How would I go about adding an audio amplifier stage to this? / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

How would I go about adding an audio amplifier stage to this?

Move Thread

LAN_403

Alex M

Sun Mar 10 2013, 03:12AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Hi, I just put together this basic AM radio circuit using an MK484 IC and was wondering what sort of amplifier stage I could add onto it, since its pretty quiet using this schematic.

Below is what I have tried already and whilst it is defiantly louder, it sounds pretty bad (LM386 sounds good with my phone as the audio source).

Its just a common emitter amplifier stage, do I need to keep the 47uF capacitor and 270R resistors in place or should they be bypassed (what do they even do?).

What would be a quick and easy solution for this sort of thing? It needs to be able to drive an 8ohm speaker.

Thanks.

Dr. Slack

Sun Mar 10 2013, 07:40AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

If you do indeed only have a 1.5v battery, this is not enough for the LM386 which needs a minimum of 4v.

The biassing for that transistor is a bit odd. You might be better off pin-strapping the LM386 for x200 gain, and ditching the transistor, while still making sure getting the 484 bias correct. If that doesn't give you enough gain, then you will need to reintroduce the transistor, but ask again before you bias it.

Sulaiman

Sun Mar 10 2013, 07:56AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I experimented with the ZN414 in the '70s
which was replaced by the MK484 and now TA7642

This ic is very sensitive to supply voltage and load resistance R3 (agc), I would start by;

use a single 1.5v battery as the supply,

connect the +1.5v to the junction of R1,R2,R3

dis-connect R2 (at either end)

dis-connect C4 from R4,TR1 base and connect it to your LM386 amplifier.

You should now get audio to your speaker but not very loud
Vary R3 (e.g. 330R, 470R, 68R, 1k) and listen to distortion at each value
lower values give lower distortion BUT lower volume too.
Tune different strength stations to check performance,
You may decide to put a 1k pot here to allow different agc for different strength stations.

Try adding a capacitor (0.1 to 1000 uF) across the battery.

P.S. I would use minimum 6v for an LM386, a rechargable 9v battery would be good
if you have the LM386N-4 you can use car battery or even 2x rechargable 9v in series for serious volume.
I've gone back from LM386 to LM380 (14-pin manhattan style circuit board)
because it seems 'tougher'

Sulaiman

Sun Mar 10 2013, 08:47AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Sorry for double post ... forgot how to insert a photo'

The LM380 section of my hf transceiver

most of the components are for filtering and bypassing 'noise' to/from the amplifier,
you probably will need half this many.
The centre six pins 'heatsink' the ic to the pcb copper.
P.S. many components from scrap pcbs so values/ratings may be overkill !

Alex M

Sun Mar 10 2013, 10:36AM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Thanks! Yeah I forgot to say that the LM386 has its own 9v battery, might use a low drop-out regulator eventually for the MK484 so it can all be powered by the single 9v battery.

@Sulaiman something like this?

Hope I have managed to follow your instructions correctly.

Hey that's a very neat looking board you have made

@Dr. Slack I am going to give the circuit Sulaiman suggested a try and see how that turns out.

Going on a bit of a tangent here, but I heard what could only be described as morse code being transmitted the other night. Only for about 10 seconds but they were very clear dots and dashes.

Sulaiman

Sun Mar 10 2013, 12:57PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Yes, the circuit looks good to me.

the 160m amateur radio band which is popular fore morse code starts at 1.8 MHz,
just past the 1.6 MHz top end of a.m. broadcast (a problem for amateurs)

Alex M

Thu Mar 14 2013, 05:27PM

Registered Member #3943 Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552

Sulaiman wrote ...

Yes, the circuit looks good to me.

the 160m amateur radio band which is popular fore morse code starts at 1.8 MHz,
just past the 1.6 MHz top end of a.m. broadcast (a problem for amateurs)

Thanks Sulaiman! I tried it and its definitely a lot clearer (slightly quieter but that's ok in this application). Although I can't really use it near my desk since the VDSL signal from the internet modem causes lots of odd noises

Just got one more question regarding the inputs on the LM386, does it matter if I use the + input or - input?

I am guessing the - input inverts the signal. If I am right when does the inverting one need to be used?

Thanks again.

Sulaiman

Thu Mar 14 2013, 07:19PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

yes, the - input inverts the audio signal,
I can't remember ever using both inputs of an LM380 or LM386
both ways sound the same to me
but there is a convention in audio engineering whereby
a positive sound pressure at a microphone should produce a positive pressure at the loudspeaker, so if there is an overall inversion of the signal due to other parts of the system then the other LM386 input could be used to maintain the standard,
but as the LM380/386 are definitely not HiFi (but good enough for my ears) it doesn't really matter.

I imagine that a balanced line (such as the transformer output from a distant microphone) could use + and - inputs simultaneously for reduction of hum pickup

I guess that in some equipment there is a possibility of feedback from output wiring to input wiring causing instabilities or oscillation, in which case inverting the power signal may help.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob