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Old July 21st, 2005, 12:26 PM   #1
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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
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Old July 21st, 2005, 01:54 PM   #2
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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.
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Old July 21st, 2005, 02:13 PM   #3
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Thanks for the reply - where do I get the resistance changers?
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Old July 21st, 2005, 02:20 PM   #4
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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.
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Old July 21st, 2005, 03:22 PM   #5
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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
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Old July 21st, 2005, 05:04 PM   #6
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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=
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Old July 21st, 2005, 09:12 PM   #7
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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.
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Old July 21st, 2005, 09:38 PM   #8
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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.
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Old July 21st, 2005, 09:47 PM   #9
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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
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Old July 22nd, 2005, 01:31 AM   #10
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Quote:
Originally Posted by Greg Boston
....
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.
...=
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.
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Old July 22nd, 2005, 04:05 AM   #11
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Steve,

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

Most sincerely,

Stephanie
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Old July 22nd, 2005, 06:57 AM   #12
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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?
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Old July 22nd, 2005, 08:13 AM   #13
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Quote:
Originally Posted by Samuel Birkan
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.
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Old July 22nd, 2005, 08:55 AM   #14
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Quote:
Originally Posted by Steve House
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 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.
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-
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Old July 22nd, 2005, 12:20 PM   #15
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Quote:
Originally Posted by Greg Boston
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.
...
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.)
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