LED noise

[idlefaction]

i have an opto compressor that is dumping loads of noise onto the supply when the LED current gets high.  does anyone know how to get rid of this/reduce its effect on other circuitry attached to the same power supply?

 :?:

[zachary vex]

i would suggest creating a separate filter for the audio power.  determine how much current it needs, and pick a resistance that won't eat too much voltage up (relatively low, say 470 or 100 ohms) to put in series with the regular power, and then put a high-valued cap (say 100uF) to ground at the audio end of it so that you isolate the noise from the led power.  the led is fed from the supply directly, while the audio is fed from that large cap being fed by the resistor.

[Rob Strand]

That can occur if you don't have enough current limiting on the opto LED, you should make sure your ckt can never drive the LED too hard.  Zach's solution will help stop the current pulse interferring with the audio but you may have a design problem in there.

[darren]

i would suggest creating a separate filter for the audio power...

thanks zach.  the designer (kev from http://recording.org/users/kev/) currently has 33R/1\
0uF filters on each opamp stage, but i will try 100R/100uF and see if it reduces it if th\
e below doesn't work.

That can occur if you don't have enough current limiting on the opto LED, you shou\
ld make sure your ckt can never drive the LED too hard.

hrm.  here is the circuit:
http://recording.org/users/kev/KROC_schem5.PDF
the LEDs are driven by a bipolar, with some current limiting to about 30mA which seems se\
nsible.  i'm only using regular diffuse red LEDs so using superbright yellow ones instead\
would allow me to set the ckt to use still less current.

it dumps the current to ground, and the PCB ground is a rail with things on it sequential\
ly in no particular order.  the left channel's LED driver dumps current into about 2/3 al\
ong the rail, so everything before that will be affected - the right channel's LED driver\
is about 1/4 the way along but the thing right before that is the right channel's input \
opamp!!!  i will try running 'star' earths from the LED drivers straight back to the powe\
r supply.

you may have a design problem in there.

fun!    i also have a hum i can't track down.  but i'm a fool and haven't used shielded\
wire anywhere  

i could say it sounds fantastic, but it doesn't sound like anything unless you abuse it. \
i took it up to a friend's studio to put over some vocals last night, and he couldn't ev\
en tell if it was working or not until he had the gain reduction meter right off the scal\
e    (i could tell, but i've been listening to this thing for weeks now hehe)

thanks rob and zach!

--
darren

[Rob Strand]

It looks like should it should work - there's something missing on IC2A, it's got no feedback.  The LED current is high but not overly excessive.  Just try judicious cap placement.  The 100uF should be OK, try one each side of R35 and make sure the -ve leads connect directly to the gnd side of R34.

If you find the threshold is too low (ie. you have to drive it hard before it works), then try increasing R11 and/or decreasing R10.

[Paul Perry (Frostwave)]

Runnng that overload led straight off the op amp output can't be a good idea, if this thng is supposed to be hi-fi! I'd buffer it for sure. The zener could be acting like a noise generator, combined with the output impedance of the op amp.

[darren]

re: IC2A, yes there is definately something missing!!  on the PCB it's hooked up as a voltage follower ie. -ve input to output.  also if you're planning on building this, the traces around the meter driver opamp are all wrong, swap pins 7&8 and hook pins 1&2 together.  you'll also notice that the -ve and +ve inputs are round the wrong way on some of the opamps on the schematic, but fine on the PCB.

re: the zener, that's a valid concern - i will put a tube cathode follower in there.  :wink:  (j/k!!!!! emitter follower!)

[Rob Strand]

the zener, that's a valid concern

Paul's onto it.  You could put a resistor between the opamp and the LED, say  68ohm or so - an emiter follower + resistor is a better idea.