The Steel Guitar Forum

I had a wakeup call today ,concerning the importance of keeping the power ground separate from the signal ground. Forum member Mark A. and I have been experimenting with a votage doubler AC power supply. The supply comes from a 120VAC wall wart that reduces power to 9VAC. Then the 9VAC is arranged in a voltage doubler arrangement with 2 diodes which produces DC, with capacitors arranged to create a common. It is then filtered, bypassed,put through voltage regulators, and filtered again. The voltage doubler arrangement has one of the 9VAC wall wart transformer lines run direct,as the ground of the capacitors. All grounds running off the capacitors must be run to one point. At that same point is where you hook the signal ground. There can be no mixing of the power grounds and the signal grounds. If there is any mixing of signal grounds with power grounds, there is "60HZ" hum. In my opinion,this is not the best type of power supply , because there is no isolation. By isolation, I mean like there is in a center tapped transformer, or a DC to DC converter, or system that has a isolation coil. Basically this is one reason some guys get a lot of hum. They have several things in line,each with it's own power supply. Case ground has a different potential on each power supply. Then the case ground mixes with the power and signal ground. Signal ground gets mixed up with the power ground, in a loop.
By the way Mark A., if you read this, you have the 1N4001 diodes turned around on the input and outputs of the voltage regulators.


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Keith, I thought you told me the last time we talked that you were going to rest you're brain for a spell and just make pedals????? You will never change!

Totally happy with my last pedals,see you in St. Louis.

I sent you a customer last week, Jeff Lampert from New York, he said he was going to call and order a pedal.

Ben

Ben, I can't help it. Two people have really helped me with my pedal design. Those two people are Hartley Peavey and Ben Jack. I want to thank you Ben, for helping me with the total elimination of noise in the pedal. Also for helping me maintain sound quality in my pedals.
Actually the power supply I described above is called a-- AC capacitor-input dual-tracking -dual polarity power supply with ground. It really does not matter what type of power supply you use, you must keep the signal ground separate from power ground. If they do join, it needs to be at one point, and that depends on the type of power supply. A lot of stuff just boils down to experimenting. There are some really open questions concerning grounding. If signal ground is tied to a medal case, and the power supply ground is not, how does the electric field of the power supply effect the metal case? Another wierd thing I ran into was where the power supply ground was separtate from the signal ground, but you "had" to bypass the power supply to the signal ground with .1uF capacitors to prevent noise. This involved the supply being split into a dual polarity supply, with a DC to DC converter, and LC network of inductors and capacitors. The thing I am experimenting on, is ground connections, "with" isolation. This is where power ground and signal ground are hooked together, but are isolated from each other in terms of noise.

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<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Keith Hilton on 13 August 2000 at 02:45 PM.]</p></FONT>

I'm confused here Keith. Why try and build a voltage doubler? Seems like it would make more sense to use an 18 volt power supply, instead of trying to double the output of a 9 volt unit.

In the "for what it's worth department," I use a rack with a Korg DTR1, Alesis Quadraverb GT, and a Rocktron Hush box (a noise gate), and a Furman light and power distribution box; all are mounted in an SKS rack with "Hum-Frees," and the tuner has a ground lift on it. Seems to work OK. Maybe the "Hum-Frees" mounting kits are the trick for some others with hum problems.

<SMALL>all are mounted in an SKS rack with "Hum-Frees,"</SMALL>

These sound potentially (pardon the pun) very useful. How much are they? How do they work? Where can you get them?

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The Unofficial Photographer of The Wilkinsons
http://members.xoom.com/dpennybaker/index.htm

Donny, a 9 volt AC wall wart will read more than 9 volts AC, when you test the tip. It will read almost 24 volts when run through a capacitor input voltage doubler. This is due to the fact of it being the RMS value ,and you must multiply by 1.4. The peak of each cycle is about 1.4 times the RMS value. 1.4 is the square root of 2. Therefore, you can wind up with a dual polarity power supply that has a plus 12 volts to common and minus 12 volts to common, or 24 volts from plus to minus.
If you try and get a dual polarity power supply out of a 18 volt DC supply, you only wind up with 9 volts on the plus and 9 volts on the minus side. Then the only way to get to 24 volts is with a DC to DC converter, which is expensive and requires a LC network, capacitor and inductors. Or you can do it with additional transformers, but this gets complicated. Next Donny, how would you like the responsibility and liability of musicians getting electrocuted with open 120 volt AC transformers? That is the "BIG" reason for wall warts---liability and personal injury. Yes, it would be much simpler, and better to use a center tapped open 120 volt AC to 24 volt AC transformer. The problem is people getting hurt. Next Donny, look in Mouser , Digi-Key and all the other electronic parts catalogs. Tell me how many 18 volt DC wall wart transformers you see for sale? There will be ZERO. 12 Volts is as high as they go.
Yes, you can get any voltage wall wart you want ,in either DC or AC outputs, but they are not as common ,and the cost is much more.
Donny, I hope this helps you understand why I was using the 9 volt AC wall wart.
I am happy that Chick has no hum or noise, and the Hum-Frees solved all his problems. Many people are not so lucky. With many things "IN" line, each with it's own power supply, you sometimes have to be creative to solve ground loops, and signal ground mixing with un-wanted power grounds. There is no cut and dried rock solid method. It sometimes takes a lot of experimenting.

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Donny, here are the exact values I measured with my meter. 9 volt AC wall wart. Output reads 10.75 volts AC loaded. When run through the capacitor input voltage doubler the value is 28.29 volts DC. This then can be meausred to a common, and the values measure plus 14.04 volts and minus 14.18 volts. This is not quite enough, in case of low voltage, if you use 7812 and 7912 series voltage regulators. Therefore if you use low drop out voltage regulators it works, because they only need .5 volts over to regulate a light load. After being run through the voltage regulators I measure the value and got: 11.91 volts DC plus, and 11.94 volts DC minus. Pretty close to 12 volts wouldn't you say? Across from plus to minus I got 23.88 volts. I told you in the post before this it was 24 volts, but I was just rounding it off. Donny, one other thing I forgot to mention is heat sinking. Due to space problems on this power supply we can't heat sink. That is another reason we cut everything close like we did, so we wouldn't have to heat sink. Donny, I hope this helps you understand.

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The Humfrees (and similar products) are not a "fix" for the ground hum problem. Merely a way to get around it in some applications. Just like lifting the ground on one amp of a two amp system - lifting the ground is a way of getting around the problem.

Keith talked about isolating the power and signal grounds. If all manufacturers established and followed a standard in this area it would minimize the problems.

In eletrical power wiring for computer equipment, the Isolated Ground system has become a "standard" and the Hubbel IG5262 (15A model) orange AC receptacles are used. They have recognized the problem with AC power grounds in buildings and now use a ground that is "isolated from the conduit ground" with an insulated 3rd wire that runs directly back to the power panel ground from the receptacle, not connected to the "conduit" ground.

I do disagree with Keith on his reason for using an external power unit (wall wart) rather than placing the entire power supply inside the unit. Using the logic that there are "dangerous voltages present inside the unit" would mean that there is a lot of electronic equipment made that's unsafe for consumers. True there is a potential for shock if some idiot opens up a unit with the ac power on and touches the ac line, but that potential is there on any unit that has an internal power supply.
Most of the external power transformers came about originally from "imported" devices and were used with battery/AC powered devices or to reduce the size of the unit or eliminate a source of heat inside the chassis and thus they did not have to consider the extra heat when designing the device or provide shielding for the power transformer. Consider where almost all external power units are made. There are also three basic types of external power units, (1) an AC voltage unit, (2) an unregulated DC voltage unit and (3) a regulated DC voltage unit.

I thought I'd clear up a few things in what I'm reading in this thread. First, it needs to be understood that the output of the 9VAC wall wart is not attached in any way to the 120VAC ground and/or neutral wire. One of the main functions of a transformer is to isolate. The power supply can actually be cleaned up by using a diode bridge instead of the two diodes. The one major problem with this design is that the "ground" you are setting up is floating because it has no reference to balance itself out. If you had a circuit that was drawing more current from either the positive or negative sides, it could unstabilize and make the voltages uneven. That is why a center tap transformer is used when making a power supply with + and - voltage outputs. By tying the filter capacitors and signal ground to the center tap, there is no a central grounding point and the two voltage will stay symmetrical.

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Jack and Michael, I totally agree with both of you. A center tapped transfomer is the best way to go. Suppose the risk of 120 volt AC had to be "totally" eliminated, and you absolutely could not use a 120 volt AC to 24 volt AC center tapped transformer. Suppose, due to product liability , with legal cost win or lose in excess of $100,000.00 you had to use a wall wart. Or-- suppose your insurance carrier said, you "MUST" use a low voltage wall wart ,or we won't insure you. You then only have a choice of a un regulated AC or DC wall wart, since regulated and filtered DC desk top power supplies can cost well over $100.00.
So, Michael and Jack, the situation is this: You are forced into a corner where you have to use a wall wart with just two wires coming in. What would you do? Remember, you can't use a 120Volt AC transformer under "ANY" condition. You "must" get a dual polarity power supply that provides plus 12 volts and minus 12 volts, with a ground, or common.
Michael, would you consider the ground to be "floating", if a DC plus and minus voltage was run through a DC to DC converter and LC -inductor and capacitor network, where you got a dual output with a ground? These DC to DC converters have a lot of stuff inside them, including a transformer. If this is true, then every computer made has a floating ground, because all of them use DC to DC converters.

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I was taking with Forum Member Mark A. today,about the 9VAC voltage doubler power supply. He advised that the signal ground and power ground , be separated by a resistor if there was a noise problem. I've already stated that they "must" come together at only one point, to prevent ground loop noise. The resistor separation, would seem to put a little stability and isolation in the floating ground of the power supply. I ask Mark A. what value he would start with ,and he said 100 ohms. That seems a little strong. When you install a resistor at this point you lower the voltage. It would be the same thing as putting resistors across the plus and minus of the dual polarity power supply, because you are cutting the ground. Therefore I think I will try a 4.7 ohm resistor, to prevent a sizeable voltage drop.

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One of the main reasons a manufacturer of electronic equipment will use a wall wart in preference to a self contained 120 v power supply is the need for Underwriters Labs approval.
The insurance industry looks to UL and similar testing organizations to guarantee the safety of the product.
Mostly this means that if it fails it will not catch fire. Personal injury is also considered.
A product like Keith's pedal could cost $10,000 or more to get the UL approval sticker.
Anything that operates on 120 v AC and is sold widely and used by consumers must have the UL sticker.
The wall warts are made in the millions...most all have UL approval so the use of them prevents someone like Keith having to get approval for a limited run product.
This is because the wall wart unit is sealed and the only thing which comes out is low voltage and they don't worry about that.
I have been in a number of situations involving large TV studio installations, electrical inspectors, and UL requirements.
On several occasions I had to convert devices with self contained power supplies to wall warts in order to use them.
Blake

Blake, you hit the nail on the head! Now, let's talk about making a dual polarity supply with ground out of two wall wart wires coming in. That means we have two wires coming in, and three wires going out of the power supply.

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Keith, here's a little blurb on wall-wart engineering: http://www.rane.com/pdf/note133.pdf

Thanks for the information Rich. Rich, here are some things that bother me; When you power a Audio circuit with a AC wall wart using a voltage doubler, one side of the AC transformer is hooked to the power ground. This "must" join the audio ground at only one point, or you could have loop noise. It just worries me that the noise from the AC wall wart will somehow get through. I am totally paranoid about hum or noise of any kind. I really like the method of using a DC wall wart, and then running to a DC/DC converter to get the dual polarity with ground. This must be done with LC network for best results-LC meaning inductor and capacitor. What bothers me about these DC/DC converters can be summed up in one word "MONEY". All of them are sealed,so the maker has his own witches brew, and all of them are high priced. The cheapest ones in the 2 to 3 watt range go for around $10.00 in quantity, to hundreds of dollars.
I have no idea why I can't find a good one in the range of $5.00,in limited quantities, but I can't. I love the isolation, and protection the DC/DC converters provide. There is no noise if you bypass them correctly, have the right inductance and frequency in your inductors, and the right filter capacitors. There is no switching noise. There is some radiated noise, but that is easly taken care of with good grounding.
The next thing that amazes me is this: So many chips ,IC's, require dual polarity, with a ground. Even though so many things require dual polarity with a ground, you can't find "ANY" wall warts with 3 or 4 wires coming out the output. "ALL" of them have two wires and they are either DC or AC.
If you do find a wall wart that provides a dual polarity power supply with a ground, they want a fortune for it. Someone could make a lot of money if they would start making "Simple" 120Volt AC to 24Volt AC transformers, with a center tap, in a Wall Wart. Since so many want and use this type of arrangment, why is no one making one in a wall wart? I know for a fact that it wouldn't cost any more to make these than it would the AC and DC units now available. This really "BUGS" me! No one is making, what everyone wants in a WALL WART. Then, if you could find one, you could never find a tip that would work. Something would be wrong. If you found tip it might not be one that was isolated from case.

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Keith, I posted the wrong link. Here's what I had intended as info: http://www.rane.com/rap.html

Rich, this last thing you suggested has some really good stuff. Thanks!

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Thanks, Keith. As to the quality of "wall warts", I don't think you can make a flat statement that they are all junk. I used to install & service security alarm systems at one point of my "electronic journey". This was on a major Air Force Base, and any instance of a random/nusience alarm was about as desireable as the proverbial "turd in the punch bowl". There were definite issues related to: 1). poor quality of the gel-cell backup batteries of the period, and 2). overheating and subsequent failure in the wall-wart power supplies which ran in extended overcurrent, attempting to charge the bad backup batteries. I decided to "post-mortem" any wall-wart which failed or was unable to provide satisfactory performance, 24 / 7 / 365 1/4! Since the warts are normally super-glued together, I had to come up with a method of opening the dead cases without further damaging or destroying the contents inside.
Most of the failed warts were due to the blowing of the internal "butterfly" fuse, due to extended overcurrent and occaisionally due to a very high initial current when first connected to the Alarm/Charging Circuit/batteries.
Indeed, most warts of the time were pretty cheezy, with relatively high internal source impedances. While audible "hum" or induced ripple was not an issue, the lack of robust construction or conservative power rating in those warts seemed to be the major contributor to system troubles.
Since I had gotten into the habit of nosing around in warts, I popped open the unit which had been provided with a Rane PE-15 parametric EQ I purchased. The difference in quality is obvious. The transformer is a much higher quality unit than usual, as is Rane's attention to detail regarding the layout of the cable supplying power from the wart to the rack unit.
Those Rane warts (for my EQ) are rated 9-0-9 VAC, and this voltage is immediately applied upon entrance to the chassis to simple positive and negative voltage doublers, and the doubled voltages supply positive and negative 15 volt three-terminal regulators. This results in rails voltages of +15 and -15 volts DC, and full differential circuit topology. Excellent headroom characteristics naturally follow in the design.
I don't know if Rane makes the warts, buys them outright, or has them made, but I'd be willing to bet that they and/or similarly made warts could satisfy your stringent requirements for the desired performance and high reliability in powering your products.
I'm always impressed with the content of the Rane on-line "technical library". They "wrote the book" on many aspects of making excellent signal processing and control equipment, and are obviously happy to share the book with all of us.
Apologies for the protracted posting, but it's technical stuff, you know? I try not to be "fuzzy" in discussing these details.

<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Rich Paton on 17 August 2000 at 03:47 AM.]</p></FONT>

Hi Keith,

I do hope you are suceeding in de-rippling your supply. Since you mention that you are using thick traces, including the ground reference point, most of your troubles will be small. Good Luck!

Keith, As Rich mentioned, there are dual output wall warts available in the audio industry. They may cost more than what you are used to.

Also there is a circuit "trick" using op amps which will enable them to work with a single ended power supply while providing the same results as with a dual one.
Several years ago I built some mobile intercom systems using this technique.
I think it is described in "The OP Amp" cook book..but if you can't find it let me know and I will look it up.

I didn't mean to imply that it would cost $10,000 to certify a wall wart. However the equipment I've worked with and the UL's penchant for "destructive" testing certainly drives the total cost of certification up.

You could ( and probably already have) hook up two wall warts to get dual polarity.
The physical arrangement is somewhat cumbersome.

Also you could have one of the wall wart companies build you a custom unit or find something in their product line which would be close to what you need.
You've probably already found out that that is cost prohibitive.
Blake

Bob M. you have some good ideas, thanks! Rich, I down loaded all the Rane information. You are correct, they have lots of good information on this subject. I especially liked their Rane Professional Audio Reference. The Amplifier Classes was interesting reading and cleared up some questions for me. Blake H. I know what you are talking about with the OP amp trick with the dual polarity power supply. Actually the op amp powers the common. I talked to Mark M. about that very thing about a week ago. Mark M. I got my power supply working and it is quite as a mouse. Each side is really close and even. I ordered about $500.00 worth of low ESR 1000uF 35 volt capacitors. The real test is a place where they have "dirty" power. Here in Branson where I live, the power is pretty clean. I just sent a unit off to a location I know has dirty power. I should know the results about hum and noise by Saturday. I bet there is no noise or hum.

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Rich, I contacted Rane about stock wall warts for sale. They have nothing stock. First, you must submit engineer drawings, and the quantity you want. I love the information in their web site. It appears they make up desk top power supplies. One thing that bothers me about these type of operations:They want everything thier way, but it is MY--MONEY. They usually want around $5,000.00 up front for design cost and "GERBER" files. Then they want want want want want want want ,when it comes to quantity. They want to do everything with wave solder and the real tiny parts. If you ask them what happens if someone sends something back. Their stock answer is, "OH, well,we will just give you a whole new board." I don't buy any of it. I really don't think if you said you wanted 6 million, that would be enough. I have another approach. Spend the money they want and buy small CNC milling machines and cut your own boards. Hire people and show them where to place and solder the parts. Wonder if 61 million would be enough enough to make them happy? No they would not be happy even if you wanted 92 million.

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Keith, were you referring to Rane as one of the "operators"? Of course, they are really not in the business of wholesaleing wall warts. Their gig is producing top-flight professional audio equipment. I merely suggested that the type of wart they use is probably the type of wart you would utilize for your product.
If you anticipate the need for large quantities of the wall warts, perhaps a good approach would be to find out who are the manufacturers of OEM warts for the many companies that provide them with their product. Chances are that an off-the-shelf item is available which satisfies your requirements is available. There are literally zillions of them made every year.