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Mike Rivers[_2_] Mike Rivers[_2_] is offline
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Default Question About Organ Service Manual Test Procedure

On 1/23/2021 3:24 AM, Paul Dorman wrote:
In the electrical test and adjustment procedure, this organ service
manual asks for an "Audio VTVM", to measure Tibia or Bourdon
channel levels, to meter readings of +5dB (1.4 VAC) for example, either
at one of the speaker voice coils, or at the crossover.


An "audio VTVM" is a voltmeter with a high (usually greater than 1
megohm) input impedance AND has a flat frequency response over the full
audio range of 20 Hz to 20 kHz.

Obviously, VTVM stands for vacuum tube volt meter, but can I just use my
regular Volt-Ohm-Meter in AC voltage measurement setting, to
make these measurements?


It depends. Is this an analog VOM, or digital multimeter? And what
frequencies will you need to measure? Most analog meters will have
reasonably good frequency response over the full audio range. On the
other hand, most service-bench DMMs tend to poop out above around 500
Hz. They figure you only need AC volts to measure power line voltage.

My meter is NOT a True-RMS meter, so I know if the signal is too far off
from an ideal sinusoid, the RMS reading will be off, but I am hoping it
will be close enough.


That's true. It depends on what you need to measure when you're doing
your troubleshooting or tune-up. The meter is calibrated to read the
correct value for a sine wave within its working frequency range. If you
need to make voltage measurements of organ notes, your meter won't be
accurate - but that may or may not matter, depending on why you're
making that particular measurement.

And I assume +5dB means +5 Decibels Above the Noise Floor, but I have
never used an audio meter that measured in dB. I would guess that with
such a meter, you would normalize 0 dB to the noise floor, with no signal?


I don't think this is talking about noise floor, but it depends on the
context.

"+5 dB" by itself is a ratio. It tells you how far above a reference
point your measurement is. For example, if you're measuring frequency
response, typically your reference is the voltage at 1 kHz.

5 dB represents a ratio of 1.78 [How do I know this?
http://www.sengpielaudio.com/calculator-db.htm] so if your reference
signal is 1.00 volts, +5 dB would be 1.78 volts.

Unless you're measuring power, in audio, for practical work, we use dBu,
which uses 0.775 volts as the 0 dBu reference. I won't go into the
explanation of that number here. But, for instance, if you measure 2.5
volts, that would equate to +10.18 dBu.

To fully answer your question, we need to know what you're measuring and
why. What does the manual say?

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