Soundtracs Topaz
VU meters & Lamps



Replacing the VU meters.

VU meters measure the volume (intensity) of analog audio signals. They display signal levels in volume units (VU), a measure of average volume level. By definition, a volume indicator (VI) reading of 0 VU describes a 600-ohm resistance in which 1 milliwatt (mW) of sine-wave power flows at 1000 cycles per second (CPS). Most VU meters have a scale from -20 to +3, and rise and fall times of 300 milliseconds (ms). If a constant sine-wave of 0 VU is applied, the meter will achieve 0 VU in 300 ms. Readings on the VU scale are approximately log to the base 10 of the power ratio referenced to that level. For a pure 100-Hz sine wave, 0 VU equals 0 dBm or 0.775-V root means square (RMS). In practice, however, VU meters contain an isolating resistor so that 0 VU equals +4 dBm for a pure sine wave.

Selecting audio meters requires an understanding of the differences between VU meters and peak program meters (PPM). Whereas VU meters display the average volume level of an audio signal, PPMs display peak volume. The difference between the reading of a VU meter and a peak program meter (PPM) is called the crest factor. VU meters and PPMs also differ in terms of acceleration and deceleration rates (ballistics). Whereas a VU meter takes 300 ms to stabilize when a 1 kHz steady-state tone is introduced, a PPM stabilizes in just 10 ms. Because of their ballistics, then, VU meters must sample the audio signal over a longer period of time than audio meters such as PPMs.

VU meters are designed to provide a very slow response, and are optimized for perceived loudness instead of peak performance. In terms of their internal circuitry, they contain a single diode, a selenium bridge rectifier, or a germanium diode bridge. Although active rectification is used to keep the diode voltage-drop from causing large inaccuracies, amplification of the signal can reduce the overall component count. To control the VU meter’s movement and minimize overshoot, some devices include capacitors.

You can replace the VU meters on the Topaz. Expensive replacement VU meters are from Sifamother VU meters will work, look for ma meters with VU scale calibration to replace the broken meters in your Topaz.


Replacing the lamps inside the factory VU meters

This procedure is for replacing the lamps in the factory VU meters
on the Soundtracs Topaz meterbridge.


* WARNING *
* bulb connections and meter are very delicate, experienced soldering technique is required. *

1. Remove meterbridge from console. You will have to take apart the meterbridge in order to replace the lamps.

2. Remove all of the outer screws holding together the meterbridge's front and rear panel. Once all of the front and rear external screws are removed, the meterbridge's front and rear panels will seperate into two parts rather easily. Check to be sure you got the screws underneath the front LED meters. No need to force or pry open the meterbridge once all of the screws are removed.


topaz meterbridge 01
 
looking at the meterbridge in it's two halves...

A. The front panel, which will have mounted to it the LED meter pcbs & the VU meter module.

B. The rear panel, the meterbridge's toroidal transformer & the XLR terminal for the external transformer.

3. Easy to follow from the transformer are the power lines, terminated with molex plugs, which are connected to the VU meter module. Commit to memory the molex connections of your front and rear panel to the VU module or take a photo if you need, you are going to need to plug it back together. Take note of the pin location for the connection leading away from the VU module to the front panels LED pcb, this specific molex only plugs in to two of the available four pins (see pics). Because we will want to remove the VU module from the rear panel of the meterbridge, unplug the molex connections from the VU meter module.


topaz meterbridge vu meter

note: This is looking at the VU meter module upside down.

4. Once you have unplugged the molex connections and removed the four screw clamps holding the VU module to the front panel of the meterbridge, the VU module should come away free from the front panel.


topaz meterbridge vu meter

5. Place the VU module on a flat surface with the meters facing you.

topaz meterbridge vu meter

Wondering if this is the meter's calibration or needle adjustment screw? "circled yellow"


6. With just your hands you can easily remove the front plastic covers to each of your VU meters on the module. If you want to be more careful you can use a small screw driver and along the base of the plastic VU meter covers you can find availble opennings to easily pry off the plastic VU covers.

topaz vu meters 

7. With the plastic covers removed you can easily see the Lamp bulb lying across the bottom of the meter, right across the needle's bridge. There are two solder connections for each bulb on each meter.

8. Heat up your soldering iron to remove the old factory Lamp from these solder connections. A light touch with your iron should be effecient enough to warm up the solder at the connection and free the leg pins of the Lamp. Each Lamp has two connections at the base of the meter.


In addition to basic soldering skills, a soldering iron, and solder, you might also need a solder wick or pump for removing solder, a small pair of wire cutters, a magnifying glass.

topaz meter bulb

Lamp Bulb Specs


12 volts 55mA - 20mm long and 4mm in diameter, axial wire ended

On top is the original factory bulb,
 bottom is my replacement bulb.


data sheet


9. Use the old Lamp bulb pins as a guide to how much length you will need from the bulb's leg pins in order to solder the bulb between the two connection points. Also, you might observe how the factory bent the connection wires of the bulb and where the bulb rests on the meter. (see pics)

topaz vu meter

10. When you solder in the bulbs, you can take two approaches. First, use a little extra solder on the tip of your iron and just solder the lamp pins right down into the existing factory solder. Seven times out of ten this will work fine and your bulbs will illuminate solid. Though sometimes this may result in an intermittant bulb, turning on and off, or flickering once all put back together. The old solder is causing a bad non continuos connection. So your second approach may be to use a desoldering wick and pull some of the old factory solder off the vu meter's connections, and thus being able to solder directly to the terminal. Both procedures were called for when I did my replacement. The yellow fuzzy plastic, carpetting the bottom half of the VU meter didn't help for a clean blulb replacement.


11. Use you DVM to do a continuity test from the far side of each of your solder connections to the bulb, to feel confident that your solder joints are solid before reassembling your meterbridge to enjoy your new  replacement bulbs.

12. Remount the VU meter module to the front panel, correctly plug back up your molex connections & bolt back together your meterbridge. All Done.



How to adjust, calibrate & test your VU meters

The meter's adjustment screw fitted into the bottom center "solid black plastic area" of the vu meter's plastic cover the vu meter is for setting zero. If your needle seems a bit off zero when at rest, you can use a flat head screw driver to correctly adjust the needle.

to correctly adjust your VU meter...

You will need a 400~1KHz constant tone (I use a signal generator) and an accurate meter good at those frequencies. Your garden variety DVM is very little use past 120 Hz. A VTVM, vacuum tube volt meter, would certainly be ideal.

To adjust VU meter, try running the same sine wave into two channels via Y cable. Use a sine wave no higher than 1KHz and adjust the VU meters to zero to match.

If your DAW is wired into two of your mixer's channels...

set the pans to center, faders to zero, play these
audio test samples

and adjust your VUs accordingly.

1kHz sine wave -20dbfs
40-80 hz pink noise -20dbfs
500-2500hz pink noise -20dbfs
pink noise full bw -20dbfs

To download, just click & save.
For best results adjust your DAW's master volume out.



How to Calibrate the VU Meter on a mixer

Why does a VU meter require calibration?

A VU meter indicates the signal level being sent from the mixer to the next device. A VU meter can be calibrated to any reference level desired. In the United States, the most common reference level is 0VU = +4dbm. That is, when the VU meter needle is lined up with 0 reading on the meter face, the audio signal at the output of the mixer has a level of +4dbm. Please note that the 0 indication on any VU meter has no pre-determined value. 0 VU is simply a point of reference.
What does +4dbm equal in volts?

+4dbm equals a signal voltage of 1.23 volts measured across a 600 ohm load.
What is required to calibrate a VU meter for a reference level of 0VU = +4dbm?

    A multimeter that can accurately measure audio signal levels at 1kHz. A good multimeter is required. Do not use inexpensive multimeters as the calibration will not be accurate.

    A 600 ohm load resistor. Create a 600 ohm resistor by using one 270 ohm resistor and one 330 ohm resistor. Solder the lead of one resistor to one lead of the other. This puts the resistors in series and their values add together to create a "new" resistor with a value of 600 ohms.


    An XLR female connector to mate with the mixer's male XLR output connector. Remove the outer metal shell of the female XLR connector as it will not be used. Solder one lead of the 600 ohm resistor to pin 2 of the female XLR connector. Solder the other lead of the 600 ohm resistor to pin 3 of the female XLR connector.

    Use a set of insulated alligator clip test leads to connect the multimeter to the 600 ohm load.



Here are the steps for VU meter calibration.

    1. Set the XLR output level of the mixer to LINE.
    2. Connect the female XLR connector with the 600 ohm load to the mixer's XLR output.
    3. Using the alligator clips, connect one multimeter probe to pin 2 of the female XLR. Connect
        the other multimeter probe to pin 3 of the female XLR.
    4. Power up the multimeter and set it to read AC voltage.
    5. Power up the mixer.
    6. Turn on the mixer's 1kHz tone oscillator, a signal generator or
        line feed : sampled recorded 1kHz tone (see samples above)
    7. Turn up the mixer's Master control until the multimeter reads 1.23 volts.
        Note: on some mixers you may also have to turn up the level control for input #1.
        In that case, set the Master to the middle of its range and turn up input #1 until
        the multimeter reads 1.23 volts.
    8. With the multimeter reading 1.23 volts (or as close as possible), adjust
        the mixer's VU calibration
        control until the meter reads 0 VU. See your mixer's User Guide to find the location of the VU
        calibration adjustments.
    9. If you have a stereo mixer, do this same calibration for the Left output and the Right output.          Making certain that the channel pan pots are in the center position.

Other common calibration levels besides +4dbm are...

+8dbm = 1.95 volts; 0dbm = 0.775 volts


Question: What is Unity Gain?

A reader writes: "I keep hearing about the idea of "unity gain" when talking about microphone preamps and my recording mixer. What is unity gain, and how does it help?"

Answer: When we talk about "gain", we're talking about one device's ability to take the lower level of one signal and bring it to a higher voltage level. A great example of this is a microphone preamp; a preamp applies gain to amplify the signal coming from a microphone.

In the idea of unity gain, the input and the output between two devices are the same level. That's to say, when a microphone is outputting 0db, a mixer will also be outputting 0db worth of signal. Unity gain is established by calibrating two pieces of equipment to talk at the same level.

Frequently, the best way to set unity gain is to adjust a microphone or line signal gain to 0db, measured both at the preamp and output stage, and then match that 0db level simultaneously on the input of the second piece of equipment -- whether an amplifier, recording software, or mixer.

Unity gain is useful for several reasons. First, it gives significantly cleaner and non-distorted signal. Microphones will sound much fuller and with much better dynamic range. There will also be a much easier time applying outboard effects, as most effects units are set to accept a unity signal.

If you're mixing sound live, getting proper gain staging is important. You'll also nearly eliminate major feedback problems, as a proper unity gain allows optimal gain-before-feedback. Keep in mind, the more gain you can get cleanly and without distortion, the better your mixes will sound!

Interested in UNITY GAIN and Calibration
interesting forum conversation with a Topaz user attempting to calibrate his meters

Some very educated answers are made in this forum conversation. I have paisted here the more learned text from the link conversation above. In the original conversation the user has a Rosetta 200 in their setup, in my editted text below I replaced this with DI ( digital interface ). The Apogee Rosetta 200 was a 2-channel 192kHz 24-bit AD/DA Converter. This theory should work for any DI device in the recording chain, and I don't own one of these DIs, so it helps me to think sometimes if I edit out the non-existent equipment.

Ok, here is an opportunity to clarify this dB business. dB is an abbreviation for decibels. Decibels are a logarithmic measurement, which means they are not a linear measurement. 1+1=2, but 1dB+1dB != (the "!=" means does not equal) 2dB. Decibel measurements are always referenced to a nominal operating level, which is defined as 0 dB. Not only are decibel measurements referenced to a nominal operating level, but they are also referenced to another measurement! To truly understand a measurement expressed in dB, you need to know...

1. The measurement that is being referenced to.
2. What the 0dB value of the referenced measurement is.

+4dBu (remember, capitalization matters) means that a device is operating at 4dB above (hence the + sign) 0dBu, which was defined above as being 0.775VRMS. The u was defined as meaning the high impedance circuit has no load connected. So if 0dBu = 0.775 VRMS, then what voltage is represented by +4dBu? This is where the log and anti-log keys on your calculator become useful. The formula for calculating voltages with logs is dBu = 20 log (base 10) Vo/Vi where dBu is the reference value, Vo is the output voltage, and Vi is the input voltage. We know that 0 dBu = 0.775VRMS, so to determine what voltage corresponds to +4dBu, we calculate it as

4 = 20 log (base 10) Vo/Vi
420 = log (base 10) Vo/Vi
.2 = 10 log (base 10) Vo/Vi

Now we take the anti-log of .2 This can be thought of as raising 10 to the power of .2.

10 anti-log .2 = Vo/Vi
1.585 = Vo/Vi

Now, we know the input voltage is 0.775VRMS, so

1.585 = Vo/0.775
1.585*0.775 = Vo
1.228VRMS = Vo

We have just calculated that audio equipment whose nominal operating level is specified as +4dBu operates at a nominal voltage of 1.228VRMS! stike cooge What are the implications of this? Well, besides learning some math, you now know that if your audio interface outputs, digital I/O (DI BOX), and Topaz inputs are all specified to operate at a nominal level of +4dBu that you could use a voltmeter at each point in the chain to verify that everything is operating at unity gain by checking to see that the voltage is 1.228VRMS.

The simple procedure from the lecture...

1. Play a 1kHz sine tone whose dbFS level you want to represent 0 dB VU. Make certain that all of your faders in your DAW are at unity (0 dB gain) with no other processing.

2. In this case you want to calibrate your DI BOX. So route the sine tone output of his DAW into the DI. Then calibrate the DI so that its meters indicate the same level as the sine tone is set to in the DAW.

3. Then set the faders on the Topaz at unity (0 dB gain) and adjusts the trim until the Topaz meters indicate the same level as the sine tone is set to in the DAW.

4. If he wants to be certain he is operating at unity gain throughout the above chain, he can use a voltmeter to measure his DAW's output and his DI's output to be 1.228VRMS.

synthpic04

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