Audio Amplifier Stage
Addendum 10/13: You Tube Video of a Test Amp.
In our last posting we gave some advice about starting from the back end and working your way forward through the build. Not only does this chunk the project into manageable pieces but also enables test as you go. Thus the completed assemblies in effect become part of the test system. You may argue with that approach; but it is a sound practice. Somehow soldering all of the project parts to a circuit board and hoping it works is only asking for trouble!
We received an input in our last post about the LM386 audio amp IC and the issue of distortion when run at 200X gain. At this point it is uncertain that this excellent input, by the way, is based upon an individuals experience or just information floating around reflectors. But we did want to further explore the input and to give our response and take on the subject.
By design the Simpleceiver is just that --a project with minimum part count and easy to replicate. The audio amp stage is what I call "Good Enough" to get you started and is a totally viable circuit. That brings up the other aspect of the Simpleceiver and that is the circuit block module approach. Once the radio is built the homebrewer is encouraged to test new circuits and devices.
Thus the Simpleceiver IS an experimenter's platform. So if a builder finds the LM386 an objectionable device then by all means substitute your favorite circuit or one that has been approved by some reflector like the BITX or EMRFD groups. The LM386 can easily be changed to an LM380 ( 8 pin or 14 pin version) which of course can put out as much as 2+ watts. You can also try the TDA7052 -- like I said I smoked six of those in the recommended circuit. Or you can use just the pre-amp stage (2N3904) and feed an external audiophile style amplifier. The important thing to note is experiment and adjudge for your self.
Thus the Simpleceiver IS an experimenter's platform. So if a builder finds the LM386 an objectionable device then by all means substitute your favorite circuit or one that has been approved by some reflector like the BITX or EMRFD groups. The LM386 can easily be changed to an LM380 ( 8 pin or 14 pin version) which of course can put out as much as 2+ watts. You can also try the TDA7052 -- like I said I smoked six of those in the recommended circuit. Or you can use just the pre-amp stage (2N3904) and feed an external audiophile style amplifier. The important thing to note is experiment and adjudge for your self.
For those who would like to build a discrete version of an amplifier a circuit is presented below. It is the same pre-amp stage followed by a complementary amp stage (straight from the Internet). I do know that several hams have tried simulating the complementary circuit in LT Spice and were unsuccessful -- I am no help there --other than I have built and it works. While it has more Pout than the LM386 I would say it is on par with the LM386 insofar as perceived or real audio distortion. You will note --lots more parts and the need for an isolated output.
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| Discrete component audio amplifier stage
In the spirit of providing alternatives for the audio amplifier for those still skeptical about the use of the LM386 then we offer the following additional circuits. These have been built and used in several radios at N6QW. But I keep coming back to "simple and good enough".
The first uses an op-amp as the preamp and the device of choice is a low noise version of the NE5534 followed by the LM386 (Oh oh here we go again about distortion at high gain.). This audio amp circuit was used in my 2009 Tri-Band Solid State version of the Heathkit HW-100. You can see this as one of the links at http://www.n6qw.com/
The only reason this is being shown is to demonstrate how this circuit was later converted to use the LM380N. Important point again about experimentation and circuit improvement. In this case the real improvement with the LM380N -- 2+ watts Pout.
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The next schematic is the same circuit as used in the KWM-4 transceiver project in 2012 and the changes involved replacing the LM386 with the LM380N. Known for its greater output power it also has less distortion than the LM386. In passing take a look at the specification sheet for the LM380N and the "innards circuit schematic" is not unlike the discrete component amplifier shown at the beginning of this post. Hmmm there is a story here.
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Below is a photo of the LM380N amplifier as used in the KWM-4 transceiver project. There are island blocks in the center of the board and this is where the 10K audio gain pot connects. (I am just heading off any questions that there doesn't appear to be any connections to some of the blocks. Having a CNC mill sure makes it easy to crank out prototype boards like this. )
To recap the audio amp stage should be the first item built and get working! We have presented options and hopefully have addressed the input about distortion in the LM-386. My take it is good enough and if you find it objectionable then you have the option of even taking the 2N3904 pre-amp stage and fitting that to the LM380N. It is all about experimentation!
Please note that unless you solder the ground pins of the LM380 directly to the circuit board and have good contact with the body of the LM-380 to the copper board you will need to add a heat sink to the LM380. (Such as the case should you use a DIP socket.)
73's
Pete N6QW
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