Howard Dawson Audio
Fixing the Decca Supertweeter Crossover
What's the problem anyway?
The Decca 'Supertweeter' contains a series 4.4µF capacitor 6dB/octave built-in 'crossover'. This is thoroughly inadequate for a traditional ribbon like this, and can only protect up to about 10W of 20Hz signal, above which the ribbon will flutter and stretch (and you will also experience bad intermodulation when this occurs).
An 18dB/octave filter can be easily implemented by adding an external inductor and capacitor on each unit. You do not need to modify the unit itself, unless you decide to fit the extra components inside for a neater job.
This will fix the problem so that it will take a couple of kilowatts at 20Hz to visibly move the ribbon (i.e. never). There will also be no need for blobs of silicone or other aftermarket bodges to prevent flutter. And you will be able to forget about regular ribbon replacement, all for less than the cost of a single ribbon!!
Most importantly, you should also notice significant improvement in sound quality due to removal of bass intermodulation effects.
How to fix it
Get yourself a pair of 2.2uF capacitors and a pair of 150µH inductors. You don't need to go mad here - ferrite rod inductors and reasonable polypropylene caps will be OK. You will be able to get them from somewhere like Wilmslow Audio.
Connect the inductor across the unit terminals (i.e. in parallel), and one end of the capacitor to the unit's positive terminal.
Your audio signal now goes to the unit negative (as before), but the positive feed should now go to the free end of the capacitor rather than directly to the unit positive.
Alternatively you can buy the add-on filters ready-made in small plastic enclosures with flying leads. Simply connect in-line with tweeter - see prices page.
For further assistance or information :
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