DEFX Lola Phase Transistor Matching

[spacekid71]

Hi everyone,

I am working on the DEFX Lola Phase and I need to match the LED transistors. Just measured about 20 2N4401 transistors using the Peak Atlas DCA55 and they all are about 210 hFE. Tried the suggested alternative 2N5551 transistors and they all stay about 150 hFE... The build instructions suggest hFE to be between 80 to 120. Am I doing something wrong or have I just hit high gain batches of these transistors?

Thanks,

Martijn

[PRR]

> I need to match the LED transistors.

Why??

These transistors do what the opamp and resistor tell them to do, no?

[spacekid71]

I am not sure why they need to be matched but perhaps to ensure that both phaser A and phaser B respond the same way.

My question still remains the same, with the recommended hFE being between 80-120, how come the recommended 2N4401 transistors
have a hFE higher than 200, and the 2N5551 transistors a hFE higher than 150? Does anyone have 2n4401 or 2N5551 transistors in
the recommended range? Do I just have some bad batches?

Just to add, this is a clone of the Mutron Biphase pedal and it has two phaser circuits in it.

[BetterOffShred]

I did not match mine and it's lush and deep AF.  You just need to tune the waves following the guide. 

[Rob Strand]

From what I can see the base of the transistors are fed directly from an opamp so the gain of the transistor is not going to be a factor at all.

Also the LEDs and transistors are in a feed back loop which removes some variation of the drive voltage.

The biggest variation is the optos, optical coupling, and fact each LED has an individual trim pot to set the drive current.  The trim takes up the slack on the variability in opto's and optical coupling.

The matching comes down your ability to set the trimpots accurately.

[PRR]

....recommended hFE being between 80-120...

I didn't see that. Where is it??

hFE of 100 versus infinity would be a 1% difference. (As Rob says) There is MUCH more variability among LEDs and photo-Rs, and even physical spacing. Why there is a trim.

> gain of the transistor is not going to be a factor at all.

That was my thought since all BJTs are high hFE today. But I postulated hFE=2, and realized that the huge base current would show in the emitter and not the collector. Paired with a high-hFE part, you might notice. (Yet it could be trimmable, at least at some reference current.)