Here is another passive amplifier that boosts the audio coming out of your phone. The Sound-2.0-BT Smartphone Amplifier can handle smartphones from 4″ to 5.7″. It can also be paired with a Bluetooth loudspeaker. It is handcrafted from 100% wood.
You don’t need any power source to get this amplifier working. woodmanufaktur is selling these on Etsy.[where to get it]