Electronic DIY hobbyists are changing out the opamps for "better" ones in the VTB1. I mention this because they are asking for schematics with no response which is very often the case for devices like this.

This is all the technical information I could dig up.

The microphone front end is a solid‑state Class AB circuit, which feeds a TRS jack insert point, allowing additional processing to be inserted into the signal path. According to the circuit description, a balanced, current‑source‑fed parallel transistor arrangement feeds into a bipolar op‑amp. The transistors provide up to 45dB of gain and the op‑amp a further 15dB. Both the mic input and line output are on balanced XLRs. The line switch is on the front panel next to the instrument/line input jack.

A FET is used in the line input circuit, to present a high input impedance when the device is switched to instrument mode. (In this mode, the input impedance is 1.5MΩ.)

Unusually for a preamp in this price range, the input also offers variable input impedance, via a rear‑panel switch, to suit both 50Ω and 200Ω microphones.

The actual input impedance of the VTB1 is 2kΩ in the 200Ω position and 300Ω in the 50Ω position, as it is normal to feed mics into an impedance an order of magnitude greater than their own impedance.

Whatever theory suggests, your best bet is to try both positions for each application and see which sounds best to you; usually, you'll hear a subtle difference at the low end.

This unit is certainly not designed to accommodate all mics but of course, it will work. I think a SM57 is about 310 Ohm actual, rated at 150 Ohms. Most reviews I have read say this is a pretty good device for a low-cost device of this type.

Without a schematic or the actual unit setting here to take apart and reverse engineer, it is impossible for me to say anything more than what I have read from a couple of electronic DIY sites.

The simple answer to all these impedance issues is to buy or build a variable impedance device. Under $50 to buy and around $20 to build yourself.

All the best,

Billy

EDIT: This describes the impedance issues with microphones in an understandable way. https://mynewmicrophone.com/what-is-a-good-microphone-output-impedance-rating/

Last edited by Planobilly; 02/20/22 09:07 PM.
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