transformer question

[sakis]

we all know very well that transformers are very critical to our amplifiers since they provide the power to the circuit .....


lets suppose for a second that we have two amplifiers that are rated like 150w per chanel and of course we expect the trafo to go down on the voltage while operating at full power .... now how much down of cource will have to do with the quality of the trafo and also the selection ......IE proper trafo for this application ...

then the question is if we have the same quality and specs of transformer in the one amplifier and the same quality and specs in the other but auto transformer instead ......

meaning : if we skip the safety issues for a minute that might ocure by the use of auto transformer which one will have the best performance ?????

normal transformer or autotransformer ????

thank you

[AndrewT]

the autotransformer will be about a half of the size for the same VA rating and regulation.
I would expect them to operate similarly.
But, that's just guessing.

[sakis]

carlos said to me that may be voltage goes down almost the same or even less but the ampers flowing from this circuit is only limited by your socket ....

i am willing to try this

thanks andrew

[bear]

On the auto transformer, one leg will be DC coupled.
On the "regular" transformer, neither leg will be DC coupled.

Could make a difference in ways that have nothing to do with voltage drop or current capability under load?

Fwiw, due to the lack of safety isolation, I'd NEVER ever use one for the power line. One miswired line cord and people could get very dead quickly...

_-_-bear

[destroyer X]

They use to say that you have a 500 watts or 500VA transformer.... VA is Volts multiplied by Amperes.... and Volts multiplied by Amperes is WATTS too.

They intend to say that you will be able to drain 1 ampere and still have 100 volts (more or less 20 percent of voltage tollerance).

But they drop.... this is not the real thing as i could see.... at least in my country (maybe third world country problem).

I think, a good idea to those folks would be to say the transformer is guaranteed to, lets say, 500 watts when the transformer is able to give you more.....but to save money into the manufacturing of transformers, they use tell you the transformer is able to 500 watts when the unit will give you LESS.
Well....of course folks, i accept this can be a regional problem...my country problem.

But the best idea is to buy, or calculate when winding, or selecting iron cores, to select twice your needs... this way you will really have what you want...and maybe a little bit more.

Auto transformers are vastly used in my country as AC stabilizers...It is a single coil with taps (many turns coil with 1.5 milimeters diameter copper wire).... those taps are controled by relays that switch the needed tap to control the AC voltage...a chip senses the AC input voltage drop and fast switch to a higher voltage tap into the autotransformer.... works great.... but has voltage drops depending the consumption alike others, more standard, traditional and common primary and secondary types of transformers.

I use to wind them in my home.... but i do not use to touch the primary..... i use to measure the secondary AC volts and lets say i could find 100 volts (for instance, example only)... then i rewound, remove, uncoil the transformer and i counting the turns i perceive was 100 turns....so...very simple... 1 turn to each volt.

If i want 30 plus 30 volts.... so, i will need 30 plus 30 turns... with the biggest wire (diameter) i can find that can be inside the coil limits (sorry this is eye calculation, cannot explain how to decide that..have to do to learn doing.... try and error basis).... then i run two wires in parallel.... long wires attached to my house doors to be straigth and then 30 turns....

The starting point will be twisted together and will be your center tap..... when finish to wind your coil, you gonna have two wires...each one will have 30 volts (1 volts each turn..very easy).

The power:

If your transformer had 100 turns, 100 volts AC and lets say 1 ampere.... a 100 watts transformer (volts multiplied by amperes result watts or VA).... you will not be able to obtain more than 100 watts.... a 100 watts transformer core, will always give you 100 watts or MUCH LESS than that because of losses...bad winding and those things...manual work is less precise to wound things....but you have two coils... this helps a little and only 30 volts...... this means your 100 watts will be distributed into your coils..... something alike 30 plus 30 volts and 1.6 amperes may be obtained, as you have used bigger wire diameter you may have nice power.

So...to manually change things... in advance select huge transformers, because the losses you gonna have...heavy ones, big ones, and another good idea is to calculate your AC voltage 20 percent bigger than you needs.... as the transformer will loose voltage when drained... will have voltage drop.... during those moments your amplifier will have the good voltage to operate, and sligthly bigger while in stand by mode.

My philosophy and small money, made me build those things, they are easy but we have things to learn doing...if you fail doing, it will turn a buzzer vibrating metal sheet.

My examples and calculations are not precise.... also i had so many surprises doing those transformers, some results better than i was expecting, that i cannot guarantee my numbers...but for sure Andrew T can help into those calculations.

regards,

Carlos

[destroyer X]

If your transformer is 100 watts, and you change it to the voltage you want, it will continue to be 100 watts.

A hundred watts transformer will not be able to make an audio amplifier to produce more than a 100 watts of audio.... in the reality, around 60 watts will be produce in your amplifier connected to that 100 watts transformer.

I know you are experienced..but there are folks that has doubts about that, beginners and very young folks.... some of them think they can connect two amplifiers and then will have bigger power...negative.... that 60 watts i have explained before will be distributed into those 2 amplifiers.... they will produce 30 watts each one.... the voltage will drop a lot under consumption and the power will be around 60 watts....so... everything is connected, and depends on the transformer power...and this depends how huge and heavy and big it is... also the material quality of course...but how huge, how heavy and how big is the most important thing.

Here, down in Brazil, my Orkut friends are all building their own transformers.... rewinding transformers (rewound?)...well.... dismounting and producing another coil to the secondary once again.... this way they are using junk transformers, from old equipments... this turns cheaper their hobby.

I have published, in 2004, instructions about that... was for a guy from Israel..... the made his own transformer and was happy....but do not know the thread name....if you want it, sorry, have to search in 2004 into Destroyer X threads.

Sorry to invade in such way your thread, but as you had mentioned my name, i felt the obligation to come and say some words.

Carlos

[coloradosound]

Auto-transformers are only used to adjust for voltage
differences and do not have separate galvanically isolated
primary and seconday. Do not use for Amp as it basically
places the whole amp hot with respect to the mains line!!
The main idea behind using transformers (either step-up,
step down, or 1/1) is really to isolate the amp from the
mains line!

As far as sound. I think the isolating transformer would
filter more dirt and grunge off the line as well. The auto-
tranny would let more line noise, harmonics, etc. get through.

[sakis]

thank you for your very usefull input ....i will keep everything in mind

though i will wind an auto transformer and see about the performance

coloradosound thank you also for your input ....i am very familiar with all this safety issues and use of auto transformers ..... still though i would like to try to see the overall performance of a device like that

thank you all

[AndrewT]

sakis,
put an isolating transformer in the mains line first.
Test both the conventional and the auto when either are fed from the isolating.

[sakis]

thanks for your advice .....

i had this in mind anyway but the circuit i am working with has isolation trafo in the signal input so if there is any danger this is limited in the speaker cables ......

the all thing is designed to operate like that and might be inderesting circuit that produces a hell of a lot of power for a device that only weights 13kg ....

the only thing is that this circuit is almost mains operated so posting a schematic or anything else will be quickly removed by moderators .....

( here i would like to state that at the age of 42 and so many electronics produced ,modified , operated..... and with probably more than 10.000.000 wats installed in clubs, theaters, and PA rentals since i started at the age of 13..... of course i put safety first .... but still a mains operated amp will be inetersting thing and very powerfull to be presented somewhere in this forum with probably very big notification that voltages related to this device might kill somebody )
but thats just my opinion

people produce or modify cars that can speed up to 300 klm/hr dont you think that this even more worst or the same fatal ????