Description

Two pairs of matched On Semi / Fairchild KSC1845 NPN small-signal transistors. 4 transistors in total. Often used in “Long Tail Pair” input stages of amplifiers. For Adcom GFA-535, GFA-545, GFA-555, and many other amplifiers. Matching is performed at 1mA collector current. Includes copper foil tape and heat-shrink tube for thermal bonding.

Transistor gain and Vbe vary with temperature, so temperature must be controlled for in finding matches. These transistors are precisely matched, on a jig that uses forced air to keep both devices under test at the same temperature. This results in more precise matches than simply measuring Hfe.

Contact me if you need transistors matched for a different collector current. Labor charges apply for special matches. In my experience, transistors matched at 1ma will also match well at higher currents.

Thermally bonding the input transistors:

The transistors should be thermally bonded, so their Vbe and Hfe will track with heat.

Wrap the copper foil tape around the pair of transistors. Be careful the foil doesn’t touch any leads.

Some transistors pairs have their faces front-to-front and others are front-to-back. Observe the component outlines on the board.

No thermal paste! It’s not needed, and might eventually seep oil onto the board, which attracts dust. The copper foil does a great job keeping the transistors tracking thermally.

Install the transistors and heat-shrink around them.

Snip the tops of the heat-shrink.

This is the jig I use to find the matching pairs of transistors. Read all about it here on this thread at DIYaudio.com. You can build your own jig from the files provided there. My version of the board mounts a standard 80mm computer fan to provide a high-velocity airflow to keep both devices under test at the same temperature.