[Jupiter]

In what range should the output impedance for the META42 be? And should there be a big difference between the left and the right channel?

I'm getting 0 and 28 ohms, so I guess something is seriously wrong.

 

[ppl]

How are you determoning output Impedance? it should be the Buffers output impedance / the amount of Feedback applyed. say you have the Opamp set for a gain of 40 dB (100) and the Closed loop gain is 20dB (11) then the amount of feedback is 40 db - the20dB of closed loop gain (11), so the nominal output impedance would be the Buffers output impedance as stated in the data sheet devided by 11.

 

[Jupiter]

I found the output impedance by using a multimeter. I connected the black test lead to ground and the red test lead to the left/right signal. Shouldn't that work?

I've resoldered some joints and I did the measurement again. This time I got around 11 and 13 ohms. That was with double stacked EL2001s. I tried with single EL2001s and I got around 12 ohms for both channels.

Something isn't right here, so I'll poke around with my meter and maybe resolder some more joints. I'll also try to figure out what values I should be getting based on your explanation.

 

[Budgie]

The meter is only going to measure the dc resistence of the circuit. The impedance is the ac resistence, and can not be measured with a multimeter.

 

[Jupiter]

Budgie, I did not know that. But shouldn't the dc resistance for the left and right channel be the same?

By the way, what equipment do I need to measure ac resistance?

 

[Joobu]

If not biased properly, impedance measurements will give misleading results. Most likely, the DMM uses a constant voltage method and measures the current flow. Since the amplifier is an active device this will give incorrect results since the amplifier is driving the ouput as well. In other words, using the DMM resistance function is not a useful figure of merit

There should be only a slight difference between DC impedance (resistance) and low frequency AC impedance since op-amp frequency response is flat until its first pole.

The simplest way to measure output impedance is to have a signal generator (or test CD) for the input so that the output voltage (AC) is known. Measure the output voltage for the unloaded case and several different loadings (32, 128, 300 Ohms). The output impedance can then be calculated from the differences between output voltages.

 

[Jupiter]

ppl, Budgie and Joobu,

thanks for taking the time to answer my newbie questions!

 

[Dev]

If you want, you may use these formulas to calculate output/input impedance of your sources/amps.

http://dev.azz.ru/pic/in_out_imp_formulas.gif

Since source (cdp as well) output impedance is very important thing to cable's sound affect (cdp-amp cord, for example) these formulas is very powerfull

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Maybe you should use more accurate AC-Volt meter to get precise result, than standart cheap multimeter...

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Hmm... I must translate some russian terms to english, for your insight

.

 

[Jupiter]

Thanks Dev! That looks pretty simple.

The input signal should be a 1 khz sinusoidal signal?

 

[Dev]

Yes, 1kHz sine.

 

[Dev]

Or 1000Hz. It's standart signal using for audio devices testing as well...

 

[Jupiter]

Thanks again, Dev!

 

[Dev]

Damn...

I don't certituded in Input Impedance formula.

I've test my CMoy Input Imp and got 600Ohms, but I know and it's working as it is about 300kOhms.

Somebody used this formulas? Get some (wrong?) results?

This formulas is not my creation, so it may be false... I think bug is what Input Imp caclulating by formula of Output Imp, only resistor have change their posture for voltage measure...

However, Output Imp formula is correct certainly

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