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Audio amp IC

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zachtheterrible

Active Member
I've been wanting to build myself a parabolic mic amplifier for quite some time and Ive got some free time. I know there's plenty of schematics out there, but I kind of wanted to design the whole thing myself. I've never really designed any kind of complicated analog circuit, and there's no better way to figure out how to design filters and what not than hands on.

I was just wondering if anyone could recommend a good high gain audio amplifier IC. I wasn't sure if the good ol' LM386 would do the job?
 

stevez

Active Member
Zach - in terms of design you might want to understand what's in front of the amplifier section or sections and what follows then work your way in to the IC. What levels would you expect from your microphone - that might be a start. How much power do you need in the output - another item to nail down.

This is not to say that starting in the middle is bad practice. Many good designs begin with the prior experience from other applications. Just suggesting a step back and a look at the whole system then work your way in gradually. At some point it's time to stop designing and start building but don't be in too much of a hurry - you'll miss some opportunities to learn.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You also need to think about different sections, and what they do. A microphone requires a preamplifier, an LM386 is a power amplifier, and wouldn't be much good fed straight from a microphone.

Assuming you want to feed a set of headphones?, then an LM386 would be fine, but following a proper microphone preamplifier.

There are numerous low-noise opamp's available, such as the NE5532 series - although even better ones are available these days. If you google for such projects a number of them actually use discrete preamplifiers.

The June 2005 issue of EPE had such a project, and actually used an LM386N, fed by a single BC109 preamp from an electret mic insert.
 

Dr.EM

New Member
For this project, you'll want the lowest noise you can get. I have found with electrets that almost all the noise comes from thier internal FET, rather than the connected preamplifier. An op-amp pre will provide good enough performance providing the cable to it is not long (unsure that anything connecting low level signals is shielded however). I would try and use a Panasonic WM61 capsule that has high sensitivity and low noise, look on digikey for them.

The circuitry if you choose to use an op-amp is quite simple, consisting of an inverting or non-inverting amplifier, the electret and its current limiting+capacitive coupling (make this a large polyester cap) and a good clean regulated power supply (batteries are ideal, 4 9v ones would give and excellent headroom, but you could probably get away with less).
 

audioguru

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Most Helpful Member
Here is the schematic of the project in the magazine:
 

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zachtheterrible

Active Member
the WM61 is an electret mic that you are talking about right Dr. EM?

Maybe I will do this with discreet components. I will definately learn a lot more. What kind of gain would I be looking for in a transistor? 100 or so? would a 2n3904 be able to handle the kind of sensitivty I need?
 

audioguru

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Most Helpful Member
zachtheterrible said:
What kind of gain would I be looking for in a transistor? 100 or so? would a 2n3904 be able to handle the kind of sensitivty I need?
Any small silicon transistor would give a gain of about 100 in that circuit with its emitter resistor bypassed. A BC109 or BC549 will have a little less noise than a 2N3904 transistor.

It would be difficult to make a discreet LM386 amp and this one with the cap between pin 1 and pin 8 has a gain of 200.
 

audioguru

Well-Known Member
Most Helpful Member
zachtheterrible said:
So a BC109 or 2n3904 will have sufficient gain?
It depends on how good is your parabolic reflector, and how loud is the sound you are listening to.
 

zachtheterrible

Active Member
What transistor than can you recommend to me w/ a high gain? I'd rather use that than the 2n3904. I'd rather overdo it than underdo it! I can always use a POT to decrease gain if needed.
 

audioguru

Well-Known Member
Most Helpful Member
zachtheterrible said:
What transistor than can you recommend to me w/ a high gain? I'd rather use that than the 2n3904. I'd rather overdo it than underdo it! I can always use a POT to decrease gain if needed.
"Any small silicon transistor would give a gain of about 100 in that circuit with its emitter resistor bypassed."
To get more gain then the circuit needs more transistors.
The circuit already has a volume control to reduce the gain if it is too high.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
zachtheterrible said:
What transistor than can you recommend to me w/ a high gain? I'd rather use that than the 2n3904. I'd rather overdo it than underdo it! I can always use a POT to decrease gain if needed.

The BC109 (as used in the EPE circuit) is a low noise, high gain device, specifically designed for small signal audio preamplifier stages!.
 

zachtheterrible

Active Member
I'm not going to use that circuit, not because it doesn't work. I want to design my own. If the BC109 is the best that you can recommend than I'll use that. Thanks for all the help :lol:
 
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