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LM380 in positive ground circuit???

wb0ldj

Nov 27, 2021
2
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Nov 27, 2021
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I have a Racal receiver that was designed with a 1 mW line output and a 10 mW headphone output. I'd like to add an LM380 stage to provide a speaker-level output. My big issue is that the receiver uses positive ground circuitry throughout. The entire receiver uses Vcc of -16V, and all capacitors have their + lead connected to common ground.

My question is -- can I use an LM-380 as an audio amp in this receiver by connecting -16V to the ground point on the PCB and grounding the + terminal? I'm not familiar enough with the LM380 internals to know whether this would work. I guess the big issue would be in interfacing the LM380 amp module to the existing signal lines coming from the 10 mW amp stage.

Help!
Thanks,
Mike, WB0LDJ
 

crutschow

May 7, 2021
851
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May 7, 2021
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Below is a typical circuit LM380 circuit diagram.
For your application just connect the Vs connection to ground, and the ground connections to -16V except the bypass capacitors and speaker ground.
Reverse the polarity of the 500µF output capacitor (+ towards speaker)
Also add a 0.1µF ceramic capacitor in series with the input 75k resistor.
The signal input ground will be your system ground.
upload_2021-11-26_21-15-23.png
 

wb0ldj

Nov 27, 2021
2
Joined
Nov 27, 2021
Messages
2
Thanks Crutschow! That will sure make my job a LOT easier!

I was kinda thinking that approach would work, but it sure was nice to hear someone else's ideas on the subject! This receiver is a very strange animal indeed. They were almost all made tor those assorted TLA (three-letter-acronym) government agencies. I think mine came from either the FCC or the CIA. Coverage is AM/FM/CW/MCW/LSB/USB from 980 kHz to 30 MHz continuous tuning!

Thanks again!
Mike Harmon, WB0LDJ
 
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