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figuredbass
Registered User Join Date: Jul 2007
Location: NYC vicinity
Quote:
Originally Posted by KJung
I always understood that the output of the LMII DI was very weak, so interesting.
I don't know a lot about DI's at all. So, I assume that the vast majority of on-board DI's are mic level, and that the line level of the LMII is unusual? Is the SD800/450, etc. also line level? I have never had a sound engineer comment regarding a super hot signal from the SD800 DI (I just keep the DI volume at noon).
Just curious as to whether a 'line level XLR' is unusual or not.
I've finally had a chance to measure the levels of the LMII's signal outputs. It turns out that the DI output is 4 dB higher than the signal going into the input jack just at the point where the clip light comes on.
What this means in the real world is that the DI was designed to output at approximately the same level as the original instrument signal going into the amp, as the optimal preamp level would be a few dB below the clip indicator point. (BTW, the clip indicator is set at a "3dB warning" point, meaning that if you get only 3dB louder than the clip light point you'll hear the actual clipping begin. This is why you don't want to see the light while playing.)
A DI at instrument level is considerably hotter than a standard XLR mic level, but nowhere near as hot as what we call a "line" level signal (or "+4"). It's at instrument level, roughly halfway between mic and line level. To answer your question, a true "line level" DI level XLR would NOT be usual. The LMII's effect send is at true line level, which I measured to be 16.5 dB above the level going into the input jack. This high level signal is to make it compatible with line level "19" rack" outboard gear. Interestingly, the effect send output can actually be used as an auxillary "pre EQ" only DI, only if sent to a PA board's 1/4" line input instead of an XLR mic pre.
The main risk is the usual potential hum problems experienced when connecting unbalanced equipment. From what I've measured, this type of connection will not run as great a risk of bass rolloff as the LMII's XLR out.
Quote:
Originally Posted by KJung
Also, I assume the input PAD on a high quality board cannot adjust for this?
K
Quite right! This hotter DI signal can actually be a good thing in most cases because the signal to noise ratio is improved, but a PA board's channel input dialed in to accept a low mic signal could easily distort from the LMII DI output! This is NOT the fault of the LMII, but usually the soundman at the PA board who doesn't (or doesn't know to) pad or trim down that channel's input level. I believe this may be why the OP hoosierdad got distortion and his sound man asked, "Is it supposed to sound like there is a distortion pedal on the bass?" Some soundmen don't even seem to be aware that the trim pot even exists, thinking (or should I say NOT thinking) everything can be handled exclusively by the channel fader. This is also why I love bad soundmen.
Ideally it's preferable to design a bass amp with a level control as in i.e. the SA450 or SD800, but most soundboards AND soundmen SHOULD be capable of doing it at the board via the trim/pad pot!
Sorry this is so long-winded but I hope it helps.
figuredbass
Registered User
Originally Posted by KJung
Another wonderful post! Thanks. Makes total sense.
OK, since you are so in the know on this stuff, there seems to definitely be something a little funny with the impedance of the LMII DI versus other heads.
Do you have any theory as to why they would 'set' the impedance at the level they did versus other heads' DI's which don't seem to ever have any issue into any board. Is it a cost saving measure, or does it have some other advantages and just the downside some have found (low end roll-off into, if I remember correctly, very low impedance board inputs).
Thanks!
K
It could very well be partly due to cost. I believe it could also have been an unfortunate oversight. The output impedance of the LMII DI per se is not really the main problem. It's actually the electronic value of the DI output capacitors, which are way too small IMO. (The reason they have to even be there in the first place is to protect the amp from damage if the PA's 48 volt phantom power gets turned on.) These output "caps" have to be fairly large in value and voltage rating in order to pass full low end when loaded by a low impedance PA board input. This makes them physically much larger and more costly, and there is additional labor mounting them as they cannot be put on the surface-mount PC board during the automated process (I think). But the cost should certainly not be prohibitive!
If you read the DI output on test equipment (which I did) it reads A-OK. But this is because the test equipment doesn't load the signal down like a real PA board can. The low frequencies a capacitor can pass are totally dependent upon the impedance of the circuit, in this case the "DI output/PA input loading" circuit. I have had excellent DI out's in much cheaper amps, and we haven't really seen this issue pop up with posters owning other amps. It can be done right, and fairly cheaply. Given the LMII's excellent sound, design, and build, I again feel the DI issue may just be an unfortunate "Oops!" I would bet that future Markbass designs not using the same preamp PC board will have corrected this problem.
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