# Squeal in amplifier



## RCE (Sep 14, 2009)

I just finished building this. I am going to use it to improve ( make louder) the sound coming out of my Digitrax sound bug.









I left the number 1 and 8 pins open. I also am using an audio output transformer on the input side of the amp. Sometimes there is a high pitched squeal coming out of the speakers and sometimes it is quiet.
Anyone know what the noise is or how to fix it?
Thanks
Richard


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## noelw (Jan 2, 2008)

Kinda curious of what kind of a coupling transformer are you using? Sound like maybe over driving it? 
Been looking for some ideas and easy here too to build for our Serria sound sys.


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## RCE (Sep 14, 2009)

Noel 

I am using an Audio Output Transformer from Radio Shack # 273-1380 hooked to the input of the amp. 
I'm using only one outside wire and the center tap on the primary. The output is 8 ohm. 
Here is the web page I get the idea from http://www.beavisaudio.com/Library/LM386/LM386.htm


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## noelw (Jan 2, 2008)

Not sure how you have it hooked up but here is the audio tranisformer in line if this show up.. .Here is the part you have I think you have..See how the sec. side of Audio Trans. is grounded.( wire blue.) Don't think you can use the center tap so have to use a fixed Resis. or pot to regulate the input.( Blue and Green wire.) Hope this helps.


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## RCE (Sep 14, 2009)

I found the cause of the squeal. After twisting the battery wires together the squeal went away.
I did tie pins 1 and 8 together ( no capacitor, no resister) and it made a world of difference in the volume. 

Here is a parts list and picture of the amp 









1 Audio transformer.... RS#273-1380---2.69 
1 LM 386----------------RS#276-1731---1.69 
1 Retention contact----RS#276-1995--- .59 
2 47uf capacitors.......RS#272-1027....2.98 
.................................................... $7.98


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## Greg Elmassian (Jan 3, 2008)

That is a M-tron 1 Mhz crystal, for oscillators, built in october 1989... 

.... not a audio transformer... google "m-tron 838-590"..... 

Pretty weird that you are getting oscillation on the battery leads, you need a bypass capacitor across the power input... try a 0.1 mfd disc cap... 

Greg


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## noelw (Jan 2, 2008)

..


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## noelw (Jan 2, 2008)

ya... Radio shack cat. show wrong part nbrs. photo. lol 
Glad you got it going..


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## Greg Elmassian (Jan 3, 2008)

Radio shack sure isn't what it used to be! Try going in and when the 16 year old kid just insists on helping you, tell him you need an electrolytic cap.... 

He'll think you want a piece of clothing. 

Then he'll try to sell you a cell phone... 

Greg


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## RCE (Sep 14, 2009)

Maybe this is a better picture


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## Trains West (Oct 4, 2008)

the drawing shows only one input lead i am guessing the other lead from the auto transformer goes to the system ground .........

also how much wattage do you think you are getting ? I thought such amps are only 1/2 watt or maybe .... and i do mean maybe twice the output of the sound bug ? loud enough to hear outside ?

I am asking becouse i have a sound bug in a little loco i did ..... as is you can't hear it at all outside so i am thinking of adding an amp


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## RCE (Sep 14, 2009)

Scott: 
I bought 3 of these http://www.bakatronics.com/shop/item.aspx?itemid=473 the IC chip went bad. If I bought a new TBA 820M chip + shipping it would have been about $4.00.Chip $0.98 + $3.00 shipping. 
I looked on the web and found this http://www.beavisaudio.com/Library/LM386/LM386.htm . 
I could buy all the parts from Radio Shack for $7.98 + tax. 
According to the specs it should be about 3/4 watt @ 9 volts. http://www.national.com/ds/LM/LM386.pdf 
When I put it in a loco I will be using 12 volt for the amp power. That should make it about 1000 mW. 

the drawing shows only one input lead i am guessing the other lead from the auto transformer goes to the system ground .........yes 
also how much wattage do you think you are getting ?  About 700 mW I think 
loud enough to hear outside ? On My layout anyway 
I thought such amps are only 1/2 watt or maybe .... and i do mean maybe twice the output of the sound bug ? I think it is more than twice the out put of the sound bug, if you tie pins 1 and 8 together. 
You NEED the audio transformer between the sound bug and the amp or you WILL smoke the sound bug. 
There are other IC chips that will putout more wattage but I got all the parts locally. 

Richard


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## toddalin (Jan 4, 2008)

Could use an LM380. They make over 2 watts and can be had cheap.
LM380


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## Trains West (Oct 4, 2008)

I like the looks of that amp 2 watts of power and a higher input voltage .... I download some info on it 

but earlyer today i went to radioshack and got the parts for the other .... which i will build now 

will post what i think in a few .... but it was cool that i could just walk into rs and get the parts


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## Trains West (Oct 4, 2008)

just got done ....... and wow 

that little amp puts out ..... I had to turn the master vol down on the decoder to keep from over driving the 2 inch 1/2 watt speaker that i had handy 

just ran it off a good 9 volt battery for the test ..... next task is to build a regulator circuit to convert dcc to 9 vots to run the amp 


may also add a 1k ohm ajustable resister between pin 1 and 8 in place of the wire bridge to make the vol ajustable 

thanks for posting what you did


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## Greg Elmassian (Jan 3, 2008)

Keith, which is the other? 

Thanks Greg


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## RCE (Sep 14, 2009)

Scott 
I did use a 3 watt 8 ohm speaker to test mine, didn't over drive it. Try to find a larger wattage speaker it may help with not over driving the speaker. 
After you build the regulator circuit to convert dcc to 9 volt dc please post a diagram of it . 
I need to do the same thing. I am thinking of using 12 volt instead of 9 volt though. 
Thanks 
Richard


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## Trains West (Oct 4, 2008)

ok all done 

I did set it up with a 12 volt regulator useing a 276-1771 that is fed with a full wave bridge 1 amp 276-1152

I put a cap on the output of the regulator to ground useing a 272-1069

then ran the power going to the chip thru a torid coil (from my junk)

the whole thing is mounted to a 276-159 board and i added a couple of turmenals 276-1388 for speaker and power in 

I also had to change the wire from pin 1 to 8 to a cap to reduce the noise used a 272-1069

I was worried when i started that the high freq dcc would put a little noise into the amp and it did but i got to a very low level and with the sound turned on you can't hear the noise at all 

I also added a 10 k verable resistor to the output of the transformer to change the volume from outside .... in this case i used the one that was in the bachmann tender for the factory chuff

all in all i am pleased the sound is very load and clear and i have only like $15.24 plus the decoder into it ..... a plus with these decoders is that i can load my own sound


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## Dan Pierce (Jan 2, 2008)

You could possibly cut your build cost to less than half of the RS parts by going to a web supplier, but only if building in quantities!!


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## Trains West (Oct 4, 2008)

since i run a small business and I know the gal that runs my rs down the street and I would like to keep it aroumd going there is just fine for me ........


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## Greg Elmassian (Jan 3, 2008)

Yep, and basically every modeller can go to RS and order the parts by RS number. I try to do the same on solutions I make public. 

Not everyone is familiar with all the electronic jargon, package styles, and ordering from Mouser or Digikey, etc. 

Regards, Greg


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