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Single Ended eXperimenter's Kit for headphones

71.207.120.209

I am interested in this kit as a dedicated headphone amp. What I don't like is it doesn't have true headphone tapes on the transformer. I would think you should have at least a 300 ohm tap. Has anyone found a true headphone OPT for this amp ?

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RE: Single Ended eXperimenter's Kit for headphones - Paul Joppa 10:33:11 10/15/09 (0)
I'd like to take this opportunity to expand on Doc B's comments (with which I fully agree).

The IHF standard calls for a 5vRMS source with 120 ohms impedance. This is supposed to be applicable to all headphones. It will produce maximum power into a 120 ohms headphone; that power is volts squared over resistance, in this case 0.052 watts. At the extremes of 600 ohm phones, or 24 ohm phones the available power is less, 0.029 watt. So for a tube amp capable of delivering 0.05 watts (50 milliwatt or mW) an output transformer with taps for the various headphone impedances in the range of 24 to 600 ohms would be appropriate, in order to preserve an appropriate load on the tube to keep distortion low.

As the available amp power increases, transformers with lower impedance outputs will be able to supply the necessary voltage. Used in this way, the amplifier always sees a high impedance load and distortion is kept low. At the same time, the noise voltage is reduced by the lower impedance output tap. For more information on this issue, look at the Bottlehead Community page; my white paper on signal and noise levels is near the top. The SEX amp produces 2 watts, which is 4vRMS, on the 8 ohm transformer tap loaded with a speaker. With the standard 120 ohm series resistor it can produce 5 vRMS easily, thus meeting the IHF standard accurately and with a very low noise floor.

There is a 500 ohm tap available on the stock autoformer. However, it would produce over 30 volts into a set of headphones, far too much for the phones or your ears to handle.

Now admittedly the IHF standard is old, and specifically it predates the widespread use of headphones with portable devices like iPods. These devices run on low voltages, and in order to obtain sufficient loudness the headphones need to be low impedance and highly sensitive. Some of these headphones do not work at their best with the IHF standard 120 ohm source impedance, being designed for a near zero source impedance and low voltages (around 1vRMS is not uncommon). Unfortunately, few headphone manufacturers will tell you this. But if you do have such a set of phones, the ideal would be a low-impedance source of even lower voltage - in other words, for the SEX amp a tap of less than 1.0 ohms, with no series resistor. It is easy enough to modify the 120 ohm series resistor to an L-pad which will reduce the output voltage and impedance for such phones, though I don't think we have documented such a modification yet. It would merely involve changing the 120 ohm resistor for another value, and adding another resistor from the "hot" headphone signal to ground. I would suggest 6.8 ohms (5 watt) and 1.2 ohms (1 watt) as widely available and suitable starting values.

I hope that helps clarify the situation.
RE: Single Ended eXperimenter's Kit for headphones - Doc B. 08:58:21 10/15/09 (0)
There is more than enough power to run 300 ohm headphones from the 8 ohm tap with a 120 ohm resistor in series to create the IHF standard output impedance. One major benefit from this configuration is that the amp will be very quiet (about 83dB S/N) because the step down ratio of the transformer (actually autoformer) at the 8 ohm tap will knock any noise down much farther than a 300 ohm tap could. I consider that absolutely critical to getting the best presentation from the headphones. Noisy tube amps suck for headphone use.

One of the favorite headphones to go with the S.E.X. amp is the HD600, which tends to indicate that 300 ohm headphones work well with the stock configuration.

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