The HVTP2 is a dual channel vacuum tube microphone preamplifier (micpre), transformer balanced on both the mic input and the line output.  The DI input has 2 impedances depending on the pad switch position (HIz = 2.2Mohms pad off, or 250Kohms pad on).  The preamp has an overall gain of 68dB, with very low noise floor (see specs below).  The HJFP2 (JFET micpre), HVTP2 (Tube micpre), and HOC1 (new compressor), all are mechanically equivalent, two will sit side by side (with some clearance to get in/out) in a standard 3U rack shelf with the handle removed.  The micpre is designed to have flat gain/phase response and low distortion, while exploiting the musical characteristics of tubes and transformers.


  1. MIC INPUT: 1.5K transformer balanced microphone input.
  2. DI INPUT: 2.2Mohms with pad out, 250Kohms with pad in.  The DI input is unbalanced, and must be used with TS (mono) ¼” cables only.
  3. GAIN CONTROL: Variable gain control.  Optional precision step attenuators available (2.00dB/step x 23 steps).
  4. PHANTOM POWER INDICATOR: Lit when +48vdc is turned on.
  5. PHANTOM POWER SWITCH: Applies +48vdc to pins 2 and 3 of the mic input via two 6.8kohms resistors (standard phantom power configuration).  Each micpre channel has an independent 2 pole filter after the regulator, further filtering the power, and providing a “soft” power on ramp to minimize loud pops when engaged.  With most phantom powered mics, engaging the phantom makes little or no noise in the audio path.
  6. INPUT PAD: Applies a -14dB pad to both the mic and DI inputs.  When engaged, the DI input changes from HIz = 2.2Mohms to 250kohms.  In general, the pad should be used with active instruments (w/ onboard preamps), keyboards… if you want the signal clean.  For passive instruments, especially those with piezo type pickups, the 2.2Mohm HIz input is desired, so don’t use the pad.  The pad value can be changed if desired, see “HVTP2 INPUT PAD MOD” below.
  7. PHASE SWITCH: Inverts the phase of the output.  Physically reverses pins 2 and 3 of the XLR output, which directly connects to the output transformer.  Switch out = normal, In = inverted.
  8. BALANCED LINE OUTPUT: Transformer balanced, low Z output.


The rear panel has the power entry module, fuse holder, and main power switch.  In addition, there are 4 x 3/8” holes designed for standard TRS jacks, that can be wired as optional parallel inputs/outputs to the micpre if rear panel connections are desired.  Each audio channel card has a set of parallel pads directly behind each XLR connector, which are labeled on the board.  The DI input must remain on the front panel.




In a few cases recording loud signals with condenser mics, customers wanted to increase the value of the pad.  Changing the pad can be done by changing the ratio of R3 and R4 on the channel cards.  The table below shows the needed values for R3 and R4 for the acceptable range of the pad.  It is not recommended that the pad value be greater than the input transformer gain (+18dB).  If further gain reduction is required, see “INPUT SENSITIVTY MOD”.  For assembled units this mod is free.  On kits, just request the other resistor sets when you order, and we will provide them free of charge.

R3 and R4 are stock 1% values
Loss dBR3R4


The gain of the first tube amplifier (there are 2 tube amplifier stages per channel) in each channel can be lowered by changing the ratio of R17 and R18 (while keeping the sum the same).  By reducing the gain, the headroom is increased.   By increasing R17 and decreasing R18 (while keeping the sum the same), the gain will decrease.  For assembled units this mod is free.  On kits, just request the other resistor sets when you order, and we will provide them free of charge.  Typical gain reduction is -6dB.