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RE: Transformers

Bits and pieces are posted here and there but there seems to be some confusion still.

The "official" headphone transformer is the Speco, same as used in the SEX amp and in the Paramour for its last several years. The Speco was selected after testing all the line outputs we could find; it is indeed a superior design with better performance than any of the other half dozen or so we studied. (Radio Shack had a pretty good one as well. I don't know if it is still the same design. It had 4, 8, and 16 ohm taps.)

These line transformers have a tapped primary and tapped secondary; the Speco has an 8 ohm secondary with a 4 ohm tap. The primary can be used as an autoformer by feeding signal to the lowest power (highest impedance, 8K ohms) tap which is labeled 0.625 watt, and taking output from the highest-power (lowest impedance, 500 ohms) tap which is labeled 10 watts.

Now pay attention here - we are using this at half the design impedance. Leakage inductance on the Speco is low enough to get away with this without losing noticeable treble, and it maximizes the inductance for better bass, which is especially important at low power levels where the permeability of the core is smaller. So it is nominally being used as 4K to 250 ohms. Because triodes are quite accommodating about load variations, this will work very well with headphones of 160 to 400 ohms, and quite acceptably with 100 to 600 ohm 'phones.

Used this way, the step-down ratio is the square root of the impedance ratio, or 4:1. The Quickie can generate 8vrms at the plate, so an output of 2vrms is available. This is less than the IHF standard of 5v, but most headphones are sensitive enough that this is plenty.

The "odd man out" is low impedance headphones. These are often very sensitive as well as having impedances around 30 ohms. This combination of low impedance and high sensitivity allows them to be used with iPods and the like, which lave low-voltage battery power. The IHF headphone specification calls for a 120 ohm resistor in series so that these headphones don't blast your ears. The resistor cuts the voltage at the headphone to 30/(120+30) or 0.2 times the specified 5 volts. Referred to the 2v available for high impedance phones with the standard Quickie arrangement, the available voltage is 0.4 volts, so a stepdown ratio of 8/0.4 or 20:1 is workable. That corresponds to 3200:8 or 6400:16. Therefor a transformer with a 4K primary and a 16 ohm output (such as the old Paramour upgrade transformer) can be used with good results. If the phones are very sensitive, or have a lower impedance like 15 ohms which some of them do, you can use the Speco on the 8 ohm secondary tap as well.

With regard to DC current, I have checked both the Speco and the old Paramour upgrade transformers, and found that they will tolerate a maximum of 2mA of DC current. That is the Quickie plate current so they can be used in series feed here even though they are otherwise restricted to parallel feed. You may have to reduce the high voltage to 27 volts (three batteries) to keep the current under 2mA if you do this, and you may or may not like the sound. But it's theoretically sound, safe, and educational whatever the outcome.

The plate load resistor (in stock or parallel feed headphone operation) can be replaced with a choke. You want 20 henries or more, a DC resistance no more than 4K ohms, and a rated current of at least 3mADC (the excess allows for bass signals as well as the DC operating point).

Finally, you can raise the plate supply voltage to get more power output. The current will rise so the series feed option is not possible with the Speco, but the other options are available. The maximum specified voltage is 67.5 volts, but I see no reason you could not use eight 9-v batteries for a nominal 72 volts. I noticed yesterday that Costco is selling a 48-pack of AA cells which would also provide a nominal 72 volts. Remember, this is well above the UL "safe" voltage so it's back to being a little careful if you do this!



Edits: 09/25/09

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