earphone amp to an etherwave theremin?

Hi!
Just got my theremin and use a friends keyboard amp with beautiful sound (eg that is the sound quality, not the intonation...)
I would need some sort of earphone amp so I can practice without bothering my neighbours. Tried a Vox-the-plug, but it sounded really awful. Do anyone have a suggestion? Don´t have the finances to buy a big keyboard amp yet.

The Samson S Amp worked OK for me.
I didn’t spend a lot of time with it. It can drive up to
4 headphones, which I know you really don’t need, but
it’s only around $50.

There is also a Behringer Ha400 for around $30 but I
have no experience with this unit. I don’t buy Behringer products
very often.

The lowest cost unit that has OK reviews is the Fiio E3 headphone
amp at $7.50. Again I have no experience with this unit.

I bought a no-name headphone amp in Germany which was originally designed for Hifi use with RCA inputs. I paid about 20 EUR (~28 US$) including a 3m (10ft) adapter cable with a 1/4" mono plug at one end and two RCA plugs at the other end.

Thank you so much for the tips! I´ll check with my music dealer.