View Full Version : Impedence

08-22-2006, 10:41 PM
What is the rated impedence for various inputs on a reciever, amplifier, etc. I know my turntable cartrage is 47 ohms.

CD player?

Tape Deck?

Audio Visual?

Well well, I'm seeing things, three of them.

-Stanley Blystone

Loopy Byteloose
08-27-2006, 12:17 AM
Audio Impedances have been subject to extensive revisions over many, many years. Just microphones are a bit complex and provide for an array of pre-amps.

You seem to be mixing items that require a pre-amp [like turntable cartriges] and items that have their own internally like a tape deck. And I really am unsure of what you clump into 'audio visual' as you·include video signals too.

I think you would do better to have a focused goal and a particular genre of hardware [like modern, 1960s, or what?] rather than to 'shotgun' the question.

Build around your main item -- the audio power amp and fit the particular add-ons.

"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [·黃鶴 ]·in Taiwan

08-27-2006, 12:57 AM
Impedance is a complex number, meaning it has two parts, real and imaginary. The real part comes from resistance, like the value of a normal resistor. The imaginary part comes from either capacitance or inductance. In a system, audio for instance, the output impedance of the preamplifier has to match the input impedance of the amplifier in order for maximum power transfer to take place. When there is an impedance mismatch there is a loss of signal power between the devices.

There is a way to manualy measure impedances but it isn't easy, you will need a variable resistor, a variable capacitor and an o'scope. Reply here and I can explain the procedure if you like. Correcting for impedance mismatches is a whole other story.http://forums.parallax.com/images/smilies/skull.gif


PJ Allen
08-27-2006, 04:48 AM

Looking at the Technical Specifications for my Technics SA-222.· It states that the TAPE·Input Impedance is 22K (that's the Input to the Amp that is the Tape Deck's Output) and the PHONO Input Impedance is 47K (that's for a magnetic phone cartridge.)· It has an AUX Input, but no specs are given for it.

Looking at a Tape Deck manual, its·LINE IN·impedance is 50K and its LINE OUT impedance is 5K (5, not 50).

Looking at a CD Player manual, its LINE OUT is 50K

Looking at a VCR manual, its AUDIO IN is 100K, and its AUDIO OUT is <10K.

So, there·you have·some impedance figures, for good or ill.

Video is supposed to be 75ohms.

Loopy Byteloose
08-28-2006, 02:01 AM
You bring up another salient point. Output impedance is NOT the same as Input impedance.
This is an area where I have always begun to loose focus.

All that talk about real and imaginary numbers can be really distracting. Those are the mathimatics behind the reality.
The electrical reality is impedance is an equivalent of resistance at a given frequency or range of frequency [which is the case with audio].
With pure DC, there is no concern about impedance.

But as the frequency climbs, so does the problem of this additional 'frequency dependent' resistance.
Audio has less concern that RF in terms of losses because the frequencies are lower.

Talk of capacitance and inductance can confuse people. Impedance does have a relationship to coils and capacitors, but that is generally applied to tuned circuitry; oscillators and filters -- not particularly to matching inputs and outputs.

The inputs and outputs are about not wasting power or wasting less power. And maybe in some cases about avoiding damage from inputing excessive power.

Generally I have just plugged things in and if they sounded okay, I accepted it.
IF I cannot hear much, I have an impedance problem. It is likely that I need a pre-amp.
If I hear a lot of distortion, I am probably 'clipping the signal' from a too powerful input. It is likey I need a volume control.

But from what I understand, a high input impedance allows for the amp to be driven by a smaller source of power.
These tend to be your pre-amps. A power amp might have a lower input impedance, but really bump up the power of the signal.
Each has its own concerns about maintaining the linearity and faithfulness of the signal.

So, I begin to think that not all mismatches have to be perfect - a ball park relationship will work.

For audio -- Would it be true to say the the output impedance should be equal or less than the input imedance?
Rather than to say that all impedances must be matched perfectly.

I really don't know.

Also, line tranmission is a completely different kind of impedance challenge. And so are antennas.
It really is about getting the power where it is supposed to be doing the most productive good. And in these cases, the kind of wire and the physical layout create the impedance problems.

The Video 75ohm is an RF cable match. It is a line transmission issue, not an amplifer input/output issue.

"If you want more fiber, eat the package.· Not enough?· Eat the manual."········

···················· Tropical regards,····· G. Herzog [·黃鶴 ]·in Taiwan