GoodSoundClub - Romy the Cat's Audio Site

In the Forum: Horn-Loaded Speakers
In the Thread: Vitavox S2 with Electromagnets

Post Subject: More on the field coil damping - and an easy way to adjust itPosted by cv on: 3/1/2010

Hello chaps,

Firstly, something I forgot before was that the high impedance coil will have more turns... so the DCR etc will be stepped down as seen by the voice coil. My guess is that overall it will still provide slightly less damping, due to imperfect transformer action and perhaps a lower copper/insulation ratio with the high voltage wire, but don't hold me to that.

Romy - I'm not sure a tapped coil is the way to go; there's only one optimal design and you may not get the flux required without full turns utilisation. But there's an easier way of varying the damping - which I'll get to in a moment.

Bud - high current CCS are a bit more difficult but shouldn't be a problem - a simple single mosfet based one should suffice. Doesn't need to have spectacularly high impedance of 200M across the audio band after all! That said, say we have a CCS (or series inductor), we can make the damping variable by putting a variable resistor across the field coil, with DC blocking capacitor in series. You could even play with the damping at different frequencies (EQ the damping network), which I'm sure only Romy would have the patience to explore and master.

Another way to do it is to undamp the amplifier - have some unbypassed cathode resistance on the output tube to raise the output plate impedance...
This will give you more predictable "undamping" across the bandwidth compared to the less predictable behaviour of a speaker motor masquerading as a transformer...

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