[netaron]

Hi everyone,
1.The impedance issue has always been a little hard for me to grasp. As I understand it, the speaker driver is more of a physical impedance then electrical, correct? Oh, I guess the voice coil presents a challenge as well, but what part of the load is usually physical and what part electrical. I know there is no sure answer, it's just so I get an idea.

2. So how does the input/output impedance of amps and preamp work, I have read that amp/preamp should have close or matching input or was it output impedance. Huh?

Thank you for your patience.


Haron

[andyr]

Hi Haron,

Re. your point #2: no, I think you've misinterpreted what you've read ... or you've simply read some of the rubbish that circulates on the Net!  Present Forum excepted, of course!!  )

As I understand it, for good signal transmission, there needs to be at least a 1:10 ratio between the output impedance of one device and the input impedance of the next device ... so, for instance, with an AKSA having an input impedance of 47K, do not use a pre-amp which has an output impedance of 1K!

Re. your point #1: a speaker driver rarely shows a uniform impedance across the frequency band which it's being used for - my own Maggies are probably the nearest thing you can get to this as they pretty much show an entirely resistive load.  Also the passive crossover needed for n-way speakers introduces it's own impedance complications.

Hopefully, the above has helped you make the picture clearer.

Regards,

Andy

[netaron]

Andy,
Thank you for your reply, ok, I understand impedance as far as driving a speaker, but what role does impedance play when it comes to amp/preamps? I am not sure I understand what poeple are refering to when they talk about amp/preamp impedance matching etc.

Thanks again

[AKSA]

Haron,

Impedance is a huge topic.  It's not so easy to understand, and definitely takes a lot of time to have an intuitive grasp of it.

Essentially, impedance is the AC version of resistance.  The higher the impedance of an amp input, then, paradoxically, the easier it is to drive.  As Andy says, you want the output impedance of the source to be at least ten times lower than the input impedance of the amp.  Thus, the AKSA with its nominal input impedance of 47k should be driven by a preamp with an output impedance no higher than 4K7.

If the output impedance of the source is too high, it will 'sag' under load, and deliver a distorted version of the waveform.  So, impedance is about how hard it is to drive something;  low impedance is tough to drive, high impedance is easier to drive.

Cheers,

Hugh

[NealH]

Matching impedance is important for maximizing power transfer.  Radio and Television transmission is a good example where maximizing power transmission is the focus.  When the signal is to be transmitted over a long interconnecting cable, maximum power transfer occurs when the amplifier output impedance equals the transmission line impedance which also equals the antenna input imedance.  

A speaker is generally designed to be driven by a constant voltage supply with the intent to provide a constant SPL output at a given distance.  Transmission line effects don't appreciably come into play here since the frequency wavelengths involved are so long compared to the interconnecting cable length.  Here we want a very low output amplifier impedance so that the speaker sees the full voltage being outputed thus enabling the speaker to provide constant SPL with frequency.  We are focused on transmitting constant voltage, not power.    

If the amplifier exhibits appreciable output impedance, such as that exhibited by a various tube amplifiers (especially zero feedback types), then the amplifier and speaker split the voltage (and power) according to the ratio of their impedance difference.  During periods when the speaker impedance dips fairly low, the tube amplifer is burning a larger percentage of the ouput power.  During periods (frequencies) when the speaker exhibits a high impedance (ie, bass resonance point), the amplifier will be burning a lower percentage of the ouput power.  Thus, as you see, the actual input power to the speaker, or voltage in this case, varies according to frequency.  Ideally you want the speaker to receive virtual all the voltage and, at the same percentage regardless of ouput frequency.  Thus a very low output impedance amplifer is generally desired.

A low output impedance amplifier is desirable for another reason too.  Since the speaker driver has mass and thus inertia, once the signal stops the cone will not stop instantaneously with the signal.  There is a settling time associated with it.  This shows up as a coloration in the sound - usually as loose bass or boomy bass.  Some midrange prominence may show up too.  When the amplifier's output impedance is very low then it dampens the driver's mass thus quickly quenching the cone ringing and bringing it to stop.  This will minimize any driver mass excursions after the input signal goes away.