Why not XLR to the instrument?

Ray Minich · From: Bradford, Pa. Frozen Tundra

Just a question that probably has it's roots in instrument/amplifier history.

Why do we use XLR connectors with balanced circuitry & coaxial cables for low level microphones putting out microvolts, yet we use unbalanced single conductor coaxial cables and connectors for instrument pickups putting out only microvolts.

Is it because a 12AX7 only has one grid?

Seems XLR from the instrument to the amp would reduce the 60 cycle hum problem. Too expensive?

David L. Donald · From: Koh Samui Island, Thailand

The Alembic basses that Stanley Clarke used to play were XLR to thier propiatrary preamp control box for the above reasons.

In general it is standardized as phono jacks and the hum issue USUALLY isn't so strong that there was a need to globally change.

I think Brad Sarnu would have a good answer for this, espcially since he has just finished his 12ax7 black box.

Paul Osbty · From: Seattle, Washington, USA

Because it was cheap! Plus, amplifier inputs in the day weren't differential.

Internally, guitar pickups could feed a transformer and be fed out differential (balanced). I'm not sure when the balanced 1/4inch connectors were initially made, though.

C Dixon · From: Duluth, GA USA

Microphones put out signals in the microvolt range. PU's on a guitar put out signals in the millivolt range.

Because of this you have an extreme "signal to noise" ratio problem when dealing with Microphone cables that are for the most part absent when dealing with PU cables.

To hold down noise; particularly in long Mic cables; you must "balance" the signal. And also send it on very low impedance cable. IE, neither conductor goes to ground. Although the conductors may be encased in a grounded shield cable. Most are incidently.

Now when you do this, it is necessary to get this "balanced" low impedance signal into the "single ended" high impedance input circuit of a tube or solid state device. Matching tranformers are used in this case to match the balanced ungrounded pair to the single ended (one side grounded pair) input to a tube or SS device.

carl

David L. Donald · From: Koh Samui Island, Thailand

I add that a pick up on a guitar IS a transformer of sorts.

But instead of a small electrical signal in a parallel winding causeing the magnetic flux,

it is the movement of the strings passing through the pickups field that creates the energy.
But essentially the same function, just a physical displacement of the motive force.

John Floyd · From: R.I.P.

XLR's are historically used for Low impedance signals, A guitar is mostly High Impedance unless it has an internal preamp. Gibson at one time made some Low Impedance Les Pauls Studio models with XLR's. I don't think price is an issue as for as the connectors are concerned.

Ray Minich · From: Bradford, Pa. Frozen Tundra

OK, so far... Now, in my EE courses from 1/2 a lifetime ago we were taught that a voltage source with an output impedance could be characterized as a pure voltage source with a resistor in series.

A low output impedance source could be characterized as a voltage source with a small resistor in series, and a high output impedance source ditto with a higher value series resistor.
I've always wondered where I screwed up 25 yrs ago when I hooked the wrong microphone up to the input of a mixer board and the results were muddy and mushy. Kinda sounded like the mike wasn't putting out enough signal. Analysis indicated low impedance mike into high impedance input. Does a low impedance microphone also mean a "low signal output" mike as well? I know some of the cheap mikes that come with today's karoake outfits put out quite a signal, even if their frequency response is rotten, and they're "single ended" or "unbalanced". Is the trade off good frequency response vs low signal output?

Thanks again to all for enlightenment on this issue that has bugged me for years. One can read all the books on this stuff but actually plugging stuff in and powering up the PA Amp is where the rubber meets the road.

Paul Osbty · From: Seattle, Washington, USA

Remember, Ray, when input/output impedances (impedi?) are matched you get maximum power transfer.

Signal strength does not have any influence on whether or not to run differential. ARINC control signals in aircraft are +/-15volts.

Hi and low impedance signals can be run differential (balanced). There is no reason a guitar or any instrument can't be done this way.

The low impedance for the mics also lowered the cutoff frequency of the cable capacitance and the impedance (1/2piRC). This gives good frequency response over long lengths of cable. Most unbalanced, Hi Z cabling starts to lose it at about 20 feet. Mics are usually good for 200 feet.

The cost is not the connectors. It is making all inputs and outputs differential (essentially double the components). Plus, all of the cabling needs an additional conductor.

[This message was edited by Paul Osbty on 06 March 2004 at 01:27 AM.]

George Kimery · From: Limestone, TN, USA

Paul makes a good point. You absolutely don't want long cables with high impedence. Your highs and clarity just go to pot. If you have a long cord and plug a Lil Izzy into your guitar, the problem goes away. That is becasue the Lil Izzy is supposed to convert the high impedence to low impedence, at least that is what Craig Baker who built them told me. I have some high Z to Low Z adapters that I got at Radio Shack. I tried them to get the same effect and they were a big dud. However, they were "passive" (no battery) whearas the Lil Izzy has a battery. I am not schooled in electronics, so I don't know if that was what made the difference or not. Anyway, BEWARE of long cords (or coiled cords, even worse). They will suffocate your sound.

[This message was edited by George Kimery on 06 March 2004 at 05:52 AM.]

C Dixon · From: Duluth, GA USA

Signal voltage and impedance are independent of each other. Although it tends to follow that lower impedance devices put out less signal voltage. But this does not have to be.

Paul is absolutely correct about cable lengths and capacitive reactance (1 over 2PiFC), or loading down of the highs.

The lower the impedance the less loss of highs and the better the "signal to noise" ratio.

DC resistance is also a separated entity when talking about impedance. Although it is sometimes used to lower the "Q" of a coil. Meaning the coil is less specific to a single frequency which spreads out its response at the expense of signal level.

Forgetting all the theory mumbo jumbo, it is best to use balanced low impedance microphones than hi impedance unbalanced microphones and cables, IF the cable length is over 20 feet. If the cable is less than 20 feet, little if any, perceptable gain is realized.

For PU's, it is much less a problem due to the close proximity most amps are in relation to the player. I doubt most any player's total cable length would exceed 20 feet, if it even totalled 20 feet.

This is why I have a BIG problem with all the hubub over guitar cables. I am inclined to believe this is more subjective than real (save noise). But when you are talking about mic cables, that is another story due to their typical lengths.

carl

Ray Minich · From: Bradford, Pa. Frozen Tundra

So, the low impedance mike arrangement lets the soundman sit in the back of tha auditorium with his mixer board whilst the performers strum and sing into their microphones 150 ft away on stage. Gotcha.

Methinks the mistake I made with the mushy mike a long time ago was having an XLR mike and cable, but a soundboard with no XLR inputs, and we "cobbled" the cable end to go single ended without success. That was before I knew about the XLR to 1/4" converter transformers. (living in the woods I didn't get out much then :>)

My thanks to all of you for the information.

[This message was edited by Ray Minich on 07 March 2004 at 08:13 AM.]

[This message was edited by Ray Minich on 07 March 2004 at 08:14 AM.]

Donny Hinson · From: Glen Burnie, Md. U.S.A.

To put it simply, a low-impedance (balanced) feed isn't required for short cables. Instrument cables have been short traditionally because the instrumentalist likes to be close to his amp, so he can hear better and make adjustments easily. On the other hand, microphones used by singers have traditionally used long cables (25 to 50 feet, or longer), and for those longer cables runs, a low-impedance balanced line is necessary, due primarily to the two factors already stated...the lower output of microphones, and the significant build up of (frequency robbing) capacitance when trying to use long cables with high impedance feeds. The benefits of using low-impedance pickups and balanced lines on instruments would probably be negligible.

Scott Swartz · From: St. Louis, MO

Its true that electrical guitars and steels are "quiet enough" for most people, but they are generally pretty noisy comparedto say any decent dynamic microphone.

For me, I find humbuckers to be barely quiet enough, and single coils unbearably hummy/noisy.

If a balanced pickup is feasible from a magnetics and manufacturing standpoint, it would be interesting to try. Even with the additional gain required it should still be better from a S/N standpoint.

You could still use 1/4 inch jacks, just use stereo or TRS jacks (tip-ring-sleeve) instead of the typical TS jacks, so no mods to the steel would be required.

I seem to remember talking to Bill Lawrence about balanced pickups at the ISGC a few years ago, I thought he said he had made them, not sure if he sold them or only as an experiment.

Donny Hinson · From: Glen Burnie, Md. U.S.A.

That's true, Scott. Also, mike manufacturers are very conscientious when it comes to shielding. Not so with most guitars, pickups, and volume pedals. In a mike, the whole assembly is usually well encapsulated and shielded in a metal cylinder. But when you look at guitars, pedals and pickups, there's almost always signal-carrying wires or jacks exposed, and unshielded. These contribute a lot to our "hum problems", also.

Since no major manufacturer is using the low-Z balanced line approach, I have to assume it isn't a big problem for most of us. You gotta admit, hum isn't evident on any professional recordings.

Oh yeah, another one of our big hum culprits is cheap incandescent lamp dimmers!

Paul Osbty · From: Seattle, Washington, USA

This adapter will give you a balanced output.

Adapter

Just plug the 1/4 inch plug into the guitar/steel and run a mic cable to a mixer. You can use another one of these to convert back to a 1/4 inch at the amplifier end.

Electrically, this is identical to what a "balanced" guitar pickup would be wired like.

Scott Swartz · From: St. Louis, MO

Something like that adapter plus a normal pickup is what was in the Les Paul Recording guitar (or whatever its exact tiltle was), but that is not what I am referring to.

I am envisioning a true balanced pickup that does not require the matching transformer. It would be low impedance with smaller output voltage (which would be OK since the noise/hum would eliminated by the balanced construction), therefore fewer turns of wire and the associated problems that causes ie limited freq response, etc.

Since a pickup is sort of a composite of a voltage generator and a transformer winding, there may be magnetic complications for a balanced pickup that I don't understand.

impedances are given for that adapter

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