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I was working on building some projects from the book Electronic Projects for Musicians and I noticed that in most of the schematics there are two symbols for what seems to be ground. One is a - (minus sign) inside a circle, which is connected to Vcc- pin of an Op-Amp (in addition to a + going into Vcc+). The other is the regular fork symbol for chassis ground, which is connected to the third leg of potentiometers and such. What is the difference between these, if there is any? Is it necessary to keep them separate, or are the - and the chasis ground both connected to the negative terminal of the battery?

Update: the op-amp I am using is an NE5532, in the datasheet it specifies a Vcc+ and a Vcc- (http://www.ti.com/lit/ds/symlink/ne5532.pdf)

I also included a picture of an example schematic, thank you for your answers.

enter image description here

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  • \$\begingroup\$ Adding an example of a schematic with these symbols would help us give you a better answer. \$\endgroup\$
    – The Photon
    Commented Apr 16, 2016 at 22:56
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    \$\begingroup\$ "Is it necessary to keep them separate?" YES. I feel that none of the answers actually answer this, but goes into lengthy descriptions of how other OP-amp circuits work while it is obvious from the illustration that this is not one of them. \$\endgroup\$
    – pipe
    Commented Apr 18, 2016 at 1:32

4 Answers 4

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Op-Amps come in two forms. Single Rail/Supply, and Dual Rail/Supply.

Your op-amp, the NE5532 accepts +15V to -15V, and can be used in a single or dual supply mode: enter image description here

enter image description here

Single Rail are typically Positive Voltage to Ground reference, while Dual Rail op-amps typically have Positive to Negative.

Single Rail are more common in newer, DC based designs, while Dual Rail are older, or AC designs.

Typical Audio is an AC signal, swinging above and below the ground reference. Single Rail designs cannot swing below it's low side reference. At best, you can add a DC offset and a virtual ground.

In your case, it's plainly a dual rail op-amp, with VCC- meaning the negative voltage rail of the power supply.

It will often be matched by a dual rail transformer:

enter image description here

This one uses a center tap between the two rails as the ground reference.

See http://www.talkingelectronics.com/projects/OP-AMP/OP-AMP-1.html for more information.

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  • \$\begingroup\$ And for this particular circuit (a classic diode-clip distortion), what does the text say about power? Does it say "two 9v batteries" or one? \$\endgroup\$
    – rdtsc
    Commented Apr 18, 2016 at 13:33
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Vcc- is only for the Op-Amp and it is consider to be the negative supply voltage. Op-Amps usually have dual power supply like +/-5V, +/-10V, so their output can vary between these voltages. The reason behind this is that the Op-Amps should operate in both polarities of the input signal, given a 2Vpp sine wave. Without a negative voltage reference the negative part of the input signal would be chopped off at ground level.

For example, with an inverting amplifier circuit we expect the following:

enter image description here

But, without the Vcc- we would only get:

enter image description here

If you do now have to deal with negative inputs Op-Amps can be operated with single-supply as well, in that case Vcc- would be GND.

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The minus sign inside a circle is a dedicated negative voltage for the op-amps, usually -3v to -18v, often labeled (Vee). Normally it is the opposite of the positive supply pin (Vcc).

The signal ground (usually 3 horizontal bars) is the true ground for the circuitry. It may tie to earth ground at a screw somewhere on the board. If the board had a mix of analog and digital circuits, the ADC (Analog Digital Converter) chip is where the analog ground meets the digital ground, because this is where the analog readings would be the most accurate.

If the op-amp had a common gnd of zero volts (some do) on it's (-Vee) supply it will connect to the closest signal ground trace. The presence of polarized capacitors on both the positive and negative supply rails is a good sign it is a bipolar power supply, and signal ground is its common center reference point.

If in doubt, always use a voltmeter to verify if (-Vee) pins are negative or common grounded. For logic circuits the -Vee voltage could be zero to -1.5v, for audio mixing boards the op-amp (usually a SSM2142 6000 ohm line-driver) could have supply rails of +/- 18VDC. The dynamic range for todays mixing boards is about 120db.

Most battery powered stuff like cell phones use 'single ended' op-amps with the Vee pin tied to common ground, and the Vcc pin will have a voltage +3.3 to +5 vdc. It is for more common daily use with a dynamic range of 60db, still much better than old vinyl records or cassette tapes.

The 2 categories of op-amps in terms of single-ended vs. dual +/- power supplies are normally NOT interchangeable. The expensive OP220EZ will accept either type of power supply (even unbalanced), but has output range and load limits. Some like the SE5539 need a gnd pin as well as a -Vee pin.

The trend toward lower voltages with a single positive supply is a matter of performance vs. cost and space, and less power consumption. Digital IC's have become tiny and work mostly with a single +1.9v to +3.3v supply. IC's for audio and RF are following this same trend, but the symbols for supplies and signal ground and earth ground have not changed for many decades.

Those and other symbols are controlled by the IEEE committee and NEC standards.

When in doubt, check your diagram for that design. If it indicates a bi-polar power supply, then your Vcc- pin will have a negative voltage opposite that of the Vcc+ pin. If it is a single-ended power supply, the Vcc- terminal will be tied to signal ground, using a IC designed for that purpose.

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  • \$\begingroup\$ I am fairly certain it is a negative supply, pin is labled Vcc-. Does that mean it should be connected to the signal ground or separate? \$\endgroup\$
    – Emmett P
    Commented Apr 18, 2016 at 0:39
  • \$\begingroup\$ No difference between using Vcc- and Vee-, except that some IC's have several positive and negative power pins and even a signal gnd pin or pad is possible. On Ethernet IC's the bottom is usually one huge grounding pad - and heat-sink. If in doubt, always use a meter on a pin listed for + or - power, just to be sure. \$\endgroup\$
    – user105652
    Commented Apr 18, 2016 at 1:15
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You actually have separate GND net and chassis net. Ground being the inverse pyramid as you probably know.

Usually how this works is that chassis is the metal enclosure as the name implies. This may or may not be connected directly to the external ground from PSU and/or mains PE. The ground on the other hand is usually is your zero-volt point for whatever DC/DC or AC/DC supply you may have on-board. This usually goes "everywhere" but is not directly connected to the chassis ground. Normally you have common mode choke at the very least between external (DC) supply and the internal circuitry and GND would be on the "inside".

Normally you do NOT connect circuitry to the chassis as it's not tightly coupled to the actual circuit GND so you'll have current loops causing noise and instability. Chassis is usually connected to whatever filters you may have on I/O going out of "the box" to short incoming and outgoing conducted noise to the metal enclosure.

There's also some differences in opinion if you should, in fact, tightly couple chassis with the circuit GND. This mainly has effect on EMI shieldind and depending on your reference this may hurt or help you. Naturally if you're using on-board AC/DC power supply, it's isolated and circuit GND should have nothing to do with the mains PE that usually goes to chassis.

To confuse matters more, on isolated I/O (ethernet for example) you have separate "chassis" which comprises of the ethernet connector shield and needs to be actually isolated from the device chassis.

Tl;dr In my opinion having chassis and GND in the same circuit is bad.

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