How to measure output impedance?

[solderdude]

For DAC XLR measurement, I don't follow what you mean by headphone plug ?

You quoted the part about headphone measurements, hence my reply and assumed you meant XLR4 out.
There you measure between hot and cold.

The same applies to balanced DAC XLR instead of 10 or 100 ohm use a 1k resistor. Same procedure.
measure between hot(pin 2) and cold (pin 3)
One should realize that the output R of a DAC is of very little concern as the load it gets is mostly resistive and relatively high. Yes, when the output R is unusally high (tube output for instance) and very long and high capacitance interlinks are used the output R of a DAC may be of some influence it the upper treble.

It is something I would never worry about myself. Headphone out and speaker out... sure.

 

[Music1969]

The same applies to balanced DAC XLR instead of 10 or 100 ohm use a 1k resistor. Same procedure.
measure between hot(pin 2) and cold (pin 3)
One should realize that the output R of a DAC is of very little concern as the load it gets is mostly resistive and relatively high. Yes, when the output R is unusally high (tube output for instance) and very long and high capacitance interlinks are used the output R of a DAC may be of some influence it the upper treble.

I have an SMSL D-6 and Amir didn't measure this and SMSL have a history of sometimes >1000ohm output impedance, so I was just curious. My SMSL SP400 input impedance is 47kohm so I'm not concerned just curious. And Amir did measure headamp output impedance and near 0 ohm so I won't bother checking that.

So how to attach resistor ?

Best to find a cheap XLR plug to use and solder 1000ohm resistor between pin 2 and 3 ?

Lets assume this DAC output impedance is 1000ohm - should i also make a 2000ohm XLR plug+ resistor to test?

For a couple data points

 

[solderdude]

find a cheap XLR plug.

solder 2 wires to it.
one to pin 2, one to pin 3.
Connect the meter to the wires.
Play a tone (suggest 100Hz) there are plenty of online tone generators.
Set meter to 2VAC setting
Set volume so that the meter reads just below 2V... say around 1.8V.
write down the voltage and call this Vopen.
Now attach the resistor 1kohm to the wires while playing the tone and while measuring. (maybe cloth pegs to keep it in place) and note the voltage, call this Vload
Now you have all the data you need.

Vload/Vopen gives you Vratio

( (1 - Vratio) / (Vratio) ) gives you Rratio.
Rratio x Resistor value in ohms gives you Rout

 

[Music1969]

find a cheap XLR plug.

solder 2 wires to it.
one to pin 2, one to pin 3.
Connect the meter to the wires.
Play a tone (suggest 100Hz) there are plenty of online tone generators.
Set meter to 2VAC setting
Set volume so that the meter reads just below 2V... say around 1.8V.
write down the voltage and call this Vopen.
Now attach the resistor 1kohm to the wires while playing the tone and while measuring. (maybe cloth pegs to keep it in place) and note the voltage, call this Vload
Now you have all the data you need.

Vload/Vopen gives you Vratio

( (1 - Vratio) / (Vratio) ) gives you Rratio.
Rratio x Resistor value in ohms gives you Rout

Thanks! A lot of people have suggested "use a resistor" but I wasn't sure of the best way to practically do it.

This makes it crystal clear now

 

[Music1969]

solder 2 wires to it.
one to pin 2, one to pin 3.

For a no-soldering option, can also use leads with alligator clips on each end?

One end for XLR pin and other end for resistor?

 

[solderdude]

For a no-soldering option, can also use leads with alligator clips on each end?

One end for XLR pin and other end for resistor?

sure.

I would use a 1k resistor.
When the voltage hardly drops (< 10%) while attaching that load you will already know that the output R is below 100 ohm.
When the voltage drops 50% the output R is 1k.
When the voltage drops more than 50% the output R is above 1k for sure.

 

[pma]

It should be understood that the output impedance of the power amplifier is complex, nonlinear and frequency dependent.

The current injection method I have shown before is the way to investigate, in case we do not want to stick with elementary textbook number. We inject a defined clean current into the output and measure voltage drop at amplifier terminals. If we omit/short the output inductor, we get an X-Y plot like this:

X-axis shows injected current (1kHz sine wave with 1A amplitude) and Y-axis shows voltage drop at amp output terminals. Elliptic shape reflects phase angle of output impedance at 1kHz. If the output impedance was linear, we would see a perfect ellipse. Irregularities near to zero current reflect the output impedance nonlinearity, namely the Gm doubling, in other words crossover distortion. This is a very effective method to investigate amplifier linearity, especially when output inductor is omitted or shorted.

 

[Music1969]

sure.

I would use a 1k resistor.
When the voltage hardly drops (< 10%) while attaching that load you will already know that the output R is below 100 ohm.
When the voltage drops 50% the output R is 1k.
When the voltage drops more than 50% the output R is above 1k for sure.

Success !

I wanted to check my SMSL D-6 given some past issues with SMSL and high output impedance DACs (over 1000 ohm)

Some theorised this helped achieve higher SINADs but I couldn't understand how.

I can now confirm SMSL D-6 has output impedance 200 ohms on XLR output

Vopen = 1.792 V
Resistor = 999 ohm
Vload = 1.491 V

It only cost me $10 for a cheap XLR plug, alligator clip leads and 1kohm (999ohm measured) resistor

Thanks for your help !

 

[solderdude]

Looks like 100ohm in series with each output.

 

[Music1969]

Looks like 100ohm in series with each output.

That's a pretty typical (and good?) number?

100ohm for single ended and 200ohm for balanced output?

 

[solderdude]

That's a pretty typical (and good?) number?

100ohm for single ended and 200ohm for balanced output?

Yes, pretty typical for an output R (source side) to be somewhere between 50 and 600 ohm or so.

 

[Music1969]

Yes, pretty typical for an output R (source side) to be somewhere between 50 and 600 ohm or so.

<1 ohm for all of Hugo TT2's outputs - definitely not the normal then (like the price) !

 

[solderdude]

That is definitely an exception and can only be done when the output stage is very stable under capacitive loads and can handle a lot of current.
Most output stages consist of a simple opamp and the output resistor 'isolates' that from the angry outside world and ensures the feedback loop does not become unstable with capacitive loads or is forced into 'output current clipping' when shorted accidentally.

Such and unusual low output R could be handy when doubling as headphone out for instance.

 

[xnor]

<1 ohm for all of Hugo TT2's outputs - definitely not the normal then (like the price) !

I wonder how happy it would be with an accidental short or some long (high capacitance) cables.

 

[jkim]

find a cheap XLR plug.

solder 2 wires to it.
one to pin 2, one to pin 3.
Connect the meter to the wires.
Play a tone (suggest 100Hz) there are plenty of online tone generators.
Set meter to 2VAC setting
Set volume so that the meter reads just below 2V... say around 1.8V.
write down the voltage and call this Vopen.
Now attach the resistor 1kohm to the wires while playing the tone and while measuring. (maybe cloth pegs to keep it in place) and note the voltage, call this Vload
Now you have all the data you need.

Vload/Vopen gives you Vratio

( (1 - Vratio) / (Vratio) ) gives you Rratio.
Rratio x Resistor value in ohms gives you Rout

This formula is even simpler:

ROUT = RLOAD x (VOPEN / VLOAD - 1)