What is the difference between common and GND?

[aballen]

As I build more and more pedals, I'm finding I'm surprised by the effects that I really love.  Compressors for example... so boring, but so awesome.  Reverb... I have really taken this one for granted, but I'm so blown away by my rub-a-dub.  I'm building a tenebrion reverb now too.

After looking at the schematics on the Belton brick I have a question.  What is the difference between common and ground?  I'm looking at the schematics here http://www.accutronicsreverb.com

I'm pretty sure there is a difference and I've reviewed the schematics for the rub-a-dub and the tenebrion reverb.  On the rub-a-dub, it looks like common is "close to GND" there is a 10k resistor between VCC and common.  Is that simply GND on a conditioned supply?  On the tenebrion, I see a Vb, also not GND, but I'm not clear on what it is.

Both examples make it clear I'm not understanding the difference between common and ground on the accutronics schematics.  What is the difference?

[jkokura]

It's a bit confusing to me too. In essence, the difference is usually "digital ground" and "absolute ground". On my reverb, I connected the common and digital grounds, no issues whatsoever. However, if you were dealing with digital separation, usually the practice is to connect all the Commons together, and then have that common make ONE connection to ground, usually through a nominal value resistor.

Not %100 sure why, but I'm not super knowledgeable or scientific.

Jacob

[midwayfair]

Common = everything is connected, the effect, the cable, the guitar, and ground.

Ground = earth. This is an actual connection to earth.

You can have "virtual common [earth]" in a pedal, but the only true ground is the one beneath you.

Edit: People got to calling "common" "earth" because (a) it eventually is; and (b) habit.

[aballen]

Thanks for explaining guys.  Let me ask for a little more clarification.  On the linked schematic from above.

• Are common and ground the same thing?  If so why mix terms?
• Are they suggesting that the brick be a treated like a digital circuit with a single connection to GND, like Jacob is suggesting?

Obviously I'll have to get a brick and experiment.  I do really appreciate the insightful feedback though.

Oh and this is the schematic I'm looking at, in case it wasnt linked properly.

[jkokura]

Well, the regulator still has to be connected to actual ground in order for it to work, so PGND connection to the Brick needs to be as well. I have a feeling that the way the brick is setup internally actually has something to do with it. If I understand correctly, the BTDR-2 is 3 PT2399's setup to create a reverb sound. the other circuitry is simply the interaction circuitry that can be altered depending on need. So, internally, that Brick has 3 digital chips, which may require very careful ground routing. They probably need that one connection to ground, so that's why they've isolated that regulator ground/PGND thing. P maybe stands for PT2399.

Take a look at the Tonepad Rebote 2.5 circuit. On the delay pot, the ground is directly connected to a Digital ground pin on the PT2399 (Gray line). Then it's connected to the rest of ground through one spot. That's similar to what Belton is asking for.

However, in the real world application, when I breadboarded and then tested the brick out, I never had any issues with simply connecting everything to the same ground plane. I imagine in other circumstances it might become more important, but with what I was doing, it never seemed to matter.

Jacob

[aballen]

Thanks Jacob, excellent explanation.

[madbean]

P stands for Power Ground, I'm pretty sure. Most likely it is connected to the ground pin through a very small resistor (~10R) like the digital/analog grounds on the PT2399s.

In the schem above, "common" is referring to the bias voltage which is not shown. Luckily in the vast majority of pedal schematics the terminology is consistent: power, ground and bias. Power is the supply, ground is common (or earth if you like) and bias is some percentage/division of the supply. In some circuits, ground and bias are the same thing (when you have a split rail supply). This may not be the most technically correct way to label things (at least not to a real engineer) but it works well enough for us.

Occasionally you will see an actual distinction between ground and earth---usually when there is a connection between ground and chassis of some type (resistor, large cap, etc).

[aballen]

Perfect, I was thinking a 10R there as well. 

When you mention bias voltage, I think I understand, but in this case I guess we simply assume bias voltage is 0 or pretty darn close because there is nothing listed?

For example in Jacobs design it looks like its 9V->10K resistor.  Not GND, but darn close.... actually as I write this I think I just realized, GND and 0V are not the same thing are they?  GND is a sink, where 0V is simply 0V, a reference voltage like you stated.  So why connect them?

P stands for Power Ground, I'm pretty sure. Most likely it is connected to the ground pin through a very small resistor (~10R) like the digital/analog grounds on the PT2399s.

In the schem above, "common" is referring to the bias voltage which is not shown. Luckily in the vast majority of pedal schematics the terminology is consistent: power, ground and bias. Power is the supply, ground is common (or earth if you like) and bias is some percentage/division of the supply. In some circuits, ground and bias are the same thing (when you have a split rail supply). This may not be the most technically correct way to label things (at least not to a real engineer) but it works well enough for us.

Occasionally you will see an actual distinction between ground and earth---usually when there is a connection between ground and chassis of some type (resistor, large cap, etc).

[madbean]

In a single rail supply, ground is 0v. Bias voltage (VB) is created by a voltage divider consisting of two fixed resistors which are usually the same values (but not always). One in series with the supply, and another right after it to ground. This divides the voltage into a bias supply. When the resistors are of equal value, the result is 1/2 of the supply voltage. When the values are mixed, it is directly proportional to the ratio of the the two values.

The bias voltage pulls up our input DC voltage to whatever is required for a particular IC or transistor we are using as an amplifier. Sometimes there is a range of voltages that this will work over, and sometimes we might under/oversupply the bias voltage for a particular effect. In the case of a non-inverted input on our ICs and transistors we also use another resistor between the bias voltage and the input. This resistor is current limiting. It also sets the input impedance into the amplifier. Inverted inputs to ICs work a little differently, and JFETS don't always require a connection to bias voltage (although it can be a benefit in some cases).

In a split rail supply, ground is still 0v, but it can also act as "bias". This is a handy trick to simplify designs, and the exact thing I used in the Kingslayer, which is a split rail design.

Just to further clarify on the reverb schem above: common is the bias voltage. In a single supply design, it will be a voltage divider (VB). If that circuit were used in a split rail supply, common would be ground. So, really what they are doing is describing the circuit without dictating a particular power supply scheme.

To get a better sense of this, take a look at some examples. For instance, look at the Green Bean schematic to see how the bias voltage is created from the supply and then run to the inputs/bases of the ICs and BJT transistors. Now look at the Kinglsayer schematic. See how bias and ground are the same thing in that design.

[aballen]

Wow Brian, thanks for that.  So much information.  That one took a but to digest but I think I understand.

  I'm going to get bread boarding.  Who knows maybe I'll come up with something that's actually works.