (including harmonies) and the capability to write custom programs for different songs. I can even make my fiddle sound like a cello. Is there any way to modulate the voltage output so I can go direct to the mixing console without the hum? Maybe some gizmo I can patch between the processor and mixer to make them play nice together? Does anyone make something like that? I suppose I could use an amp and mic it, but I really hate to pack the extra gear.
Any advice from any of you techies would be muchly appreciated.
Thanks,
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Johnny Thomasson |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 6 Nov 2011 4:44 pm
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Otay, Johnny, I will take a shot at it for you...
Most microphones generate something like -50dB into 150 ohms (referenced to 0.775 VDC = 0 dB), generally referred to as a "Mic Level" and almost always is output as a balanced signal to a 3-pin XLR conncctor, with pins 2 and 3 signal 180 degrees out of phase, or exactly opposite polarity. Pin 1 is connected to ground to ensure adequate shielding against ambient electro-magnetic interference.
The output of most guitar pickups is in this same -70dB to -50dB range, but nearly all of these are very-high-impedance devices, designed to achieve best tone and level when facing 450KOhms or more, and the signal is output to a 1/4" unbalanced jack, which means that the tip of the cord plugged into it carries the entire signal and the sleeve is connected to ground, providing BOTH the return component of the pickup's electrical path AND the ground for shielding.
Consumer stereo gear is designed around a nominal -10dB level intended to face 10K Ohms or more and utilizing unbalanced RCA connectors, with the same wiring conventions as guitar gear above.
Professional line-level is a nominal +4dB at 600 ohms and is almost always output via our old friend the 3-pin XLR, wired in the same balanced manner as pro microphones discussed above. Some inexpensive gear uses 1/4" TRS (tip, ring, sleeve) connectors rather than XLR's, with the tip = pin 2, ring = pin 3 and sleeve = pin 1, otherewise identical to the XLR configuration.
Because an XLR may be mic or line level and a 1/4" jack may be not only mic or line level but also balanced or unbalanced, lots of issues can arise if you are not conscious of what is going on electrically.
The 1/4" jacks of a guitar amp are intended to see a low-level, very-high-impedance (~500KOhms), unbalanced signal. If the level is too high it will distort, if the impedance too low frequency response will be haywire with a tendency for uncontrollable feedback. If you plug a balanced 1/4" TRS signal into it, even if the signal itself is not balanced, the ring on one end or the other may short to ground, and this partial ground or compromised signal path can cause loss of signal or, yes, lots of 60Hz HUMMMM.
The XLR jacks of your mixer are designed to see a low-level, low-impedance, balanced signal. Again, signal too hot will distort badly, impedance too high frequency response is shot and level is down, and if you somehow get pin 2 or 3 connected to ground things can get really unpredictable, depending on the design of the mic pre's.
If your mixer has 1/4" line inputs these will most likely accept unbalanced or balanced signals (read your manual to find out more) but they must be at line level as this buss has no preamp.
SO, now to your specific issues with using the Digitach Studio 5000 with the mixer at your church. The 5000's manual does not mention balanced I/O so we will assume that it is not. It lists the maximum level in or out at +16dB, which places it squarely in the pro line-level realm, but with unbalanced wiring protocol and 1/4" jacks.
If you are using 1/4" unbalanced cables of 15' or less in length you should be able to connect directly to any 1/4" line level input on the mixer. IF you get a 60 Hz hum when connected thusly it is not a level or impedance mismatch but a bad cable, or ground loop, we will get to in a momeent.
If the mixer does not have 1/4" line inputs OR is at the other end of a long signal snake you will need a decent direct box to convert your unbalanced line level to a balanced mic level which you can then run to any mic input. Using 1/4" TRS cables, a simple 1/4" to XLR adapter, or any other variant of this specific hookup will not work properly.
IF you are already hooked up according to this protocol and you are getting a 60 Hz hum then the issue has to do with the input facing multiple paths to ground, for instance your rig is plugged into a wall socket on stage and the mixer is plugged in at the back of the room and there is no ground lift between them. In this case you must use the ground lift switch on your DI to interrupt one of these ground paths while preserving the shielding properties of the cable.
Again, if the distance between your rig and the mixer is more than 15' or so forget about trying to use the 1/4" inputs as the grounding and shielding WILL be compromised no matter what else you do.
I was unable to find a manual online for the Baggs Studio Pre, but with a name like that I suspect that it may have balanced XLR output jacks and/or a ground lift incorporated into the design, which would explain why it is quiet when the 5000 yields ground noise. |
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Clete Ritta
From:
San Antonio, Texas
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Posted 7 Nov 2011 12:17 am Re: Line Level vs. Pre-amp Level Signals?
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| Johnny Thomasson wrote: |
| ...I used my Digitek Studio 5000 to go direct to the PA...I got a really annoying hum. I switched back to my LR Baggs Studio Pre with an RV-5 looped in, which of course eliminated the problem... |
Have you tried looping the 5000 instead of the RV-5 with the LR Baggs to the PA? Is that possible? Im not familiar with the units, nor am I an expert, just a thought.
Clete |
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Johnny Thomasson
From:
Texas, USA
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Posted 7 Nov 2011 11:09 am
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Dave, thanks for such a detailed response. I can see now that I did not provide adequate information for the problem to be diagnosed.
The LR Baggs Studio pre does have a balanced XLR output. My signal chain went like this: LR Baggs bridge pickup (piezo) to 1/4" unbalanced input to Studio 5000; Studio 5000 1/4" unbalanced output to LR Baggs Studio pre 1/4" unbalanced input; LR Baggs Studio pre balanced XLR output to mixing console balanced XLR input across a 100' snake. Sorry for leaving out such pertinent information.
Clete, that is definitely something I can and will try. I just didn't think of it at the time. The thing that has me puzzled about this is that I can go straight through the Studio 5000 into a NV400 without the hum. That's what leads me to believe I have an impedance mismatch somewhere. The LR Baggs Studio pre has a one-plug TRS effects loop, and I can patch in my RV-5 and go to the board via the XLR output with no problem. I'll try it with the Studio 5000 and see what happens.
Thanks for the help guys.
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Johnny Thomasson |
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Charlie Thompson
From:
South Carolina, USA
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Posted 7 Nov 2011 11:20 am
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| Dave thanks for taking the time to provide all of that information...very valuable! |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 7 Nov 2011 11:58 am
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The problem that I see with your hookup is that you are putting the 5000 between the pickup and its preamp, since the 5000 wants to see a line level input which the piezo pickup cannot supply and the Baggs pre wants to see a very low level input which the 5000 cannot produce. As you have surmised, inserting the 5000 into the effects loop of the Baggs should solve your problem.
Keep us posted on your progress. |
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Johnny Thomasson
From:
Texas, USA
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Posted 7 Nov 2011 6:58 pm
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Dave, thanks again for taking the time to provide so much detailed information. I'm sure others will benefit from it just as I did. Tonight I looped the 5000 into the Baggs pre, and ran its XLR out to a Mackie recording mixer, and listened through headphones. Not surprisingly, you were exactly right... DEAD quiet. Success!
Guys, if Dave tells you something about this gizmo stuff, take it to the bank. He knows what he's talking about. Thanks again Dave.
Clete, maybe you're not an expert, but you sure made a dang good guess! My thanks to you too. Now I can use my Studio 5000 at church.
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Johnny Thomasson |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 7 Nov 2011 8:38 pm
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| I'm glad it worked out, it's all those years playing with electric trains I guess... |
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Clete Ritta
From:
San Antonio, Texas
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Posted 8 Nov 2011 5:07 am
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Nice to hear its working fine for you now with nothing extra required. Sounds like a nice rig you've got to play through! Id like to hear that 5000. I have some LR Baggs and Digitech gear as well. A couple Ovation acoustics, a 6 string and a 12 string, I had stock electronics replaced with LR Baggs active pickups with a noticeable improvement straight to the PA with no effects. I also have a really early Digitech DSP 128 along with an old Yamaha SPX 90 that I probably used for decades. Its still in a rack out in the garage somewhere. Every once in a while I miss the speed and flexibility of digital presets and one neat midi foot control (I had a Korg A2 for a while too). Im using slightly more modern software based recording at home now, but for playing electric guitar live, I have gone back to an old-school board of MXR, Boss, Fulltone and Dunlop pedals again. 
Clete |
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Johnny Thomasson
From:
Texas, USA
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Posted 8 Nov 2011 10:21 am
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| Dave Grafe wrote: |
| I'm glad it worked out, it's all those years playing with electric trains I guess... |
Lucky for me... 
Clete, it is a nice rig. Over the years I've gone back and forth from using my rack unit to traveling light, just using a pedal or two, like a lot of people I guess. I've been "traveling light" for the past 2-3 years, then got to thinking about all the cool stuff the rack system will do. The Studio 5000 is a really slick unit, but Digitek didn't make them for very long. Mine is about 15 years old. They have tons of capability, but were just too expensive for most working musicians. I paid about $1500 for mine new, way back then. I saw one for sale on the Forum about a year ago; I almost bought it for a spare.
Along with the Studio 5000, I have a power conditioner, a BBE Sonic Maximizer and a UHF wireless receiver in my rack. It's a fairly big chunk of signal conditioning gear to haul around, but it packs a lot of punch. There's also something to be said for throwing the Baggs pre and an RV-5 in a bag and calling it good. That's what I've been doing for the past few years, and I'm kinda excited about using the big rig again. I really like LR Baggs stuff, too. I have two of their Para Acoustic DI Studio preamps and carry two fiddles fitted with their bridge pickups. That comes closer to a "natural" amplified fiddle sound than anything I've tried in the past 35 years. After struggling with tone for years (decades) I'm pretty happy with the fiddle rig I have now.
For steel, my Rains, a Goodrich pot pedal, an RV-3 and a NV400 seems to get the job done quite well. Oh yeah, and a BJS bar...
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Johnny Thomasson |
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