Noticing many microphones are 600 ohms, that mean anything?

[TalismanRich]

Cables have capacitance, but it's low relative to a what you normally think of with a capacitor.

Here's the spec for Belden 1192 star quad microphone cable. Notice the conductor to conductor capacitance value of 39.2pF/ft. The longer the cable, the higher the resistance, and the higher the capacitance. It really comes into play when you have a high impedance signal to start with. That's why you don't want to use 50 ft cables on your guitar, the capacitance will change the tone, just like a tone control.

 

[ecc83]

Ignoring the Flat Earther, cable capacitance is as real as a Mullard Mustard! It has the same effects as any component capacitor.
Hung on a passive electric guitar it will cause a roll off of high frequencies, although this is more complex than just HF loss as guitar circuits are a mix of resistance, inductance and capacitance +cable C and this can cause resonant peaks.

The output circuits of things like mixers and interfaces have to be designed to cope with cable C or they can become unstable. The usual fix is a low value resistance of some 68 to 100 Ohms. This is important to know because sometimes one might delve into a device and 'pinch' an output from a point not so designed.

On 'professional' grade kit steps have to be taken to ensure the outputs can drive very long lines with minimal treble loss or distortion, that means the stages have to have sufficient current capability to drive the cable capacitances.

I include the above because 'a joke's a joke' but many people here do not have a grounding in electronics and IMO it is important that we don't confuse them.

Cable RESISTANCE is very low even for quite light signal cables and can be ignore most of the time and in any case, as said, there is normally a resistance in the output anyway of around 50 times any cable R.

Dave.

 

[LazerBeakShiek]

I include the above because 'a joke's a joke' but many people here do not have a grounding in electronics and IMO it is important that we don't confuse them.

right. That is not my intention either.

 

[ecc83]

Back to ~600 Ohms microphones! 50 odd years ago I had the problem of amplifying a line of kids singing in an amdram production. Could not be onstage, would get in the way and no way to get them off. Naturally there was little or no money in it!
We found some very cheap, plastic 'cassette' mics at our equivalent of Rad shack. We modded them to balanced op' and ran a mic cable down a length of metal tube to weight them. We hung about 6 of them on a lighting barrel and 'flew them in' for the kids part. All the mics were wired in parallel and were sent up to the 'gods'. to a mixer then out on 100V speaker lines to speakers. Worked pretty well as I recall...

Dave.

 

[gianluca68]

As already said, 600 ohms is only an old standard. Today, microphone's impedance is about 250 ohms and standard Shure microphones are designed with a low impedance/low output coil and a little impedance transformer in the body to get higher signal with an impedance of about 250 ohms. Latest design microphones can use neodymium magnets, then they can achieve a higher signal without using a transformer.

 

[gianluca68]

About impedance and sound: the input impedance of a preamp or a mixer should be some times higher than the mic impedance to have a low signal attenuation (it is a voltage divider), but a too hogh input impedance raises noise. Then, common input impedances are about 2.5kohms.
I noticed a little difference between Sennheiser microphones on an old Sennheiser catalog:

Sennheiser revue 13 - Katalog 1988 - Seite 24-25

Dynamisches Mikrofon MD 918 Das MD 918 ist ein dynamisches Mikro fon mit Nieren-Richtcharakteristik. Es zeichnet sich besonders durch sein her vorragendes

bibliothek.echoschall.de

for some microphones I read Impedance: 200 ohm; Minimum load impedance: 1kohm, but for other ones I read Nominal load impedance 200ohm or something similar!
I think that some microphones are designed to work on 5 times internal impedance, but many old design ones are designed to work in a impedance matched condition. I.e., SM58 with a low impedance load is less sensitive to handling noise and it is also less boomy. It sounds better! In my SM58, I put a 150 ohms resistor inside across teh capsule, before the transformer: it sounds better! Other microphones are designed to work on an impedance of some kohms: try and hear!

 

[ecc83]

"600 Ohms" is one of those numbers that is responsible for a shed load of confusion and misinformation,not least because it gives rise to the level system "dBu" which is historically related to "dBm" the latter being a reference to one mW across 600 Ohms. That give 0.775V for dB 'unloaded'. There is the far more logical "dBV" with one volt rms as the reference voltage (which gives of course 1mW across 1k but nobody bothers with that!) Those sensible chaps that deal with guitar related electronics stick to dBV. I can only conclude that somebody made 600 Ohm mics because they did not understand the situation or/and thought it looked 'technically sexy'?

The noise level produced by a mic pre amp is almost wholly determined by the input devices, the currents they are run at and the terminating resistance. For a vast range of devices and currents (1 to 3mA) 1000 Ohms is pretty much bang on but 1/2 a k either way makes little difference. So, yes we don't want to attenuate our hard won mic signal so make the mic one tenth that resistance i.e. 150 to 200R. Making it 600R leads to signal loss and touch of extra noise but the extra turns on the mic coil 'might' make up for that?

However, the makers of quality microphones KNOW the input R of yer common pre is going to be 1 to 2k and so they build their mics to have the optimum response when so loaded. People might LIKE a lower or higher load resistance but "you can't pl.........!" Capacitor mics rarely use transformers these days so they are going to be pretty load agnostic. Many upmarket pre amps have much higher input Zs than 1k5, some as high as 15k especially for ribbons as some people say it sounds better. Yer pays yer money. Such pre amps have to be without phantom power since the 6k8 feed resistors would shunt the mic.

Do note that I have mentioned RESISTANCE more than impedance. This is because it is the resistors that cause the noise. Does not mattter what the open circuit 'impedance' of a pre amp is, once shunted by the mic's resistance it is THAT which determines the noise floor almost completely.

Dave.

 

[gianluca68]

one mW across 600 Ohms. That give 0.775V for dB 'unloaded'. There is the far more logical "dBV" with one volt rms as the reference voltage (which gives of course 1mW across 1k but nobody bothers with that!) Those sensible chaps that deal with guitar related electronics stick to dBV. I can only conclude that somebody made 600 Ohm mics because they did not understand the situation or/and thought it looked 'technically sexy'?

That's right. They started with power levels, then 0.775V was the voltage of 1mW/600 ohms, but afterwards they used the voltage regardless of the impedance, then they kept the same voltage level: 0.775V; neverthless, the standard level of consumer audio is -10dBV (316mV).

The 600 ohms standard for mics comes from telephone lines.

 

[ecc83]

"The 600 ohms standard for mics comes from telephone lines." Probably where the confusion started. Phone lines are very long and become "transmission lines" and have a "characteristic impedance" of about 600 Ohms (look it up in Wiki but the math is waaay beyond me!). This means that the device that drives the line must have the same impedance and the load at the far end the same otherwise you get 'reflections' and a messed up frequency response*. Anyone who has messed with CCTV will no doubt have seen these reflections as decaying vertical line on the screen if the co ax is not properly terminated at 75R. CCTV of course runs at mega Hz and so the problems show up with much shorter cables.

But there is no need for "600 Ohms" in studio sized audio systems and it would be great to get rid of dB bloody u!

*Before digital links, radio and TV sound was sent over dedicated GPO lines from concert halls etc. Very complex CRL equalizers were used to get a reasonably flat frequency response to 12kHz. IF you were lucky. The coming of stereo FM meant TWO perfectly matched, equalized lines...Tricky! (I think though that someone had the bright idea to send the signals as 'L+R and L-R' and matrix stereo at the TX end which eased things greatly?)

Dave.

 

[Slouching Raymond]

If the cable is 600 ohms, it does not matter how long the cable is. If it was twice as long, it woud still be 600 ohms.

 

[rob aylestone]

There's some really good YouTube videos on the first transatlantic telegraph cables where the impedance and losses meant sometimes speeds were down to a few words a minute! One of the pioneers thought about the terrible losses, so increased the voltage a bit, which improved the minuscule voltage the other end, so he turned it up and at 2000V the insulation broke down and the cable had to be scrapped. It turned out when they tested it that the laying was a bit rough and the cable insulation had tiny damage holes which let the inner arc to the outer, letting the water in!

There is also a great video with Dave Rat in his audio warehouse store - he plugs a 58 into a mic cable, then gradually adds every cable he has in the building, looking at the results and listening to the mic - the distance he gets is amazing. he even gets 6 miles with a phantom powered Neumann U89!

 

[ecc83]

If the cable is 600 ohms, it does not matter how long the cable is. If it was twice as long, it woud still be 600 ohms.

Yes, "characteristic impedance" is the same for an assumed infinitely long cable.

That YT was very interesting but I am sorry to say flawed in one respect. Coiled cable pairs cancel inductance* so all that is left is resistance and capacitance and we saw the results of those in the response loss. The fact that phantom power was maintained was impressive, pity he did not measure it at the 'six mile mic end'?

I worked at a network company who were developing CAT x products, jacks, and modules. They had to string cable around a large wooden structure in the lab about ten mtrs long and 1.5 high and the cables not get closer than about 200mm on each 'wind'. This was so that the cable represented a 100mtr 'straight' run. A 100mtr drum would have made life a LOT easier!

*A transmission line is 'modeled' as an infinite string of series inductance and shunt capacitance. Lose most of the inductance and you don't have a transmission line any longer.

Dave.

 

[drtechno]

Was there ever an industry standard for microphones to have 600 ohms resistance?

Some rack equipment I use recommends the use of a 600 ohm microphone. Whats the reasoning?
View attachment 133070

If I build a passive resistance box with XLRs and a rheostat to change the ohms, how would that change the vocal sound? If I dialed in more resistance what would that sound be? High cut or low cut? or somehow lo fi'd... ? It would have to adjust the volume somewhat..overdrive it?

a tonestack like guitar controls is capacitance. Changing resistance would effect tone less than leveI, I imagine..

Was there? not really, but mics that are 600 ohms and higher are usually condenser mics. Which a lot of the newer interfaces are engineering their mic pre on purpose that way.
Putting a resistance across or in series before the mic pre might help, but its not the correct way of changing the impedance. As this changes the relation to the coupling capacitors frequency dependent series resistance and effect the low frequency roll off point. Putting a mic pre transformer in front of the corporate mess would be the best non modification solution as well as their solution of adding a dc coupled mic pre in front (cloud lifter, fet head) Which is a $3-5 circuit they sell for $100. A more permanent solution would be replacing the phantom blocking capacitors with a mic pre input transformer.

The print saying " The load impedance of the preamp should be at least 5 times that of the output impedance of the microphone." is total garbage. What they are referring to is the load range of the input impedance that pass through the coupling caps at the lowest series resistance known as capacitive reactance. This is commonly referred to as "the ten times rule" by audio techs, but for an electronics guy like me, its a tolerance.

I knew something like this would happen eventually with the mass corporate built stuff. As over the years the audio designer is not the same type of people. These corporate people originally hired Electronics Technology people who specialized in audio electronics. They applied standards in equipment and find the best methods to build it. These were the guys who worked on equipment for decades before even going into the manufacturing sector. The degree that they had (Electronics Technology is an Associate in Applied Science) the corporate world (which came from a different industry) decided that an associate degree was not a real degree and started not considering anyone who doesn't have a bachelor's degree and laying off people they hired. Well there isn't a Bachelor's or Master's degree with Applied Sciences degrees. Because most of applied science is about applying the technology available while maintaining standards. And a lot of these people like me got booted out of the corporate world even though at one time they leaned on us for design guidance and referred to us as electronics design specialists. So, a lot of us either got out of the field, went back to repair, or like me being hired by boutique and high end manufacturers for consultation on a contract basis. But the corporate mass producers don't. But I'm glad they are screwing up their product. It leaves room for others like me to make better things on our own for people.

I have ecc83 on ignore because he is an audio tech and most of them only have small relative information that only applies in certain situation and not all and they misapply theory all the time. So I rather not see it than argue over something that they convinced themselves that is loosely based in theory.

As for input impedance, its actually better to be the same impedance than higher or lower, because otherwise it changes the input sensitivity, resulting gain & operating noise.

 

[ecc83]

"As for input impedance, its actually better to be the same impedance than higher or lower, because otherwise it changes the input sensitivity, resulting gain & operating noise."

That flies in the face of standard audio practice of the last 50 years at least. I suggest you expound your theories over at the Sound on Sound forum.

Dave.

 

[drtechno]

That flies in the face of standard audio practice of the last 50 years at least. I suggest you expound your theories over at the Sound on Sound forum.

Practice does not define correctness. Rather, just a settlement of actions and solutions in an application.
But since you don't know, any impedance mismatch is going to effect the overall gain formula in the stage and input noise figure.

 

[ecc83]

Practice does not define correctness. Rather, just a settlement of actions and solutions in an application.
But since you don't know, any impedance mismatch is going to effect the overall gain formula in the stage and input noise figure.

That is why we use "voltage matching" these days and not "power matching"...if I understand you correctly which is not easy.

Dave.

 

[Papanate]

Of course cables have capacitance! Whenever two conductors are separated by an insulator you got a cap! Look at any cable mnfctrs specs. Never heard of the guitarist's nemesis? Tone Suck?

High Impedence cables have some 'capacitance' - but Low Z cables don't exhibit capacitance at all - that's why you can run a Mic Cable 200' and not have an issue.

 

[ecc83]

High Imprudence cables have some 'capacitance' - but Low Z cables don't exhibit capacitance at all - that's why you can run a Mic Cable 200' and not have an issue.

High Imprudence cables have some 'capacitance' - but Low Z cables don't exhibit capacitance at all - that's why you can run a Mic Cable 200' and not have an issue.

I'm sorry? All cable have capacitance. The reason low Z mics don't lose HF over 100mtr or so is because the very low source resistance does not cause the capacitance to shunt HF. A mic + cable is a low pass filter.

Sorry! Son called from France. If you consider a mic cable with a capacitance of 150 picofarads per mtr and a '200' Ohm mic then the 3dB turnover frequency will be about 53kHz for a 100mtr run, i.e. not a problem. However, a dynamic mic is not a purely resistive source and so the response will likely fall a bit lower in frequency than that. Still not going to bother anybody but bats!

Dave.

 

[drtechno]

That is why we use "voltage matching" these days and not "power matching"...if I understand you correctly which is not easy.

Transformer to transformer coupling still fallows power matching rules. Its when it goes into a capacitor coupled input frequency response starts going awry a little because the transformer has resistance lower than the termination resistance on the other side of the coupling caps.

 

[ecc83]

Transformer to transformer coupling still fallows power matching rules. Its when it goes into a capacitor coupled input frequency response starts going awry a little because the transformer has resistance lower than the termination resistance on the other side of the coupling caps.

As Mr Bates said "I don't believe you".

Dave.