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Khan Audio


Q: What is the thru jack? Where is it in signal flow?

A: The ¼” thru jack is an output that is after the 12AU7 tube gain stage and cathode follower.  It has unity gain and is intended to plug “thru” to your stage amplifier. The tube stage in the VTDI will enhance the sound of whatever stage amplifier you use.

Q: What is the “off bypass” feature?

A: The VTDI has a feature that bypasses the unit when the power is turned off. Both ¼” thru and XLR balanced outputs will revert to the direct connection when the power is disengaged. An internal relay accomplishes this task so comparisons can be made, or as a fail safe in case power is lost.

Q: Can I use the VTDI to enhance the sound of other instruments?

A: YES, the VTDI can be used on acoustic guitar, violin, cello, upright bass, mandolin, basically any instrument with a piezo pickup will greatly benefit from the input stage of the VTDI. The combination of a capacitive piezo element and a high impedance tube stage is the ideal match. (on paper and in sonics)

Q: What is the level at the XLR balanced output? 

A: The XLR balanced output is at a low line level. There is some gain in the VTDI so an instrument level translates to a low line level signal at the XLR. Hotter signals from an active bass for example, will approach a 0 dB signal.  The mixing board input should be set to a line input for maximum headroom and lowest noise. Adjust the gain needed from there, less than 10dB will be needed.

Q: Does it get hot? Do I have to worry about airflow?

A: The VTDI uses a preamp tube which generates very little heat. It will not run hot under most conditions. It can be mounted in any location, at the amplifier or even on a pedalboard. We have quite a few users who have integrated into their pedalboard setup.

Q: When should I change the tube? 

A: Preamp tubes are under very little stress when used in circuits. The main cause of preamp tube failure is microphonics, so if you start to hear high pitch squeals or general sensitivity to vibration, it may be time to change the preamp tube. In some circuits sudden failure can occur where the tube simply ceases to function. This will be obvious. We do like the Chinese 12AU7 sold by Ruby, TAD and others, as well as good NOS offerings.


-Use as a processor for prerecorded tracks. Send to VTDI, return from ¼” thru or XLR output and enhance drum tracks, vocals, synths anything in your DAW.

-You can use the VTDI in a post signal application by connecting to your send jack on the amplifier. This will use the preamp signal from your amp and then into the VTDI so you entire sound will be processed.

Use the ground lift switch if you experience buzz or hum when connecting the XLR. Never defeat the safety ground connection on the power cable.

-Acoustic guitars sound incredible through the VTDI. The high frequencies are smoothed out and the bottom end is thickened. It has a very natural sound, uncommon for a piezo system. The harshness is removed and replaced with natural reproduction. The proper impedance match of the pickup is critical to achieving this.


Active basses may benefit from the -6db pad. If you are boosting frequencies at the guitar, it may overload the input stage. Distortion will be evident and then the pad can be engaged. The pad will lower the input impedance from 2.2M ohm to 47K ohm, which is more compatible with an active output instrument.