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Old 2nd July 2013, 07:45 AM   #1
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Default Tube Headphone amplifier suggestions 2x(1:40) transformers

Hello,

Recently I got two 1:40 transformers that were used in a school PA system. They are for monitoring a 100V output used in schools and small villages.
Since I got two of them and the ratio is pretty high I imagine I could build a headphone amplifier. I have 32 Ohm Sehnheiser headphones. So from my calculations the transformartion ratio would look like this: 51200:32 Ohm. That is 40*40:1=>1600:1=>51200:32.
What tubes would be good for this? What would you suggest to do with these nice transformers?

Thanks!

Raz
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Old 2nd July 2013, 09:01 PM   #2
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Not sure how "hi-fi" these would be, it certainly wasn't a primary design criterion. Plus, to get even a moderate output level of 1 Vrms out of 'em (which is about what a portable player does) you'd need a 40 Vrms signal somewhere - almost 120 Vpp with low distortion and stuff. I hope you are comfortable working with at least the classic 250 V anode voltage.

Then there still is the question of where/how to get enough gain. Most headphone amps have at least unity gain, so a gain of 40 (32 dB) would be the very minimum. This is almost begging for some input transformers, like 1:4 or so. (That still gives >10k of input impedance with a 250k pot.) Good ones traditionally aren't growing on trees. Of course, you could "cheat" and use some solid-state circuitry instead... which means extra supply voltages. Then we still have 20 dB left - I'd use at least a 2-stage amplifier for that.

You may want to follow up the output transformer with an autotransformer (self-wound, presumably) with multiple taps... 1:1, 1:2 and 1:4 or so, so you get some more flexibility for different headphone impedance ranges (32, 128 and 512 ohms nominal).

Hollow state done right means a fair bit of "iron", I'm afraid.

Just some random thoughts. Been thinking about coming up with a tube design myself. Seems like a bit of a pain though.
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Old 10th July 2013, 08:41 PM   #3
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Maybe drive via cap from anode of ECC81, with choke or current source to HT supply to give about 250V on the anode. That might turn out around unity gain? Assuming the trannies aren't gapped and have no centre tap, there aren't many simple options. If you want feedback you'll need two stages.

Just a suggestion.

I would connect tranny to sig gen via resistor equal to envisaged source impedance, with 32 ohms on secondary, and see what small signal bandwidth I got. I'd also check at what voltage the primary begins to saturate with 32 ohms on the secondary, for a range of low frequencies. Driving an impedance other than that designed for will shift the bandwidth, so if it was barely sufficient it may not remain so. Also true of primary saturation current.
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