Help me understand impedance
 

I recently bought a Rode VideoMic for my TRV950. I found that I have to take the "volume " control down to -20Db and that Auto Gain is impossible to use (I know I should not be using it but I was in situations where I could not put on a headset).
The Rode Specs say - Output Impedance 200 Ohm, Then "MIcrophone will operate satisfactorily int a load impedance as low as 1k Ohm.

The TRV950 specs say "Mic Jack" low impedance,,, output impedance 6.8k Ohms.

The Sony Ecmms908 Mic says Output Impedance 1k Ohm. (This Mic works fine on Auto Gain)

Can someone comment on the compatibility of these devices and maybe explain why 1k Ohm is "lower" than 200 Ohm

Thanks
Sam

4 ohms is less resistance than 8 ohms.

Current is limited by resistance, the higher the resistance the less than can travel the circuit.

So basically by introducting a lower resistance mic into a circuit designed for a higher resistance mic you are letting more current (volume) travel through the input circuit. Thats why it's so loud.

You can get a crap load of resistance changers for mics for pretty cheep. I have an entire grab bag of load changers for any occasion. They are really helpful when changing from a camera input situation to a mixer input situation.

Thanks for the reply - where do I get the resistance changers?

B&H sells a bunch of them. Resistance changers isn't the proper term for them. You should at very least be able to find a inline device to bring your resistance up without clipping the signal at all.

Hello,

Are you speaking of a Line Input Adapter? These convert a line level audio signal to a mic level signal. I bought a 600 ohm to 150 ohm adapter at B&H.
model # SHA15LA.

Sincerly,

Stephanie

These are commonly called 'impedance matching transformers'. They can be built into an XLR housing and attached inline. You could also find them at your local music store such as Guitar Center.

When the specs say 'will operate with an impedance as low as 1K', it means that your 200 ohm impedance mic is 800 ohms too low and as was stated, it will allow too much current to flow through the mic circuit and could possibly blow the input much like a short circuit which is zero resistance.

For the record,

Resistance, is measured in ohms and is a straightforward calculation in a purely resistive circuit where the voltage and current are always in phase with one another. Applies mainly to DC circuits.

Impedance, is also measured in ohms but is a more complex calculation because an AC circuit which includes coils (inductance), or capacitors (capacitance), will introduce a 90 degree phase shift between voltage and current causing the voltage and current values to be different, depending on where you measure them in the circuit. Frequency of the AC plays a part in calculating the inductive reactance(coil) and capacitive reactance(capacitor). These two values are then applied to the basic formula for DC resistive circuits to arrive at total impedance.

Probably more than you wanted to know but hey, knowledge helps. That's why we're all here anyway.

regards,

=gb=

You know, I've been installing automotive electronics for years, and I never understood in such concise terms how impedience worked with AC voltages. Thanks, lol. I guess you learn something every day...things make more sense.

Thanks...and what he said.

You don't want to use an impedance matching transformer between the Videomic and your camcorder. Just use an inline attenuator or headphone volume control. See this: http://www.shure.com/support/technot...dance.html#top.

I would like to offer a slightly different take on this.

A general rule of thumb is that the input impedance should be at least 10 times the output impedance of the preceeding device. The input impedance of the TRV950 is only 6.8x the output impedance of the Sony mic and this will load it some.

The other difference is the sensitivity of the Sony mic is -51dBm and the Rode is -38dBm, a difference of 13dB. You need a 10dB to 15dB pad. They are available from several manufacturers. I noticed B&H has them from ProCo, Shure and Whirlwind.

Sam

The mic specs say it will "...operate INTO a load impedence as low as 1k ohm ..." That mean the impedance of the mic input you plug into should be at least 1k ohm and the 6.8k input of his camera is more than high enough. The rule of thumb is that it's ok to plug a LOW impedance output stage into a HIGH impedance input stage next in line. If you reverse the scenario and plug a high impedance output device into a low impedance input, the circuit you're feeding will impose a higher load on the source than it was designed to deliver, the consequences depending on the device. In the case of the Rode mic, the manufacturer says the result of plugging into a low impedance input is reduced output levels from the mic.

The low->high rule is appropriate for modern, solid state equipment. Vacuum tubes are more picky and more attention needs to be paid to matching output and input impedances.

Steve,

Thank you SO MUCH for all your wonderful contributions to this site. Your posts are always so educational.

Most sincerely,

Stephanie

Thanks for all your replys. As always people on this board have a huge amount of knowledge to share. So I now understand why the mic is so "sensitive" and there is no danger of doing any damage.
Now what will I lose by using an inline attenuator, there must be a trade off somewhere?

Not really, a decent pad will cut the mic level back without introducing any noise or distortion. If your camera has a built-in attenuator see what switching it on does. If the mic is still too hot, add attenuation between the mic and the camera. The object is to send the camera a signal that gives normal recording levels with the manual gain control somewhere around 50%-60% or so of full up. What are "normal recording levels?" Turn on the level indicator and the automatic recording gain, record something reproducable like a steady tone, and see where the camera puts it. Aim for the same level on the meter with the auto turned off and the manual control around 50% of full.

Quite right, Steve. I mis-read or mis-interpreted his statement as being the camera's input requirement.

I respectfully disagree with you on vacuum tubes vs. transistors. I can assure you that in high power applications, transistors will self destruct much sooner than a vacuum tube will. Why? Because transistors are current driven amplifiers, whereas vacuum tubes are voltage driven. That's why high power RF transmitters using transistors all have an SWR bridge built into the output. Impedance mis-match between the xmitter output and the antenna system will cause high SWR and destroy a transistor in short order. The SWR bridge monitors for this condition and applies a negative feedback signal to reduce the drive level to the final stage to 'protect' the transistors. Tubes, on the other hand, will sit there with their plates glowing cherry-red from excessive SWR and take the abuse although it will shorten the useful life of the tube.

regards,

-gb-

You're correct when referring to amplifiers that must deliver power into a load. But such things as mic inputs on solid state, transformerless equipment are voltage driven inputs in contrast to the power driven inputs of vacuum tube amplifiers. Impedance matching is still very important where power is being transferred from one device to another such as an RF amplifier feeding into an antenna load but that's not often the case with most equipment along today's production audio chain. Where it does matter still is when using dynamic and ribbon mics, classic tube-type preamps and equalizers, and of course power amplifiers feeding speakers. (paraphrased from Rose's book on audio production techniques.)

Hi Sam,

An interesting thread here.

Don't confuse output and input impedance. If the TRV950 specs say ' "Mic Jack" low impedance,,, output impedance 6.8k Ohm', that's very misleading. Inputs don't have an OUTPUT impedance, they only have an INPUT or load impedance.

Further, impedance and mic sensitivity are separate issues. Just because a mic has a higher impedance doesn't mean it will be more sensitive.

More to the point, the VideoMic is more sensitive than the other mic. Unless you can back down the input at the camera (controls which exist on all pro cameras and many consumer cameras), your options are to use inline pads or (well not in this case, but otherwise) use a mixer between the mic and camera that allows you to control the level more finely.

You are moving from a one person band to a use which really requires more than one person paying attention to things. How do I know this? Because you say you are using Auto Gain.

A good sound person does more than sit on a stool and watch the meters. Among a LONG list of audio related things, a good sound person is constantly making minor adjustments to make sure the sound is as good as it can be; even if the shot is just talking heads.

I'm not suggesting you hire a sound person. I'm just pointing out that as the complexity and quality of your work becomes more demanding, there's just too much for one person to pay attention to. Something will suffer.

Welcome to the world of professional audio/video. :)

Regards,

Ty Ford

PS: I'm giving another Audio Bootcamp Seminar next week end here in Baltimore a my studio. We'll spend 4 hours going over nothing but audio; which mics, where, when, mixers, levels, camera inputs and, as amazing as it seems, four hours goes by in a flash.

Do you ever make up Toronto way for your seminar or into upstate New York? Would love to attend some day but BWI is just a bit too far.

Saw your stint as a taxi driver last night. Ain't it a dirty shame! <grin>

Steve House

Steve, I quite understand. I haven't taken it on the road yet other than down to DC several times. I'm trying to keep costs down. The event here only costs $75, includes the Bootcamp Book and is limited to six people so I can give each person some direct help with his or her rig. Some say that's a ridiculously low price.

I can't afford to fly anywhere running that sort of deal. If I came to Toronto (lovely city, BTW) I'd probably want to do two sessions; a Sat. and Sun. to make it worth my while.

Airfare, hotel and a space to do it in would all have to be added up and I'd have to make a bit to keep the house, studio and groceries back home.

I've done seminars with 30-40 people. Not quite the same as doing it with six, but everyone seemed to get a lot from it. Find me a video users group and lets see if we can figure out how to make it work! ....um, before Winter sets in, please. :)

Regards,

Ty Ford

Hey Ty,

That's when you bring the tour down here to Dallas and the southwest like Phoenix. You get to stay nice and comfy warm while everyone is freezing in Baltimore. (Hehe).

Actually, I was going to ask the same question that Steve did about taking the seminar on the road. Final Cut Pro user groups are pretty strong around the bigger cities and would probably love some help tweaking audio both hardware wise, and software wise (Soundtrack Pro).

You may have an untapped revenue potential if you can handle the travel.

regards,

=gb=

Hey Greg,

Dallas is very doable. I haven't had the need to do Soundtrack Pro yet because I use Pro Tools LE and a Digi 002 Rack for most of my stuff. I have been doing some work in the old Soundtrack and in Garage Band.

I just created a bunch of beds for a Mockumentary as part of a 48 hour film festival up here. Garage band has some nice sounding software instruments. The piano is great. I'm using it on a singer/songwriter project for a client. You can hear what that sounds like by clicking on Debbie Bilezikian's track "Threads" here: http://home.comcast.net/%7Etyreeford/Library.html

SW Airlines might actually make it possible for me to do Dallas. Don't know about Toronto. I'll check.

Regards,

Ty
410.296.2868

I use and love Garage Band. I am also a musician and it compares nicely to my flagship Yamaha ES7 workstation synth. I'm also able to use softsynths with it such as Arturia's beautiful re-creation of the Arp 2600 and Yamaha's CS-80(a great synth but it's a 200lb behemoth). Will give the soundbeds a listen.

-gb-