please help newbie clip LED passive

[garydmd]

Hi there,
I just built an amplifier with 2 lm3886's. I am just a hobbiest, and haven't worked on stuff like this for 10 years or more. I had an old power amp that was blown up with a good power supply and I thought I would give it a try. Worked out really well. I now have a power amp with a switch, 1 input, 1 output and volume pot that I guess runs about 100 watts for ~$10.00 and sounds fine. The only the feature the amp doesn't have that I think would be really nice is some kind of led meter to indicate when the amp is being driven too hard. These seems standard on most power amps. Now, I have searched and seen led meters etc talked about, but to be honest, they are more complicated than my amp! (which is more or less the 3 resistor version). Is there something simple like a zener diode with a resistor and led that I can setup just to run like a clip indicator. I mean, I think I'll hear the amp clipping, but I just think it seems standard to have such a feature.

In short, I would like an LED that is off when the signal going into the amp is below a certain threshhold, and on when the signal is above a threshold.

I also am hoping to find something passive that doesn't require me to power an op amp or something, and hopefully only has a few parts.

Thank you for helping with this question, sorry if it is so novice.

I really would be proud if the amp was kept ultra simple.

[richie00boy]

These are a waste of time and just for cheap gimmick. Your ears will tell you if you are overdriving anything.

[garydmd]

I see what you mean, but I think they can be very useful under certain situations. For example, if you are running a mixing board into a compressor into and eq into an amp and you are hearing clipping, you would like to be able to glance at the equipment and see what is being driven too hard. Also if you are in a room with 500 people and your amp is 50 ft away, and are not sure how much headroom you have left to drive the amp, this could be useful. But I agree, use your ears, not your eyes.

I came up with a circuit, and any critique by anyone would be appreciated. I first started with a parrallel circuit at the speaker output. I first knocked the voltage down with a resistor. I put a regular silicone diode to only allow current 1 way. This feeds into a zener diode in reverse, which in a perfect world would not conduct at all until a threshold is reached and then would conduct at a constant voltage drop. The LED is placed across the zener diode, which I used a 2.4 volt. I adjusted a potentiometer to get the light to come on when the amp is being driven at the maximum power I want. The resistor ended up being 3.3k.

Here is a picture of the circuit:




The parts are: 3.3 k resistor, 2.4 volt zener diode, !N4007 silicone diode, and LED.

For as simple as it is, it works ok. The zener diode leaks and lets through some faint light when them amp is not clipping, but with large peaks it comes on brightly.

Any critique suggestions would be helpful.

[beerman]

I thought about making a gain setting tool with a switched LED circuit for each channel, then I realized I already have a DMM.

[garydmd]

Sorry Wrong circuit, forgot 1 diode, here is correct circuit.

[garydmd]

Here is an idea on a circuit which is better I think. What type of transistor should I use for this?
Thanks, Gary

[I_Forgot]

What you really want here is a comparator that compares the output voltage from the amp to a clipping reference voltage set by a trim pot. It should also have some circuit to keep the LED lit for maybe 100 ms so that you will actually see it even if the clipping is a very short peak. You can use a 555 timer wired as a monostable (aka one-shot) for that.

If the waveform isn't symmetrical, or the amp isn't, it may clip one rail before the other so you may want to set up two references and use a "window" comparator.

A window comparator's output is high when the voltage input is within the set range, and it goes low when the voltage goes outside that range (such as when clipping either the positive or negative rails). The low output can be used directly to trigger a 555 monostable that drives the LED.

I_F

[impsick]

this is interesting. i need a clip light also for a mic pre im building. any schematics of this circuit?

[garydmd]

Right now I am experimenting with 2 more circuits and I will place a post after I finish. The first is a circuit using the "transistor as a switch" concept to turn on the led. The second circuit is more basic, but I think it might work. I have a dual led, red one way, green the other. I am seeing if I can get something cool to work that way. The 555 timer is top of the line, but I am going to try some simpler things first. I will post what I find out soon. Thanks, Gary

[I_Forgot]

Originally posted by impsick
this is interesting. i need a clip light also for a mic pre im building. any schematics of this circuit?

Look up "window comparator" and look up "555 monostable". Connect the two and you have a clipping indicator.

If you can program uCs, a chip with an A/D converter can be programmed for the same function. The comparator and one-shot can be implemented in software.

I_F