Reddit reviews Traveler Guitar, 0-String TGA-1A Acoustic Headphone amp

Not the most attractive instrument, but in terms of functionality, you really can’t beat Traveler guitars. Their Acoustic-Electric is a full scale guitar, can be plugged into an amp or headphone amp (like the the Traveler Acoustic headphone amp), and is otherwise very quiet. There’s a full electric version for about twice the price, but the only major difference is that the Acoustic-Electric sounds like an acoustic guitar when plugged in while the Electric will sound like an electric.

While the JamStick might work for some practice, it isn’t a real guitar. The Traveler is, and it stands to reason that practicing playing guitar on a guitar will be more effective than practicing guitar on an expensive toy.

Instrument level signals are not intended to be plugged straight into consumer playback equipment. There needs to be something in between first to make the signal stronger and make a signal with the right impedance to be fed into a playback system's low-impedance input. Imagine plugging headphones into an electric guitar's output jack straight - this is more or less what you're trying to do right now.

The cheapest external preamp box for piezo pick-ups to do this which I know of is the Behringer ADI21 (a clone of a Sansamp Acoustic D.I. according to Google). A device like this outputs a line level signal, which you can then plug into any number of things such as effects units, instrument amplifiers, mixers, or other equipment, and they often come with various other features buit in such as multiple outputs and EQ.

Guitar amps, or acoustic guitar amps can have headphone outputs as a practice feature, and ones intended for acoustic guitar generally have a preamp circuit designed for piezos so you can plug a passive violin into one directly. These are fine too, and are easy to use since you only need a single unit to directly plug your passive violin into to get sound, whether you're using the speaker, or the headphone output. I recommend this option personally, as you sound like you're looking to plug and play and aren't familiar with technology.

There's also products intended for the specific thing you're trying to do with this consumer headphone amplifier - small dongles that plug directly into an acoustic instrument and provide a preamp circuit and headphone output and tone shaping features in a single box specifically for headphone practice in a small form factor. Using these on a violin may be awkward depending on where the instrument's output jack is, but shouldn't be impossible depending on the exact use case: https://www.amazon.com/Traveler-Guitar-TGA-1A-Acoustic-Headphone/product-reviews/B075VTWHYG/ref=cm_cr_arp_d_paging_btm_next_2?ie=UTF8&reviewerType=all_reviews&pageNumber=2