Hipshot D-Tuner H 20600B

High quality product. Beautifully made, looks like an original Warwick part. Fitting and adjusting it was a simple, 15 minute procedure. It works exactly as advertised. I can now flip my low E string to a low D and back, at will. A little expensive, but still highly recommended if you need to switch to a Drop-D tuning occasionally.

So you love your 4-string but want to play a low D occasionally? Whack this baby on and you're away! Literally one screw and 5 minutes to install! Really useful and extremely well made - a bit pricey, but cheaper than a five string!!

I use it with my Spector Bass.

turns my low B into a low A and everything in between. easy to install, super clear instructions. very stable tuning and super high quality. the low A combined with the regular A played as an octave on a 37'' scale make me feel funny in my tummy.

the only bad side is its eye wateringly expensive for what it is. but if you have a low A and a 54'' nut, your eyes will water anyway for other reasons.