damnyankee said:
[quote author=David Kulka]
Your table of currents and meter readings is correct, but practice repeating these words 100 times "Audio is AC. VU meters are AC. dbx meters are DC."
Forget this "audio output amperage" stuff. dbx used a DC meter, driven by a 3 mode metering circuit. In Gain Change mode it basically displays the DC control voltage going to the VCA. In Input mode you are seeing the DC output of the RMS Detector. Output mode (as I recall) is a sum of the RMS Detector DC and the VCA control voltage (input level plus VCA level equals output level).
There is never AC or audio signal connected to this meter. It only measures, depending on mode, different DC voltages within the side chain circuit.
Ok, I wrote it on the board Bart Simpson-style 100 times. ;D
Depressing one of the buttons on the 3 button gang allows us to select DC voltage going to the VCA, DC voltage out of the RMS, or sum of the voltages of the VCA & RMS - which is being read as "decibels" on the dbx
DC meter.
So here's a few stupid questions..
1. Why is current (mA) being measured at various points along the meter scale instead of voltage? I know when I troubleshoot pedals, I look for voltages...
2. Bridge rectifiers are used in AC applications and true VU meters, correct?
3. Would it be safe to say DC voltage meters do not have these rectifiers?
4. As the dbx meter is a DC amperage meter, we would measure the mA rating along the meter points, correct?
Thank you again!
DY
[/quote]
These current / voltage questions are very good, and the topic is an important one that many experienced techs (especially self taught ones, like me!) tend to overlook. For anyone who's really interested, it might be worth grabbing some basic electronics book and reviewing ohms law. I don't want to go way off topic, but it helps to understand that as a general rule electronic circuits are based on current, not voltage. Without current flow there is no circuit, and nothing can happen. Consider a flashlight that is switched off. There is voltage potential at the battery but the open switch prevents any current from flowing, and nothing happens. When the switch is turned on, current begins to flow and the circuit operates.
So circuit operations depend on current, not voltage. However, for most troubleshooting and for much design work, thinking in terms of current is a pain. Compared to voltage, current is hard to measure and hard to visualize. (Ever try to "look at" current with a scope? You can, but it's a big production.) Fortunately, in most cases, voltage measurements tell us what we need to know about power supply and audio circuits. Most audio circuits are capacitively coupled between stages so even though it's current that's doing the work, we can easily "peek" at the AC and DC voltages in different places to know what's going on and locate faults.
Direct coupled circuits are harder to troubleshoot and understand, especially when multiple feedback loops are involved. With amplifier and summing amp circuits "virtual ground" inputs may be involved. These are current inputs, and there is no voltage to measure. Many dbx products use direct coupled, current based designs. (If this 160 project goes forward, I suspect that some people reading this thread may find themselves losing hair while de-bugging the VCA and sidechain circuits. At many of those circuit points, voltage readings tell you little, and the currents can't easily be measured. It can be a challenge!)
Back to the meters, remember that ALL mechanical meters measure only current, not voltage. Meters are "adapted" to measure voltage by adding a series resistor. Current meters are almost always "shunted" with a parallel resistor, which reduces the meter's sensitivity and calibrates the meter's printed scale to the circuit. A panel mount volt meter will always have a series resistor inside it. The dbx 160 has an external network of several resistors and a trimpot in series with the meter.
To answer your other questions:
2. Bridge rectifiers are used in AC applications and true VU meters, correct? YES. I "believe" they usually use copper oxide rectifiers, which have low forward bias.
3. Would it be safe to say DC voltage meters do not have these rectifiers? YES.
4. As the dbx meter is a DC amperage meter, we would measure the mA rating along the meter points, correct? If you have a meter on the bench and want to know it current rating (usually referred to as "full scale current" you connect it to a low voltage DC source in series with a protection resistor and a pot. You carefully increase the DC voltage and/or decrease the pot resistance until the meter reads full scale. You can then measure the full scale current by placing a DVM or VOM, in current mode, in series with the meter under test. (You could also calculate it by adding the values of the pot, the protection resistor, and the meter coil, and dividing the DC source voltage by the total resistance. But it's easier and more accurate just to measure it.) If you have a complete 160 in front of you you could simply connect an external VOM or DVM in series with one of the wires to the Beede meter, and directly measure the actual current that is driving the meter.
Hope that helps!
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