Not so good noise figure on that input pair 2N5551/2N5401. Probably an artifact of the era it was designed in.
Calculate as such: I(led) = (Vin-Vled) / (Rled). You want "I(led)" to be less than the specified current (If) of the manufacturer. Vled is the forward voltage of the LED, and Vin is the applied voltage. Yes, newer LEDs are more efficient, they have a higher luminosity rating with the same current, so in many cases you can use a much higher dropping resistor and get good brightness, and longer LED life.
Reviving this old old comment. I received a severely underbiased sample of this amp and tried to use a 6.2V zener but got the inverse of the results (ie a set of 4735 diodes actually made the output stage lower in bias but a 4730 got the amp to run marginally hotter. R19/20 gets me 5.XV with a 6.2V zener 4.XV with a 5.1 and only 2.XV with a 4730. The lower the voltage across the 39Ohm the hotter the output stage gets (!) I am curious if anyone has adjusted the base resistor R22 (20Ohm) to the driver to a higher or lower value to get to desired bias points.
Last edited:
Never mind... I ended up increasing R21/R22 to 40Ohm on 1 channel and 50Ohm on the other channel and got the output stage to bias up properly. That amp must have some severely under-spec'd 3281 and 1302s.
I got 2 units that measure very differently until I tweaked one of them. The harsh and bass shy one was severely underbiased. Does your unit run hot or cold to the touch?
Mine runs quite hot but it’s not bass shy at all. Quite the opposite. My amp is supposed to have closely matched transistors though I haven’t measured them.