Naive question from poor educator

[Frank Maggi]

We have a program in our elementary district that helps teachers and students build curriculum based dv videos. Some are at the point where they want to use better (XLR) mics, but the cost of, say an AT 897, with a Beachtek thrown in is for us poor public school employees prohibitive. I've seen XLR to mini cables (up to 20') for a lot less than a Beachtek. Is that idea a non-starter? Some of the cables in the ad-talk say "perfect for getting good mics into simple systems". I know that's not true, but is it a viable option at all? Appreciate any help you can provide.

[Jon Fairhurst]

At a minimum you need a transformer to go from XLR to 1/8", and that assumes that the mic runs on a battery. A mixer with phantom power is better, but you can always upgrade later...

This gets you from XLR to 1/4". http://www.radioshack.com/product/in...ductId=2062443

You can get a second adapter/cable to go from 1/4" to 1/8". Keep that unbalanced cable as short as possible. Use as long an XLR (balanced) cable as you need.

[Ty Ford]

not to be disagree-able,

but you don't need a transformer nless you're trying to match impedances or need to isolate something.

I don't get what you want to mic or from what distance.

Regards,

Ty Ford

[Jon Fairhurst]

Ty,

I've never tested it, but doesn't the transformer deliver more voltage (and less current) to the high-impedance camcorder input, compared to coming straight out of the low-Z mic?

Also, you can run a longer balanced cable this way, and be somewhat immune to interference. If you run a long unbalanced cable from a low-Z mic, you're asking for problems.

Then again, I've had a transformer adapter for about 25-years, so I've always used one when adapting an XLR mic to a high-Z input when I don't have access to a pre-amp. For all I know a simple adapter would sound about the same.

Someday, I'll do an A/B for the heck of it...

[Ty Ford]

Hey Jon,

Yes, a transformer can, if chosen properly, step up the voltage, but that's not always needed. All transformers are not the same. Most color the sound to some degree. The cheaper ones more than the expensive ones. That can be a problem.

Yes, running long high impedance cables is asking for trouble for two reasons; high impedance circuits lose energy and frequency response after more than 25 feet or so. And, most high impedance circuits are also unbalanced; unprotected because they don't have a protective shield around two signal conductors as balanced circuits do.

Converting high impedance sources like most electric guitars and keyboards to low impedance as quickly as possible to low impedance, then running long distances to a low impedance input is the "best practices" way to go.

You can run unbalanced low impedance cables a greater distance, but running long unbalanced cables is asking for trouble as well because they are not shielded.

Impedances between low impedance mics and low impedance mic inputs don't need to match. That's a practice that went away back in the 600 Ohm telephone days when current transfer was a consideration.

Typically there's a 1:10 ratio or more between the impedance of the mic and the input impedance of the mic preamp. 10 k Ohm mic inputs are pretty typical for 50 even ohm mics.

What high impedance mic inputs are you referring to?

Regards,

Ty Ford

[Jon Fairhurst]

Originally Posted by Ty Ford

What high impedance mic inputs are you referring to?

I've generally assumed that any 1/8" input is going to have a relatively high impedance. I believe that's the case for PC soundcards. I'm not sure on camcorders. I'd have to look at the specs for a given model.

BTW, my transformer is a Teac Model 109A 200 ohm to Hi Z job. Made in USA. Definitely vintage!

[Doug Lange]

I produce a weekly news magazine at my elementary school. Here's what I've found:

I used a 12' foot XLR-1/8" stereo mini with a Shure Beta condenser. It worked with good sound. Not very hot, so we went handheld with it. The cable wasn't very robust so it has a short now that needs repaired.

Used an inline transformer with adapters to fit to 1/8" stereo mini. Still not a very hot connection, used handheld.

Mackie 1202VLZ worked great with adapters to 1/8" stereo mini. Good hot signal so the mics were out of the frame. However, the ventilation and computer in the back corner was also heard. I had better luck with the mic on a boom overhead. This worked with some group shots and kept a couple more kids busy ;-)

Best solution yet, I use two cheap Audio-Technica ATR35s wired lavs that are battery operated. I also have a Beachtek (actually, it's an XRL Pro, Studio 1 Productions) on ebay for $100 that has 2 1/8" mono plugs for the mics. Running 1/8" mono directly into my little Canon DV camcorders results in having audio on one channel only. The XLR adapter puts the signal into both channels. An 1/8" mono to stereo adapter would also work and would cut the cost down. But it is nice to have the option of going handheld with the Shure Beta for group interviews.

Having lavs is by far the best option. The students are audible without too much ambient noise. The lavs don't have the sound quality for broadcast news but it's clean and clear compared to the onboard mic. These lavs are cheap enough that if one breaks and isn't repairable, no great loss.

What do you do for lighting?...

[Frank Maggi]

This has all been very helpful to me.

John, below is the cable that the B&H catalog said was perfect for bringing xlr to consumer cameras. Any comment?

http://www.bhphotovideo.com/bnh/cont...=423360&is=REG

Ty, thanks for all of the elucidating comments. It really helps.

Doug, sounds like you and I are in similar boats. We've developed a program called Project LIVE that gets teachers working with dv with their students:

http://www.eusd4kids.org/edtech/project_live.html

Typically the cheapy wired lav route is our usual solution (we use Sony WC 999; how do you like the AT lav you're using?). Most don't use other than ambient lights although we do have a Lowell DVCreator 55 kit for checkout that some of the more advanced try now and again. The Rifa 55 is the one that gets used the most. We're able to provide a small stipend each year that a teacher continues with the program ($500-$1000). Microphones have been the most popular "big" purchase (including Blue Snowball and Samson USB mics for v/o, AT 897, Sennheiser ME66).

Again, thanks all for your help. We really appreciate it.

[Steve Leverich]

Ty, I have to take exception to your comment, "running long unbalanced cables is asking for trouble as well because they are not shielded. " -

I've been working with audio since 1960 when I bought my first Webcor reel-to-reel recorder, have taught electronics in both military and civilian environments, been working with video gear from cassette to 2" quadruplex machines since 1970, have been an engineering tech, tech writer and/or service manager for various companies including Memorex, Ampex, Sony, etc, and until now I have NEVER heard anyone claim that un-balanced audio cables were not shielded.

If you really believe this, I recommend that you buy two connectors the same kind as any commercially made high impedance audio cable in your system and build a new cable using lamp cord (the kind where you can see both conductors/insulations running side by side) - then, with the commercial cable plugged in, turn your speaker level up just to the point where you can hear the program material.

Now, power down the system without changing any settings - replace the commercial cable with your "zip" cord cable, and power the system back up.

Assuming that both cables were longer than about a foot, if this doesn't convince you that the commercial cable was shielded, NOTHING will.

The actual reasons that balanced line audio is quieter and cleaner than unbalanced -

Balanced lines use TWISTED pair conductors, surrounded by the shield - the actual electron path for sound is ONLY through this twisted pair of conductors. Even without a shield, this will reduce any hum the cable might pick up, because the receiving input normally has a DIFFERENTIAL amplifier that sees both conductors, and only amplifies the DIFFERENCE between the two wires (the audio) - any outside interference is imposed on BOTH conductors, therefore no DIFFERENCE, therefore the receiving circuit ignores the hum.

An unbalanced line, even though it is almost always co-axial (a single conductor inside of a shield) has to rely on the SHIELD for its return path (it takes two conductors to transmit even an UN-balanced signal)

In order for a shield to work to maximum effectiveness, there must be NO CURRENT FLOW in that shield; especially current flow from UN-desired signals. If you connect two chassis together thru the shield and the two chassis aren't properly grounded (few are) you get AC current flowing in the shield - this can produce hum even in BALANCED wiring, but nowhere near as MUCH because of the self-canceling effect of the TWISTED wires.

Anyway, having AC ground current in ANY shield is BAD - but with un-balanced lines, you have no choice because the shield is being used not only as a shield, but ALSO as a return line (which is necessary for the signal to get there)

So the main reason unbalanced lines are noisier is that their SHIELD is also being required to act as the signal RETURN, so isn't as effective as a STATIC shield.

The main reason unbalanced lines hurt signal quality is CAPACITANCE.

Any cable, besides having resistance, also meets the definition of a capacitor - IOW, each wire acts as a "plate" of a capacitor - the insulation and/or air BETWEEN these two "plates" becomes the dielectric - this "dielectric", or insulator, affects capacitance between two conductors in the same way as an actual capacitor - different materials used for insulation have a greater or lesser effect on capacitance (all other factors being equal) because of their effect on "dielectric constant", which is one of the variables that define the capacitance of a given device - these are (1) the area of the "plates", in this case the size/length of the wire, (2) the distance between the "plates", in this case the thickness of the insulation between two conductors, and (3) the dielectric constant - air is rated at a dielectric constant of "1", other materials such as pvc insulation, foam, teflon, etc, all have constants HIGHER than air.

The reason this becomes important with high impedance lines is BECAUSE of the impedance - typical hi-Z cables have characteristic capacitance values in the range of 30 to 90 pico-farads per foot of cable - so do low impedance cables, but in order to "filter out" high frequencies (treble) you need to form what's known as a "high pass filter" between the two conductors of the cable - a high impedance circuit presents a typical input impedance of around 100 K ohms, expecting to see a source impedance of around 10K ohms (a 1:10 ratio if source/input impedance keeps the voltage from being loaded down)

In the case of high impedance circuits, this 100k input impedance coupled with a cable capacitance of 600 picofarads (60 pf per foot, 10 foot cable) gives an RC time constant in the range that will start to attenuate audio frequencies somewhat. Longer runs = worse attenuation. If you then "turn up the treble" to get it back (like in post, for example) you will also "turn up the NOISE" - as in, any OTHER high frequency noise that may have been recorded - self-noise from resistors, transistors, IC's, etc...

Lower impedance circuits have less of this "filter" effect, because although the same capacitance is there from the cable, the lower impedance of the circuit puts this "high pass filter" at a much higher frequency, and although you could likely MEASURE the treble loss with lab instruments, you won't HEAR it since it would likely be a small fraction of a dB.

This is already getting longer than I intended - one of the best references I've found on the web to explain much of this is here -

http://www.rane.com/note110.html

and here -

http://www.rane.com/note151.html

If anyone's still awake, sorry for the ramble - this is a complex subject, and some of the more condensed classes I've taught on it were 2-3 hours long... Steve

[Jack D. Hubbard]

Hi Frank,

If the issue is a 1/8" mini pin for an on-camera mic, you could use the Rode Videomic. It does a very good job and costs $149 at B & H.

Jack

http://www.bhphotovideo.com/bnh/cont...goryNavigation

[Bill Davis]

Originally Posted by Steve Leverich

Ty, I have to take exception to your comment, "running long unbalanced cables is asking for trouble as well because they are not shielded. " -

I've been working with audio since 1960 when I bought my first Webcor

<SNIP>

This is already getting longer than I intended - one of the best references I've found on the web to explain much of this is here -

http://www.rane.com/note110.html

and here -

http://www.rane.com/note151.html

If anyone's still awake, sorry for the ramble - this is a complex subject, and some of the more condensed classes I've taught on it were 2-3 hours long... Steve

Steve,

While your reply is accurate, detailed, and spot on in it's technical analysis,
I expect Ty was doing the same thing I've been guilty of in lots of posts in public forums, trying to take a subject that is inherantly technically complex and "simplify" it to a level where someone starting out can grasp the essential concept easily.

His overall point (using long unbalanced cable runs is generally to be avoided) is sound advice.

Your higher level technical explanation provides MORE details on specific circumstances where the exception proves the rule, but note it took many paragraphs to explain in depth, where Ty's response was contained in all of 14 words.

Sure, he might have worded it more precisely. And if he was writing for publication, I'm positive he would have taken more time to proof what he was writing and be more precise.

But this is free public discourse. Nobody's getting paid to pitch in - so we all tend to shoot from the hip in our posts. That's what makes these forums so dynamic and useful.

But. really, if the end result of his post is that is that someone investigates and adopts using balanced audio over unblanced in the kind of general work we're discussing here - that's surely a very good thing.

As good a thing as your taking the time to share your expertise and letting everyone here develop a higher level of technical understanding.

I spent a decade of writing about tech for a consumer trade publication audience and I know that even with a team of editors and technical proofreaders backstopping me, occasionally this kind of miss-phrasing would creep into one of my column submissions.

It's inadvertant - but it's also a great chance for people to develop a further understanding of the depth of the subject - something I know a lot of folks here will appreciate that your taking your time to help with here.

[Jon Fairhurst]

Originally Posted by Frank Maggi

Jon, below is the cable that the B&H catalog said was perfect for bringing xlr to consumer cameras. Any comment?

http://www.bhphotovideo.com/bnh/cont...=423360&is=REG

That cable looks perfect. It has the transformer and a short unbalanced cable in a single unit. Use a nice XLR mic cable to get your mic close to the subjects, and you're set.

Two considerations: 1) Figure out a way to secure the XLR transformer to the camera, so it doesn't pull the 1/8" plug out, and 2) Keep the shor unbalanced cable away from all power cords. If next to a power cord, it *will* pick up hum.

Again, this won't work with a mic that needs phantom power and doesn't take a battery. And be careful not to use a battery that is dying.

All the best...

[Frank Maggi]

Again, thanks to all who have shed light here.

Jon (I spelled it rightn this time), assuming all of the caveats you listed are taken into account, what benefit would I get going to a $200 Beachtek besides obvius stuff like multiple inputs, volume control, mic/line switch? In other words, if all I need is a single feed from a mic that doesn't need phantom (e.g. AT 897), is there anything inherently different or better going through the Beachtek than going through the cable mentioned above?

Again, we poor educastors greatly appreciate the generosity this forum has provided. Thanks.

[Steve Leverich]

Frank, first off I agree 100% with Jon's assessment. The 897 (I have one) works fine with a simple transformer and connector adapter such as the one linked.

Second, most of the Beachtek devices are passive til you spend quite a bit more - a couple of the upper end ones are active devices, at least one offers phantom power. The rest are also transformer devices, not sure about the difference in transformer quality though - Hosa tends to be kind of a middle-ground product for the most part so the Beach may have better transformers.

The advantages of using a phantom power device for your 897 are that the 897 actually has more headroom by 14 dB when using phantom power - so if you were recording rock bands you might see a difference in how close you could mic without clipping. Also, the 897 has a slightly lower impedance (better drive capability) under phantom than with battery power.

The disandvantage of phantom is batteries - any battery powered device that offers phantom power will tend to eat batteries faster when supplying phantom than when NOT.

Overall, for a situation where multiple students will be using the gear I'd go for simple over elegant; IOW, the cable you asked about. I'd also pay particular attention to Jon's recommendation about rigging some sort of strain relief for your adapter cable; those mini jacks on cam's aren't exactly "tank-proof", you don't really want the cam being carried around with that connector used as the handle...

Bill, Ty, I meant no disrespect in my comments whatever; on the contrary, I've seen nothing but excellent advice in all things audio from Ty - that's probably why it surprised me so much to see something "glossed over", or whatever - my only goal in posting was to prevent any more "internet mythology" from spreading - I moderate two other sites on studio design and acoustics, and have seen enough myth/confusion in that equally complex field to fill several books. I'm sure I've been guilty of it myself no matter how hard I try NOT to. You're right about things "creeping in" - sometimes even after every single word and punctuation mark is exactly what I've intended, some well-meaning "editor" changes two words in good intent and it's no longer saying the same thing :=(

Peace all - Steve

[Jon Fairhurst]

Originally Posted by Steve Leverich

Frank, first off I agree 100% with Jon's assessment. The 897 (I have one) works fine with a simple transformer and connector adapter such as the one linked.

Thanks Steve,

We run an AT815b into a transformer using a battery and it also works surprisingly well. (Except when the battery is dying, the cord is near a power cord, and the connector isn't in all the way - we've experienced and learned from all of those...) The 815b is really bright, so I think the transformer actually helps soften it a bit.

At NAB we were looking at various mixers, recorders and cobbled together solutions. No decisions yet.

One hairbrained solution I've been toying with would be to run a Tascam FireOne into an Apple (6-pin Firewire) laptop.

http://www.tascam.com/Products/fireone.html

Such a solution would give us a simple controller for editing, a nice 2x2 soundcard with preamps for desktop work, and *might* be usable in the field.

There are a number of things I'll have to sort out first: can the routing be set correctly? are the knobs adequate for field mixing? Is it practical to see when recording is on and off? Can the meters be used as input monitors? Can we use the wheel to control the levels, etc...

Sure, it's not ideal for the field, but if the questions above can be sorted, it could really work. My biggest concern, though, is that we can easily tell when the Mac is recording. There's nothing worse than recording people standing around, and turning off the recorder for the actual scene!