Clarification on power supplies - bipolar specific

Frequently asked questions regarding powering your pedal.
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Post by sullywoo »

Greetings.

I have some questions, and hopefully there is a composite answer here.

I have a moderate amount of experience with elec theory. No schooling, just learning as an interest. In the past I messed with fan control circuits for my pc, and dabbled in some other things like DAQ stuff. Last year I decided maybe I would like to play a musical instrument, so I started messing with a guitar. One thing led to another, and now I am making amps, mainly those from RunOffGroove and the cMoy. I am going to start messing with some effects pedals too, as I am handy with a dremel and a soldering iron, so why not.

Most of the basics I understand to a decent degree. I understand what OPamps are and what they do, basically. I realize some call for a bipolar power supply. However, most of what I have dabbled in either comes in battery form or used an existing power supply. I am interested in learning more about the bipolar units.

I have googled and then googled some more. I find a good amount of schematics on making different amps with many different ICs, some even to make bipolar without some component or another. But in the end, I want to understand, so I was going to have a go at one of a few I found good schematics for. I sort of understand the theory, but need help.

Specifically, I am confused about the transformer. Putting toroidals aside, and just focusing on a transformer, there are many to choose from. But, some convert the AC to AC outputs and some to DC outputs. None of the articles or schematics I have seen address this. I sort of understand what the circuits are doing, where the caps and resistors and diodes basically split the sine wave giving 1/2 the voltage positive, and 1/2 negative. So in the case of the GGG bipolar psu, they call for 18volt +-.

Some research leads me to needing 36 volt for the transformer, where 18v needed per output. This has been expressed like 35-0-35. I more or less understand what the center tap is, providing a ground where the primary and secondary output provide the +- voltage. I have seen other verbage that the transformer only needs to be able to supply 18v, like say a 24-0-24 transformer.

I want to make it clear that I am not trying to make a razoo amp, only some of the smaller projects, like the 8watt ones. I realize that when you start pumping out some decent wattage there are spec designs what should be followed. I think I can get that down if it progresses that far. It is just that I am not able to find what I need to fully understand what is going on. Nothing tells me what I evidentily should know, such as if the transformer is outputting AC or DC.

I don't expect anyone to do my work for me. Far from it actually, I like to learn. But, I am stuck on this transformer bit and would appreciate any guidance and tips anyone might impart.

I realize I don't have all the terminology down to properly describe what I want to do. Like I said, I know some basics and can usually figure things out by reading.

Thanks to any takers.

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Post by Mr.Grumpy »

Transformers can only pass AC - alternating current. They block DC, and they are sometimes used to couple signals from one amplifier stage to another, but capacitors are more commonly used for this purpose because they are smaller and cheaper. Every transformer has two windings - the primary and the secondary. There is no direct connection between the two windings.

The most common use of transformers in music electronics (especially pedals and pedalboards) is for POWER SUPPLIES. Typically a transformer is used to step-down the mains voltage of 110/200/220 down to 9 volts, 12 volts or whatever's needed. The lower voltage is still AC of course, and to be converted to DC for use in a circuit, the AC current must go through a RECTIFIER (usually a 2 or 4 diode bridge), and then be FILTERED with capacitors and resistors. Some power supplies will additionally have a voltage regulator to provide a very consistent voltage that doesn't vary with the current load.

Most pedals that use op-amp circuitry don't use a true bi-polar power supply - they fake it by making a "virtual ground" at half the supply voltage, sometimes marked as V/2 on schematics. The +9 volts becomes the "positive", the 0 volts becomes the "negative supply" and V/2 is the virtual ground. There will always be two resistors connected in series, one side to +V, the other to ground, and the V/2 tapped off the junction between the two resistors. Often there will be an electrolytic cap on between the V/2 to ground to stabilize and filter the V/2 voltage.

I really recommend getting a book on basic electronics at a library or bookstore.

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Post by sullywoo »

Thank you for the reply.

I was confused because I have been reading a lot of different infos on different designs.

I do understand that the 120vac is being most likely stepped down to something like 12vac. And further that half wave rectification gives the positive side of the wave and full wave gives both sides, and that the caps are smoothing out the peaks of the waves to create a more dc like form.

The schematics just don't declare this. I would assume that the ones with a bridge rectifier would be expecting to chop the ac into dc, but I have also seen some which use only 1 leg so to speak, with only 1 or 2 diodes. It looks like it is missing the rectification. I have seen some mention of using an existing transformer, like a wall wart to get the bipolar supply, and some that say to possibly use a transformer that gives the +- and the center (ground).

For example, the GGG 18v bipolar schematic does not show a center tap at all, but I have seen many examples and advices to use a center tap. In the GGG example, there is no bridge rectifier. I assume that the caps are smoothing it out enough for, what is that, a full wave rectifier?

Anyway, while trying to learn something new, a lot of the educational sites I visit give a very different (and basic) view on what is going on, and these schematics are (obviously) a bit more advanced. The GGG example states that to get 18v output, the primary needs 18v and secondary needs 24-30v. What do I look for then in the overall output of a transformer? 30-0-30? Further, since there are designs for both center tapped and non ct, is there a reason why one would be advantageous over the other?

A lot of topics are easy to find good infos on. OPAmps and biases are somewhat complicated, but enough infos exist, especially in regards to projects like amps/effects. How caps/diodes/resistors work and why and how you use them, also a lot of good info. Even ICs have topic related infos. This whole bipolar transformer bit though has been much more elusive to (at least for me) getting some good explanations that are topic specific and also geared a little more towards teaching how and why rather than just the application of.

I do appreciate the time you took to respond. I do understand pretty much all you said. Perhaps my use of transformer is too generic. I understand that a transformer outputs AC, but isn't a wall wart called a transformer? I guess I always called it that. I get small 120vac to 12vdc transformers for things I do at work periodically. They call those transformers. I suppose that they are not really transformers per se, as they have already rectified to dc.

Is there any info you can impart in regards to what size/type of transformer I would be looking for? For example, "if you get an unregulated one, and you want 18v output, remember to get a 10% over, so go for a 24v. Also, if you plan on pushing XYZ with it, you should get a 2va model, remembering that each blah blah will have a blah blah".

I am not completely without knowledge or the drive to obtain it, I am just finding it difficult to procure infos on this topic.

Thank you again.

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Post by mozwell »

OK
http://www.generalguitargadgets.com/pdf ... lar_ps.pdf
this uses a single secondary winding, two diodes, two filter capacitors.
Lets make the "bottom" of the secondary to be 0V.
The "top" of the secondary will alternate from positive to negative with respect to (wrt) 0V.
When the "top" of the secondary is positive wrt 0V, current flows through D1 & charges C1 to the "positive peak" of the AC volts. C1 will be charged positively.
When the "top" of the secondary is negative wrt 0V, current flows through D2 & charges C2 to the "negative peak" of the AC volts. C2 will be charged negatively.
So now we have a positive & negative DC supply. In the GGG supply, this is then regulated with voltage regulator ic's to get a nice smooth DC. All this from a singel AC winding.
You can think of this as two separate half wave rectifiers, only half of the AC waveform is used to charge up the positive cap C1 etc.....
Note, the LM78xx & LM79xx series voltage regulators have much more output noise than the LM317 / LM337 series variable voltage regulators.

In the pdf, this a very nice 100W into 4 ohm power amp from an australian electroncis magazine.
You can see the 28-0-28 transformer (or 56V CT). There is also a bridge rectifier & two filter cacacitors.
When the AC is "positive polarity", current flows from the top secondary connection through D1 into the top capacitor & back to 0V & center tap (CT), it also flows from the bottom secondary connection through D4 into the bottom capacitor & back to 0V & center tap (CT)

When the AC is "negative polarity", current flows from the top secondary connection through D2 into the bottom capacitor & back to 0V & center tap (CT), it also flows from the bottom secondary connection through D3 into the top capacitor & back to 0V & center tap (CT)

So each capacitor is charged up, on BOTH polarites or half cycles of the AC secondary.

Generally the GGG type supply is used for low power supplies, and requires more filtering capacitance.
The supply in the pdf is more suited to higher power.
There is of course nothing to stop you building a low power supply of the type in the pdf.

In the end, if you are building for yourself, we use whatever we have lying around, and make the power supply to suit.

have a look through the other pdf, from the same electronics magazine, there are several pwoer supplies.
1. a low current +/- 15VDC supply from a single AC secondary winding
2. a high current +/- 52.5VDC supply for the pwoer amplifier with a CT secondary, a bridge & lots of big filter capacitors
3. a high voltage, low current supply for the power amp input stage, using LM317 variable voltaeg regulators
4. a single 12VDC supply using a bridge rectifier & a single secondary winding, this is for a fan & led's so is higher current than needed for the +/- 15VDC supply, hence the bridge rectifier, to keep ripple on the 100uF filter capacitor low.
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Post by sullywoo »

mozwell wrote:OK
http://www.generalguitargadgets.com/pdf ... lar_ps.pdf
this uses a single secondary winding, two diodes, two filter capacitors....
Ok. I understand all of that.
You can see the 28-0-28 transformer (or 56V CT). There is also a bridge rectifier & two filter cacacitors.
When the AC is "positive polarity", current flows from the top secondary connection through D1 into the top capacitor & back to 0V & center tap (CT), it also flows from the bottom secondary connection through D4 into the bottom capacitor & back to 0V & center tap (CT)

When the AC is "negative polarity", current flows from the top secondary connection through D2 into the bottom capacitor & back to 0V & center tap (CT), it also flows from the bottom secondary connection through D3 into the top capacitor & back to 0V & center tap (CT)

So each capacitor is charged up, on BOTH polarites or half cycles of the AC secondary.
Ok. With a CT, expressed as 28-0-28, the transformer would be 56VCT. If the CT was not used, would it be used (or could it be used) like a single secondary winding transformer, as your first paragraph describes?

I do understand that with a CT, the 56v is going to yield 28 + and -. I assume this makes a CT somehow "better" for such applications? Is it becuase the waveforms are "smoother"? Or is it simply a matter of which type of transformer is needed for a given design, and you use what it calls for. I like to understand things, and I wonder if it matters or not.
Generally the GGG type supply is used for low power supplies, and requires more filtering capacitance...
Why is this? Due to the drawbacks of a single secondary transformer?

I feel I have a decent grasp of the circuit. I have spent a good number of hours reading up on transformers in general, and also have a better grasp on those as well. The circuit doesn't bother me much at this point. True, I don't fully comprehend it, but I will the more I mess with it. What still confuses me at this point is how I am going to quantify what I need for a transformer.

For example, suppose I want to build a circuit with a TDA2030/2040/2003 IC. They are not super output models, and don't call for much. But, they do call for a bipolar psu @ 18vdc. If I went with the single secondary like GGG suggests, would I need only a 18vac transformer, because I am rectifying each side of the AC into pos/neg DC? Or, as in the case of the CT models, do I want a 36v or better? Amperage wise from what I have read a 2va model should suffice. I am not to the point yet of calculating all this myself because I am just learning.

Can anyone then give me clarification on this? If I go the GGG design, do I look for an 18vac output transformer, or do I need 36vac? I know the secondary usually has like 1.4x the output, but do I even worry about that with the GGG design. Or would I be better off using a CT design (which I have 2 versions of)?

Like I said, I don't expect anyone to do my homework for me. I was not expecting such detailed answers either (not complaining though :) ). I really was hoping for some tips and advices, so I have gotten more than I wanted.

Thank you.

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Post by mozwell »

If you want a power supply, high power, with positive & negative voltage, then it is best to use a secondary with a center tap. On each mains half cycle you are topping up both the positive & negative filter capacitors.
For 50Hz, one mains cycle is 20ms. So with this arrangement you keep topping up the charge in the capacitor every half cycle, or every 10ms. The result, less ripple voltage, and you can use a smaller capacitor than with the GGG circuit.

If you had a single 56VAC winding, then you must use the GGG circuit. To get +/-40VDC though, you would only need a 28VAC winding, remember the capacitor is charged to the mains peak, 28VAC rms = 40V peak.
If you used this sort of supply for a power amp, then we are not making the best use out of the transformer winding, the diodes or the capacitors. Each diode in the GGG only works for one half of the mains AC cycle, so we can only charge up the capacitor for one half of the mains AC cycle. So we only top up the charge in the capacitor every 20ms, so for the same ripple voltage & same load current on the power supply, the GGG circuit needs DOUBLE the capacitance of the secondary with CT type supply.
For low power supplies the extra capacitance does no cost much maybe 10c, but for big power amplifiers, the capacitors can easily cost an extra $50 or more.

Basically the GGG circuit is much less efficient than the secondary with CT circuit, but for low power supplies, who cares. It is ok to build a power supply for a power amp with the GGG circuit, but, the transformer will need to be have a 28VAC 4A secondary instead of 56VCT 2A secondary, you will need to use bigger (more expensive) diodes & filter capacitors with the GGG circuit.

There are many arrangements of power supplies, each has their own advantages & disadvantages, but each has an application they are "best" suited to.....

May i suggest, that you find a book on basic electronics & go through that. Explanations are often so much better with simple diagrams. You can still ask questions here, and i am happy to help, its just i dont always have the time to spend half hour blocks of time.... work unfortunately, comes first...

Good luck, i hope i explained it a bit better this time.

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Post by mozwell »

back to the original purpose for your post..... what power supply do you want to make

Do you need a bipolar 18V supply, single 9V supply, what are you going to power with it, a guitar pedal, a small power amp.

Most data sheets for power amps will include a recommended power supply circuit.

OK, bipolar +/- 18V or +/- 15V supply, low power for pedals & op amps.
Use a 18-0-18 transformer (actually up to 22-0-22 is ok), for the mains filter caps use 1000uF 35V (50V is better), for 18V out use LM7818 & LM7918, for 15V out use LM7815 & LM7915.
A 5VA to 10VA transformer is fine

For a power amp needing +/-25VDC, use a transformer of 18-0-18 adn teh bridge rectifier & capacitors. Power rating, lets say add an extra 30% to the power amp rating, so if the amp is rated for 40W, lets use a 60VA transformer.

How did i get to the 18-0-18 for a supply of +/- 25VDC
Ok, we know with 18VAC, the peak is 18 x 1.414 = 25.6V. The filter capacitors will be charged to this level LESS the voltage drop of the diode which is 0.6V, so we end up with 25.6VDC - 0.6VDC = 25VDC.
This assumes we have enough capacitance to give us low ripple, but that calculation we can leave for another day.

All chip amp data sheets should have suggested pwoer supplies.
Follow what they suggest.

As a general rule, for a single power amp, up to 100W 4 ohm output, 4700uF is all you need, well you will two, one for the positive supply & one for the negative supply.

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Post by sullywoo »

Thank you very much for your reply. I understand the time it takes for such things, as I often help others on security and computer related forums with such detailed answers.

My goal with this thread was not so much to get help with a specific project, but to better understand the nature of a bipolar power supply and to understand why the circuits I have seen have a tendency to not spec out a transformer. Further, the differences in CT vs NON CT don't seem to be as "easy" to get a good "laymans terms" type of understanding.

I don't have many elec theory books, but I have over the last 6 years maybe read way way too many articles and project sites, from beginning theory to more advanced op amp stuff. I certainly don't claim to know much, but I do understand just enough to stumble along behind people like you who use terminology I have to look up then find I "basically" understand what you are saying.

Unfortunately elec theory is a very complex topic, and like you I simply don't have enough time for everything, so I focus on what I am currently seeking to do - although that usually leads to learning more than I need to lol.

Anyhow, having the input here and reading dozens of other sources of infos, I think I do understand what you mean. The GGG design was something I was looking at because I have been making some 386 type amps and some effects pedals, and I wanted to see what some of the other IC amps might sound like. I would love to make a tube amp, and there are lots of kits/schematics/projects available for that, but I realize too it isn't just something you decide to do one day on a whim. I have a great respect for "coming up to potential" and don't really wish to do that lol. So I thought some 2-20 watt IC amps might be a good place to get a little more advanced designs under my belt.

However, when looking at the schematics I seen 12 and 18v bipolar supplies needed. I have lots of wall warts (I keep any I find), but rather than just plug something in, I wanted to learn something. Whether I make my own or not in the future to house in an enclosure really isn't a concern. I could probably buy a wall wart cheaper than what I can make a psu for, not even including the time spent. But, I am just a curious sort, always wanting to know why and how.

I will attempt to explain what I think I know. Then, any who would like to correct me can help clear this mess up for me. My initial posting was based on a day or two of trying to find an explanation, without a result. The info given here has both helped explain and given fresh search terms to learn more.

If I need a bipolar power supply for a given circuit, I need to rectify it from an AC source.
A transformer is needed to step down the 120vac main voltage to something near what I will need, lets say 24vac for my target 18vdc.
There are two basic ways to do this - a non CT transformer, with one secondary, that is half wave rectified, one on the pos and one on the neg cycle. The neg side of the output would be the ground. (or is it the neutral?) The other way is with a Center Tap, where there are two outputs from the transformer, each producing X voltage but one pos and one neg. The CT would be the ground (or neutral?).

In half wave rectification, one diode allows top wave form to charge cap1. The other diode, on the bottom of the wave form, charges cap2. These caps store the energy and release it on thier respective wave forms 180 degree fall/rise. The combination of the two caps output will create a direct current. In many designs another set of caps will filter out this dc to eliminate as much ripple as possible since there is still some waveform left in the dc.

In full wave/bridge rectification, the diodes do much the same as half wave, except the voltage is already coming in as neg/pos. They still charge the caps 1 and 2, which store and release creating a direct current. Another set of filter caps is commonly used to again smooth out the signal. One advantage of a CT with full wave is that since the voltage is being "chopped" more often, on both cycles, the output dc will have less ripple - it will be smoother. Thus, smaller caps might be used because the signal will be more dc like.

In the GGG application, the dc signal reaches the respective voltage regulator, which dissipates excess voltage as heat to maintain a constant output. The end result is a regulated voltage, according to the 3v minimum on the regulators, and the output is per regulator parameter, including sizing it correctly for the heat required.

When choosing a transformer, a half wave design, not using a CT, I need to choose one that is going to provide the voltage/current needed. I don't implicitly understand yet what you mean when you say
If you had a single 56VAC winding, then you must use the GGG circuit. To get +/-40VDC though, you would only need a 28VAC winding, remember the capacitor is charged to the mains peak, 28VAC rms = 40V peak.
The 28vac rms=40v peak part that is. I will research that a bit more to see what that means.

I do however understand that of the output for a CT transformer, the primary output is lets say 18v and the secondary will be 18 x 1.414. At least I think that is how it works. But, if you had an 18-0-18 (a 36v transformer?), do both outputs produce 25v? I thought only the secondary output did, and the primary was only going to be 18v. Maybe I read that wrong, like a dozen times lol.

Anyway, will will study that some more.

As for the rest of the psu design, why the caps are there, what they are doing, I feel I get that. But today I was looking at different designs (I found quite a few more) and I am wondering, is it OK to use a higher voltage cap than is needed? I was looking this up (with no quick answer, just more thoery ;) ) because some schematics spec a much higher voltage than others, which got me to thinking, will it be important. When I order parts, would I need to get a high voltage cap and use it for all projects (as long as they are within the specs) or is it more important to have the correct voltage? Correct being something like a 16v cap for a 12v application, instead of say a 63v cap, which would be way over kill.

Another thing, I put this thread here in requests. I don't know if that was the best spot or not. Should I label this "solved" or something when I am done?

Thanks again for the info. Much appreciated.

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Post by mozwell »

Ok to higher voltage caps than needed, but NOT lower voltage.
A capacitor will have a rated current, so the higher the voltage rating the lower the current. If you put too much current through them, they can over heat. The most cost effective solution is to use a capacitor with a voltage rating a little bit over what you need. No point spending extra $ when you dont need to.
Also keep in mind that mains supply voltage is not constant. a 230VAC mains can vary by as much as +/- 5%, and sometimes, even +/- 10% depending on where you are.
So always allow for this extra 10% voltage when choosing a voltage rating for a capacitor.
Lets say you have a 28-0-28VAC secondary with a bridge rectifier and a filter capacitor of 4700uF. This woudl be suitable for a 100W into 4 ohm amplifier.

Also CT is NOT connected to neutral, dont ever do this.

The mains voltage is a sine wave. For the first 180 degrees it is "positive", for the next 180 degrees it is "negative". Voltages are always quoted in RMS. We know that peak to rms for a sine wave is teh ratio of square root of 2 (1.414). So a 28VAC voltage is a sine wave with a positive peak of 39.6V. We charge a capacitor through a diode. We lose 0.6V (approx) across the diode when it conducts, so in effect, the capacitor is charged to the peak of the sine wave. We only charge the capacitor at the peak of the sine wave, because as the sine wave falls away after the peak, the diode charging the capacitor is reverse biased & does not conduct. As we only top up the charge on the capacitor at the peak, we have to store this energy during the time in between peaks so as to get a smooth DC supply.

For an amplifier we need a smooth DC voltage supply. If we have a lot of ripple on the DC supply, we can often hear this as hum on the output. If our power supply capacitor is too small, as we provide current into teh speaker, we draw this out of the capacitor & its voltage falls. Its voltage is only topped up at the next mains peak, 10ms away for a full wave bridge power supply.

The capacitors are there to store the energy during time between sine wave peaks.

Back to our example, 28-0-28, with peak of 39.6V.
We need to allow an extra 10% in voltage and then take off a diode drop of 0.6V, we end up with 39.6 x 1.1 - 0.6 = 43V approx. We coudl choose a 45V cap, but nobody makes that voltage, so we choose the next voltage rating up. In this case i woudl sue a 50VDC rated cap, or a 63VDC is also ok.
If i used a 35VDC cap, we woudl be exceeding its rating, and i woudl not expect it to have a long life.

Check the ripple current rating on the cap you choose, 100W into 4 ohm speaker is 20VAC at 5A.
Sound is basically sinusiodal in nature, so we can assume that the speaker output voltage will be positive & negative at down to 20Hz, so a capacitor in the positive rail woudl provide current for only half the time.
For a 5A load, i would try to use a capacitor with a ripple current rating of more than 3.5A, the higher the capacitors ripple current rating, the cooler it will run & the longer it will last.

We could use multiple capacitors in parallel, so we get very little ripple, but we get to the point of diminishing returns quickly, where extra capacitors really give us no benefit.
As a very general rule, 4700uF woudl be suitable for the power supply for amps up to 100W

A lot of times scheamtics will not specify transformer secondary voltages, but if the DC rail voltage is there, its simple to work it back & calculate what the transformer secondary will be.

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Post by sullywoo »

Wow. Nice reply. There is more in that reply than in pages and pages of books and tutorials. You give great working examples, and complicated enough that I can only vaguely follow along and have to now do some research to fully understand what you wrote. Just the way I like it :thumbsup

Now I shall strive to assimilate the portions I don't understand into my collective consciousness ;)

So there is no detriment to using a 100v cap when 16v is called for? I ask because I have ample supply of things to desolder, so I can play without buying many parts. And, when I buy parts, not all circuits state what rating should be, so as I am learning, if I can use overrated caps, it might be better until I can figure out what would be the proper ratings.

Thanks again.

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Post by sullywoo »

Maybe a few more items. I researched a lot of what has been written here and have learned much. Beginning to understand things.

Here is something though. If I look at the GGG schematic, it appears that one leg of the secondary output is rectified to dc, but the other leg is used as the ground of the circuit. Would this not be the neutral? I mention neutral because I am familiar with AC line voltage leg, neutral leg and earth ground. It appears that there is a line voltage leg and neutral leg in that GGG schematic.

But, I see on a CT transformer, one leg is pos voltage, one leg is neg voltage, and the CT is the output (in parallel ?) used as the neutral. Perhaps my terms are not accurate. In a schematic for DC you reference ground as the neg side of things. A common ground ties together and the pos side winds here and there depending. In AC you have a neutral and a true earth ground. Isn't the neutral the counterpart, in AC to the ground in DC? This is where some laymans terms would help, as I have read lots of tutorials and such, but I guess I haven't found the relationship from AC earth ground and neutral to DC ground.

Anyhow, my comment prior were attempting to translate to you that the CT leg would be the ground or neutral in a twin output, and that in a single output, one of the legs would be the ground or neutral. Again, not meaning the neutral of the primary AC of the transformer. Just how does one label this anyway? Is it a nuetral since it is still AC prior to the rectification?

So I get how the 100w @ 4ohms translates into 20vac 5a. Ohms law and all. I get the load impedance will effect the output, where higher impedance gives greater amp but less wattage available. I get that 18v x 1.414 = 25.6v and the diode drop of maybe .6v gives us a max of 25v output, therefore a properly rated cap would likely be 35 volt.

Now for a little more if you will indulge me.

You mention 5va to 10va transformer. How does that relate to the 5a the 100w @ 4ohms refers to? Would the transformer need to output up to 5a current? When I look at specs for a transformer, and I find lets say a power of 10va 115 pri 18vct sec, the output is like 500ma current. If I look at a transformer that is 115 pri and 18vct, rated at 2a, the power is 36va. So which do I look at, the current needed for the load (the speaker at 4 ohm) or the amp rating of the transformer, or the va?

Slowly but surely, with the infos given here and the help of mr google, I am learning what is going on. Too many questions maybe. I could just start soldering, but would really like to understand why I am using part X Y and Z.

Thank you again.

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