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XLR Output Quieter Than Line Out?

Discussion in 'Amps and Cabs [BG]' started by bgavin, Jan 4, 2002.


  1. I normally run my Alembic F-1X using the XLR out (post EQ) into a Rane line level crossover and EQ, then into the power amp. Everything is balanced XLR and hum free. I have to crank all the Rane gear up wide open, and the F-1X at 7 to get any real volume out of the rig. Granted my subs are inefficient, but... Running the F-1X volume above 7 causes preamp tube farting something awful.

    Last night, I built and tested an unbalanced 1/4" to XLR cable and drove the Rane signal chain using the F-1X Full Range 1/4" line out jack. I was able to get stage volume on the F-1X at 3 ~ 4 on the volume control and without farting.

    Q: Has anybody noticed if their XLR Out is markedly quieter than using the standard 1/4" jack for Line Out?
     
  2. Steve

    Steve

    Aug 10, 2001
    When I first got my SWR/Crown set up, I ran unbalanced for a while. Too lazy to make cords.
    When I finaly went to balanced, it TOTALLY changed the gain structure of the rig. It was way louder. I've never tried to go "Cross Platform" unbalanced out to balanced in. Sounds like you have an I/O impedence/ sensitivity issue working in your favor. I'm sure an electronics guru could explain it.
     
  3. The loudness issue is clouded by my use of inefficient subwoofers. I attributed the lack of volume to them, but now I'm not so sure. What was sticking in my mind was the lack of lighting either the clipping or overdriven LEDs on my Rane gear or the QSC power amp.

    I suppose the proper method would be to create a pair of dummy load cables using the correct resistance for both balanced and unbalanced, and put a DVOM across them while feeding pink noise into the F-1X.

    It's easier to just leave the rig the way it is now, put up with the little bit of hum and enjoy the extra gain.

    :D
     
  4. XLR is always quiet-er (?) than normal line-out..

    the XLR has 2 signals, one normal and one that is the exact opposite of the normal signal.

    then the recording-unit/whatever substracts the signals from eachother. 1 - (-1) = 1 + 1 = 2

    but because the static, etc is equal on both channels is will be : 1 - 1 = 0

    the static/interferrence (sp?) is canceled out.

    because of this phase-thingie ( balanced output ), it has about 1/2nd the volume of a normal line-out.

    :)
     
  5. I suspect that the reason your XLR output is quieter than the 1/4" is because the XLR is not a "line level" output. The XLR is primarily intended to send signal to a mixing console. Most mixers (even high end ones) don't like line level signals on their XLR inputs. A hot line level signal will overload most inputs even with a pad. The XLR output is probably buffered so that it's output isn't so hot, to make it more compatible with mixer inputs. The 1/4" fullrange out is designed to drive power amps which need line level signals. If you take the 1/4" out and run it into a DI, the effect is basically the same. The signal is buffered and transformed into a mic level balanced signal.

    This has zip to do with whether the outputs are balanced or unbalanced. It's just the nature of the two outputs. Actually if you looked at two line level outputs, one balanced and one unbalanced; the balanced one will be hotter (by 6 dB). What AllodoX posted is only partially right. What happens in a balanced system is that a copy of the output signal from the a device is created by a balancing transformer or active circuit. This signal is 180 degrees out of phase with the original signal . So the two signals are out of phase during transmission but when they reach the input of the next device, the out of phase signal is flipped back in phase with the other one via transformer or active circuit. The two signals are then added to each other which gives double the voltage than if there was only one signal. Any noise picked up in transmission is picked up equally in both signal carrying conductors if the cable is decent. (This is why good mic cable has the conductor wires tightly twisted: so both conductors will pick up equal amounts of noise.). The beauty of balanced is that when the signals are flipped back in phase, the noise is flipped out of phase with its copy and cancelled out. The effectiveness of this is dependent on the quality of the cable and on the common mode rejection ratio (CMRR) of the downstream device. If you connect a balanced output to a balanced input with an unbalanced cable you will lose about 6dB of gain and be left open to severe noise problems, because any noise picked up in transmission is amplified along with the signal. In a balanced system you have the ability to lift the grounds between devices (i.e. not have the shield in the cable connected at one end) because the signal is not referenced to ground (shield) but to its copy. This is very important in eliminating ground loops between devices. This has turned into quite a rant, sorry for veering off topic...;).
     
  6. I understand the concept of differentially balanced lines. During my IBM career, we used differential SCSI control lines for noise rejection over long cable lengths in our industrial laser printers.

    You confirmed what I suspected about the XLR/DI outputs of bass preamps not being true Line Out levels, but a bastardized version intended strictly for mixing boards. There is a noticeable increase in loudness when I use the 1/4" Unbalanced Out on my F-1X.

    I'm running a 12-inch 1/4" TS to XLR cable that I built to Rane's specifications for matching an unbalanced output to a Rane balanced input. The hum increased a bit, but cannot be heard in a live environment.

    This leads to the next question: since the XLR on bass preamps is designed for the board, what impact occurs when using Direct Out for the amp, and XLR Out for the sound system? Any drop in loudness, or are these two isolated circuits?
     
  7. Glad to have been helpful!
    I would guess that they are isolated and have no effect on each other. The XLR output is transformer balanced (also helpful in noise reduction) so it's physically decoupled from the other output for sure. I'd suspect that the 1/4 out is driven seperately (maybe even from a different point in the circuit) but I'm not 100% sure. From what I've read on the Alembic website they seem to be independent. I'd have to see the schematic to be sure. That being said, it would be pretty stupid of them to design a preamp where the loading of one output affected the other in any meaningful way. I don't understand why the manufacturers don't used balanced outputs to drive power amps. It would make sense because 90% of amps have balanced inputs. It would certainly reduce noise.
    I kind of figured that you understood the balancing thing given your background, the main reason for the rant was to correct some previously posted misinformation;).
     
  8. Good point on many preamps not offering a true Balanced Out. All the major power amps use balanced lines, and a preamp requires a power amp... A 1/4" TRS jack would not consume much real estate on the rear of a preamp, and would still leave the bastardized XLR for connecting to a mixer.

    According to the schematic, the SWR Interstellar Overdrive offers both an XLR DI out plus an XLR Balanced Out. There is a Pad level control on the DI Out connector to control the loudness going to the mixer. The DI Out is not affected by either the tone controls or the volume control.

    The XLR Balanced Out is entirely controlled by the front panel volume and tone controls. It has a maximum output of +24dB according to the manual.

    Damn, but I've got fierce GAS for an IOD...
     
  9. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    Is there really a noise problem to begin with? Bass preamps are normally situated in a rack with the power amp, and the patch cord only needs to be about a foot long, not 50 or 100 or 200 feet as you'd find in a PA system. Maybe the preamp designers realised this and omitted balanced outs to keep the cost down?

    This is an interesting thread! It's becoming clear to me that "XLR" on a bass preamp has nothing in common with "XLR" on a power amp (or the Rane) other than having the same connector. The signal levels and impedances are not the same. Is that what you're all saying?

    On my Crown there are 1/4" TRS and XLR input jacks, and both inputs have the same sensistivity and both are balanced inputs. But on my Alembic, the XLR and unbalanced 1/4" out have different output levels (which is what Bruce has discovered).
     
  10. Steve

    Steve

    Aug 10, 2001
    FWIW, on the back of my Grand Prix ar two xlr outs.
    One is fixed, post effects for driving a power amp. This one is markedly louder that using the unbalanced out. The other XLR is switchable pre or post effects with a PAD for use as a DI out.

    At the risk of sounding like a complete moron...You mean to tell me they aren't all like that?

    I've only had a couple of pre's and they were both pretty much the same on the backside. Anything less than the aforementioned seems kinda like a rip especialy when you hit the $500+ mark.
     
  11. Yes there is. The balanced (differential) wiring scheme is highly effective at hum avoidance, which is why it is used in pro sound gear. Rane goes into this in great detail in the Interconnection Tech Notes. I keep all the Rane Notes on my site.

    Link: Rane System Interconnection Tech Note

    In my particular situation, I have the F-1X, GE30, MX22, and MX-3000a (amp) in the signal chain. My cabling is very short, but introducing a single unbalanced component (F-1X) into the chain results in the entire chain being unbalanced. I noticed the increased amount of hum as a result.

    IMO the bass preamp designers were tightly focused on a single concept (DI to a mixer) and designed their XLR accordingly. It appears no thought was given whatsoever to the other end (amp), as all amps I've seen are Balanced ready.

    I dunno if there is an impedance mismatch, or if the XLR Our just has a reduced voltage. I suspect the latter, but it sucks for my needs. That is why the SWR IOD is looking better and better all the time.