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Topic: How do I measure output impedance? (Read 15967 times) previous topic - next topic
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How do I measure output impedance?

Reply #25 –
Quote from: pdq on
Quote from: Arnold B. Krueger on
Transformers require careful engineering to avoid adding audible nonlinear distortion (particularly at low frequencies) and frequency response aberrations.

Which is why it is so difficult to design a tube amplifier with very low distortion and flat response.



Precisely. To that end any number of transformerless tubed power amps have been developed. Some of them are pretty good.

They typically run into severe economic and reliability problems because of the vast number of tubes required to deliver large amounts of power into low impedance loads.

How do I measure output impedance?

Reply #26 –
Quote from: Arnold B. Krueger on
Transformers require careful engineering to avoid adding audible nonlinear distortion (particularly at low frequencies) and frequency response aberrations.

    The cut sheet that you linked neatly avoids dealing with both issues, right? ;-)  Ditto for the Niles volume control, right?


The Niles link I gave says "Frequency Response: 20Hz To 20kHz (+/-) 2dB " which I could probably live with especially since I expect it is more like +/- 1 dB over the more limited central range I'm more concerned with that doesn't include the frequency extremes of 20 Hz and 20 kHz.

Intact Audio only has an animated graphic, no written spec, showing their autoformer based volume control has a flat response with less than 1 dB alteration out to 100kHz (and beyond) for any attenuation level from 0 to -42 dB:

How do I measure output impedance?

Reply #27 –
But that's for the line-level control. Also, these things are not pure inductances. There's also resistance that will change with position. So it might not satisfy a 1/8th rule with certain headphones either. And we've still ignored the nonlinearities of cheap speaker-level autotransformers.

As for resistors, you wouldn't just put 1 in series. You'd e.g. put 180 ohm || 30 ohm for a >250 ohm headphone.

But again, why not just get a headphone amp?
"I hear it when I see it."

How do I measure output impedance?

Reply #28 –
Quote from: xnor on
But that's for the line-level control. Also, these things are not pure inductances. There's also resistance that will change with position. So it might not satisfy a 1/8th rule with certain headphones either. And we've still ignored the nonlinearities of cheap speaker-level autotransformers.


I notice the more complete of the performance specs that have been shown for transformer based solutions only went down to 1 KHz. It is below 100 Hz where you separate the men from the boys in transformer land.

Quote
As for resistors, you wouldn't just put 1 in series. You'd e.g. put 180 ohm || 30 ohm for a >250 ohm headphone.


I think that the recommended solution might be something like an 8 ohm resistor in series with a 1 ohm resistor in parallel.

That follows the 1/8 rule for headphones with impedances as low as 8 ohms which seems to be pretty good coverage.

Quote
But again, why not just get a headphone amp?


Letsee, 2 of these for $10 shipped:

1 ohm NI power resistors

Two of these for $5 shipped:

10 ohm NI power resistors

Put them on a scrap piece of aluminum or other good heat sink.

And you have a zero-added distortion, as good as possible frequency response solution for any AVR.

Due to the built-in attenuation, problems with hum and noise will be far less likely.

How do I measure output impedance?

Reply #29 –
And that whole thing would probably go up in smoke with a bit over 1V output.
"I hear it when I see it."

How do I measure output impedance?

Reply #30 –
Quote from: xnor on
And that whole thing would probably go up in smoke with a bit over 1V output.



I've experimented with those little Caddock resistors, and when properly heat sinked, they seem to do quite well.

Proper heat sinking is not trivial. They are usually in TO-220 packages, and so are the output devices in many of the power amps that they are used with. Therefore, an old power amp heat sink might be in order.

This is a good time to remember that we are using them with music and dialog, not test tones, so their heat stress is vastly reduced by the crest factor of music and dialog.

With a 100 wpc receiver, my proposed design is limited to 3 volts by power amp clipping.

How do I measure output impedance?

Reply #31 –
I'd still consider raising the parallel resistor value. It will reduce the power dissipation and/or you can get higher output voltage for hard to drive headphones, which I think are the targeted thing to drive here.

Can't imagine someone building such an adapter for in-ears or sensitive low impedance headphones.
"I hear it when I see it."

How do I measure output impedance?

Reply #32 –
Forty years ago I worked in audio equipment repair. One trick that we used to provide a high-wattage load on an amplifier was to take a 8 ohm ceramic resistor and insert it into a container of water. This kept the temperature of the resistor at a safe level while delivering many times its rated wattage. I don't recommend this in general, but it could be used as a temporary solution until you can supply a proper heat sink.

How do I measure output impedance?

Reply #33 –
If you're already ordering parts anyway then just add proper heat sinks and maybe even a case and headphone jack etc. to build a nice small adapter box.
"I hear it when I see it."

How do I measure output impedance?

Reply #35 –
Not water, use mineral oil (or maybe light motor oil).  Ham radio operators do that to dummy loads for their radio transmitters.  Heathkit even sold oil filled dummy loads.
Kevin Graf :: aka Speedskater

How do I measure output impedance?

Reply #36 –
22 AWG nichrome wire has a resistance of about 1 ohm per foot. A 25 foot spool (less than $5) would make two nice resistors, which you would tap at a point that gave you a 10:1 divider. Wind each around a non-conductive, non-flammable cylinder.