On Mon, Feb 17, 2025 at 01:03 PM, Howard wrote:
I completed the ABS build and everything is working well except for the speaker.? I added a 750 Ohm resister between pins 2 and 3 of the LM386.? The resistor had little effect in curing the awful audio coming from the speaker.? The receive and sidetone sounds are excellent when using headphones.? Also, when attempting to use the speaker, I have to turn the volume up many turns to hear sound through the speaker.
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Howard N8HO
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Howard,
I also initially had problems with the speaker, but now it sounds quite nice.
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Did you do the 12V speaker mod that I suggested in my build notes?? That helped mine dramatically by getting enough voltage to the speaker so it would have a good volume.?
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Note also that if the speaker is touching any of the other components, or is attached directly to the circuit card, it will vibrate against them and create a very annoying sound.? You need to make sure your speaker is mounted with two or three layers of foam tape, or suspended from the circuit card using stiff wires for the connections.
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The ABS design already has a 750 ohm resistor between pins 2 and 3 of the LM386 - did you add another one in parallel?? I think that would make things worse.? The two resistors R4 (7.5K) and R6 (750) form a voltage divider of the sound source going into the LM386, giving about 1/10th of the voltage to the LM386, so it doesn't overdrive from the QMX+ audio signal (normal output from the QMX+ at comfortable headphone audio levels is too high for the LM396, and causes distortion, so this voltage divider was put into the circuit).? If your QMX+ audio signal is already way too low, you would want to increase the value of that 750 resistor, or remove it entirely.? But then I would ask why are we getting such a low level from your QMX+, if it sounds so good in the headphones - doesn't quite add up, so I would be looking for an issue.
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Cal and I did have very low volume in the ABS speaker using the 5V supply that is on the card as delivered, and when we turned it up to a nice listening volume, it was very distorted.? Using the 12V supply voltage to the LM386 fixed this, by allowing the LM386 to have a much larger output voltage swing without distorting.? It now only starts distorting if we turn the volume up higher than about 86 (using the 'absolute' setting on the sidetone level).
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If you are already using the 12V mod, then something else is wrong.? The following measurement method may help in diagnosis, either way, but requires a simple oscilloscope:
1) turn the QMX+ volume down to a low level (we don't want the receive signal/noise interfering with this test)
2) set the 'sidetone abs/rel' setting to 'absolute' (this gives us a level with an easily visible setting)
3) key and listen to the sidetone in the head phones and adjust the sidetone level to a comfortable level (not weak, but not blasting loud) - note this level - you may want to do this with the QMX+ in 'practice' mode so you are not transmitting, we just want a controlled audio level.? Make the following measurements while keying the sidetone (set the cw speed to 0 so you get a constant tone):
4) measure the peak-to-peak voltage level at the LM386 input (pin 3) - it should be a clean 700Hz sine wave from the sidetone
5) measure the peak-to-peak voltage level at the LM386 output (pin 5) - it should be much larger, but still a clean sine wave (the LM386 has a gain of 20).? If the sine wave is getting clipped (flat-topped), note that, too.
6) now do both the LM386 input and output voltage measurements again, while adjusting the absolute 'sidetone level'.? Note the sidetone level setting and the peak-to-peak voltage at which the input sinewave starts to clip or distort, and do the same measurements for the output.
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With those measurements, we will be able to see how the circuit is operating, and compare it to values I measured on mine, and hopefully pinpoint the source of the problem.
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Stan KC7XE
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