EHHA I Device Matching


The EHHA is a differential, complementary high performance amplifier. Although matching devices is not absolutely essential, performance will benefit from some critical device matching. For example, the EHHA uses a symmetrical VAS driven by both NPN and PNP devices. Matching the hfe of these two transistors helps to balance the gain on each half of the audio cycle. Other device matching opportunities are shown in this section.

Tubes

The 6GM8 twin triode tube is used as a differential pair in the EHHA. Consequently, just as with solid state differential pairs, it is preferrable if the two triode sections are matched. Unlike the solid state devices, however, the triodes are not in independent packages. They cannot be swapped out for another triode with a better match. The best option, therefore is to purchase tubes that have been matched by the supplier. But, the EHHA will operate very well with triode sections that have some degree of mismatch.

If you use 6922 or 6H30 or similar tubes, triode matching is more important. These tubes, designed for higher voltage operation, will show their triode mismatches more at lower voltages. The better the sections are matched, the closer they will be at 30V. However, the EHHA has a cathode trimpot, R6, that can partially compensate for out of balance triode sections.

Transistors

If you choose to match devices, this is how to do it.

  • Q3: Simply pick a device with a high β. Do not match.
  • Q7: Simply pick a device with a high β. Do not match.
  • Q8 & Q9: These devices should be matched. For Mosfet output, match for Vgs. For BJT output, match for β.
  • Q5 & Q6: Match for β. Lower β devices will reduce the open loop gain.
  • Q1, Q2, & Q4: Match for β. This is not particularly important because of the large (1kΩ) degeneration resistors.
  • Q10, Q11, Q12, and Q13 do not need to be matched. Mismatches in the servo current feedback are reduced by the open loop gain of the servo opamp. Moreover, the 100Ω emitter resistors further reduce mismatch. Match for β if desired.

Resistors

  • R12 & R13: These set the closed loop gain, match across channels.
  • All others: 1% resistors are good enough.

 

How to Match Devices

Methods for matching devices are available on the web. One of the best descriptions can be found at AMB Audio - β22. The following information is a summary of that section

The matching process uses a 12VDC source. This can be a power supply or batteries. 

Device properties are temperature dependent. Even the temperature of your fingers can affect their behavior. Handle the devices with needle-nosed pliers and take the readings immediately after you apply power before the device begins to warm up. To get good matches it is important to be consistent going from one device to another. If you will be measuring different devices at different times, do so when the ambient temperature is about the same for each so that the results will be comparable.

The P-type device test setup in each case is the same as that for the N-type, except the power supply and meter polarities are reversed. Using the wrong polarity can destroy the devices.

Matching BJTs

BJTs are matched according to their hfe (DC Current Gain). The test arrangement for matching BJTs is shown in this diagram.

Cavalli Audio EHHA I BJT Matching

Set the DMM to DC mA mode. Note that most DMMs have a different input connection for measuring current. The BE junction drop of a BJT is typically 0.65V. This arrangement, with a 1MΩ base resistor, sets the base current to approximately (12V - 0.65V) / 1,000,000 = 11.35μA. The current reading in the DMM gives the hfe according to the formula: hfe = (Imm / 11.35μA). For example, if the meter reads 3mA, then the hfe is (3mA / 11.35μA) = 264.

The 1MΩ value was chosen to suit the hfe range for the 2SC2705/2SA1145. Note that these devices are ECB pinout arrangements. Please see the 2SC2705 and 2SA1145 and MJE243/253 data sheets for pinout arrangements and other information.

Matching MOSFETs

MOSFETs are matched by their Vgs (gate-to-source voltage) for a given drain current and drain voltage. The test arrangement for matching MOSFETs is shown in this diagram.

EHHA I Mosfet Matching

AMB chose 150Ω to flow about 50mA into the drain. The DMM should be set to DC Volts with a range of 30V. Match Vgs, which is read directly on the meter. See IRFZ24N and IRF9Z34N data sheets for pinouts and other information.