As some of you know....
Dino (DIGI2T), me, and a cast of other characters are working on a MN3007 version of the Mutron Flanger.
We're getting close to building the prototype. (parts in the mail)
There is one thing that is bugging me a bit, so I turn to you great forum members for some knowledge so I may understand this anomaly.
The power supply section outputs 15 volts to "power" ground. There is also a "virtual ground" circuit that feeds the signal/delay sections and a few points in the LFO section.
The buffered virtual ground circuit sits at 1/2 VCC (7.5 volts) in relationship to power ground.
(https://dl.dropboxusercontent.com/u/53299166/DIYstompboxes/Mutron%20Flanger%20virtual%20ground.jpg)
Now all of this is good with nothing out of the norm.......except that the virtual ground is connected to the sleeve of the input/output jacks thereby making a connection to the enclosure! (non isolated jacks)
This connection is depicted in the schematic, and verified with voltage readings by Dino in the actual unit.
Now this is the part I can't seem to wrap my mind around......7.5 volts on the sleeve and enclosure!
This means that 7.5 volts will be "sent" through the sleeve connection to your guitar and amplifier and any effect unit in between!
Can someone explain how this works?
Here is the schematic for reference:
(https://dl.dropboxusercontent.com/u/53299166/DIYstompboxes/Mutron%20Flanger%20redraw.jpg)
I just found some info in "Electronics for Guitarists" by Denton J. Dailey which outlines the mixing of virtual ground circuits and "standard" 0 volt ground circuits.
It seems as if there is a potential for short circuit and damage to the virtual ground op amp when sharing a common wall wart supply.
Check out pages 128-130:
http://books.google.com/books?id=HccWz4_ym7YC&pg=PA129&lpg=PA129&dq=virtual+ground+connected+to+input+jack&source=bl&ots=JcYgyTR6QJ&sig=ZslYhyjaOsIjeQYom8_d5qgfjzY&hl=en&sa=X&ei=jOs1Utv1B7DXiAKC04DQDg&ved=0CF0Q6AEwBQ#v=onepage&q=virtual%20ground%20connected%20to%20input%20jack&f=false
I was thinking that the original Mutron Flanger utilizes an AC power cord to a transformer. This transformer would provide isolation and the only return path for the virtual ground.
The build we are working on has the potential to share power with other effects.....so.....I believe that the input/output sleeves and the enclosure should be connected to "power ground" 0 volts.
Any thoughts?
I would think that the virtual ground at the input of the opamp (R73 and R74 in your first schematic) shouldn't be connected to the sleeve or the enclosure , it should only be used to bias the inputs of the opamps , the sleeve and enclosure should be connected to the Power ground not the virtual ground ......
Quote from: Minion on September 15, 2013, 02:35:44 PM
I would think that the virtual ground at the input of the opamp (R73 and R74 in your first schematic) shouldn't be connected to the sleeve or the enclosure , it should only be used to bias the inputs of the opamps , the sleeve and enclosure should be connected to the Power ground not the virtual ground ......
Yes....this is what I think as well.
As I stated....I believe that the original Mutron design "worked" because the transformer created an "isolated" return path for all aspects of the power supply that wasn't "shared" with any other effect.
"Ground" is whatever you say it is - except you only get to call one net "ground".
Remember, there are three kinds of "ground" - reference ground, which tells your circuit what Real, True Zero Volts is; then shield ground, which keeps radiated nasty signals off your signal lines; and finally "sewer ground", which is what returns used electrons to the power supply.
Reference ground is needed so your signal can tell how the signal is moving. "Sewer ground" only happens when one combines the power supply return lines with the reference ground. This is an unfortunate side effect of trying to make single-sided power supplies. When you mix sewer ground and reference ground on the same wire, you can get ugly things happening. Sometimes you get away with it.
The use of single power supply circuits has confused issues so that many people thing one side of the power supply *is* reference ground. This is not necessarily true, and it leads to some conflicts, not least being the difference between attaching the negative side of the power supply to ground in one pedal, the positive side to ground in another pedal, then trying to connect both of these grounds with a piece of wire (i.e. the shield of a guitar cord) and slamming the power supply.
It is perfectly OK to call some voltage between V+ and V- on a unipolar power supply "reference ground", and attach it to the input jacks and enclosure. But you can NOT then successfully connect either of the power supply leads to "ground" either intentionally by internal wiring or unintentionally by external connections or paralleling something else to the power supply that attaches one or the other power supply lead to this third-party-circuit's "ground", and then hooking the two ground together. This is just another version of the positive/negative ground tragedy.
Quote from: R.G. on September 15, 2013, 03:47:46 PMThis is just another version of the positive/negative ground tragedy.
Also known as "The tragedy of the commons."
R.G.
Thank you for the reply! I was hoping that you would "magically" appear for this one.
Quote from: R.G. on September 15, 2013, 03:47:46 PM
It is perfectly OK to call some voltage between V+ and V- on a unipolar power supply "reference ground", and attach it to the input jacks and enclosure. But you can NOT then successfully connect either of the power supply leads to "ground" either intentionally by internal wiring or unintentionally by external connections or paralleling something else to the power supply that attaches one or the other power supply lead to this third-party-circuit's "ground", and then hooking the two ground together. This is just another version of the positive/negative ground tragedy.
Just so I have a better understanding, why does connecting the rail splitter ground to the sleeve work (assuming it doesn't cause problems with some pedals) in this Musitronics design? Is it as I surmised...not sharing the power supply among multi effects?
The transformer providing some form of isolation?
Since the "clone" circuit might be powered by a supply supplying more than one pedal, I'll connect the sleeve/enclosure connections to power ground so this build will "play nice" with other pedals.
Quote from: armdnrdy on September 15, 2013, 04:14:53 PM
Just so I have a better understanding, why does connecting the rail splitter ground to the sleeve work (assuming it doesn't cause problems with some pedals) in this Musitronics design? Is it as I surmised...not sharing the power supply among multi effects?
The transformer providing some form of isolation?
The transformer provides complete isolation. It's secondary is completely floating. That is, the transformer secondary and rectifier/filter/regulator generates a completely floating 15V supply. The rail splitter generates a signal ground that is halfway between the two sides of the power supply. Notice in the lower right corner of the schematic, there are two different symbols for "ground", signal ground and power ground. As long as these are not connected together with a wire, all is well. The closed-triangle symbol for power ground must always remain NOT connected to the three-bars signal ground symbol net, otherwise the the rail-splitter opamp IC11B has its output shorted to the negative power supply for the chip.
There is an inside and an outside to this circuit. The inside of the circuit uses the negative power supply rail (closed triangle) for both reference and power return/sewer. Presumably they did the wiring well on the boards so that this does not cause noise issues. The outside of the circuit is connected to the "signal ground", which performs the same function as a "Vbias" in many pedals. However, instead of tying the signal ground to the power supply minus side with a wire like most pedals do, the designer wired it to the chassis and only connected the "signal ground" to the minus power supply with a capacitor, so they could be a fixed DC voltage apart. It's a confusing way to set things up. Perhaps the idea that someone would ever try to use this without its own power supply never occurred to the designers.
It has the side effect that you cannot use the same power supply for this pedal and other pedals without fixing the grounds somehow.
QuoteSince the "clone" circuit might be powered by a supply supplying more than one pedal, I'll connect the sleeve/enclosure connections to power ground so this build will "play nice" with other pedals.
That won't work. What you have to do if you want to power this pedal and others with a single power supply, you have to
(1) Go through the schematic and everwhere you find the three-bars ground symbol, replace that with "Vbias". This step is to help keep your head straight.
(2) Check each place there is a "Vbias" from step 1 and see if it needs any work; at least two places do, those being the input and output jacks, which need pulldown resistors. There may be others, I haven't checked exhaustively.
(3) Hook the jack sleeves to the negative side of the power supply.
This in effect converts the power supply back to +V and combined sewer/signal ground, and renames the previous "signal ground" to "Vbias". Now you can share the sewer/signal ground with other circuits.
Thanks for the guidance R.G.
Here is the redraw with the (confusing) virtual ground symbols replaced with Vbias symbols, input/output jacks connected to the negative supply, input/output pull downs added, and new power supply depicted.
(https://dl.dropboxusercontent.com/u/53299166/DIYstompboxes/Mutron%20Flanger%203007%20Vbias.jpg)
Looking...
R37 isn't needed.
Places where a capacitor is connected to Vbias could be connected to ground/V- if you have noise issues with the finished product. This ought to be the same, but the more places connected to Vbias, the more it must be **pristinely** quiet and well bypassed. C2 and C8 are examples, but for different reasons. C2 is to bypass high RF, C8 bypasses current peaks from the envelop detector. Normal signal connections to Vbias don't matter all that much. This is a nit.
If your power supply is fed with 18Vdc, the negative side of the 18Vdc must NOT be shared with another pedal that can tie it to signal ground. This is one version of the "univibe problem".
I didn't catch any, but any place there's an electro cap that's not NP, it should have a DC voltage across it the correct way, not nearly 0V.
You're working hard on this.
Thanks for the reply R.G.
Yes...I am working hard on this one....sometimes good things don't come easy!
After I posted the above redraw, I took a look at a few somewhat similar single supply circuits (Wow! say that fast 10 times) and I did see that there are other things I'll have to change. I noticed certain caps that probably want to go to ground, the feedback and rate pots, T5 trimmer, etc.
I was wondering if you had noticed the link that I posted earlier. The exact sleeve ground connection issue is outlined in this article and an unbelievably easy fix is given as well.
I'll post it again. Check out just above page 128-130:
What are your thought on this?
http://books.google.com/books?id=HccWz4_ym7YC&pg=PA129&lpg=PA129&dq=virtual+ground+connected+to+input+jack&source=bl&ots=JcYgyTR6QJ&sig=ZslYhyjaOsIjeQYom8_d5qgfjzY&hl=en&sa=X&ei=jOs1Utv1B7DXiAKC04DQDg&ved=0CF0Q6AEwBQ#v=onepage&q=virtual%20ground%20connected%20to%20input%20jack&f=false
If the enclosure is only connected to 'ground' at the jacks, then as long as the input and ouput are not DC coupled, the question become: whether (aside from potential noise issues; the elephant in the room) connecting the jack sleeves and pull-down resistors to 'sewer' ground is a good idea?
Is the plan to power this with DC and use a charge pump to generate the bi-polar supply?
Sam,
The schematic posted shows the power supply that I intend to use.
The supply basically provides 15 volts to the existing circuit in a different fashion than the original. ( No bipolar supply)
Quote from: armdnrdy on September 16, 2013, 01:32:50 AM
I was wondering if you had noticed the link that I posted earlier. The exact sleeve ground connection issue is outlined in this article and an unbelievably easy fix is given as well.
I'll post it again. Check out just above page 128-130:
What are your thought on this?
It's pretty much what I told you. In figure 3.51, he shows using the minus side of the power supply as ground in a single-power-supply pedal to the left, and using the negative side of the power supply as *both* the minus-side power for a bipolar circuit, and the rail-split middle voltage as the ground for the circuits inside the right-side effect. The only difference is he calls the internal virtual signal ground in the right-side circuit "ground" instead of "vbias".
He also notes the issue with offset grounds from the same single power supply in figure 3.50.
Sorry - big schematic, little phone.
One thing - if I'm reading it right, your 'ground' is a diode drop above the DC power supply 'ground'. Is this going to play nice with other pedals? I don't know.
Right - that was the issue with the full wave rectifier that I alluded to here:
Quote
If your power supply is fed with 18Vdc, the negative side of the 18Vdc must NOT be shared with another pedal that can tie it to signal ground. This is one version of the "univibe problem".
Quote from: R.G. on September 16, 2013, 10:46:21 AM
Right - that was the issue with the full wave rectifier that I alluded to here:
Quote
If your power supply is fed with 18Vdc, the negative side of the 18Vdc must NOT be shared with another pedal that can tie it to signal ground. This is one version of the "univibe problem".
I'm using 1N5819 Schottky diodes to lessen the voltage drop.
Can you point me in the direction to some other configuration that will solve that problem?
I could always drop the AC/rectifier option, or the DC option. I wanted to include the AC option for builders because a 12VAC wall transformer is much easier to source than an 18VDC. On that note, some people already have 18VDC on their pedal boards.
Not a great solution, but would a half-wave rectifier work?
Quote from: samhay on September 16, 2013, 11:02:14 AM
Not a great solution, but would a half-wave rectifier work?
It would with heavy filtering.
You're right - it's only going into a regulator. I guess if it's quiet enough, then it's good enough.
A diode drop above ground using a Schottky is only .3 volts......I wonder if that is enough to cause any issues.
I used 1N5819s for the bridge rectifier for the least voltage drop possible when the 18VDC passes through.
Also, since there is no guaranteed voltage rating of an unregulated 12VAC wall supply, ( I have a 200ma that measures 16 volts with no load) I wanted to be sure that I was above the minimum voltage input rating of the regulator.
The issue with using signal reference ground and also power supply sewer ground as a source of 0.000000000000000000 volts is that any voltage away from ground is a problem. The classical example is that noise generated from currents flowing form one end to the other of a solid copper wire can be a problem, and that can be in the millivolt range.
Not that you can't get away with it sometimes. But you *will* hear from the folks where it didn't work. :icon_lol:
If you're designing a project for random people to make, or for commercial production, you are going to have problems in some cases when someone wants their 18V DC power supply to power this pedal and also another pedal that doesn't have the ground shift issue. DIY hackers can cope with it, but not the technically illiterate.
If you simply must have both 12Vac and N volts DC be inputs, the simplest thing to do is go half wave on the rectification. This is not as big an issue as you may think. The filter caps only need to be twice as big (about) for half wave as full wave, as there's twice as long between charging intervals. Make C50 be 1000uF, or make it two 470's in parallel and you're at the same ripple as you would be with a full wave bridge.
Some years ago, I spent some time trying to figure out how to make a pedal immune to whatever voltage and polarity was stuffed into it. I could do this with a bridge and a regulator and some messing about, but I could never solve the ground shift problem. I went so far as to make the bottom-side diodes in the bridge be synchronously-enabled MOSFETs for low drop, and got the ground shift down to a few millivolts, and that was fine for most low gain pedals, but given that the battle cry for distortion pedal users is "MORE GAIN!!", there's always a pedal somewhere which will go nutso over a ground shift. True, I was solving a different problem, including reversed polarity, but that's a subset of AC stuffed into the power jack.
Once again, Thanks for the guidance R.G.
It just so happens that I have a ton of room left at the top of the board where the power supply is located. Fitting larger a larger cap and half wave rectifier will be a breeze!
I always find that the frustrating issues with the general genre of pedals are the things we can't do anything about: the history which has given us 1M single ended signal cables to 1/4" phone jacks, a difficulty dealing with reversed power supplies and AC on the power inputs, sensitivity to DC offsets and noise, and so on.
Most of these are a legacy of the time when tubes and transistors both cost about $5-$20 in 1960s dollars, which would be $50-$200 dollars in today's diminished currency. The electronics designers were often so focused on saving a part that the designs were cut to the bone, sometimes INTO the bone.
Consider what the guitar pedal world would be like if the first guitar amps could have used a dual triode input as a differential amplifier reading a balanced, shielded input cord. Triodes were too expensive to use both halves for an input amplifier.
Quote from: armdnrdy on September 16, 2013, 12:56:03 PM
It just so happens that I have a ton of room left at the top of the board where the power supply is located. Fitting larger a larger cap and half wave rectifier will be a breeze!
Having tons of room can fix a lot of problems! :icon_lol:
Quote from: R.G. on September 16, 2013, 12:57:33 PM
Quote from: armdnrdy on September 16, 2013, 12:56:03 PM
It just so happens that I have a ton of room left at the top of the board where the power supply is located. Fitting larger a larger cap and half wave rectifier will be a breeze!
Having tons of room can fix a lot of problems! :icon_lol:
This is a rare but welcome occurrence!
One thing that threw me about this rail splitter is the fact that +V is 7.5 volts and the ground is -7.5 volts but only in relationship with the virtual ground (bias).
If +V is used in conjunction with "true" ground, then we have +15 volts to ground and bias to "true" ground is 7.5 volts.
If there was a emoticon with a spinning head.....I would use it!
it's all relative, and probably always will be.
Just wait till we start designing quantum mechanical pedals.
Well....
I feel on track again!
Thanks again R.G.
I made some adjustments to the schematic, drew a 13X26 7.5 lead spacing horizontal capacitor in Eagle, and made the proper rectifier/capacitor changes on the board routing.
It's a good day! ;D
Just waiting on the main order of parts.
Quote from: armdnrdy on September 16, 2013, 02:29:19 PM
One thing that threw me about this rail splitter is the fact that +V is 7.5 volts and the ground is -7.5 volts but only in relationship with the virtual ground (bias).
If +V is used in conjunction with "true" ground, then we have +15 volts to ground and bias to "true" ground is 7.5 volts.
If there was a emoticon with a spinning head.....I would use it!
This is a really good lesson in what "ground" is and why I so very much overuse quotation marks about it. The only "true" ground is a conductive rod driven into wet dirt or submerged into a body of water on the planet's surface. Anything else is a "ground" of convenience - that is, a point we choose to call "ground" and by that we mean only a relative reference point against which all other measurements are made.
Is it more proper to call the negative side of your power supply "ground" or the mid-point that much of the circuit is biased to? The only correct answer to this is that for this circuit in isolation, it does not matter at all, as long as you're consistent with your reference point. Utterly no difference, because the point you hook your meter's black lead to makes no difference to the readings on the red lead, other than a shift in reference point. The differences between voltages on all other points will come out the same.
It's when you hook up something else with a "ground" and want the two to agree on the same connected nets being ground that things start getting ugly and complicated.
Remember: it doesn't matter what point you call ground as long as (1) you're consistent about it and (2) other equipment/circuits you hook up to the "ground" you've just designated can work OK with that point being "ground" for both of them.
Quote from: R.G. on September 16, 2013, 12:56:46 PM
Consider what the guitar pedal world would be like if the first guitar amps could have used a dual triode input as a differential amplifier reading a balanced, shielded input cord.
Well, I wouldn't have to explain to students why we build unbalanced instrument cables and why the instrument inputs they feed are high impedance. The guitarists in the room get it when I relate the historical sequence of events but nobody else really follows what the hell I'm talking about, especially after I've just spent so much time explaining why low-impedance, balanced inputs are the way to go. Would it have killed Leo to throw a transformer in there?
And if I do it in the opposite order it doesn't really make it any better.
You should have heard me today talking about Victrolas and the Radio Corp. of America so I could justify the RCA phono plug. There's another one there's very little excuse for now.
How did you explain RCA?
RCA was a company that was in existence from 1919 to 1986....you know....three years before you were born.
They manufactured electronic parts and equipment.....what was that?......No.....RCA never made iPhones.
Excuse me....No.....they did not make the iPad.
No Sir....that was quite a while before iTunes.
I'm sure it's a "blast" teaching kids about things that they don't give a rat's AAss about!
We live in a very different world now! People wonder why certain outlets don't carry DIY goods anymore. We're a dying breed.
We have turned into a world of consumers!
Well, I guess I didn't explain it so much as refer to it. I kind of hoped that the word "radio" still meant something to them - I supposed I should have called it "terrestrial radio" but I didn't have the heart.
Actually, in a classroom setting, it's a great entre to the real world note to them that they're engaged in a furious effort to learn the theory of how things work, and that as soon as they exit and try to support themselves on that, they'll run face-first into the wall of real-world things they can't change just because something theoretical would be better.
Rather than making the theory less important, it makes it more crucial, because some things can't be changed for historical, psychological, and economic reasons, so anyone wanting to really understand what they're doing has to look beyond the pure theoretical and into how what they do works economically. Then you REALLY need theory to patch around what the dismal economic world has done to you. :icon_lol:
Very few of my professors hinted that some things that are perfectly possible by the laws of physics are absolutely forbidden by economics, and that this was just as important and no less real.
It might make a great extra-credit question to mention something about economic feasibility and then ask it on a final. :icon_lol:
Quote from: armdnrdy on September 16, 2013, 07:44:55 PM
We live in a very different world now! People wonder why certain outlets don't carry DIY goods anymore. We're a dying breed.
We have turned into a world of consumers!
Actually, we may be past the DIY Dark Ages, at least the most recent one. In the 80s, there was utterly nothing. I'm still blown away on a regular basis that this forum has nearly 30k registered members and massively more watchers.
It was pretty notable to me when I found a few folks on usenet that were interested in building effects. I figured there was one of me until then. To go from that to a world where people go from "Which end of a soldering iron do I hold?" to "We have released our new Belchfire Mark IV, with its utterly new dynamic tone architecture build with entirely NOS germanium point to point parts in the USA" in less than a week is pretty amazing.
To me. But then I'm easily impressed. :icon_lol:
Quote from: R.G. on September 16, 2013, 07:50:01 PMnote to them that they're engaged in a furious effort to learn the theory of how things work
True only for very small values of "effort," "furious" and "engaged."
Point!
> The only "true" ground is a conductive rod driven into wet dirt or submerged into a body of water on the planet's surface.
Yeah, butt....
The only thing a dirt-rod is really good for is Lightning.
Audio systems with NO dirt-rod: every car on the road. Also PigNose battery-amps. iPods. Pocket radios.
The concept we should be groping for is "common".
Current flows in loops.
These loops "can" all be isolated. Separate battery for each tube/transistor. Transformers between stages.
Ah, but tube/transistors have 3 legs and three current-loops, so MUST have some loops "commoned" on one leg. (Photo-devices, also carbon mikes, can evade this, but perform poorly.)
Instead of a dozen or more separate loops, it is MUCH simpler/cleaner to "common" many loops. In a car, most lamps and switches return through a common frame/body.
In a pedal we have input, output, power, and internal loops. We usually common some part of each loop to all other loops. Somehow the mis-leading name of "ground" is used for this common bus. It might be better to just say "common".
Quote from: PRR on September 17, 2013, 12:19:22 AM
> The only "true" ground is a conductive rod driven into wet dirt or submerged into a body of water on the planet's surface.
...
In a pedal we have input, output, power, and internal loops. We usually common some part of each loop to all other loops. Somehow the mis-leading name of "ground" is used for this common bus. It might be better to just say "common".
Actually, that's what I was going on about. The only "true" ground is pretty useless for our purposes and what pedal hackers like to call "ground".
Well, there's that die-if-you-kiss-the-microphone thing, but that's something else again. :icon_lol:
I agree, "common" would be a less-bad word in many senses, but we're stuck with "ground", just like we're stuck with high-impedance unbalanced signal lines, 1/4" phone jacks and 9V battery clips. :icon_lol:
Quote from: PRR on September 17, 2013, 12:19:22 AM
It might be better to just say "common".
In the line of work that I do, the word "common" used to describe a path to earth, or a return could be a bit misleading.
The word common is used to describe just that....anything that is common. You can have common marked on a control switch that is actually where a "hot" connection is made.
Besides...what's in a name? How about color codes?
We'll make +V red, GND black.......oh wait...that's already been done and it's still confusing! :icon_wink:
Quote from: tubegeek on September 15, 2013, 04:05:22 PM
Also known as "The tragedy of the commons."
I have been sadly remiss in award a coup-count on this one!
It isn't as much fun when you have to beg for it.
But I'll take it anyway - I have as little shame as the next guy.
Quote from: tubegeek on September 17, 2013, 09:25:36 PM
It isn't as much fun when you have to beg for it.
But I'll take it anyway - I have as little shame as the next guy.
But it really, really was good! :icon_biggrin:
> make +V red, GND black.......oh wait...that's already been done and it's still confusing!
I had a Real Bad Month when I repaired a radio-mixer and the factory had wired it Red=Minus Black=Plus.
> we're stuck with "ground"
Granted.... but it's still a bad word.
> that die-if-you-kiss-the-microphone thing
Are you standing on dirt? (Concrete on dirt?) THEN you wanna know where Earth Ground is.
In many-many zap-lip situations, the stage is non-conductive. The wall-outlet 3rd-pin may even be near dirt-potential, and the Neutral(*) may be close-enough. Or maybe the stage is on one neutral, the PA system on another. (Had that at work.) Just one ungrounded chassis, 2-pin plug, 3rd-pin cut-off, can throw enough leakage to sting.
(*)"Neutral" on a 120V circuit is another poor word, but I'll leave that fight to another day/lifetime.