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Old 7th May 2006, 01:10 AM   #11
lndm is offline lndm  Australia
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I typically assess my power transformers for this duty before sorting the rest for landfill

You can put a resistor in series with the primary, equal to about your output stages desired load. Sweep a sinewave across the spectrum and find the frequencies where the resistor drops all the voltage.

Some power transformers don't have enough inductance to be useful at the bottom end, and some have too much interwinding capacitance to be useful at the top end.

Of all the power xfmrs I look at, I find more that are useful as chokes, and some that could maybe be used as interstage xfmrs
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Old 7th May 2006, 01:45 AM   #12
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For 300 ohm headphones, EL34 seems like overkill. You only need about 1/2 watt of OP power, which I think you could get from a commonly available double triode, such as a 12AU7 or 6SN7, operating in class A push-pull. That would enable you to use a toroidal power transformer as the OP transformer. I don't know how good the bandwidth would be but it would certainly be better than an EI tranny.

The toroidal transformer would need a split primary, such as you can find in a "universal" power transformer rated for 115v/230v, with a secondary voltage rating selected according to the ideal plate-to-plate loading of a 12AU7/6SN7 in class A push-pull. Perhaps someone can suggest a suitable ratio?
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Old 7th May 2006, 03:57 AM   #13
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Here's what you do. Cathode follower 6AS7 biased at around -30V Run 75V PSu with a choke load or 150V with a big 15W resistor load. Bandwidth will be DC to light. Get a good (not botique) film cap for output. Don't even think about a 'lytic.
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Old 7th May 2006, 09:54 AM   #14
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I'am inclined to agree with people when they say that no output transformer is the best with a headphone amp. With a 300ohm load this should be fairly easy. Look to the Headwize site for good circuits.

However mains transformers can be used for outputs with a few provisos.
EI can be used but sound crap (to much interwinding capacitance).
Toroidals can sound excellent but in SE can only cope with AC, therefore Parafeed is essential. As said before, think 4-6x the required watts in VA rating. Even then there will be acertain amount of roll off in the bass. Because parafeed is been used then the roll off can be compensated by forcing the transformer- parafeed capacitor to resonate at about 10Hz, this will pick up the bass nicely. I have done this in my main 807 amp and it sounds very good.
I have also built a simple headphone amp with ECL82's. I used 240:24V transformers at about 30VA, again there is low frequency roll off, but again this can be compensated for. I used 200V B+ with a 3K3 resistor as the anode load.
In all cases you can expect a better high frequency response than from a conventional airgapped transformer. They would make an excellent companion to horn loaded speakers where the bass is already missing.
I am currently experiementing with toroidals in a 6AS7 PP amp. The higher the ouput current, and lower the output voltage - the better the expected results are likely to be.

Shoog
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Old 7th May 2006, 12:22 PM   #15
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Friends,

A friend have alerted me who I'm not need a output transformer to driver my headphone.

I was win two ECC82, it isn't the best tube. I will mount a amplifier for my Sennheiser and put some photos and questions in here.

ilimzn, what topology you think is the best to I use with a ECC82 to driver my fones?

Shoog, have you a schematic of this amp with ECL82?

Thanks in advance for all replyes.
Felipe Navarro.
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Old 7th May 2006, 01:35 PM   #16
Merlinb is offline Merlinb  United Kingdom
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This headphone amp can be used with an ECC82 without any major changes http://headwize.com/projects/showfil...cmoy5_prj.htm.
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Old 7th May 2006, 06:18 PM   #17
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Heres a link to the original thread in which I designed it.

Ideas for using a pair of ECL82 in a headphone amp.

I since modified the load on the triode to 100K and the cathode resistor to 1K, this allowed the plate to plate feedback to be increased a little without swamping the driver triode. You could probably reduce the load and cathode resistor further.

Shoog
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Old 25th May 2006, 08:31 AM   #18
Tuliman is offline Tuliman  Romania
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If I manage to separate the E-s from the I-s in a power transformer (E+I) and put an air (paper) gap, I could use it as an OPT?

The transformer I intend to use looks to be very responsably built; each layer in the primary/secondary is isolated with paper, the output voltage is 40V (from 220), so I cand modify the ratio to 20:1 or similar.

The downside I see, is that the primary/secondary are wired in one piece and not P=4*800....
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