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Old 20th January 2010, 07:21 PM   #6691
AndrewT is offline AndrewT  Scotland
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most transformers are rated at a limiting internal temperature that guarantees a reasonably long life into a resistive load. i.e. AC current into a resistor.

We hang a capacitor input filter on our transformers that forces them to run hotter than the manufacturer's rated maximum temperature unless we de-rate using the manufacturer's cap input de-rating factor. This is around 70%.

This means your 300VA transformer cannot deliver more than 210W continuously from a DC supply after the capacitor input filter.
But fortunately transformer feeding ClassAB amplifiers do not run at maximum power all the time. They spend very little time at peak current because the capacitor bank does the job of supplying the peak transients sent to the speaker. The transformer then recharges the capacitor bank. Transformers are also very good at tolerating short term overload. They will even deliver ten times rated current for a very short period, without damage, if you are courageous enough to try it.

You will find that even taking these various measures into account that an 300VA transformer can power ClassAB amplifiers producing a total maximum power from 150W to 300W reasonably well.
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Old 20th January 2010, 09:08 PM   #6692
amp_guy is offline amp_guy  United States
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If I am not mistaken F5 is class A not AB
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Old 21st January 2010, 12:45 AM   #6693
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actually depends on the impedance of the load, the number of output devices and their bias point...

but Andrew's point asks "will the F5 ever draw 210watts?"

Anyone know?

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Old 21st January 2010, 12:53 AM   #6694
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Hairball,

seen the 10A in 1R piccy that Papa posted ?
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Old 21st January 2010, 07:17 AM   #6695
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Quote:
Originally Posted by tinitus View Post
Hi,
Please, I need input on how to choose between these transformers
I will buy two trafos to build seperate monos
I dont know much about R-core trafos, quality etc

SELECTRONIC ::: L'univers électronique :::

230V / 2x 18V 300VA

230V / 2x 18V 500VA
Tinitus.

In my opinion the 300 VA trafo is more then enough for a F5 monoblock.
For a stereo F5 class A amp 500 W is perhaps better, but as written here before, one of the most important parts of a amplifier is the powersupply.
Also Amplimo is a very good trafo

and.. dont forget a fuse on the primary side

Last edited by rikkert1978; 21st January 2010 at 07:22 AM.
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Old 21st January 2010, 10:29 AM   #6696
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if can help this R-core , my single 500va 30vac in a borbely amp with 40000µf and 800ma bias for channel work like a dead !
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Old 21st January 2010, 12:30 PM   #6697
AndrewT is offline AndrewT  Scotland
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I'm going to give this response in question format, so that everyone who wants to read and think about it can apply the same logic to any amplifier.

What is the VA rating of the transformer?
What is the maximum continuous DC power that the PSU can supply?
Is the amplifier ClassA or ClassAB?
What is the output bias current?
What is the total of all the other front end currents?
What is the supply voltage +V + |-V|?
What is the wattage that the amp draws continuously in quiescent operation?
If the transformer runs @ about half maximum continuous power it usually runs cool enough to not suffer reliability problems.
Is the transformer big enough to not run too hot delivering that power?
If the amp is ClassAB then continue here.
What is the total maximum output power that the ?VA transformer can supply?
What is the total maximum power you expect from you amplifier design?
Is the transformer big enough?
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Old 21st January 2010, 12:49 PM   #6698
tinitus is offline tinitus  Europe
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Thanks, thats all very good and relevant info
I rate double 300Va to be fine with F5
Fore BA-1 and BA-2 I might choose 500Va

As I raised the trafo question, this time, I have to say I was interested in info about the R-core from selectronic
As alternative to toroids, from Amplimo, to have a quality reference

We know now the R-cores in question are made in India
Well, they can do fine things, if they set their mind to it
French people are usually critical enthusiasts, and they use them
Question is still whether quality is consistent, with no humming etc

So far 3 posts about the R-core, two fore and one against, thanks

Another alternative I have yet to try
I might contact AutomaticEurope and ask fore price on custom UI-core, 10A
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Old 21st January 2010, 03:08 PM   #6699
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Quality of the Silchar transformers is OK, no different from other R-core manufacture i'm familiar with, Silchar has been producing these transformers for quite some time.
Also neatly packaged in comparison to toroids, possibly convenient for postal ordering, i tend to pick 'm up myself when i go shopping in the Lille area.
R-cores are wound by rotating the bobbins around the wire instead of rotating the wire, plus the bobbins are round and straight, much easier to get tight windings.
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Old 21st January 2010, 03:16 PM   #6700
Tea-Bag is offline Tea-Bag  United States
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I have a couple of the Silchar R-Core trannies. I played with them a bit in an F5 - I could not get them to not generate some hum. The Plitron and inexpensive Antek toriods I have had no problem with.
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