the OVERLOADB pin on the TA2020

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vtpeaknik

Member
2013-03-23 12:21 pm
I posted this question to an ancient Lepai T-amp thread, but better to start a new thread I suppose.

I want to add a "clip" (not "fault") LED to the LP2020, but have not found an actual circuit. Has anybody done it? What is not clear to me is whether the OVERLOADB pin on the TA2020 goes high when there is no fault, or does it float? The chip's doc only says that it is "5V logic" and goes low when there is an overload. If it normally goes high, I'm thinking of a circuit like this:

OVERLOADB -> 5K resistor -> base of an NPN bipolar transistor. The Emitter to ground. The collector to the positive end of one of the diodes that lights up the volume knob, between the LED and the resistor the supplies it with power. (Can also replace the LED with a red one. And can also add more resistance to that supply line to reduce the LED current somewhat.)

The idea is that normally (when there is no overload) the transistor will be driven to conduct and bypass the LED so it will not light up. But that depends on that OVERLOADB pin going high normally. If that's not the case then a more complicated circuit would be needed. If that pin is an open collector, can it take 12V when open-circuit?
 

vtpeaknik

Member
2013-03-23 12:21 pm
Well I can now answer my own question, since I went ahead and tried it out. That pin does output +5V (with no load) normally, and the circuit I proposed above works nicely. I used a 15K resistor and a 2N3904 small NPN transistor. Also added a 475 ohm resistor in series with the supply line to both LEDs to make them somewhat less bright. (The two LEDs have individual 1300 ohm resistors to start with.)

I must say though that replacing the LED was a pain. The leads of LEDs that I have don't fit in the tiny through-holes on this board. Improvising a way to mount a red LED, and the transistor, in that location was no pretty. Also the LEDs I have are much dimmer than the original blue ones. I added some black ink from a permanent marker onto the remaining blue LED. If I were to do this over I'd leave both LEDs in place, although then the overload condition would only be signalled by a slight and flickering increase in the blue light level.

I'm working with this amp for use as part of a small battery-operated PA system. In that light I made the following mods, besides the clip LED:

* Added two 1/4-inch phone jacks in the rear for easier connection of speakers. These jacks must be of the type that isolates the sleeve from the chassis! Room is very tight there but I managed.

* Modified the preamp circuit so that the bypass switch in the "direct" position puts the amp in a mono mode. This makes it easier to connect mono output from a mixer, using a standard cable, for amping into both speakers. I did this by cutting open the PCB traces from both output pins of the volume pot, inserting 3.9K resistors into each, and shorting them together on the end leading towards the switch. In mono mode with a single input this means a 6 dB drop, but you can simply up the volume a bit.

* Added the missing 4 Schottky diodes, since I might push the amp to its limit, sometimes at supply voltages up to 14.5V.

* Swapped the right and left inputs from the "MP3" jack, just to make it right!
 
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