aion SKYWAVE Handmatig - Pagina 5

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BUILD NOTES

Silicon transistor selection

The Broadcast uses a BC549C for Q1, the NPN transistor. The Skywave layout uses the USA E-B-C

convention, so the 2N5088 is specified as an equivalent, but if you want to use the BC549C then just

rotate it 180 degrees from the silkscreen.

Germanium transistor selection

The manufacturer has used different types of germanium transistors for Q2 across the different models.

The most common type is the Russian M

20 (MP20), which are typically in the range of 60 to 80 hFE

with less than 100µA of leakage current. Others including the Dual Broadcast use the NTE 2N404A, a

new-production germanium that is anywhere from 50 to 250 hFE. The 2N404A taken from a Broadcast

this video

measured at 216 hFE with 125µA leakage.

With this said, given the ranges in the originals, the gain of the germanium transistor likely does not

make a big impact on the sound. Just use what you can find, taking care that the leakage isn't too high

(perhaps 200-300µA maximum).

330µF capacitor

The Broadcast uses a 330µF electrolytic capacitor for C9, the low-cut section. It's unlikely that there

would be an audible difference between this and a lower value such as 220µF, but we'll assume they

chose it for a reason.

330µF is a less common value than 220µF or 470µF, and in this application it needs to be 35V or higher

since it connects directly to the positive supply, which may be as high as 24V depending on the Voltage

slide switch.

The

Mouser parts list

provides a part number that will fit the Skywave layout, but if you source a

different one, it may be physically larger than the one we designed around. If this is the case, there is

plenty of space for it to be folded over and lay flat on the board if needed.

Gain trimmer

The internal gain trimmer sets the gain of the silicon transistor in the first amplifier stage. This gain

setting applies to all three toggle switch modes. As you turn it up, resistance is reduced and the

transistor gain increases.

This is purely a set-to-taste control, not a bias, so there is no calibration procedure or target voltage. In

the original Broadcast, it's set to the minimum position by default, but is intended to user-adjustable.

Listen to all three modes of the gain switch across the range of the Gain knob, and adjust the trimmer

until each mode is useful.

SKYWAVE GERMANIUM DRIVE