Measuring preamp output impedance, resistor termination etc.

ln76d · Nov 13, 2015

Since i have no idea what's the output impedance of my preamp i want to measure it.
I tried potentiometer method.
Line input, level scale set to 0dB.

I replaced 100k termination resistor with 100k potentiometer, generator (600Ω output) 2.2Vpp

Output:
Without termination and with 100k - 1.9Vpp
Drop to 0.9Vpp with potentiometer set to 5Ω

With 1k termination - 1.9Vpp
Drop to 0.9Vpp with potentiometer set to 15Ω

Changed input level to 1Vpp

Output:
Without termination and with 100k - 0.8Vpp
Drop to 0.4Vpp with potentiometer set to 2.88Ω

With 1k termination - 0.8Vpp
Drop to 0.4Vpp with potentiometer set to 23Ω

Right now what i measured gives me 15-23Ω impedance range.

Is this method gives proper results?
How i should properly measure it?
Should i measure it with 100k terminating resistor and additional potentiometer load?

JohnRoberts · Nov 13, 2015

Not sure what you expect.

If you are measuring the low impedance output from an active IC that source impedance will change with frequency (due to loop gain margin).

Prudent design generally includes resistors in series with the output . At LF you'll measure the impedance of the caps, at HF the IC.

I am not familiar with that driver perhaps look at it's spec for internal source impedance resistors?

JR

PS: I'd generally like to see 50-100 ohms in series with each output

RuudNL · Nov 13, 2015

The easiest way to measure the output impedance of a device:

- Measure the output voltage of the device without a load. (With a signal generator connected to the input.)
- Connect a resistive load to the output and observe the output voltage.
Lower the load resistor until the output level is half of the output without a load (-6 dB).
Observe that this does NOT cause distortion!
In this case the load resistor is the same as the output impedance of the device.

The easiest way to measure the input impedance of a device:

- Connect a signal generator to the input and measure the output level.
- Connect a resistor between the signal generator and the input.
Increase the value of the resistor until the level at the output of the device is -6 dB compared to the situation without the series resistor.
The input impedance of the device is now the same as the series resistor.

(The output impedance of the signal generator should be low compared to the input impedance of the device under test,
Input and output impedances may be frequency dependent.)

ln76d · Nov 13, 2015

JohnRoberts said:
Not sure what you expect.

If you are measuring the low impedance output from an active IC that source impedance will change with frequency (due to loop gain margin).

Prudent design generally includes resistors in series with the output . At LF you'll measure the impedance of the caps, at HF the IC.

I am not familiar with that driver perhaps look at it's spec for internal source impedance resistors?

JR

PS: I'd generally like to see 50-100 ohms in series with each output

Thank you John!

Unfortunatelly i have no spec for the driver and no luck with email contact to the manufacturer...
Best part!
Preamplifier specification - "Module Line Output - Electronically Balanced" ;D
It's the only information from my Technical Handbook".

Am trying to trace the driver circuit and draw schematic.
It's not so easy, because resistances are "printed" on the board (i don't know how that technique is called).

In attachment is the board without opamp - NE5532.

Measured resistance on both outputs - 21.21Ω and 21.32Ω

ln76d · Nov 13, 2015

RuudNL said:
The easiest way to measure the output impedance of a device:

- Measure the output voltage of the device without a load. (With a signal generator connected to the input.)
- Connect a resistive load to the output and observe the output voltage.
Lower the load resistor until the output level is half of the output without a load (-6 dB).
Observe that this does NOT cause distortion!
In this case the load resistor is the same as the output impedance of the device.

The easiest way to measure the input impedance of a device:

- Connect a signal generator to the input and measure the output level.
- Connect a resistor between the signal generator and the input.
Increase the value of the resistor until the level at the output of the device is -6 dB compared to the situation without the series resistor.
The input impedance of the device is now the same as the series resistor.

(The output impedance of the signal generator should be low compared to the input impedance of the device under test,
Input and output impedances may be frequency dependent.)

Thank you RuudNL!

Great post, and i think some other users will definatelly enjoy this

So the method which i did is correct?
Or did i miss something in your description?

JohnRoberts · Nov 13, 2015

That picture look like some smd caps and screened (?) resistors that have been laser or machine trimmed.

JR

ln76d · Nov 13, 2015

Yes definitely those are "trimmed" resistors!
Happy to know the technique name

It's hard to measure, i'll try at the day.

Now i know why the manufacturer support is so generous:

"Your message was deleted without being read on 14 November 2015 00:05:02 UTC."

Yay!

Somebody knows what happend (again?) with Audix Broadcast Ltd.?
Notification came from "ultra-woodanddouglas.com" domain...

Measuring preamp output impedance, resistor termination etc.

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