regen stage output level? - Page 1
Forum section: electronics / beginners

author: questron
author_rank: Regular Contributor
timestamp: April 11, 2016, 02:48:16 pm
content: i'm messing with a regen radio circuit now, 40m band. the audio power amp is an LM386, and there is a darlington pair AF stage before that. the volume is ok, but sometimes i wish it can deliver a little more power, especially for the amateur band.so i'm wondering if i build my own AF chain, what input signal level does it have to be for the LM386 to sound loud and also have some extra power in reserve. then i figure i'd need to build an AF stage to meet that requirement. and in order to do that i'll have to know the output level of the regen stage, which in turn will require me to know the usual signal level received from air.i have no equipments other than a multimeter. would some nice people give me some ball park numbers for:-1) what input signal level is needed for LM386 to be loud and with some reserve? (desktop listening, 3" 8ohm speaker.)2) what's the output level of a regen stage normally?3) what's normally the signal level received from air, weak but workable stations?just some rough ball park numbers from experience will do, so that i'd be able to figure out what AF stage gain i'll need. using the 200 gain configuration for LM486 is fine with me, but if i could get away with something lower than that, then so much the better.thank you guys!

author: danadak
author_rank: Super Contributor
timestamp: April 11, 2016, 02:59:12 pm
content: Here are some ap notes that should help -http://www.ti.com/product/LM386/technicaldocuments?keyMatch=lm386&tisearch=Search-EN-EverythingIf you need a scope, here is a PC based freebee that handles AC coupling only, but very usefull. Note in themanual you have to limit input to sound card on the order of a volt, but still application, with FFT and Sig Genmakes a very usable freebee instrument. Build an interface for the signal gen, a simple opamp buffer would do,to buffer sound card output, and an attenuator/clamp on sound card input and you should be good to go. Acouple diodes paralled shopuld work as a clamp.https://www.zeitnitz.eu/scope_enhttp://www.daqarta.com/dw_0all.htmhttp://homediyelectronics.com/projects/howtomakeafreesoundcardpcoscilloscope/?p=3http://www.build-electronic-circuits.com/sound-card-oscilloscope/Regards, Dana.

author: Audioguru
author_rank: Super Contributor
timestamp: April 11, 2016, 03:17:42 pm
content: An LM386 produces a puny little output power of only 1/2W if it has a 9V supply and an 8 ohm speaker. 1/2W is loud if you hold the speaker to your ear.10 times the power sounds twice as loud so maybe you need 50W.A regen circuit does not have automatic volume control (AVC) like all normal AM radios have.It sounds like you want more "sensitivity", not more power. The LM386 has a voltage gain of 20 times but its datasheet shows that its voltage gain is 200 times if a 10uF capacitor is connected between pin 1 and pin 8.

author: questron
author_rank: Regular Contributor
timestamp: April 11, 2016, 03:32:55 pm
content: wow? thank you danadak!great details, great links, just what i need.hope i won't burn my computer

author: questron
author_rank: Regular Contributor
timestamp: April 11, 2016, 04:13:25 pm
content: audioguru, very good point! haven't thought about it that way yet, learning.now, how do i get more sensitivity...sensitivity and gain...uhm... are they the same thing?no, no AGC, some strong stations are quiet loud, which is fine with me now. i'll deal with AGC later, lower priority relatively.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 11, 2016, 06:39:15 pm
content: Quote from: Audioguru on April 11, 2016, 03:17:42 pmFYI, I've heard that the LM386 is a lot more well behaved if you leave the gain at 20. Probably ideal to make up the gain with a jellybean opamp before it.

author: Audioguru
author_rank: Super Contributor
timestamp: April 12, 2016, 12:28:20 am
content: Quote from: mmagin on April 11, 2016, 06:39:15 pmOf course it will oscillate if you have an input wire beside an output wire and the capacitance between the wires feds some output into the input.But ANY amplifier with negative feedback will oscillate if it has too much negative feedback. The datasheet for the LM386 says its minimum gain limit is 9 (if the pcb layout is perfect).

author: vk6zgo
author_rank: Super Contributor
timestamp: April 12, 2016, 06:47:10 am
content: Quote from: Audioguru on April 11, 2016, 03:17:42 pmYou must use incredibly insensitive speakers!Lots of old style transistor radios make plenty of noise with that output power,or significantly less.

author: mmagin
author_rank: Frequent Contributor
timestamp: April 12, 2016, 09:38:42 pm
content: Quote from: Audioguru on April 12, 2016, 12:28:20 amI wasn't merely talking about oscillation, I was given the impression that the distortion was a lot worse at the higher gain level due to various imperfections of the LM386 design.