here's how you do it for ALL amps.

Posted by Ken Gilbert (A) on September 25, 2002 at 12:39:21

In Reply to: How to measure output impedance of SE amp ? posted by gurevise on September 25, 2002 at 11:52:45:

the old standby is to use a sig gen, a dmm, and a dummy load.

first, hook up the sig gen, set it to zero signal, attach NO LOAD to the output terminals of the amplifier, but hook your dmm set to ACV across the output.

power up the amp.

slowly--CAREFULLY--increase sig gen output until you read something like 5vac across the output terminals. DO NOT READJUST YOUR SIG GEN OUTPUT FROM THIS POINT ONWARDS.

power down the amp.

now, strap your dummy load across the output. for illustration let's assume it's 8r.

power up the amp.

read the resulting Vac across the output terminals. let's assume it's 4.5vac.

power down the amp.

what have we learned?

with no load attached, you are reading the open circuit output voltage of the amp, with no current flowing. with the load attached, you experienced a drop in output voltage. the drop in output voltage is proportional to your internal amplifier's resistance... the higher the output Z, the higher the voltage drop.

with the load attached, what is the current flow in the output section of the amp? ohm's law v = ir, we get 4.5v = i * 8, or i = 0.563a.

now take the DIFFERENCE in the two measured voltages: 5.0 - 4.5 = 0.5v

ohm's law again to find the internal amplifier's resistance which solves for both a current flow of 0.563a and a voltage sag of 0.5v...

0.5 = 0.563 * r

r = 0.89 ohms

hth
ken gilbert

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Follow Ups Full Thread

Topic - How to measure output impedance of SE amp ? - gurevise 11:52:45 09/25/02 (8)

here's how you do it for ALL amps. - Ken Gilbert 12:39:21 09/25/02 (7)

Re: here's how you do it for ALL amps. - Scholl 17:58:10 09/25/02 (2)

Re: here's how you do it for ALL amps. - Ken Gilbert 18:38:42 09/25/02 (1)

Re: here's how you do it for ALL amps. - Scholl 16:55:26 09/26/02 (0)

Re: here's how you do it for ALL amps. - gurevise 14:34:09 09/25/02 (3)

Re: here's how you do it for ALL amps. - Ken Gilbert 18:19:36 09/25/02 (1)

Re: here's how you do it for ALL amps. - PÃ¤r 22:46:00 09/25/02 (0)

Re: here's how you do it for ALL amps. - jdarg 15:38:44 09/25/02 (0)

Follow Ups

here's how you do it for ALL amps. - Ken Gilbert 12:39:21 09/25/02 (7)

Re: here's how you do it for ALL amps. - Scholl 17:58:10 09/25/02 (2)

Re: here's how you do it for ALL amps. - Ken Gilbert 18:38:42 09/25/02 (1)

Re: here's how you do it for ALL amps. - Scholl 16:55:26 09/26/02 (0)

Re: here's how you do it for ALL amps. - gurevise 14:34:09 09/25/02 (3)

Re: here's how you do it for ALL amps. - Ken Gilbert 18:19:36 09/25/02 (1)

Re: here's how you do it for ALL amps. - PÃ¤r 22:46:00 09/25/02 (0)

Re: here's how you do it for ALL amps. - jdarg 15:38:44 09/25/02 (0)

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