40 µH bandpass choke - how to?

[Kay Pirinha]

Hello,

the 4430 crossover requires a 40 µH choke in it's variable 17.8 kHz bandpass section that features remarkable improvement in the 2421a/2344's high frequency response. Air inductors of this value are hard, if not impossible to source these days. So what to do? As far as I see, there are the following options:

- Buying 50 µH air core inductors and modifying them by decreasing the turns number to N_required = N_stock/sqrt (50/40).
- Combining a 50 µH stock inductor with a capacitor of reduced value (2 µF -> 1.6 µF).
- Modifying stock 100 µH or 50 µH chokes on toroidal ferrite cores by reducing their turns number according to the abovementioned formula.

Which one would you prefer?

Thanks and best regards!

[Malefoda]

I had the same issue with a crossover and ended up using a good LCR meter to unwound. Change the frequency of the meter to be in the band of interest. As a bonus you get a tight matched pair...
Cheap'n good:
https://www.deree.com.tw/de-5000-lcr-meter.html

[Kay Pirinha]

Thanks, Malefoda! I appreciate it.

Where did you buy this meter? I followed your link, but it didn't give me it's price, nor did it tell me how to source.

Best regards!

[Malefoda]

I bought it on eBay years ago, was the best bang/buck at that time and should still be. Mind the accessories you may need such as crocodile clip for your coils and parts, included in this bundle:
https://www.ebay.fr/itm/DER-EE-High-...2/273073305291
Peak has some tiny great ones but AFAIR the DE-5000 is a better deal.
https://www.ebay.fr/itm/Peak-Atlas-L...w/392249339760