Just bought the Elenco kit and some old test equipment - these videos have inspired me to give the kit a try. Thanks for your thorough teaching presented here!
I am completely new to electronics work and test equipment, and picked up this project kit last weekend. These videos have been a godsend in acting as measurement examples that the manual doesn’t have. Thank you for these.
Thank you so much for doing these videos. I bought and started this kit sometime back, and had to table the project. I may get a chance to pick it back up soon, and was thrilled to discover your video series. Also very happy to see such competency, knowledge, and skill. Time to learn! Thank you.
The analysis of the LM386 internals was great! I don't recall seeing that anywhere else. Two things: when measuring bandwidth with a digital scope, make sure to increase the sweep speed as you increase the input frequency so the digital filters know what you're trying to look at. Otherwise you might be measuring the frequency response of the scope! Also, on the low end you may want to be DC coupled, otherwise once again it's the scope input that you're measuring. I'm glad to see you making videos again!
If you really don't want to touch the dc bias of the input differential amplifier you should also don't short the 50 kOhm resistor at the inverting input of the LM386 by a short between this input and the ground. You should use an electrolytic capacitor (I think 10 to 22 µF is ok to get a small enough ac impedance compared to 50 kOhm at 50 to 100 Hz) between the inverting input and ground so only the ac path goes to ground.
Thanks for the video! Dumb question: in this circuit, the volume pot is attenuating the input signal to the LM386 audio amp, which has fixed gain set by R44. Is that preferable to having a fixed input signal to the LM386 and using a pot to vary the gain of the amplifier? Or is either approach fine?
Changing the gain of an amplifier has an effect on its bandwidth. Putting the signal thru the pot and tapping that instead of varying the amplifiers gain presents a constant load to the detector stage and its I-F amplifier and would not have any practical bandwidth limiting effects. That said, in the earlier RF circuit there is an AGC control which controls the gain of RF chain so that the signal out of the detector stage remains relatively constant even if the RF signal strength varies up and down.
There are a number of complications with varying the gain that make it more difficult to control the volume in that way. The biggest risk is that the gain is controlled through negative feedback which in the presence of capacitance (both designed and parasitic) will introduce delays (phase shifts) that could cause instability. The analysis of the phase margin of an amplifier feedback is somewhat involved and it is simpler to do this analysis about a fixed gain. Attenuating the input signal ultimately is much simpler to design as by design the input impedance (resistance) of the amplifier is relatively high and can be viewed from an analysis of the volume control as an open circuit.
I don't like the LM386 that much, because it's modification possibilities are not very useful. The bypass pin 7 is nonsense. Instead of bringing this connection out of the housing it would be better to cut the positive end of the current source of the output driver from the positive power supply and bring it out of the housing in order to enable bootstrapping to get the same headroom for the positive half wave as for the negative one.
Just bought the Elenco kit and some old test equipment - these videos have inspired me to give the kit a try. Thanks for your thorough teaching presented here!
I am completely new to electronics work and test equipment, and picked up this project kit last weekend.
These videos have been a godsend in acting as measurement examples that the manual doesn’t have. Thank you for these.
Great to hear!
Thank you so much for doing these videos. I bought and started this kit sometime back, and had to table the project. I may get a chance to pick it back up soon, and was thrilled to discover your video series. Also very happy to see such competency, knowledge, and skill. Time to learn! Thank you.
Great video, I should have my Elenco radio kit any day. Looking forward to the rest of the videos.
Thanks for these videos, I've just come across them; great descriptions throughout
Glad you like them!
The analysis of the LM386 internals was great! I don't recall seeing that anywhere else.
Two things: when measuring bandwidth with a digital scope, make sure to increase the sweep speed as you increase the input frequency so the digital filters know what you're trying to look at. Otherwise you might be measuring the frequency response of the scope! Also, on the low end you may want to be DC coupled, otherwise once again it's the scope input that you're measuring.
I'm glad to see you making videos again!
Thanks for the feedback - I agree with you - need to keep an eye on the sweep.
Great video - thanks!
If you really don't want to touch the dc bias of the input differential amplifier you should also don't short the 50 kOhm resistor at the inverting input of the LM386 by a short between this input and the ground. You should use an electrolytic capacitor (I think 10 to 22 µF is ok to get a small enough ac impedance compared to 50 kOhm at 50 to 100 Hz) between the inverting input and ground so only the ac path goes to ground.
Thanks for the video! Dumb question: in this circuit, the volume pot is attenuating the input signal to the LM386 audio amp, which has fixed gain set by R44. Is that preferable to having a fixed input signal to the LM386 and using a pot to vary the gain of the amplifier? Or is either approach fine?
Changing the gain of an amplifier has an effect on its bandwidth. Putting the signal thru the pot and tapping that instead of varying the amplifiers gain presents a constant load to the detector stage and its I-F amplifier and would not have any practical bandwidth limiting effects. That said, in the earlier RF circuit there is an AGC control which controls the gain of RF chain so that the signal out of the detector stage remains relatively constant even if the RF signal strength varies up and down.
There are a number of complications with varying the gain that make it more difficult to control the volume in that way. The biggest risk is that the gain is controlled through negative feedback which in the presence of capacitance (both designed and parasitic) will introduce delays (phase shifts) that could cause instability. The analysis of the phase margin of an amplifier feedback is somewhat involved and it is simpler to do this analysis about a fixed gain. Attenuating the input signal ultimately is much simpler to design as by design the input impedance (resistance) of the amplifier is relatively high and can be viewed from an analysis of the volume control as an open circuit.
Where do you talk about the headphone dummy load? All I can find online are people building large dummy load boxes, but nothing simple like this.
I like to videos I wish I can be like u
Nah, be yourself. Thanks for watching!
7:49 to 18:17 LM386 + gain calc
GOOD PRESENTATION BUT, A LITTLE CONFUSED ON THE WAY YOU CAME UP WITH THE GAIN CALCULATION.
I don't like the LM386 that much, because it's modification possibilities are not very useful. The bypass pin 7 is nonsense. Instead of bringing this connection out of the housing it would be better to cut the positive end of the current source of the output driver from the positive power supply and bring it out of the housing in order to enable bootstrapping to get the same headroom for the positive half wave as for the negative one.