Old Wollensak tape recorders often had nice SE audio circuits with 6EU7s and
7591s. Some of them also had shock-mounted tube sockets. If the mechanics
fail you can recycle quite a few of the parts.

Anyway, I'm planning to build a headphone amp using a 6T9 Compactron and
saving the 7591A for something else.

The Wollensak was originally rated for about 5W "music power" output. I
plan to use the SE tranny at a fraction of that power for headphones.

1) Is it GENERALLY true that a SE transformer . . . . particularly a small
one . . . might have a better bandwidth at lower power? (well below
saturation).

2) A nominal headphone impedance is 32 ohms (vs. the 8 ohms of the original
design). Obviously, this would change the reflected load seen at the plate
of the tube.

a) Should I put a resistor in parallel with the headphone so that the
resistance on the secondary side is closer to 8 ohms?

b) If I do nothing (and drive the 32 ohm phones) will this improve
damping factor? What would be the adverse effects (e.g. more distortion,
etc.) I understand the maximum power is transferred when the impedance
matches, but power is not important with the headphone load.

Any comments & related suggestions would be appreciated.

TIA

- Jon

"Jon Yaeger"


a) Should I put a resistor in parallel with the headphone so that the
resistance on the secondary side is closer to 8 ohms?


** Yep - at least 33 ohms / 2watt.

But place a 120 ohm 1/2 watt resistor in series with the 32 ohm
phones - to limit the dBs to a safe level for your ears and reduce
background hum etc.




............ Phil

Jon Yaeger wrote:

Old Wollensak tape recorders often had nice SE audio circuits with 6EU7s and
7591s. Some of them also had shock-mounted tube sockets. If the mechanics
fail you can recycle quite a few of the parts.

Anyway, I'm planning to build a headphone amp using a 6T9 Compactron and
saving the 7591A for something else.

The Wollensak was originally rated for about 5W "music power" output. I
plan to use the SE tranny at a fraction of that power for headphones.

1) Is it GENERALLY true that a SE transformer . . . . particularly a small
one . . . might have a better bandwidth at lower power? (well below
saturation).

The saturation effects at low power won't restrict the LF response,
but the primary inductance may, regardless of level.
HF response will be similar from low to above medioum levels.



2) A nominal headphone impedance is 32 ohms (vs. the 8 ohms of the original
design). Obviously, this would change the reflected load seen at the plate
of the tube.

For SE designs, the rise of load value
reduces power output, but may lower thd and
better the damping factor.
Triode thd gets lower with higher load,
pentoad thd gets higher with too high a load,
but so does gain, and if there is NFB, the
thd may reduce with higher load value.



a) Should I put a resistor in parallel with the headphone so that the
resistance on the secondary side is closer to 8 ohms?

Nope, unless the map goes unstable.
12 ohms might be better, if at all.



b) If I do nothing (and drive the 32 ohm phones) will this improve
damping factor?

Yes.

What would be the adverse effects (e.g. more distortion,
etc.)

see above

I understand the maximum power is transferred when the impedance
matches, but power is not important with the headphone load.

If you find the level is way too loud at clipping, with 32 ohms,
then you have enough power.



Any comments & related suggestions would be appreciated.

TIA

- Jon

Patrick Turner.