preamp to amp

crivoli · May 22, 2006

Question 1:
I built the circuit on the LM386 datasheet with the amp at a gain = 20. Everything works fine.
I then adjusted the gain to 50 and it works correctly.
When I adjust the gain to 200 the speaker just squeals horribly.
I have built this circuit before and used a gain of 200 with no problems. Any idea of what I could be doing wrong?

LM386 datasheet: https://www.electro-tech-online.com/custompdfs/2006/05/LM386-1.pdf

Queation 2:
I hooked up a preamp circuit using a LM741 and send the output to the amp I built with the LM386. Needless to say the output to the speaker is very bad quality.
Is there anything special I need to do with the output of the preamp before it goes to the input of the audio amp?

I have attached the LM741 preamp schematic

Thanks all
-newb

chemelec · May 22, 2006

crivoli said:
Question 1:
I built the circuit on the LM386 datasheet with the amp at a gain = 20. Everything works fine.
I then adjusted the gain to 50 and it works correctly.
When I adjust the gain to 200 the speaker just squeals horribly.
I have built this circuit before and used a gain of 200 with no problems. Any idea of what I could be doing wrong?

Do you have Proper Filter Caps on both pins 6 and Pin 7?

LM386 datasheet: https://www.electro-tech-online.com/custompdfs/2006/05/LM386-2.pdf

Queation 2:
I hooked up a preamp circuit using a LM741 and send the output to the amp I built with the LM386. Needless to say the output to the speaker is very bad quality.
Is there anything special I need to do with the output of the preamp before it goes to the input of the audio amp?

I have attached the LM741 preamp schematic

Thanks all
-newb

If your using a Preamp, Reduce the Gain on the 386 or you will overdive its input.

crivoli · May 22, 2006

I do not have the exact values of capacitors for pin 6 and pin 7.

I am using a .68uF cap on pin 7 and a .47uF cap on pin 6.

I'm also using a 220uf cap frpm pin 6 to the speaker.

Thanks

chemelec · May 22, 2006

crivoli said:
I do not have the exact values of capacitors for pin 6 and pin 7.

I am using a .68uF cap on pin 7 and a .47uF cap on pin 6.

I'm also using a 220uf cap frpm pin 6 to the speaker.

Thanks

With a Gain of 200, I would recommend a 10uF Tantalum for Pin 7.
And I assume you mean 220uf cap from pin 5 to the speaker, not pin 6.
There should also be a 0.047uf to a 0.1uf cap to bypass the supply, connected between Pins 6 and 4.

Gary

crivoli · May 22, 2006

I tried your suggestions but I still got the same results.

When I place a 10uF cap on pin 7 it mutes the speaker.

The cap between pins 4 and 6 also had no effect.

I built the circuit before just like it is shown on the LM386 datasheet and it worked fine. Now the speaker squeals with the gain at 200. I am wondering if something ielse may be wrong.

Thanks

chemelec · May 22, 2006

crivoli said:
I tried your suggestions but I still got the same results.

When I place a 10uF cap on pin 7 it mutes the speaker.

The cap between pins 4 and 6 also had no effect.

I built the circuit before just like it is shown on the LM386 datasheet and it worked fine. Now the speaker squeals with the gain at 200. I am wondering if something ielse may be wrong.

Thanks

There is "No Way" that a cap on pin 7 to ground should mute the speaker.
(Unless your cap is shorted.)
It purpose is to create an RC filter circuit for the input transistors in the IC.
(Check out the Schematic of the IC)

crivoli · May 22, 2006

Well, I actually noticed that I replaced the alligator clips I was using previously with new ones. I think the new ones are not able to handle the load of the IC when the gain is at 200. I used the old clips and the speaker did not squeal at a gain of 200, but when I got close to the mic the speaker went bazerk.

I must be using some type of vaslues of caps somewhere to cause that.

Also, the cap on pin 7 sure made it sound like it muted the speaker. Maybe the value was to large.... I need to investigate it.

I am hoping to place an AM receiver into this amp and listen to the radio wish me luck.

thanks for the help

chemelec · May 22, 2006

crivoli said:
Well, I actually noticed that I replaced the alligator clips I was using previously with new ones. I think the new ones are not able to handle the load of the IC when the gain is at 200. I used the old clips and the speaker did not squeal at a gain of 200, but when I got close to the mic the speaker went bazerk.

I must be using some type of vaslues of caps somewhere to cause that.

Also, the cap on pin 7 sure made it sound like it muted the speaker. Maybe the value was to large.... I need to investigate it.

I am hoping to place an AM receiver into this amp and listen to the radio wish me luck.

thanks for the help

Check out the Schematic at the bottom of Page 6 on that PDF file.

Getting too close to the Mic, can cause Feedback. Especially at a gain of 200.

crivoli · May 22, 2006

I am not sure what the "ferrite bead" is on the schematic, but if I take the output from an AM receiver and feed it into the amp should I get good results out of the speaker? This is assuming I can get a good signal from an AM transmitter.

thanks

Roff · May 22, 2006

when I got close to the mic the speaker went bazerk.

Do a Google search on "acoustic feedback".

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