Question on transformer specs

[aarond]

I've been wondering aobut this for a while...

Why are transformers rated in voltamps instead of simply watts? If I'm not mistaken either spec would tell you the same thing (the total power output of the transformer).

Just to make sure I am understanding this properly, a 500 VA transformer would be capable of putting out 5.55 amps if the output voltage is 90V, correct?

Thanks,
Aaron

[aarond]

My question is realtive to power supply transformers for audio amps if it makes any difference. If I wanted to supply 90 volts would I look for a 90+90 volt output or a 45+45 volt output?

Any good online links that would get me up to speed on this subject? I don't want to take an EE course so a practial version would be great. I'm going to go check out Wiki now...

Thanks,
Aaron

> I've been wondering aobut this for a
> while...

> Why are transformers rated in voltamps
> instead of simply watts? If I'm not mistaken
> either spec would tell you the same thing
> (the total power output of the transformer).

> Just to make sure I am understanding this
> properly, a 500 VA transformer would be
> capable of putting out 5.55 amps if the
> output voltage is 90V, correct?

> Thanks,
> Aaron

[danielhaden]

Okay, let's take a look at that question, economies of scale, and some amp errata. . .

A 36v center tap at 1 amp rating, is 36va.
It is also 2 amps if used parallel at 16v (a single 16v line--1 hot, 1 neutral).
It is also 1 amp per side if used +-+ for two 16 volt channels with a common ground.

A 28v center tap at 2 amp rating is 56va (much more useful at exactly the same cost).
Used in parallel at 14v, its 4 amps (a single 14v line--1 hot, 1 neutral).
It is also 2 amps per "side" if used +-+ for two 14v channels with a common ground.

It is unlikely that an amplifier project would require 90v+90v. However, its probable that the specs call for a 40v to 48v center tap that can provide a pair of 20v to 24v.

Look up the reference board diagram and have a peek at the power output vs voltage chart. There's usually a variety of voltages available for use.

Here's some help. Usually, amplifier designs are available for single, ct, or dual power. Those are usually all the same voltage, whether its one 24v (single), two 24v with one rectifier (ct), or two 24v with two rectifiers (dual).

Two of these designs can be powered by the $4 24v sale transformer--single (one in series)--and dual (takes two of them and two rectifiers).

One of these, Center Tap (+-+) takes the 48v ct transformer (more price on transformer, less price on supporting parts).
Most of the circuit layouts that match up with your example take that one; however, that's voltage maximum for most amplifiers, with no room for safety. So, that would need big heat sinks, be restricted to 8 ohm speakers, and need a "snubberized" or regulated power supply (avoid peaks at 60v).

Safety margin? That's probably a 36v ct. Check the amp's spec sheet--you want "centerline" tolerances on voltage, not maximum.
Then you can run 4 ohm speakers, use bridged layouts and have the amp last longer.

The amplifier produces a tiny percentage less power output on centerline tolerances for voltage. But, it lasts longer and sometimes has nicer dynamics (results vary--check the specs).

Most amps will run on a variety of voltages. If your power supply doesn't have a regulator, surges could be at 140% (40% again), so plan accordingly (don't buy a maximum spec transformer if the plans don't call for a regulator--1 step less is usually enough for safety).

A factor to consider while looking at the voltage versus power output chart. . . Speakers gain 3db every time the amp doubles in power.
So, the difference between 50 and 60 watts is about as loud as tying your shoe.
However, the difference between a 2 amp and 6 amp transformer can be tremendous.

The other question on VA? That's sheer marketing goodness. For instance 12v 1 amp is 12va, but more expensive to make than 36v 1 amp, 36va. However, since VA makes more impressive numbers, the lower cost item can sell for more. That gets the bills paid for people who are winding and winding and winding and winding. . .

Now, if your example of 45v CT is 160va, 3.55a, then if you were to use a 40v ct, it would need to be 142va, or a 36v ct would be 128va--all of which are 3.55 amps.
Real life differs. . .
Big trouble awaits if the manufacturer speced 160va at centerline voltage and then advertised maximum power output at maximum voltage input.
This is all quite normal.
In the above example, and with the above typical amplifier marketing, your power supply would actually need 4-1/2 amps (216va minimum) strength, and 6 amps would be more appropriate than a minimum--just in case the transformer product is also peak-rated. ;)

Most non bridged class A-B amps like LM3886 set up plain stereo, can handle wide variances in voltage but do like a bit of amperage for whenever you do some enthusiasm with the bass beat.

For Class-D, those don't need as much amperage, and most of them blow up immediately if run unregulated at advertised voltage.
To run Class-D on a homebrew power supply (unregulated), just multiply its advertised maximum by 0.7, and there you have a safety margin, plus probably better sound. ;)
For Tripath, a form of Class-D, multiply the advertised max by 0.85 (it is more tolerant).

Next thing, its easier to purchase the kit board of caps and diodes than it is to do one by scratch on the first try. Just do a kit first and then you'll be comfortable with it.

And lastly, you're going to think that your power supply is nuts if you try to use an amp without a volume control. Also nuts is 100k and some of Alps.
Before searching for the buzzing noises. . . Try a regular 50k pot if using a preamp, or 20k if you'd like direct line level and/or preamp options. Stereo models are called "dual gang" and so an "A20k dual" is a general purpose stereo volume control for a powerful amp. The "A50k dual" is also usable for general purpose applications.
Without one of those, you might think that the amp sounds like a guitar amp wide open. ;)

See also, Gainclone, Tripath, Bridged Thompson, LM3886, chipamp.com, 41hz.com, and see the difference between diodes, fast switching diodes, and ultra fast switching diodes (a difference of quality).

Hey aaron! have fun!

> My question is realtive to power supply
> transformers for audio amps if it makes any
> difference. If I wanted to supply 90 volts
> would I look for a 90+90 volt output or a
> 45+45 volt output?

> Any good online links that would get me up
> to speed on this subject? I don't want to
> take an EE course so a practial version
> would be great. I'm going to go check out
> Wiki now...

> Thanks,
> Aaron

[daryl]

Provided Link: Daryl's Speaker Stuff. (http://www.villagephotos.com/pubgallery.asp?id_=144711)


VA or Volt-Ampere is the uncorrelated product of EMF and current where power (Watts) is correlated.

Power is the actual delivered power and VA is what is reffered to as 'apparent power'.

The signifigance of this is that if you were to connect a capacitor or inductor to a power transformer that causes the transformers maximum rated current to flow the actual power delivered will be virtually zero but the apparent power tells you that the transformer is maxxed-out even though no power is delivered.

Regardless of how much power you are actually getting from a transformer the manufacturer wants you to know apparent power is what matters.

> I've been wondering aobut this for a
> while...

> Why are transformers rated in voltamps
> instead of simply watts? If I'm not mistaken
> either spec would tell you the same thing
> (the total power output of the transformer).

> Just to make sure I am understanding this
> properly, a 500 VA transformer would be
> capable of putting out 5.55 amps if the
> output voltage is 90V, correct?

> Thanks,
> Aaron

[AJ]

<A HREF="http://www.tnt-audio.com/clinica/ssps1_e.html">http://www.tnt-audio.com/clinica/ssps1_e.html</A>

[Wolf]

> A 36v center tap at 1 amp rating, is 36va.
> It is also 2 amps if used parallel at 16v (a
> single 16v line--1 hot, 1 neutral).
> It is also 1 amp per side if used +-+ for
> two 16 volt channels with a common ground.

It won't be 16V, but 18VAC. And- if the TxF is 18-0-18 as you state, then is is .5A per secondary, not 1A.

> A 28v center tap at 2 amp rating is 56va
> (much more useful at exactly the same cost).
> Used in parallel at 14v, its 4 amps (a
> single 14v line--1 hot, 1 neutral).
> It is also 2 amps per "side" if
> used +-+ for two 14v channels with a common
> ground.

No- the current capability cannot increase more than rated. It's 1A per side when you divide it.

> It is unlikely that an amplifier project
> would require 90v+90v. However, its probable
> that the specs call for a 40v to 48v center
> tap that can provide a pair of 20v to 24v.

Multiplied by 1.4 post-DC rectification of course.

> Look up the reference board diagram and have
> a peek at the power output vs voltage chart.
> There's usually a variety of voltages
> available for use.

> Here's some help. Usually, amplifier designs
> are available for single, ct, or dual power.

Dual-Secondary/Dual-Primary, not dual-power. If you tie the closer points on the windings together you change your input voltage requirements, and result in a CT secondary.

> Those are usually all the same voltage,
> whether its one 24v (single), two 24v with
> one rectifier (ct), or two 24v with two
> rectifiers (dual).

Rectifier is not the CT. The rectifier is the diode bridge or combination.

> Two of these designs can be powered by the
> $4 24v sale transformer--single (one in
> series)--and dual (takes two of them and two
> rectifiers).

Again- not rectifiers.

> One of these, Center Tap (+-+) takes the 48v
> ct transformer (more price on transformer,
> less price on supporting parts).
> Most of the circuit layouts that match up
> with your example take that one; however,
> that's voltage maximum for most amplifiers,
> with no room for safety. So, that would need
> big heat sinks, be restricted to 8 ohm
> speakers, and need a "snubberized"
> or regulated power supply (avoid peaks at
> 60v).

It is better to avoid running transistors at peak voltage, I agree.

> Safety margin? That's probably a 36v ct.
> Check the amp's spec sheet--you want
> "centerline" tolerances on
> voltage, not maximum.
> Then you can run 4 ohm speakers, use bridged
> layouts and have the amp last longer.

Not always. Just because the amp is safer in PSU voltage does not mean you can bridge or run lower impedancel loads. There is more to it than that, which is why I don't design amps.

> The amplifier produces a tiny percentage
> less power output on centerline tolerances
> for voltage. But, it lasts longer and
> sometimes has nicer dynamics (results
> vary--check the specs).

However, you can clip an amp without enough rail voltage.

> Most amps will run on a variety of voltages.
> If your power supply doesn't have a
> regulator, surges could be at 140% (40%
> again), so plan accordingly (don't buy a
> maximum spec transformer if the plans don't
> call for a regulator--1 step less is usually
> enough for safety).

> A factor to consider while looking at the
> voltage versus power output chart. . .
> Speakers gain 3db every time the amp doubles
> in power.
> So, the difference between 50 and 60 watts
> is about as loud as tying your shoe.
> However, the difference between a 2 amp and
> 6 amp transformer can be tremendous.

Later,
Wolf

[aarond]

I'm going to read through the info a couple of times to see if I can make sense of it. Looks interesting.

In hind sight I probabably should have asked:

"What kind/size of transformer would I need to but to upgrade the power supply in a Hafler DH500/P500?"

I know that musical concepts rates their upgraded transformers @ 500 watts and they use two of them to build a dual mono amp. The voltage going into the p/s caps is 90V (100 volt rating on caps). The MC upgrade costs $229.

I am also considering getting some big p/s caps from Apex Jr.

All that is the motivation behind my question. But a simple question usually gets a simple answer and I prefer to know the "why behind the what" on subjects like this. You guys are usually more than willing to entertain such questions and I appreciate it. I don't have any non-internet friends in my life that would even understand such questions.

In the future I will try an remember to give more background on such questions. It was late when I posted it and I simply did so because it was something I had been wondering about for a while. It's fun to ask a question, go to bed, and wake up to see what you can learn.

Thanks again,
Aaron

[AJ]

Hey Aaron, Why are you changing the power supply in the DH500? The stock transformer is big enough for the job, and to replace it you would need a 1000va toroid or two 500va toroids. That gets expensive quick.

The positive and negative voltage seen at the filter caps is DC voltage, as thats after the bridge rectifier. That means to get to the +/-90vdc you need to divide the 90vdc by 1.414. That gives you the AC voltage you need from the secondaries on the transformer.

So, dividing 90 by 1.414 you get 63.7 or so. That means you need a transformer that has a 65vac - 0 - 65vac winding. The "0" is the center tap.

If you want to start learning about amps consider building one from scratch. Keep reading, and applying the knowledge. Leave the Hafler alone as a sort of benchmark for comparison. Like i said, the stock transformer is big enough, no real point in installing new ones. Also, Toroid transformers by design draw more current at startup than the EI core transformer that the stock amp uses. This can stress the "slow start" circuitry in the amp.

If you do want to replace parts in the Hafler, consider replacing the electrolytics on the amplifier circuit boards, and the large power supply caps (these won't be cheap).

If the Hafler is working fine now, don't work on it. "Don't fix what isn't broke"

[AJ]

I still say you missed your calling as a teacher Wolf. ;)

[Wolf]

> I still say you missed your calling as a
> teacher Wolf. ;)

I suppose that's possible, but it's the older ignorant rebels that would be hard for me to tolerate. Little kids would probably not be a problem for me. Thanks for the cred! I try to 'splain some things when I see that it's in error.
Later,
Wolf

[AJ]

> I suppose that's possible, but it's the
> older ignorant rebels that would be hard for
> me to tolerate.

What? Like me? :)

[Wolf]

> What? Like me? :)

I would not think of you as ignorant.
Later,
Wolf

[danielhaden]

Okay 36v ct does 18+18. ;) Not enough coffee this morning.

> No- the current capability cannot increase
> more than rated. It's 1A per side when you
> divide it.

I did phone Allied Electric in Ft Worth, TX, and they state the amp rating is using the entire length of wire.
The transformer will not fall below the rated amperage unless you add more wire to it.

I have two of these product here. Allied # 226-2060 (36v ct 1a) and # 227-2030 (28v ct 2a).
This is rated to produce that amperage at that voltage.

Using half the wire length produces the same or more amperage (depending on options)--never, ever, less amperage.

I had the 2 amp deal in my lap, but it was too heavy on my stomach. Maybe transformer advertisments differ? Stomach? Oh heck no!!! I gotta go work out! See ya later!!

[philiparcario]

> Hey Aaron, Why are you changing the power
> supply in the DH500? The stock transformer
> is big enough for the job, and to replace it
> you would need a 1000va toroid or two 500va
> toroids. That gets expensive quick.

> The positive and negative voltage seen at
> the filter caps is DC voltage, as thats
> after the bridge rectifier. That means to
> get to the +/-90vdc you need to divide the
> 90vdc by 1.414. That gives you the AC
> voltage you need from the secondaries on the
> transformer.

> So, dividing 90 by 1.414 you get 63.7 or so.
> That means you need a transformer that has a
> 65vac - 0 - 65vac winding. The "0"
> is the center tap.

> If you want to start learning about amps
> consider building one from scratch. Keep
> reading, and applying the knowledge. Leave
> the Hafler alone as a sort of benchmark for
> comparison. Like i said, the stock
> transformer is big enough, no real point in
> installing new ones. Also, Toroid
> transformers by design draw more current at
> startup than the EI core transformer that
> the stock amp uses. This can stress the
> "slow start" circuitry in the amp.

> If you do want to replace parts in the
> Hafler, consider replacing the electrolytics
> on the amplifier circuit boards, and the
> large power supply caps (these won't be
> cheap).

> If the Hafler is working fine now, don't
> work on it. "Don't fix what isn't
> broke"
go to <A HREF="http://www.apexjr.com">www.apexjr.com</A> or <A HREF="http://www.tedss.com">www.tedss.com</A> both have caps that will work. match the volts and raise the capacity 10000uf go for 15000uf or 22000uf. you will get the most improvement that way. the caps are 15 to 30 bucks a piece.

[AJ]

The stock DH500 uses 20kuf 100v. Unless they are dead or failing there is no point in changing them. I have three Haflers here that all have the original power supply caps in them. Unless stressed these caps last a long time. The smaller electrolytics on the main circuit boards do go bad.

[Wolf]

> Okay 36v ct does 18+18. ;) Not enough coffee
> this morning.

Okay.

> I did phone Allied Electric in Ft Worth, TX,
> and they state the amp rating is using the
> entire length of wire.

Correct!

> The transformer will not fall below the
> rated amperage unless you add more wire to
> it.

But you are misinterpreting the info. If you have a VA, and you split the secondary, then you halve the voltage per secondary, AND halve the amperage per secondary. For the full-secondary, you have 2 Amps, for half the secondary, you have 1A. You can't have more VA than the supply is rated, and cannot make more current than it can supply.

> I have two of these product here. Allied #
> 226-2060 (36v ct 1a) and # 227-2030 (28v ct
> 2a).
> This is rated to produce that amperage at
> that voltage.

Correct!

> Using half the wire length produces the same
> or more amperage (depending on
> options)--never, ever, less amperage.

No it does not. You have the same current flow in the secondary, relative to the primary. If you shorten the secondary length, the current does not have to travel as fast to match time with the primary. Therefore, the current is moving slower. You do not have full-amperage rating in both secondaries, or CT usage. You get half.
Later,
Wolf