Hello, I've been trying to build a mono amplifier for a scanner (Uniden BCD396XT) note that said scanner's output is stereo. I don't care if the audio is mono or stereo. The factory audio is too low for me for my usage. I'm trying this little microchip to boost output enough to be audible. The output speaker is wanting/needing to be a mono CB type speaker. Note I have tried to use different speakers both mono and stereo and different inputs.
Now onto the problem. I get a buzz when supplied 12 volts by conversion of mains 120v. That's not really a major problem as it will be a mobile operation but any help is appreciated. It's a noisy buzz but not the primary problem. My primary problem is that audio is not being amplified without distortion. Audio comes through muffled to the point or almost to incomprehensible. This is with the audio gain at 20 so pins 1 and 8 have nothing connected. If gain is amplified to 200 the distortion gets worse. If I reduce to volume down by a pot or on the input unit, distortion gets better and audio gets legible but the audio is not amplified anymore. It's simply put no better than as if the board was nonexistent between the speaker and audio making device. I've built the circuits several times on PCB and a breadboard with no luck. I'm getting similar results on both circuits. I've changed the chip as well with no luck.

Below is the links to both circuits I've used. The minimal gain on the first and tracecom's schematic on the second link. Thanks, any help is appreciated.

Chris


http://www.hobby-hour.com/electronics/lm386-power-audio.php
http://forum.allaboutcircuits.com/showthread.php?t=73725

 

Are you getting the audio from the scanner's headphone jack? or where?

I am afraid the LM386 just does not have enough power for your purpose. It is best used for amplifying a headphone level signal to a speaker level, and even that speaker level is not extremely loud. How loud do you want it, i.e., what size room is it in, and how far are the listeners from the speaker?

 

So, did you put a capacitor across the power and ground?
Try 10μF in parallel with a 470μF electrolytic capacitor.

 

The LM386 is mostly for amplifying low level signals to be heard through a speaker.

If trying to add gain to a large signal, the result will be a lot of distortion.

There are other ICs around for amplifying a low power audio output, such as headphone jack to speaker amps.

What loudness are you looking to achieve? What is the rated wattage of your speaker?

Hi-Fi ICs such as the LM3886 and TDA2030 are... Overkill.

ESP Is an Excellent resource to find the project to fit your needs. Spend some time browsing and you'll find all manner of power amplifiers, pre-amps, headphone amps, etc.

 

The LM386 should be plenty loud enough from a 12v DC supply.

If you used the low gain circuit shown in your first link it should work fine, but I think you should try to capacitor couple the input wire (input signal goes through a 47uF electro cap).

Otherwise you might have a grounding issue, is the 12v supply for the LM386 used for anything else? If it is a little 12v plugpack just used for the amp that should be fine.

 

I don't know what kind of scanner the OP is using, nor do I know whether he/she is attempting to amplify the audio from the headphone jack or the speaker. Based on my experience with scanners and with a variety of LM386 amplifiers - homebuilt and otherwise, if the audio from the scanner speaker isn't loud enough, neither will the output from an LM386 amplifier. BTW, I like the LM386 a lot, and use one to amplify the headphone output from my Sansa Clip+ almost every day.

 

We don't know if the "CB Speaker" is 8 ohms. The article shows a higher output power when the amplifier is clipping badly with horrible-sounding 10% distortion.

The datasheet shows that the maximum output of an LM386 tiny amplifier IC with low distortion and a 12V supply is only 0.53W into 8 ohms which is like a cheap clock radio or like the original scanner output. A higher output car radio amplifier IC should be used with a better 4 ohm speaker.

 

First, thanks for all the replies. I apologize I haven't been able to get back sooner. All test speakers I'm using are 8 ohm. The output on the scanner as well as other test outputs are headphone jacks. Other objects used to test are 2 way radio headphone jacks and mp3 player jacks. The audio output of all test units vary greatly. The primary reason of me needing an external amplified speaker is the factory internal speaker of the scanner (which is a handheld unit) is approx. 2 inches and sound very tinny so when audio level gets up toward its maximum, audio can be hard to make out. If I hook up any external speaker the bass improves quality however the output volume level just isn't enough to even bring the external speaker up to the decibels of the factory speaker. By no means am I trying to get the output level of say a car stereo but just enough to override background noise in a cab of a truck with a diesel engine which for me isn't too much. This is why I chose the 386 for the job. Hopefully all is not lost.

Also the 12v supply run off the mains is exclusively used for the circuit. As a way to attempt to help the IC, I also use a resistor in between the 12v and the positive rail on the breadboard bringing the voltage down to approx. 7.8 volts. I did this also to help with the fluxing of voltages between alternator output and standard battery to provide a better buffer between the max voltage.

One final note is all I am trying to do is get the output speaker to be just a few decibels louder than the internal factory speaker on said scanner. I will try any suggestions listed when I get a little bit of downtime at work.

Thanks for all your help,

Chris

 

The easiest solution would be to use a set of external computer speakers (generally 5 or 12 volt powered by an AC adapter). You could clip the power wires and add a cigarette ligher adapter (or what ever else is available in your truck).

You can generally find some old external computer speakers at the Goodwill store for $5 or less (good brand names with decent sound). A pair of these powered computer speakers from Harmon Kardon, JBL or Bose will sound a lot better than an old LM386N.

 

I suppose I could try that (and will if I can get the ideal solution) but for the mounting and such it would be rather difficult to mount computer speakers. Ha. This is just a little project for me if it doesn't work like I need it to then oh well. I'm trying to get it working for general purpose and just for giggles if it doesn't work as planned. I like building circuits so I figured this would be a great opportunity to try something new. A square box speaker is really the ideal for mounting in my case. And just for reference I have just about every method of power in my truck. From 120v to 12v and got a 12v rail in my radio console. (I am an emergency vehicle.)

 

Look for a standalone "mono" center speaker with amplifier. They are meant to go under a monitor, so about 16" wide, 4" tall, and 4" deep, with a volume control.

With all the other variables of the hostile environment you'll be in, it will need to be designed well. A resistor of small value in the power line is a good idea. Temperature ranges and vibration are what will kill most solutions for your circuit.

 

If you are just doing it for fun, then welcome to our world. The 386N is a fine place to start and you can move up from there. If you are not satisfied, the chip below will give you about 10 watts into a 4ohm speaker at 12 volts. It can also handle higher input voltage for more power (or less distortion). Under $2 with shipping.

http://www.taydaelectronics.com/ic-...da2050-18w-hi-fi-amplifier-35w-driver-ic.html

 

The datasheets show the output power.

The LM386 has an ouyput when clipping pretty badly of 0.25W into 8 ohms with a 6V supply. Its output is 0.56W when the supply is 9V and the output is 0.66W when the supply is 12V. With a 12V supply it gets very hot when playing near max output.

The TDA2050 produces about 1.6W into 8 ohms with a 12V supply or 7W with a 24V supply. It produces only 3.1W into 4 ohms with a 12V supply (not 10W). It produces 10W into 4 ohms when its supply is 22V.

You probably need 5W. Then use a TDA2050 with a pretty big 8 ohm speaker in a big enclosure and a 20V supply.

 

Datasheet Fig 10 shows +/- VS, so @ 10 watts it's using ~22 VDC supply.
With 12v (13.8) battery voltage you only have about 4.9 Vrms to work with.
The amp circuit won't drive rail to rail, so you just can't get more than 3-4 usable watts or so at 4 ohms.

 

Ordinary low power car radio amplifiers are bridged so they produce about 15W per channel into 4 ohms when the supply is 13.8V. My car was built with 2 ohm speakers that produce about 25W per channel from a bridged amplifier.