Audio impedance meter (schematic)

Frequently asked questions about tools and instruments used in stompbox development.

Audio impedance meter (schematic)

Postby apollomusicservice » 16 Feb 2013, 15:26

For all those who are making the effects this is very useful tool.
The basic features are:
-measurement input impedance of effects, various preamps and amplifiers
-measuring the output impedance of effects, preamps
-generating sinusoidal signal with variable amplitude
-use of vu instrument meter for measurement, comparison of two signals simultaneously

Audio impedance meter is useful for measuring the input / output
impedance values ​​0-1K Ohm and 0-1M Ohm, generating sine waves
670Hz frequency variable amplitude over 0-3 V pep voltage and level measurement
signal up to 0 dB (+3 dB)

The precision of the instrument depends on the instrument of used (vu meter), his
calibration as well as the precise gradations potentiometers.
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Re: Audio impedance meter (schematic)

Postby apollomusicservice » 16 Feb 2013, 22:31

For DIYers,
first check sinusoidal waveform at the end of C7. Potentiometer R9 "SG out" can regulate the level of the signal from 0 to about 2v, then
switch S1-switch in position "to input" set the resistors R16 and R18 that the instruments show 0dB. AC voltage from the signal generator
can be measured with a DVM and it should be 0.775v on the reference level 0 dB. Then with R9 reduce the voltage to half value (0.3885V)
and the instrument will show 4dB on the scale. If you are using some other instruments it is necessary to mark its reference level and the
half-value position. Position of potentiometer should be indicated by means of the DVM. First find 500k Ohm value on R13(x100k) and
draw line and No. 5 in the middle of the scale, then on the same way all mark values from 1 to 9.
On the same way draw values for potentiometer R14 (x1k).
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Re: Audio impedance meter (schematic)

Postby ppluis0 » 17 Feb 2013, 01:45

Thank you for post this design, but... Is really the upper part of the diagram a sine oscillator ?

I don't see any feedback loop to force this dual op-amp to generate a signal. (For example a resistor connecting pin 7 with pin 2)

Perhaps is a drawing mistake... :hmmm:

Cheers,
Jose
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Re: Audio impedance meter (schematic)

Postby apollomusicservice » 17 Feb 2013, 09:58

Yeah Jose you right, there is resistor 6k2 between point R6-R7-C4 and pin 2 of IC1. Thanks :D
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Re: Audio impedance meter (schematic)

Postby ppluis0 » 17 Feb 2013, 16:46

You're welcome !!

And thank you again for post this nice design.

Cheers,
Jose
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Re: Audio impedance meter (schematic)

Postby apollomusicservice » 18 Feb 2013, 11:44

procedure for measuring
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Re: Audio impedance meter (schematic)

Postby apollomusicservice » 17 Jul 2014, 19:47

For those who prefer DIY tech.
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Re: Audio impedance meter (schematic)

Postby DrNomis » 18 Jul 2014, 12:44

Cool, I've got to build one of these, cheers for this design...... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.
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