Measuring preamp output impedance

[kacernator]

Hi,

I have aikido preamp and I would like to measure its output impedance. I am using 6SN7 output tubes. According to the table provided by John Broskie, output impedance at 300V and 10mA (240R cathode resistors) should be around 500 Ohm.

So I load the preamp with 15k resistor, generate 300Hz sinewave. Then I measure AC voltages at output with 15k load resistor and without and calculate output impedance. But I do get different results (almost 1k of difference) with different input voltages and/or different load resistor. I am measuring just with DMM. Why do I get such different results?

Thanks.

[palmas]

What voltages you get with and without the 15k load resistor?

[kacernator]

Here are two sets of measurements:

15k - 16,54 ac volts
no load - 17,58 ac volts

and for higher input voltage

15k - 33 ac volts
no load - 36,8 ac volts

Calculating output impedance like this:
Output impedance=(Load resistance*(Vunloaded minus Vloaded))/Vloaded

[palmas]

Originally Posted by kacernator

Here are two sets of measurements:

15k - 16,54 ac volts
no load - 17,58 ac volts

and for higher input voltage

15k - 33 ac volts
no load - 36,8 ac volts

Calculating output impedance like this:
Output impedance=(Load resistance*(Vunloaded minus Vloaded))/Vloaded

Ok, calculating you have an output impedance of 943Ohm and litle more than 1mA.

Fot higher output you have probably reached the current capacity of the circuit, therefor it cannot supply more current and impedance seams to increse a lot.
15k is quite a high load for tube amp. Try using 100k for impedance measurement.

[kacernator]

I think 15k should be ok, because there is cathode follower at output with about 500 Ohm impedance. And I am using 15k because aikido will drive 15k impedance of amplifier.

Cathode follower is bias at 10mA, so even with 40V input voltage it should not be current limited.

Also when I use lower input voltage (say 6V), then calculated output impedance is again different.

[kevinkr]

You should probably be doing these measurements at more reasonable amplitudes IMO, something in the range of 1 - 2Vrms given the intent to drive a subsequent power amplifier.

[Pano]

Agreed. I was a little surprised to see the test voltage listed above. It's nice that tube preamps can do this, but they aren't often asked to. Keep the output below 5V rms for a better idea of what you have.

If you have a DMM that will measure small AC currents, you can directly measure the output current into a load. That's how I do it.

[kacernator]

Well, the preamp is intended to drive 100W class AB unity gain power buffer, so preamp output should be around 40V at full 100W output.

I do not know how to measure it properly, I mean why so different results? Is it the problem with DMM?

[kevinkr]

Originally Posted by kacernator

Well, the preamp is intended to drive 100W class AB unity gain power buffer, so preamp output should be around 40V at full 100W output.

I do not know how to measure it properly, I mean why so different results? Is it the problem with DMM?

You might want to check with Broskie, I don't think the Aikido was really intended to provide such large amplitudes into such a low load impedance. You are heavily modulating the transconductance of the tubes in the white follower stage by asking for so much output current, and make no mistake 3mA is a lot for a follower running at 10mA, more than it can handle and stay linear in all probability. This probably explains the inconsistent output impedance measurements, I would not ask for much more than 1mA in this scenario. Broskie should be able to advise you as to the validity of my comments. (His should be the last word)

Can you change the input impedance of your power amplifier to a more benign 30 - 50K ?

[kacernator]

Aikido output is not configures as white cathode follower, but single ended cathode follower which should deliver max its idle current into the load.

I can change the input impedance of the power buffer to say 50k.

Aikido should drive 15k without problem. Look for example at this hybrid amplifier by Broskie: "http://www.tubecad.com/2007/06/blog0111.htm"
There you have some power buffers too (one with IRFP has even 11k input impedance).