What will raising the Va of atransformers provide?

1. Unless you are upgrading and altering the circuit designs within your amp, the new transformers must have the same turns ratio for voltage in and out. In other words, whatever the original input and output voltages are in your current transformer, that is what it must be for your new transformers. With Higer VA rating, that means for the example I stated above, that the current output will be different.
2. Current in-rush must be considered. you would need a slow start circuit.

I have found more benefit by upgrading my power supplies, larger capacitors, separate power supplies, etc. and upgrading the output transistors and pre-drivers to newer, more linear powerful transistors. I have also added Threshold style heat sinks to my amps and altered the bias accordingly to more class A operation. Amps such as Bedini, Threshold, etc. really benefit from this.

By-the-way, you can bias for various levels of Class A. What most people feel is class A is bias whereby the output drivers are producing rated current with no signal. you can bias to lower amounts also. Say, for example the amp is a 100 WPC amp. Nelson Pass on his web site gives several examples of heat sinking, transformer characteristics, amp design, etc. so take a look at his web site, but, for full class A, this means that the output drivers are biased for current output that equates to 100 WPC for the amp. you can bias for 5 WPC, 10 WPC, 50 WPC, etc. This totally depends on the transformer capacity, other circuit considerations and mostly proper heatsinking. To have true full output class A, you need rediculous heat sinks and they cost a ton of money. This is why most amps are not true full output class A, but are biased to class A for lower wattage.

So, if you really like your amp and want to upgrade it. Try upgrading the power supplies first. Then look into new more linear output transistors and pre-drivers. for tube amps, that is a totally different story.

enjoy

Firstly lets get this Class-A and AB thing sorted out.

Class-A: This is where the transistors that operate both the negative and positive going parts of the waveform are ALWAYS on. The reason why Class-A amplifiers get hot is because the transistors are always ON.

Class AB: is where the transistors are only on half of the time, they switch on and off, cycling between the two halves of the waveform. The good thing here is that the transistors are OFF half the time hence less heat.
The problem now is crossover distortion! This is where the 2 halves 'don't quite meet up properly'.

Stanwal - it has nothing to do with full power or AB varying with demand.

So the question to whether a larger VA rating can improve the audio is YES. This is partly because as the VA rating increases the transformer 'regulation' becomes more efficient. Also (and remember VA = Volt Amperes) as you increase VA your current 'tap' increases allowing your amplifier to draw more current when needed.
If you imagine - a standard desktop computer draws approx 25-35A for a few milli secs when asked to perform a BIG number crunch. - this is the equivalent of a large transient in music (kick drum, bass guitar pluck, timpany on drugs)

How much current does a preamp need????

Well, I'm not a Naim Audio lover BUT if you listen to any of there preamps with their smaller power supplies and go to a bigger power supply the improvement is BIG. This is the same for any amplifier design (A,AB,B etc).
Class-D (again not a fan) improves with larger VA transformer - just ask Hypex

Remember, to get the best from a larger VA rated transformer you must upgrade the rectifiers and filter caps accordingly or else it's bottleneck time.

Dcarol, why do you think it does not? A true chass A amp draws 4 watts ALL THE TIME for each watt of output. So a 100 watt a channel amp will draw a constant 800 watts from the wall. An AB amp idles at a few watts and rarely draws its maximum current. Therefore the transformers in an A amp have to be up to ALWAYS supplying full current as opposed to doing it once in a great while. It is obvious that class A amps require more robust parts to deal with the constant heat generated; they actually run cooler at full output than at idle. Class A amps never see a sharp increase in current draw because they are already drawing their maximum; how then would bigger transformers improve the performance? Yes you could increase the power rating but that would mean everything else would have to be bigger also. I thought the origional question was just changing the transformer and I have seen nothing in what you said to convince me that it would.

Stanwal

high-end audio reproduction doesn't work like that. If it did work the 'theoretical way' then a bigger power supply or larger VA rating on a transformer on a preamp would do absolutely nothing!

As we all pretty much know though, it does make a difference.

Your calculations do not take into account instantaneous/ transient current draw - the current draw during fast, large musical passages which happen in a few milliseconds. This is where a larger VA rating is needed. If you have ever built an amplifier whether Class-A or AB and calculated the exact power requirements and even been generous adding an extra 1.4 multiple, you will still hear an improvement doubling the VA rating.

If the theory and maths could explain everything then how does high-end audio work? Theory and maths does not explain differences in audio nor does it explain the reason why a larger supply helps the audio reproduction in just about every piece of audio equipment. Ask any audio designer...he'll/she'll be able to get so far but then after that it's all 'guess work'.

stanwal: I thought the origional question was just changing the transformer
Dirk: Changing the transformer to something larger is changing to a larger VA rating.

It is your original explanation to class-A and AB that is questionable. Class-AB 'varies with demand' and Class-A 'runs at full power' and also the idea that they are all large and expensive - they are not all large and expensive.

I'm not asking you to take anything I say onboard, just that it is not as simple as you make out - if it was then we would all have the same system.

Dcarol, it appears that you are not addressing Stanwal's point about a class A amplifier. I agree with him- if the transformer for such an amplifier is up to the task in the first place, a larger transformer is not going to make a difference, high end audio or no. If the transformer is not up to the task, then the amp is either not class A or its one with a reputation for eating transformers. In any case it may be moot as a class A amplifier is 'high end' anyway...

Unless you are upgrading and altering the circuit designs within your amp, the new transformers must have the same turns ratio for voltage in and out. In other words, whatever the original input and output voltages are in your current transformer, that is what it must be for your new transformers.

This comment is not entirely true. Power transformers can have voltage drops across their windings, a lot has to do with the internal temperature rise of the part during operation- transformers with more temperature rise have higher internal voltage drops. If you go with a transformer with more VA, you may well have to *reduce* the turns ratio to obtain the correct output voltages under load.

In any case, while on the surface it might appear to be a simple upgrade, the fact of the matter is that doing such a change is an R&D project and should be viewed in that light, especially when the initial results fail to satisfy :)