EHHA I Instructions

Before you start

You can print silkscreen images of the Amplifier board from these links:

EHHA I Amp Silk Screen PNG EHHA Amp Silkscreen (PNG)  Cavalli Audio EHHA I Amp Silk Screen EHHA Amp Silkscreen (PDF)

It's a good idea to study the board and components. Also, look at the Schematic diagram to locate each part on the layout. Measure each resistor before you install it to make sure that you're reading the code properly.

Component Choices

The EHHA has been designed to offer a lot of flexibility for the builder to select output devices, gain settings, and frequency response compensation.

Output Devices

  • IRFZ24N/IRF9Z34N Mosfets - see Parts for components unique to these devices.
  • MJE243/MJE253 - see Parts for components unique to these devices.

Feedback Resistor R13 - Setting the Closed Loop Gain

  • Use the tables in the Tweaks section to choose this value to set the closed loop gain.
  • For example, if you want a gain of 11 use 1kΩ for R13.
  • If you are not interested in making this choice, then set R13 to 1.2kΩ.

Compensation Capacitors C2 & C3 - Frequency Response Optimization

  • Review the slew rate tables in the Performance section.
  • Choose a value for C2 and C3 that corresponds to the closed loop gain category you've chosen. This will compensate the amp for no ringing or overshoot.
  • The values for C2 and C3 also set the slew rate of the amplifier.
  • For example, if you've chosen a CL gain larger 7.8 then don't use C2 and C3.

Resistors R22 & R23 - Setting the Open Loop Gain

  • Take time to review the Tweaks section to determine open loop gain that you want to set.
  • The open loop gain determines the amount of effective negative feedback.
  • For example, if you want an NFB value of 35db then choose 62kΩ for the Mosfet version or 100kΩ for the BJT version.
  • If you don't have a particular choice, then set R22, R23 to 33.2kΩ.

If you plan to experiment with these resistor values, you may want to socket C2, C3, R13, R22, and R23 on the board.

Board Assembly Instructions

Have the components specific to your build in the parts box.

Solder the components to the board from lowest profile to highest, roughly in this sequence:

  1. Resistors and diodes.
  2. Small signal transistors.
  3. Film capacitors, trimpots, opamp socket, and connectors.
  4. Tube socket and large electrolytics.
  5. Output transistors and their heatsinks.
  6. Install the he DC servo Opamp. Be sure to get the correct orientation.
  7. Insert the tube.


Clean up the solder flux residue from the board with isopropyl alcohol (or electronics flux remover) and a brush.

Inspect all solder connections carefully, using a magnifying glass, to make sure there are no solder bridges or cold solder joints. Use a multimeter in ohms scale to check for short circuits. Correct any mistakes before proceeding.