Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Sun 16, 2018 2:57 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 10 posts ] 
Author Message
 Post subject: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 1:09 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13265
In this schematic, is the ferrite bead just on the wire or maybe looped a few turns?? I found nothing in the design procedure pertaining to it's spec..

http://www.ti.com/lit/ds/symlink/lm386.pdf

Image

_________________
Tom


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 3:08 pm 
Member

Joined: Jul Wed 22, 2009 3:07 pm
Posts: 731
Hi Tom,

The NS datasheets gives the specs for such a coil as 3 turns on a Ferroxcube K5-001-001/3B bead.
The application examples there also show "regular" Zoebel-networks with a 50nF cap and a 10 Ohm resistor, in case you do not have the proper bead.

Regards, Peter


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 3:13 pm 
Member

Joined: May Tue 30, 2006 4:46 pm
Posts: 9515
Location: Santa Rosa, CA
See page 4-31 in the old National Linear Handbook.

ftp://bitsavers.informatik.uni-stuttgar ... ndbook.pdf

I forget the name of that R/L network, but it is there to suppress RF noise in AM radio amplifiers.

Rich


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 3:53 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1327
Location: Jackson, TN
Rich,

Thanks, and as a bonus I now know what Floobydust means!

Tim


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 4:53 pm 
Member

Joined: Aug Thu 06, 2015 2:20 am
Posts: 151
Rich, W3HWJ wrote:
See page 4-31 in the old National Linear Handbook.

ftp://bitsavers.informatik.uni-stuttgar ... ndbook.pdf

Rich


Thanks for the link. It contains a wealth of information the most significant to me is the fact that National Semiconductor made the LM389 IC which contained an LM386 audio amplifier and 3 NPN transistors good to VHF.

Perfect for making simple regenerative or superregenerative receivers!

Although this forum is primarily a tube domain, I thought some might find interesting my latest LM386 project. It is a spectacularly performing AM regenerative receiver using only an LM386 audio amplifier. Earsplitting volume on locals with no external antenna and good for DX.

A number of years ago I discovered that the LM386 made an excellent envelope detector. The circuit below uses one section of the amplifier as a Colpitts oscillator. By connecting both inputs Pins 2 and 3 together, significant demodulated audio output takes place.

Regeneration is controlled by varying the amplifier's current through gain determining port Pin 8. Once set, the regeneration is flat across the band and does not have to be adjusted. If the regeneration is advanced just above oscillation so that heterodyning occurs, the heterodyning is suppressed as the station is tuned in.

The 10 mH choke and 10 uF cap to ground from Pin 1 shunt the internal gain determining feedback resistors which significantly increases audio gain.

The 10 pF and 220 pF capacitors form a Colpitts oscillator on the non-inverting section (Pin 3) of the amplifier.

By using the LM386 in a feedback oscillator configuration instead of the Colpitts circuit shown, it is possible to build an simple 80 and 40 meter shortwave regenerative receiver with reasonable volume receiving SSB with no external antenna.


Attachments:
lm386 regen choke Pin 8 regen.png
lm386 regen choke Pin 8 regen.png [ 62.73 KiB | Viewed 803 times ]
Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 7:36 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7984
Location: Beautiful Downtown Burbank CA
Rich's link.

Image


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 10:00 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 13265
Thanks everyone!!

I was thinking it was for noise/oscillation suppression... No issues at present with the one I have boarded up but chip gain is set to maybe 25-30% of max..

BTW with the mention of Floobydust, I'm thinking one could get in on ground floor of marketing a filtration system... :lol: :wink:

_________________
Tom


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Sun 01, 2018 10:31 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7984
Location: Beautiful Downtown Burbank CA
Is there a reason I see different output e-caps on these `386 amp schematics?


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Mon 02, 2018 2:36 am 
Member
User avatar

Joined: Jun Sun 22, 2008 2:03 am
Posts: 1336
Location: Burke, VA
I built a similar circuit with a three-turn ferrite bead and a 0.1uF capacitor. The bead's inductance measures 25uH which makes it series resonate with the cap at 0.1 MHz. To critically damp the LC network, I parallel the bead with a 15 ohm resistor. That's equal to the bead's reactance at the resonant frequency.

Mike, the value of the output cap dictates the lowest frequency passed to the speaker. With 470uF cap and 8 ohm speaker, the lowest cut off frequency is about 42 Hz. With 220uF, it's 84Hz.

_________________
Binh


Top
 Profile  
 
 Post subject: Re: LM386 Amp Question
PostPosted: Apr Mon 02, 2018 3:40 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 7984
Location: Beautiful Downtown Burbank CA
bb.odin wrote:
Mike, the value of the output cap dictates the lowest frequency passed to the speaker. With 470uF cap and 8 ohm speaker, the lowest cut off frequency is about 42 Hz. With 220uF, it's 84Hz.
So bigger IS better. Thank you. :lol:


Top
 Profile  
 
Post New Topic Post Reply  [ 10 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 5 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB