Output impedance list

[Goodrat]

Who can add to this list I've stared. I'm collecting for headphone purposes.
Most have this resistance when the headphones are connected to prevent damage if shorted. This is either from speaking to the manufacturer or schematics posted here. Although a few of the schematics leave the headphone output out for some reason.
This of coarse would be at least the minimum impedance.

KNOWN OUTPUT IMPEDANCES (OHMS):

TESORO:
CUTLASS II/BANDITOII uMAX: 150
SILVER SABRE PLUS: 150
Silver saber: 150
GOLDEN SABRE: 150
ELDORADO (NON-uMAX): 180
GOLDEN uMAX: 150

WHITES:
6000DI: 470
3900: 300
GM III: 168
CLASSIC I: 220

FISHER:
1260: 220
1210: 100

TROY:
SHADOW X2: 150
SHADOW X3/X5: 277

NAUTILUS:
150 ON HIGH CHANNEL, 47 ON LOW CHANNEL.

[chemelec]

Just HOW are you determining these Output Impedances?

The Series Resistor to these Headphones is NOT the actual output inpedance. This Resistor just Adds to the impedance.

Typically it is just a Series Resistor to Limit the current (Also Volume) to the headphones, as well as reduce noise.
Without it, it could burn out the headphone and the volume could hurt your ears. As well the noise could cover up the signals your trying to hear.

[Goodrat]

Yes...

Exactly correct.
So far in actual testing (measuring at no load, then using a decade box to find where the rms voltage of a constant battery test tone is half that) , is very close to that resistor. The resistor would be the absolute minimum.
Although with xfrmr coupled machines like the sov, this does not work well.
So collecting these values is a just a good starting point if you don't have access to the actual machine.

I believe though, if there is a drive transistor in series, you will have a voltage drop (Vce), but the highest output voltage to the phones will still be when the phones match the limiting resistor if no other passive device is in series.

[Goodrat]

Correction...

...highest output POWER when I match the resistor.

(Why is there a 15 minute limit to edits?)

[chemelec]

...highest output POWER when I match the resistor.

(Why is there a 15 minute limit to edits?)

In most cases on these metal detectors, the highest power out will be with No Resistor.
Your Resistor test also needs to be done with AC, Typically its done at 120Hz or 1Khz.

In the case of my detector. For a pizo buzzer, I don't use a resistor because It s not necessary to limit current. There is a pot shown on the schematic, just to control volume, if wanted.
I don't use Either on my own detector

For speaker or Headphone operation, a Minimum 50 ohm resistor must be inserted to limit current or it can:
A) Burn out the 555
B) Burn out the Speaker or Headphones.

Actually for headphones a 150 ohm resistor is better.

**As to the 15 Minute Only, Edit, I Agree.
I would like at least a few hours.