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Old 30th August 2005, 10:20 PM   #1
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Default Need help - O/T trannies for future project

Hi guys!

I need some enlightnment from all you tube gurus.

I'm on the middle of a small headphone project, all written down on the paper, but need some help regarding the o/t tranny.
The design is single-ended, 12mA bias, and 5K primary (i hope i don't have to tell much, i want it to be a kind of surprise ).
I was thinking about using regular power trannies, 220V/18V (5K:32Ohm), at around 3VA, as the total circuit power is 1,08W (though flea-power output). Would this be enough to delay core saturation and give a quality output?
I don't want to spend a handfull of euros for some nice o/t trannies...

Thanks a million!
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Old 30th August 2005, 11:04 PM   #2
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Quote:
I was thinking about using regular power trannies, 220V/18V (5K:32Ohm),
Should be very do-able...but you'd have to parafeed.

I.e. use a choke or CCS and add a capacitor in the transformers path to block the DC. A 220/18V (3va) would saturate so much I doubt there would be "quality output" at all...

Good luck!
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Old 31st August 2005, 01:18 AM   #3
PRR is offline PRR  United States
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Try it and see.

> power trannies, 220V/18V (5K:32Ohm), at around 3VA, as the total circuit power is 1,08W

Think MUCH bigger. Distortion will be significant at 1/3rd of rated AC voltage and frequency (power transformers work with HUGE distortion; only the power company cares). 50Hz is a reasonable bass-power limit for audio, but you should not ask your 220V winding for more than about 70V at 50Hz. 70V^2/5K is 1 Watt, so you are at the AC limit for low THD at 50Hz.

The DC limit is nominally zero. For torroids the real limit is very-very small. For EI stampings, most constructions that have an air-gap no matter how small, you may get away with DC current about 10% of rated AC current. You want 12mA DC, you probaby want more than 120mA AC rating, 0.12A*220V= 26VA rated core!

Fortunately 25VA cores are not much more expensive than smaller cores.

Also: it is hard to put numbers on it, but a little core wants to have low inductance. In power use, this means large idle current. But for small transformers, who cares if 3 Watts flows to the load and 5VA sloshes through the winding? On big transformers, idle current can overheat the windings, and big cores tend to give more inductance, so a bigger core will give more bass.

Response will be good from 50Hz or below (depending on plate resistance and NFB) up until the leakage inductance gets in the way. I did some tests once. A 2-bobbin winding, 115/230V on one leg, 12V on the other leg, gave terrible response, falling by 1KC. Another tranny, similar price but with both windings on one bobbin, was smooth to past 7KHz, so a trice of NFB would have flattened to 20K.

As Bas says: choke-coupled solves some problems, mostly by moving them into another (more money) part.
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Old 31st August 2005, 05:17 AM   #4
wa2ise is offline wa2ise  United States
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There's always the advice to disassemble the transformer lams and restack them to create the continious gap. Doing this can be a PITA though.

Thought that comes to mind is to attach a magnet to the lams externally to try to counteract the DC magnetic field. Say a pair of magnets salvaged from a dead hard drive. But I'm not sure if I could get the magnetic fields into the lams right....
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Old 31st August 2005, 06:02 AM   #5
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Huh, that's interesting. I'd have never thought of using a fixed magnet like that.

Is it reasonable to make an SE output transformer on a conventional non-gapped core, using a bifilar primary and a DC-only on one of the windings to counteract primary current in the other?
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Old 31st August 2005, 06:28 AM   #6
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> DC-only on one of the windings to counteract primary current in the other?

Needs to be high-impedance DC, not a battery. (Think about it.)
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Old 31st August 2005, 09:40 AM   #7
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by PRR
Needs to be high-impedance DC, not a battery. (Think about it.)
OK, a CCS. But wouldn't that imply that a permanent magnet might cause the same effect?
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Old 31st August 2005, 09:58 AM   #8
Giaime is offline Giaime  Italy
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Exscuse me if I'm slightly OT, but what about PP output stages with power trannies as outputs? Will the balanced current in it limitate the DC saturation? I saw some PP amps made with mains toroids OTs... The rule is still about 10x the audio power to get the needed power rating of the mains tranny?

(obviously one needs 2 primaries in the mains tranny... or a centertapped one... not easy to find here in Italy)

And are EI cores better than toroids in this?
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Old 31st August 2005, 12:26 PM   #9
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That sometimes works and is worth trying. But most power transformers have pretty terrible coupling at high frequencies.
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Old 31st August 2005, 12:28 PM   #10
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Quote:
Originally posted by PRR
> DC-only on one of the windings to counteract primary current in the other?

Needs to be high-impedance DC, not a battery. (Think about it.)
Indeed. But the cost of a CCS and a second primary winding might be less than the oversized core needed for a conventional SE transformer.
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