Hi guys,

I'm just looking for a little help with an amplifier circuit I'm building. I can't seem to amplify an audio signal past a certain point .

I've got a microphone input into the NE5534 and I'm outputting from there into the MC34119.

The understanding is that the microphone signal is a small signal and feeding it into the NExxx will 'pre amp' it before it runs into the MCxxx. The issue I'm having is that if I input the microphone straight into the MCxxx it is outputting a higher signal than both IC's together.

I ran the signal (just to test) out of the MCxxx through a couple of transistors set up in a Darlington pair and got a very slight (0.02a) increase in power.

I would just use the one chip, but I need more power as the whole system is a relay system for quite a large room. When the room is in use we need to be able to hear what is going on whilst we are in the office.

I hope I make sense, I have no idea where I'm going wrong...

I've uploaded the datasheet for both IC's.

Edit: I know the NExxx is working as I ran the Mic signal into it and got an amplified output as I expected.

 

Thanks for providing the datasheets, but the real question is about your circuit. Can you post your schematic?

ak

 

I'll draw it up when I'm on my break (I'm currently at work) and post it.
It'll be done on paint or something so you'll have to excuse the crudeness of it, I never usually draw my circuits up.

 

Or draw it on paper and post a cell phone shot.

I never usually draw my circuits up.

"Well *there's* your problem." - Mythbusters season 1, 2, 3, 4...

 

Ha! Just done that, figured I couldn't computer draw it in 10mins... See attached...

 

I usually just mash components (in a controlled manner) into a breadboard, making vague reference to a datasheet along the way.
On this occasion, I actually took my time and tried to figure things out properly...
...No prizes for guessing which method yielded best results...
...imma go get my component hammer...

 

Some issues I see right away:
Your drawing does not show power supply bypass capacitors. These are very important in a circuit that has both small signals and high currents. Add a 100 uf or larger electrolytic cap from +5 volts to ground near where the power is connected to the MC34119.

There are missing components around the MC34119. See the data sheet for an example circuit. Don't try to save parts here, they are all needed.
The Chip Enable pin of the MC34119 should be connected to ground if you are not using it. See the application information in the data sheet.

The NE5534 is not specified to run on a 5 volt power supply. They imply that the lowest voltage allowed for proper operation is 6 volts.
The NE5534 needs to have the input pins biased to near the center of the power supply. For a single 6-volt power supply this would be 3 volts.

My laptop battery is getting low so I will let others fill in the details.

 

No more details needed. Like the other chip, the 5534 datasheet has example app circuits that are good for this project. But every part in every circuit is necessary.

OK, one thing - the gain resistors are not correct.

ak

 

Thanks guys, you've both been a big help!

Couple of questions though, with regards to supply voltage, for the NE55xxx the recommended operating conditions say min 5v max 15v. Where did you see 6v? Am I using the wrong bit of the sheets?

pin 2 labelled 'FC2' on the MCxx, info about that pin says the cap isn't needed if the cap at pin 3 is sufficient, does this still need connecting to ground then? Have I been a numpty?

I'll go back to the bench and reassess this now, I honestly thought the example circuits in the datasheet were just for reference and could be tweeked and messed with.

Cheers guys!