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Old 19th June 2014, 03:06 AM   #1
Einric is offline Einric  United States
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Default Balanced F4 With Input Transformer

I previously built an F4 and I really love it.
The other day I was dismantling some old tube gear and I found some input transformers with a CT on the secondary.
They are 5:1 (10k:50K) step up and are potted in nice little metal cans.
This got me thinking about people using transformers as phase splitters in tube gear and I was thinking why not apply the same principle with my F4.
I have already rigged up one transformer between my Aikido and my F4.
With 680mV out of my iPod driving the Aikido I get 17V out of the mono channel F4.
Upon listening it sounds really good so I figured I'd give this a go.
With a full 2V input signal I should be able to put the F4 into clipping on a mono 100W channel.
I did all this testing on my F4 biased at 400mV which is double the specified bias.
I built my power supply to handle the extra load and I'm still holding strong at loaded 21V rails.
My heatsinks are fan cooled and even at 400mV bias my middle pin temperatures are barely touching 60C.

So, my new set of F4 boards showed up today and I have begun the stuffing of those boards.
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Old 19th June 2014, 03:11 AM   #2
Einric is offline Einric  United States
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I did something a little different with my F4 boards that I haven't seen any body else do.
There is an extra hole on the 220uF spot so you have flexibility on the diameter.
I took this opportunity to place Vishay MKP1837 polypropylene caps as a bypass on the electrolytic.
NOW, before anybody starts jumping up and down........I have no idea whether it makes any difference at all, but hey that's why it's DIY folks.
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Old 19th June 2014, 03:15 AM   #3
Einric is offline Einric  United States
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Tomorrow I will match the rest of my source resistors and get them stuffed.
I expect to have the two F4's inside this single chassis running standard or maybe a little more than standard bias.
The input transformers will take the input from the RCA and the CT will go to ground with the + on the secondary going to one board and the - of the secondary to the other board.
I think this is a clever idea and I really hope it works out in the end.
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Old 19th June 2014, 04:02 AM   #4
6L6 is offline 6L6  United States
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I love the idea!

It's amazing how you can take a pair of vintage transformers and make 2 stereo 25WPC amps into one stereo 100WPC amp.

Magnetics are magical. (Or something like that...)

Eagerly awaiting more photos and results.
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Old 19th June 2014, 07:54 AM   #5
Zen Mod is offline Zen Mod  Serbia
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that's 10K CT : 50K

which means , as written on it , 2K5+2K5 :50K

if you rotate it , to have CT at output , that's 50K : 2K5+2K5

recalculating to voltage ratio , that's 1:0.22+0.22

disclaimer - morning coffee just arriving
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Old 19th June 2014, 10:01 AM   #6
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Funny FE will be more suited to the task...if i ever finish it.
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Old 19th June 2014, 12:32 PM   #7
Einric is offline Einric  United States
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The print on the side shows a CT on the Primary at 2.5k.
The print does not show the CT on the Secondary.
However, the schematic for the amp it came out of shows the CT on the Secondary and it is there, I verified this with my inductance meter, it's just not printed on the can.
So I will have 10k:25k-0-25K configuration which will be 1:2.5-0-2.5
This should (in theory) give my Aikido the ability to swing +- 36V (minus any losses) into the Balanced F4.
I'll draw a schematic for what I plan and post it later.
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Old 19th June 2014, 01:23 PM   #8
Zen Mod is offline Zen Mod  Serbia
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re-read #5

voltage ratio is √ of impedance ratio

also , impedance to No. of turns is behaving by square law ....... so :

if we assume that 50K is having CT , that means 50K side is 12K5+12K5


10K : 50K is 1:2.23 [V/V] ......... which is , then , 1:1.11+1.11 [V/V]
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Last edited by Zen Mod; 19th June 2014 at 01:27 PM.
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Old 19th June 2014, 01:48 PM   #9
Einric is offline Einric  United States
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Thank You Zen for the clarification.
SO, I get 8V RMS out of the Aikido & 16.7V RMS out at the speaker terminals from a 580mV source (iPod) with the transformer between the Aikido output and F4 input.
I should have plenty of V out with a 2V source to drive the Balanced F4 into clipping???
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Old 19th June 2014, 02:07 PM   #10
Zen Mod is offline Zen Mod  Serbia
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if Aikido is capable to receive 2V input , then yes
or , better to go backwards :

balanced F4 will produce , say , 80Vpp at output

then you need 80V/(2x 1.11) at xformer input

that's roughly 36Vpp

divide with (2 x √2) , that's 12.74Vrms

btw. I'm always thinking in pp terms , translating to RMS only when speaking about Watts
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Last edited by Zen Mod; 19th June 2014 at 02:11 PM.
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