Impedance matching questions

On reception, the problem with open wire transmission lines is the external interface especially when entering a building. Even a balanced line with a large spacing between conductors will pick up a lot of interference.

I have been planning to install a remotely controlled antenna tuner unit directly under my 3.5 MHz antenna feedpoint in the middle of my garden and connect the antenna with a open line feeder and use ordinary coax to connect the indoor transceiver.

On a sunny day (Wed, 15 Oct 2014 11:29:04 +0300) it happened snipped-for-privacy@downunder.com wrote in :

Yes, I am working on 2 antenna - , no make that 3, systems now. One is the high end fed for 7 and 14 MHz, I wound the coil, tuned it, and a nice plastic waterproof box has arrived from ebay:

This wil be in it:

The other is the inductive loop:

And the third is the 2.4 GHz dish I stil have to purchase for sat uplink, but that one still has time to 2016.

Anyways looks like the high end fed will be first when I have time, and will be fed with a relative short piece of 50 Ohm coax. Does not take much space and not much 'return' current or radiation from the cable, as the high impedance is shorted by the slightest capacitance to ground. I do not have space for high antennas here, I do have a 27 MHz GPA that I can tune to the bands next to it by shifting the tube out and in a bit.

The plascic box for the high end fed goes outside ...

On a sunny day (Wed, 15 Oct 2014 02:58:37 -0500) it happened John S wrote in :

Oh I know it exists, but not anybody here who uses that.

Agreed.

Voltage drive makes the loudspeaker cone behave more like a servo.

Current drive makes it behave like a bell! In particular hit a resonance frequency of the speakers and the voltage rises.

A current source supplies current into the load and in the ideal case approximates an infinite source impedance. The mechanical components are undamped and can flop about all over the place after a transient.

The impedance mismatch between the loudspeaker(s) cone, the air and enclosure resonances dominates the movement of the speaker cone.

A voltage supply forces the driver to a position and in the ideal case approximates a zero source impedance. The mechanical components are heavily damped and sit roughly where the signal tells them to be.

R is actually Z(w) and due to mechanical resonances inside a finite loudspeaker enclosure not quite the same. The power delivered is not very different but the sound output is much more closely related to the driving waveform with a low impedance voltage source amplifier.

The mass of the moving cone dominates at higher frequencies (and the stiffn ess of the suspension dominates at subsonic ones).

The result is that for a constant applied drive force, cone movement drops by a factor of 4 for each doubling of frequency, above the resonance band.

The idea of driving loudspeakers from an AC voltage source is simply a *CON VENTION* adopted by makers of high quality speaker drivers and speaker syst ems when "hi-fi" arrived. They simply assumed purchasers would own high qua lity amplifiers that employed a goodly amount of NFB.

The vast majority of audio amplifiers made in the tube era used no NFB and so had HIGH output impedance - approximating a current source.

FYI:

It is perfectly possible to make loudspeakers that have flat impedance acro ss the whole audio band and hence derive NO benefit from voltage source dri ve. Most headphones are an example of this and so are many flat panel hi-fi speakers.

In the 1970s, KEF fitted a conjugate network to their 104-2 model to give i t a near constant 6 ohm impedance - rendering "amplifier damping" completel y moot.

... Phil

I can't state how all tube audio amps were designed over the last 80 years for all applications, but it is a fact that guitar amps (Marshal, Fender) of the 60s onwards have of the order of 20dB feedback. I would say actual output impedances were of the order of 10 ohms or so. Marshal actually make transistors amps with an adjustable current feedback loop to adjust damping in order to simulate tube amp response. My Marshall MG100FX has this facility.

It is also a fact that millions of tube telephone amps use large amounts of feedback. This goes back to Black inventing feedback to reduce distortion for the 100s of repeater chains in a telephone system. I am aware that there are a significant number of audio buffs that have tube amps with no global feedback, however, I have not seen any evidence that that is the norm for the masses of commercial tube radios and tube record players used in the

Kevin Aylward

A tube stage typically used cathode resistors to generate the negative control grid bias (relative to cathode). Unless bypassed by a capacitor, this also created some negative feedback.

Also the "ultra-linear" configuration with screen grid voltage taken from a tap at the output transformer _primary_ created some NFB.

Except for some AC/DC powered televisions, there was also a global feedback from the output transformer secondary to the input stage (e.g. 1950's QUAD II). With AC/DC powered televisions, in which the chassis could sit on the mains phase voltage, you either had only an internal speaker with global feedback or a separate speaker without feedback. "De-luxe" TVs had a separate secondary winding for an external speaker and an other for feedback (sitting on the mains voltage).

Most used very little or no NFB at all - the effective output impedance of a VOX AC30 or AC15 is 80 to 100 ohms.

.... Phil