Buying Plitron Transformers?

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#41
ticknpop said:
If you insist on a torroid then you want an electrostatic shield from primary to secondary to reduce the capacitive coupling and lower the bandwidth of the AC line noise it lets through and an external shield to reduce the radiated field. You can only get those 2 options if you get custom made toroid transformers, not off the shelf series. And you still have to deal with the torroid inrush current, either with a soft start circuit or at least a properly chosen thermistor

Or you could buy an R core and skip all that crap, but no one here seems to be listening. Seems odd for an audio oriented web site.

Torroids were popular in the 1980s, but we’ve learnt things since then. Look at Audio Research, the new Parrasound JC 1+ and JC 2 preamp, Constellation Audio Etc. There's a reason their all using R cores in thier best products.
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I don’t think it’s people not listening to you, just a lot of people are beginners and want to follow the schematics to the letter. If I were to design my own amplifier I definitely would try an R Core. The ease of finding a torrid in the voltages they need from trusted source is attractive to a beginner or someone not wanting a hassle to find parts.
 
#42
Could also be that existing R core transformers are only suitable for preamps, and we are more focused on Class A power amps in this forum.
If there is a good example of a 25W Class A power amp built with an R core, one could try posting some pictures or links to such.
 
#43
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#44
Do you know a good source for the R cores? I can never find the specs I want in a R core.
ticknpop said:
Yes R cores have lower inrush- no thermistor or soft start required in the dac or preamp - Fuse is 2 amps for the 4 x 100 va in the preamp chassis - load is about 100 watts class A in the preamp / phono -yes 100 watts class A for a balanced jfet/fet preamp and phono. There are almost 100 TO247 fet and bipolar devices running at 45 C, most are in the power suply - Jung super regulators, then the fet shunt regulators, the output of each analog stage is Hitachi fets running at 80 ma and 24 VDC - 2 watts per device - very deep class A
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#45
I’ve never seen a normally priced R-Core above 200VA. These R-Core are normally used in pre-amp. Like everyone be happy and use a nice toroidal xtrm.

Why are we talking about R-Core for Pass Class A amps ?

BR
Eric
 
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#48
You realy dont NEED a softstart for a 400VA toriod transformer 🙂 500+VA you will often need it.
But all would need to remember, it's not as much the transformer that make the need of a softstart. It's the combo transformer/PSU cap bank.

R-core transformers for big poweramps is allmost never used. And it's not whitout reason. try to get a 1000VA R-core.

I just had a look at ebay now. 200VA is the biggest i find. And about $120 a piece. About $240 +tax for 400VA transformer. What do you get at antek or toroidy for that amount? At toroidy you can get a 400VA supreme audiograde for about $100. Or a standard audiograde 400VA for $50. Or a standard grade powertransformer 400VA for about $27. You sure can get a lot of softstarts and other transformers for the money left over 🙂
 
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#50
You can get R cores up to 2000va, but above 200 va R core get expensive.

R core transformer_James transformer-custom toroidal transformer|R core transformer|O core transformer|C core transformer|EI transformer

These 200 watt R cores are $99 to $112 each on ebay

200W R-Core Transformer for Audio Amplifier Power AMP - Selectable Input Outputs

Yes you can add soft start to a torroid, the real problem is the torroid has too much primary to secondary capacitive coupling and lets too much AC line noise through. If you add a primary to secondary shield to the torroid to reduce the capacitance and noise you are in the price range of the R cores. The problem with thermistor soft start is once the thermistor is warm and you turn the amp on again before the thermistor cools (maybe a minute) it then has low resistance and the next turn on inrush isn’t reduced and you burn out your power switch. Thermistors are cheap and easy to use, but turn an amp off and then on too quickly and they don’t work at all. You want to add a relay to bypass the thermistor once the amp has started, that cools it off and its ready for the next start up cycle. But this all adds up

my next class A monoblock amps will have 1500 va R cores with resistor soft starts and the wife will have to get a second job, I win both ways......
 
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