impedance.. ohms.. help!

  • Thread starter Thread starter richardosim78
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richardosim78 said:
i just wanna doublecheck... so can u tell me ?

some info bits:"Ultra High Impedance: Piezo transducers sound best when their input load is within the 1-10 million+ ohms (1-10 megohms) range.If the loading (amount of resistance) is not optimal for the type of pickup, problems result. The reason that this happens - without getting too technical - is because a "non-optimal" resistance load creates a sort of unintentional (and usually undesired) filter which may decrease the level of certain frequencies.Piezo-based (most common) and similar acoustic instrument pickups are rather different from those magnetic pickups, in that they react differently to low impedance values."

so what is the impedance of such sources???
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The best answer is, it depends. Different units will most likely have different impedances. If you have a specific unit in mind, that is a question that can be answered.

As far as the active ones go, the preamp acts as a buffer, so it will act like any active circuit and not react to whatever load would normally be put to it. (guitar amp, line input, passive DI to mic input, etc...) So if you get an active one, it's a non-issue.
 
JUST for the sake of completeness....This is no longer a happy place.

Piezo transducers are insulators, i.e. essentially infinite DC resistance but behave as small capacitors. Thus any load you put on them forms a High Pass Filter and because guitar piezos are so wee they need a very high resistance load. One meg is not enough, for decent LF you need at least 5meg*.
This is why most acoustic electric guitars are active.

Magnetic pups are essentially inductive but also have significant resistance which limits the "Q" they are also always associated with a load of pots and often capacitor junk and thus the ACTUAL source impedance is both indeterminate and variable! Note too that cable has some tonal influence whereas for a pure piezo, cable capacitance just causes aperiodic signal loss.

*Even 100 meg is easily done with our old friend the TL07X!

Dave.
 
richardosim78 said:
this is not something i wanna discuss about. good day
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Well ... I was legitimately interested but, since you want to be that way, I whipped up a little song called Off the Grid!
[MP3]https://dl.dropboxusercontent.com/u/65316259/Off%20the%20Grid%20(Master).mp3[/MP3]
 
thanks guys,

so what kind of signal is from that active pickup on the acoustic guitar? is it instrument level or mic level?? what r typical characteristics of such sources????????
 
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propman said:
Well ... I was legitimately interested but, since you want to be that way, I whipped up a little song called Off the Grid!
[MP3]https://dl.dropboxusercontent.com/u/65316259/Off%20the%20Grid%20(Master).mp3[/MP3]
Click to expand...

nice.. good intonation.. sweet guitar... cool,...
 
OK, I need to speak up. Bottom line up front "Run away!" Go find a better mixer. First, as so many people already mentioned, a 600 ohm line level mixer would never work without spending gobs of money on external mic-pres. Quick lesson, you have two basic types of inputs for audio, line level and microphone level. I won't get too geeky, I will just say that line level means the signal was already amplified before it hit the mixer, like from a CD player. A microphone (or guitar pickup) does not produce a high level so it needs to be amplified first. That's called a Mic-Pre and is usually built into a quality mixing board. It brings the level from microphone level to line level. Now the tricky part, a LOW LEVEL IMPEDANCE microphone (you don't want to even consider a high impedance mic) will have a rated impedance between 150 and 600 ohms. The impedance of the mixer must be much higher, a ratio of about 10:1. So a 150 ohm mic needs a mixer with an input impedance of about 1,500 ohm. So most quality mixers will have an input of around 1.5-2K ohms.

BTW, if you ever want to have fun, google the history of the 600 ohm standard. Bottom line is, nobody clearly knows where it came from. Some say it goes back to telegraph. It is a standard, just like railroad track gage going back to Roman chariots. Who knows, who cares.
 
People always seem to want information before they do anything nowadays. when I had my first guitar I had no idea what impedance even was. I connected item a to item b, and sometimes it worked perfectly, sometimes not. I learned that if you had to have the level control way down, then you were putting something too high in level into something expecting far lower, and vice versa - if you had to have the gain right up and it was hissing away, then again, you messed up. If you plugged the xtal mic into one gizmo it was a bit dull, yet in another it sounded brighter - I just knew what my kit did best, but not why, or how. Did it matter? Nope - not a jot. I'd plug a keyboard into a cassette deck mic input, because the sockets were the same, and I knew that turning the cassette gain down, and the keyboard up sounded best. Turning the cassette up and the keyboard down was hissier. Then I went to college and learnt what was actually happening. I used these passive mixers and battery amps and all sorts. I could hear nice distortion and nasty distortion. I could hear hiss, and learnt to detect and define eq.

Nowadays, everyone seems to want to define everything in absolute terms. I just can't see why experimentation and self-learning is bad?

Do I care if I plug a very high impedance source into something, if the sound is fine? No. Have I ever run a signal the wrong way through a passive DI and solved a problem? Yes. Plug them in and see what happens! everyone gets so cross about everything and bad tempered. life is too short.
 
By the way, the reason 600 ohms became one of the audio standards has to do with old telegraph lines. Here is some history from Bill Whitlock of Jensen.

"Thought you might be interested to know a little history behind the venerable "600 ohms". The first telephone lines that linked cities miles apart were actually existing telegraph lines. The wise engineers at Bell knew transmission line theory and realized that, even at audio frequencies, the lines were long enough to be true transmission lines (a pair of conductors behave as a transmission line when their physical length becomes more than about 1/10 wavelength at the highest frequency of interest). Therefore, they needed to know the characteristic impedance of the existing telegraph line pairs. They were typically #6 or #8 AWG wires spaced about 1 foot apart. If you do the calculations, an average value is about 600 ohms! To eliminate reflections (echoes), any transmission line must be driven from and loaded by a resistance equal to its characteristic impedance. So all telephone filters, transformers, etc. were designed for a 600 ohm system. And this hardware found its way into the first radio stations, and later into the first recording studios. Today, it is rarely necessary (or desirable) to "terminate" an audio cable unless it is a vintage passive filter or tube gear with transformers. Incidentally, audio cables begin to exhibit slight transmission line effects only when they are over about 4,000 feet long!"

Bill knows his stuff!

Dave.
 
guys, the only question i have is still the same question:

whats is the impedance and other properties of an active AND passive piezo pickup systems' signal, often found on electroaoustic guitars etc..
thats all i need. gracies!
 
richardosim78 said:
guys, the only question i have is still the same question:

whats is the impedance and other properties of an active AND passive piezo pickup systems' signal, often found on electroaoustic guitars etc..
thats all i need. gracies!
Click to expand...

OK! A passive piezo pickup has an entirely capacitive impedance and you can only quantify it if the makers tell you the equivalent capacitance THEN you can work out Zload for any given turnover frequency.

An active piezo pup is amplified so the output impedance depends on the amplifier's configuration. Again, only the makers spec will tell you the value, but I very much doubt they will!


Dave.
 
ecc83 said:
OK! A passive piezo pickup has an entirely capacitive impedance and you can only quantify it if the makers tell you the equivalent capacitance THEN you can work out Zload for any given turnover frequency.

An active piezo pup is amplified so the output impedance depends on the amplifier's configuration. Again, only the makers spec will tell you the value, but I very much doubt they will!


Dave.
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ok thanks, but what r typical, most common and most often found values of these sources??
r these values so random and different that we cant draw even a small line in the sand??
 
richardosim78 said:
ok thanks, but what r typical, most common and most often found values of these sources??
r these values so random and different that we cant draw even a small line in the sand??
Click to expand...
They aren't random, they are design dependent. If you don't specify an exact model or design, the question can't be answered.

Your question is like asking "how many horsepower is a car?" If you ask how much horsepower a specific car has, the question can be answered. If you ask how much power cars in general have, the range is too wide to make a useful answer. (volkswagen beetle= 45bhp, bugati veron=1100bhp)
 
Even if you had a specific model in mind, it's pretty unlikely they'd give you the relevant specs anyway. Best you can do is assume that a raw passive piezo is a "very large" capacitive source impedance that needs a load with in-Z as big or preferably even bigger than a typical "instrument input" The output of most active piezo guitars is "very low" and can easily drive "line input" type impedances or higher, but might have trouble with lower "mic inputs", but you can't probably know for sure without trying.
 
ashcat_lt said:
Even if you had a specific model in mind, it's pretty unlikely they'd give you the relevant specs anyway. Best you can do is assume that a raw passive piezo is a "very large" capacitive source impedance that needs a load with in-Z as big or preferably even bigger than a typical "instrument input" The output of most active piezo guitars is "very low" and can easily drive "line input" type impedances or higher, but might have trouble with lower "mic inputs", but you can't probably know for sure without trying.
Click to expand...


... and i could try using DI box or preamp with DI capabilities if any problem arises.. plan b.. if someone brings a guitar with "strange" piezo levels..
i could take that signal and try to convert it to the preamp's output, or balanced xlr.. right?

and thanks farview for giving me a depression..............damn bugatis.. ;)
 
richardosim78 said:
... and i could try using DI box or preamp with DI capabilities if any problem arises.. plan b.. if someone brings a guitar with "strange" piezo levels..
i could take that signal and try to convert it to the preamp's output, or balanced xlr.. right?
Click to expand...
If someone brings in a guitar with piezo pickups, just run it through a DI. If that's all you are worried about (some guitar that might show up sometime in the future), don't worry about it at all. 90% of them have active electronics, which will allow you to use a DI box or simply plug it straight into the interface/mixer you are using. You are normally better off using a mic to record it anyway. The pickups are for live, where it goes into an amp designed for acoustic guitars or it's simply DI'd into the sound system.
 
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