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Old 20th March 2010, 11:41 PM   #1
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Default VA, how do you size?

I am going to build a SKA GB300D based power amp soon and I was wondering how to size the transformer stage correctly? I will be building in dual mono mode ad a pair of transformers. This will be 40-0-40 and I was thinking approx 500VA but I have no science to base that guess upon.

The SKA amp modules have a very high PSRR and so less power required at the transformer if I understand correctly. Therefore a pair of 300VA transformers would be fine?

Any advice on how to size the transformers?
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Old 21st March 2010, 01:25 AM   #2
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The PSRR has no influence on the tx VA.

Doesn't this guy Greg Ball give any advice on this?

You don't say how you intend to configure the amp. The ad says: 'may also be configured in pure Class A to 100Wrms at 8 ohms on +/-43V supplies.'

Your tx is 40-0-40. This is AC, it has a peak value of 56.6V and after rectification and filtration will give you a DC voltage of ~55V

You're going to run the amp at 55V, dual rail. This will give you ~200W in 8ohms. If it's class A the efficiency will be no better than 50% so the actual power required will be 400W, when outputting 200W RMS, but this is rarely encountered except under sinewave testing.

Most people feel bigger is better in power supplies so 500VA will be fine. You can cut back a bit if the budget is tight, 300VA is a bit mean, what class A means is that it will draw 200W at idle.

If you change the voltage, the output power will change and so will all the other numbers.

w
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Old 21st March 2010, 09:06 AM   #3
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Hi Mike,

Yes Greg does provide very good advice for his amp kits, with his own forum and very responsive to emails.

I was asking on a more general basis of has does one come to the right size of the transformers for their amps? How do you calculate in theory the correct amount of VA and then by what factor do you scale up within the audio world for unstressed performance.

So no reflection on Greg leaving me on my own, just me trying to develop my own knowledge within this journey of mine.

With this project I am not going to run class A so they will be in class AB mode. My speakers are approx 6 Ohm nominal load from the specs.

Thanks for the help
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Old 21st March 2010, 10:32 AM   #4
AndrewT is offline AndrewT  Scotland
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It is normally recommended that a ClassAB amplifier be fed with a transformer VA between 1times and 2times the maximum output.

The 40-40Vac will allow about 200W into 8ohms or 350W into 4ohms.
This will require a VA rating of 200VA to 400VA for 8ohm use or 350VA to 700VA for 4ohm use, per channel.

For a stereo (two channels) amplifier the transformer rating could be 200VA to 1400VA.

Greg's support is great. His own website and Forum are great.
We will also support you here, but regretably, expect some flack from those that bear a grudge to a former Member.
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Last edited by AndrewT; 21st March 2010 at 10:34 AM.
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Old 22nd March 2010, 11:33 PM   #5
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Thanks for the help Andrew. Seems like my thought of a pair of 450-500VA transformers isn't far out then. My speakers are about 6ohm so with 40-0-40 seconday the amps should be good for 300w

Shame there should be some bad feeling on here, guess I shouldn't ask why
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