Lm311 output

[Hero999]

Is it still in print?

If not how old is it?

How much of it are you posting?

Not naming the magazine also helps?

Copy the schematic rather than scanning it and don't copy any of the text apart from part numbers and component values.

 

[confounded]

Ok here it is, i'm so frustrated with this not working i feel like i've tried everything.

 

[audioguru]

The minimum supply for the LM833 is 10V, you have only 6V.
I think you need more gain for the photo-diode.

Your lowpass filter cuts all high audio frequencies then the 4.7nF coupling capacitor feeding the volume control cuts all low audio and medium frequencies leaving no sounds to be amplified.

 

[confounded]

crikey i would of thought as its from a magazine it would be ok.
Am i wrong or are these fundamental design floors?

I think im gona post the transmitter as well, i cant trust it now.

 

[confounded]

Here it is

(they are 2 phono input sockets on the left)

 

[audioguru]

How are you monitoring and adjusting the transmitter's output level?
Maybe the level is way too low.

 

[confounded]

The aprox 100Khz triangle wave on pin 2 of the lm311 is shown on pic 1. It peaks at about 11V. It has a DC bias of about 6V.

The audio signal i looked at pin 3 of the lm311 and i adjusted the 50k pot until it was not clipping.
It clips at about 8V and about 0.5V. It has a DC bias of about 5V.

Is the difference in DC bias an issue? Or the fact the audio signal doesnt peak as high as the triangle wave?
Or do you think it is just my reciever thats the problem?

Thank you for your help guys

(ive used x10 probes)

 

[Hero999]

How are you monitoring and adjusting the transmitter's output level?
Maybe the level is way too low.

What do you mean by that?

Do you mean modulation index?

It's PWM, not AM so that question is meaningless.

 

[audioguru]

Your transmit level is high enough.

The 4.7nF coupling capacitor to the colume control in your receiver is attenuating most audio frequencies. It should be at least 34 times larger at 160nf or 220nf.
The gain ogf the receiver should be increased. Its photo-diode should be operated in the dark.

 

[confounded]

I have replaced the lm833 in the reciever with a lm358.

I have replaced the 10k resistor on the inverting input on the amplifier with a 5.1K for more gain.

I am operating the photodiode in the dark.

which capacitor do you refer to as i dont have a 4.7nf capacitor on my schematic?

I still cant get this to work

When i turn on my tv i get massive white noise on the headphones of my reciever where as when my tv is off i have none. This happens if my tv is within about 2 metres of the reciever. Its not being recieved by infra red either as the transmitter was off and the photodiode covered up.

Further than 2 metres and i still cant get any audio transmitted anyway

Also the led on the reciever is always on when turned on, even if i completly cover the photodiode. Surely it should only be on when the transmitter is recieving infra red signals?

 

[audioguru]

I have replaced the lm833 in the reciever with a lm358.

Look at the specs in their datasheets:
1) The LM833 is low noise, low distortion and has an output bandwidth to 100kHz.
2) The LM358 is very noisy (hiss), has high crossover distortion and has an output bandwidth to only 1kHz or less.

Use an OPA2134 low noise, low distortion and wide bandwidth dual opamp that works with a supply as low as 5.0V.

I have replaced the 10k resistor on the inverting input on the amplifier with a 5.1K for more gain.

Then the noise from the lousy old LM358 is almost doubled. If a low noise opamp is used then the range is almost doubled.

I am operating the photodiode in the dark.

Good.

which capacitor do you refer to as i dont have a 4.7nf capacitor on my schematic?

Between pin 1 of the lower LM358 and the 1k resistor that feeds the volume control.

I still cant get this to work

When i turn on my tv i get massive white noise on the headphones of my reciever where as when my tv is off i have none. This happens if my tv is within about 2 metres of the reciever. Its not being recieved by infra red either as the transmitter was off and the photodiode covered up.

Further than 2 metres and i still cant get any audio transmitted anyway

Also the led on the reciever is always on when turned on, even if i completly cover the photodiode. Surely it should only be on when the transmitter is recieving infra red signals?

Did you make the receiver circuit on a breadboard with many long wires that act like antennas? Use a compact pcb in a grounded metal box as a shield.

The LM833 has a fairly high input bias current that might need to be nulled so that the LED is turned off without signal. Or the replacement LM358 is producing too much noise that lights the LED.

 

[confounded]

I will have to order the opa2134.

Would you recommend a different IC to the lm311 or lm386n as i have to spend a certain amount anyway or pay extortionate delivery fees?

Thank you for your advice

 

[confounded]

The Opa2134 just arrived and I switched it for the lm833 and my reciever now works

I'm now working on the box to put it in, i'll post pics of it when completed.

Thank you for all your help guys