[sts9fan]

Can this be done with a DMM?   I am audtioning one of the Eastern Electric BBA units which has a variable output impdance to aid in matching componets.  The impedance is adjusted with variable feedback gain and attenuation.  I am not big on guessing games so I would like to "set it and forget it".  Thanks for your help.   

Kris

[ctviggen]

I don't think so, at least if you want to measure by putting the DMM on the output terminals.  A DMM typically uses a battery to pass current through the unknown resistance.  Based on how much current passes through the unknown resistance (and, really, on a known resistance inside the meter), the DMM can determine a value for the unknown resistance.  You'd have to measure resistance of a live circuit, which is a recipe for blowing the DMM.

I have several amplifier books at home.  One might tell me how to measure the output impedance.  I will try to look tonight to see what they say (you might want to PM me to remind me).

[JoshK]

DMM's measure "resistance" not "impedance", at least not the standard ones.  Impedance is the AC equivalent of DC resistance. 

[amplifierguru]

Hi sts9fan,

Perhaps I can help - if I knew what a "Eastern Electric BBA unit" is?

Using a sig gen with a known output level through a resistor R into the output concerned while reading the level on the output, then calculating the output Z as that fraction of R as the fraction of signal level. This is good when Zout << R. Do it for a number of frequencies.

Greg

[Steve Eddy]

Perhaps I can help - if I knew what a "Eastern Electric BBA unit" is?

Damn, Greg. Ain't y'all got Google down there? 

Here ya go:

Eastern Electric Vacuum Tube BBA

se

[mgalusha]

The easiest way is to get a cheap 10K pot from Radio Shack and feed a 1KHz signal to the BBA, via a DAC or preamp or whatever. There are plenty of signal generator programs on the web for free.

Measure the voltage of the BBA output with no load then connect the pot to the output, the wiper to one side and either end of the pot to the other (doesn't matter which). Adjust the pot until the BBA output voltage is exactly 1/2 of what it was without a load. Disconnect the pot and measure the resistance of the pot between the wiper and the end you connected to the DUT and this will be the output impedance of the BBA (or preamp/CD Player etc). No math required and if you use a 10K pot you can measure output impedance up to 10K. Not suitable for measuring amps however.

Repeat for any frequencies of interest. Note that many DMM's aren't accurate at high frequencies.

[sts9fan]

cool thanks I will try that. I knew I could not just attach the DMM and measure i was wondering if the was a method using one that was suitable.