F2 PSU with 2 EI transformers? - diyAudio
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Old 12th December 2009, 05:05 AM   #1
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Default F2 PSU with 2 EI transformers?

hi,

i want to build an F2 (or should that moniker be reserved for the originals made by the big man himself? i should say F2 clone). i have a question regarding the PSU. the original schematic in the service manual shows a toroid with dual 18v secondaries. can replace this with two 0-18 EI transformers? is it possible to use a single, centre tapped EI transformer? i am asking because toroidal transformers are very expensive in india, about 2.5x the cost of a good EI transformer. if i use two transformers i am thinking of going with 350VA each. do you think i need to increase this?

thanks
mymindinside
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Old 12th December 2009, 10:43 AM   #2
Zen Mod is offline Zen Mod  Serbia
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according to my tricky memory - F2 have 75W dissipation per channel , so even 250VA xformer per channel is approaching overkill ...... but more is always merrier , at least in xformer department ;

you can use CT xformer - connecting CT to gnd and using just two diodes (one from each sec. end to + ) to have full-wave rectification

or even better - use two xformers ( from 250VA and up) , each having just one 18Vac secondary

Papa's PSU is sort of dual mono PSU with common core and primary
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Old 12th December 2009, 10:46 AM   #3
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thanks Zen mod,
if i go with two transformers (which is not more expensive for me than going with one, larger one) what type of rectification would i use? a bridge with the - connected to ground?
Also, with this xformer size, will i need a separate soft start circuit aside from what is already in the schematic (in the form of the thermistors)?

Last edited by mymindinside; 12th December 2009 at 11:10 AM.
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Old 22nd December 2009, 05:32 AM   #4
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Going out to buy the parts today.
was a bit confused about how to connect the thermistors...do they go across the primaries?or in series with the primaries?
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Old 22nd December 2009, 05:45 AM   #5
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If you are building an F2 then you will definitely want to
acquire a pair of the SemiSouth Jfets the next time a group
buy occurs. Just drop them in, adjust the offset, and be
happy.

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Old 23rd December 2009, 04:31 PM   #6
masag1 is offline masag1  Germany
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Hello Mr. Pass,

thank you for the Info.

Are there more useful mods for the F2 + SemiSouth Jfets?

Happy X-mas to you and your familiy!

Maybe you have a small gift for us DIY'er ?


With kind regards

Sam
Greding / Bavaria
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Old 23rd December 2009, 04:43 PM   #7
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hello Mr Pass,
will definitely try the Jfets once the amp is up and running.
my enthusiasm has taken a hit in the past few days though, when i was told in another thread that none of the heatsinks i have available are good enough for the F2. just to confirm, the dissipation is 75w for 2 channels?
apparently the heatsinks i posted did not have a thick enough base so the thermal conduction from the device was effective only across a small area.
i would really appreciate it if you had a look at the thread and shared any insights,
and of course, merry christmas to you and your loved ones!

mymindinside
p.s. the thread is located here
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Old 24th December 2009, 12:03 AM   #8
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I read the thread in regards to the heatsinks you are looking at. One thing you can do to help spread the heat more evenly across the heatsink would be to attach the mosfet directly to a piece of copper that covers most of the flat face of the heatsink. Then put the insulator between the copper plate and the aluminum heatsink. Copper has a higher thermal coefficient, so it will pull heat away from the mosfet (or any other device) quicker. And having the transistor tied directly to the copper, then isolating the copper pulls the heat from the transistor faster.

Peace,

Dave
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Old 24th December 2009, 05:18 AM   #9
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by mymindinside View Post
thanks Zen mod,
if i go with two transformers (which is not more expensive for me than going with one, larger one) what type of rectification would i use? a bridge with the - connected to ground?
Also, with this xformer size, will i need a separate soft start circuit aside from what is already in the schematic (in the form of the thermistors)?
yes - full Graetz , with - to gnd
you can use one NTC for both xformers

Quote:
Originally Posted by mymindinside View Post
...... just to confirm, the dissipation is 75w for 2 channels?
......
yes ...... 75W for two channels ........ and double it

I wrote already - 75W per channel
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Old 25th December 2009, 05:21 AM   #10
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uh oh, sorry
thanks for the patient help

mymindinside
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