Stepdown transformer value/size VA

[whoaru99]

In the US, modern residential electrical services are typically 200 amps at 240 volts (with the 240V comprised of two seperate 120V lines fed in tandem with a common neutral and earth ground).

Just FYI, the service is not 240V from two separate 120V lines. The service is from a 240V center tapped transformer. 240V is fed to the house and two 120V legs are derived from that to the center tap (neutral).

By rights, it's a 240V split phase service.

 

[Sansuiman]

^ yes, my off the cuff explanation was a bit technically lacking as to the center tap arrangement on a single phase 240V residential service. Not being an eletrician by trade, my explanation was not in that mindset.

I am surprised by the transient specs on a Square D QO series breaker, I'd not have expected them to be that high. What I was trying to convey is that if one is running an amp hard enough to dim the lights, they are very likely heating the internal contact which over time will weaken the breaker mechanism. That's also not particularly good for the amp to be running on sagging line voltage like that.

The best way to settle these questions is by measurement. All of these factors can be quantified by test and measurement equipment, so there really needn't be any guess work involved. While one may not wish to invest those tools, this is the only way to be certain.

 

[GrisPato]

This was discussed before, and I am not 100% convinced by the fundaments for the preferences or suggestions offered which appear a bit exaggerated. The input transformer in the audio amplifier does not know whether it is plugged into a wall plug or in a transformer. Having a transformer powering a second one, if there were no losses then it is sufficient to have identical volt-amp ratings for the step down as that you have on the transformer inside the amplifier. A factor of 2 or 3 in this case would have no consequence, and if the VA rating of the step down transformer is 10% bigger than the internal one, then you are limited by the internal transformer. You can not draw current into the amplifier at a higher rate than its internal transformer can. The real issue is that there are losses. But the loss is not 50% in a transformer, or it would be useless. Some loss is heat (resistive Joule heating of the winding, eddy currents in the core etc.), some is sound(by the transformer vibration) and some is a magnetic field that radiates out of the transformer. So all that is needed is to compensate for the losses of the transformers.
It was mentioned above that some G- series amplifiers cause the lights to dim with the music transients (i understand that no step down transformer was being used). In that case, the amplifier would have likely been distorting as a result of the voltage variations in its rails. My take on that is that playing music like that defeats the purpose of using a G series Sansui or AU-919 or an AU-X1.
Coming back to the step down transformer, you do not want to be limited by the step-down power rating+losses. Having extra capacity is handy because allows you to plug other pieces of 100V gear. I use a 750VA transformer to bring the 230V into 100V for my AU-D907, with no issues so far. I did testing with square and sine waves and did not notice any anomaly at low frequencies.

Thanks Jose!
Really think you give a rational explanation to your opinion. This where about the reasoning I had starting this topic with a lot less experience and knowledge about these things. I can understand the reasoning for taking a 2-3 times bigger wattage step down transformer just to be at the safe side for a 20-30 % more money to feed your favourite amplifier. But I think there was offered little to none proof that it's really necessary to do that.
The only wurry I still have is about the mostly used autotransformer that doesn't offer any separation between the primary and secondary windings which would give a safety risk as well to person as to equipment in case of failure. Although what information I can find they also offer benefits in being more efficient and more direct because partly they deliver the voltage/ current electrically, so more directely. Maybe for this safety reason, you want to be 99% sure you are not tasking the transformer close to his limit, came the 2-3 times rated va of your equipment.

 

[JoseHH]

autotransformer that doesn't offer any separation between the primary and secondary windings which would give a safety risk as well to person as to equipment in case of failure

That is right. And at school I had close encounters with an accident happening with a variac (variable autotransformers), because of the lack of isolation. Some autotransformers are better built and are safer than others.
On all this discussion about step-down transformers size, I believe that one reason people here give you a broad range of recommendations, is that we are missing information about what is the actual Volt-Ampere rating of the transformers that were factory installed in the amplifiers. While the back faceplate may state 360VA, that may be some average figure and the transformer may have been over-dimensioned. I do not know if this is the case in general and I speculate that -just to be sure there is no bottleneck in the supply- people prefer to use a rather large capacity transformer, just like you say.
Judging by the weight of the transformers, I suspect that an amplifier like the AU-D907 has a transformer with a rating on the order of 400 or 500VA (because somewhere i read that it weights like 13kg, and it is also a toroidal type which tend to be higher power for the same weight compared to the EI core). I am now holding the transformer for an AU-D707X decade and it feels like a10-12kg and it says (on a sticker on the transformer) that it is 356VA (while the back plate of the amplifier states 320W). It does not hurt the amplifier to use a larger transformer, it does hurt your wallet though.

 

[slimecity]

Im glad I saw this post.

I have a 2kW stepdown, and also an X1. Interesting to see its power consumption posted above.

One thing about large stepdowns - My one frequently throws the main board breaker when switched on, with nothing connected to its outputs. Causing me to just leave it on all the time. Ive had it happen in new and older houses.

So does this mean that there can be "inrush current" in something with no capacitors, that doesnt in itself "consume" power?

Ive never been able to work this one out.....

 

[JoseHH]

I have a 2kW stepdown, and also an X1. Interesting to see its power consumption posted above.

One thing about large stepdowns - My one frequently throws the main board breaker when switched on, with nothing connected to its outputs. Causing me to just leave it on all the time. Ive had it happen in new and older houses.

So does this mean that there can be "inrush current" in something with no capacitors, that doesnt in itself "consume" power?

Ive never been able to work this one out.....

Transformers have a transient at power up. There is an inrush current on the first cycle of the AC waveform. It happens because the core is still unmagnetized, so there is a transient magnetization flux which is equal to twice the steady state flux. If you visualize the AC voltage applied from the wall plug as a sinusoidal signal say V_AC=220V*Sin(omega*t) there is a maximum flux when (omega*t)=180 degrees. At this point the transformer core causes a very large transient current which many times larger than the maximum rated transformer current. This is known in the electrical engineering jargon as "Doubling effect". This is over my head, but I hope I pointed you in the right direction to figure out what happens with your 2KW transformer when you turn it on.

 

[johnny_fever]

Tube/valve amplifiers will be less demanding in step down situations (unless they are monsters) than SS amplifiers. The AU-X1 power figures given here suggest that my 1.5kW step down could be close to 'only' 1.5 times capacity for that load, but I never intend to test that limit while listening, so maybe the whole question is more 'academic' than I initially thought.

Update
However, being someone who repairs amplifiers, I might need to run an amplifier at maximum power or beyond albeit briefly, which could mean that plenty of step-down headroom is highly desirable.

Thanks. I know the little tube amps are less demanding. I chose to see for myself what the Huge 35watt per side AU-111G was demanding. I also run a 100V Alpha CD player at the same time.

 

[johnny_fever]

It would be nice to see some tests comparing tag ratings and actual demand numbers.

 

[JOMARK911]

Thanks jomark911, I was hoping you would bring your experience to the table. Your system inspired me to go looking in Japan for a cd10. I would love to go activ with my B 2102 for the low/mid and a d907 limited for the highs. The cd 10 just turned out to be to popular and all went above my set budget. Looking now for a cd5 because I have 2-way only so would be sufficient. Anyway...
I like it you bring some numbers.
So 17 A at 230 V is 3910 watts ( assuming peak). 70% is bass, so 2737 watts for one B2301 with a 1400 VA transformer!!!
This shows the overcapacity the Sansui transformers have but just on peaks. This will never be continues.
I think myself your middriver will use some of this too, depending on crossover setting. Because the B2301 is known to push out over 600 watts a channel not 1300 watts.
But you have really inefficient audiophile loudspeakers? Because having this need for power driven active!? Or they dip easily under 2 ohm at low frequencies?
You will be using at least 2kva stepdown for each, maybe 3.
But you know your system.
I have 3 sets of loudspeakers I change out. One set 8 ohm, never goes under 4 ohm, 86 dB sensitivity, another 4 ohm, 92 dB sensitivity, the third 8 ohm minimum ( Linn Kan )83dB sensitivity. A listening room of 24 m². I pushed the B2102 a little for showing my son up to volumepot sign -28dB ( C2101), was deafening and not a level to enjoy music.

Probably i forgot to put in my count the JBL MPX1200 that was powering the two subs , but that is only at the very low end , subs run to complement the main speakers below 30hz or so.
I don't think there is much difference , although JBL is a beast 2x800/8, 2X1200/4.
As far as the listening levels , sometimes i realy like to hit club levels for some minutes . Somethimg like 120db on 1m . The room is realy big 12x6 m .
The corals are pretty sensitive , 95db's woofers 97db mids and 98-99db tweeters.
If we think that 120db's on 1m turn down to 114 at 2m , you easily get the point.

 

[Bill Ferris]

You mention motors and start up load, what do you think is happening in an audio amplifier on bass peaks? - well I'll tell you, LOTS of current demand. Which is why figures of at least Double, but 3 to 5 times rating for step downs is not unusual. Do you really want to spend all that money on a 'to die for' amplifier only to throttle it with a cheap step down transformer? - but its up to you and what you want to spend.

Plus, if you are buying more JDM units do you want to be forced to buy Another transformer to power it because you 'cheapened out' on the first one?

The rating plate on my AU-X1 says 400W, so I bought a 1.5kW step down, but I was seriously considering the 2kW model.

,

 

[GrisPato]

So, what did I learn up to now?

First this:

Do you really want to spend all that money on a 'to die for' amplifier only to throttle it with a cheap step down transformer?

Don't buy a lower VA just to be 20-30 € cheaper if you're not absolutely sure you know what your amplifier power use is. Also for safety reasons.

Consider this:

Then again, I detect a faint whiff of embellishment of the sort that usually accompanies stories about catching a record breaking trout with a bamboo fishing pole and some 20 pound line, or the time we took on 8, I mean 12, bikers in a bar fight walked away with only a snag in our favorite shirt. And yeah, admit it, we've all played that card a few times.

I think we all know when we do it . . .

I think this response is based on logic reasoning from someone with experience and knowledge:

The input transformer in the audio amplifier does not know whether it is plugged into a wall plug or in a transformer. Having a transformer powering a second one, if there were no losses then it is sufficient to have identical volt-amp ratings for the step down as that you have on the transformer inside the amplifier. A factor of 2 or 3 in this case would have no consequence, and if the VA rating of the step down transformer is 10% bigger than the internal one, then you are limited by the internal transformer. You can not draw current into the amplifier at a higher rate than its internal transformer can. The real issue is that there are losses. But the loss is not 50% in a transformer, or it would be useless.
.......
.......
So all that is needed is to compensate for the losses of the transformers.

This leaves me with the opinion that when you can decide within a reasonable margin ( within say 15% ) what the power usage from your amplifier is you add some margin for losses and safety you get between 1.5-2.0 times power of your amplifier.
So for an au d707 the power consumption is stated 465 watts with a transformer rated 360 VA, I should be up from 750 VA to 1000 VA.