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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! :D
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
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.
 
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. :hphones:

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.... :magnet: :crazy:
 
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?
 
EC8010 said:
OK, a CCS. But wouldn't that imply that a permanent magnet might cause the same effect?

The CCS needs an output voltage range at least equal to the maximum AC swing (PP), which means it will also need a DC power supply of about 2/3 B+, which in turn means the power dissipated on it will be on the order of the anode dissipation of the tube. This almost certainly implies a MOSFET.
There is, however, a nice way of implementing this that guarantees pretty much perfect DC balance, you use a BJT current mirror, with the reference leg being in the cathode circuit of the tube, and the mirror leg being cascoded by a MOSFET. The connection point of the bases of the BJTs can be bypassed to ground via a cap to make this act as a mirror only for LF and DC, but as a current source for AC. This actually isn't strictly required as the cathode of the tube will typically be bypassed already so the reference leg of the current mirror will not see AC.

Regarding setting up magnetic flux in a transformer via a permanent magnet, the only way to do this properly would be by inserting the magnet into a gap in the iron wight about midway on the bobbin, which automatically implies a gap. Sticking magnets on the outside will just make a locally higher flux, which means that part of the transofrmer will saturate more easily. Re-assembling the lamination with a gap is a better idea, though a real PITA with impregnated transformers (and ransformers suitable for audio should be impregnated).

Another alternative was discussed previously on this board, here:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=56916

This is, for all intents and purposes, an asymetric push-pull. There was a lengthy discussion on it, and the circuit is actually quite clever in that it develops usable power unlike a CCS to DC balance the core, which just presents a power loss, but does not cancel out even order harmonics unless the triode is replaced by a pentode, when the circuit degenerates into a regular push-pull.
This brings me to a question I would like to see discussed about such circuits: considering the triode only sees reflected load impedance (because pentode output impedance is very high), would this circuit require a transformer that has ONE half of the primary equal to a primary for a SE amplifier? Indeed, a discussion on how to chose a primary impedance for pentode mode would be nice ;)