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I'm building a kit for the LNR precision mountaintopper 3 band qrp radio. I'm having problems wiring a panel mounted volume knob and speaker to hear the cw output. I'm new to home brew electronics and wanted some advice. My questions are as follows:

  1. When testing my speaker and volume knob(with music from my computer via audio cable), I the volume is too quiet. The potentiometer seems to only give me audio at the far end of its range, and still it's very quiet. What could be causing this? Wrong pot? Wrong speaker? How do I match a speaker and pot so they work well together and give good volume control?

  2. The radio has a spec that says "headphone output 16ohm min". How do I know what speaker and pot combination would get the best result with this specification for good volume control?

  3. I've got 3 ring audio cable that I'm wiring to my small speaker. Do the left and right rings of the cable get wired to the 2 prongs on the speaker?

Please excuse my ignorance, I'm just trying to figure things out. Thanks for any help you can provide!

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    $\begingroup$ To help answer the first part, please edit your question to provide a schematic and/or photograph showing exactly how you've wired up the potentiometer to the speaker, and also what type ("taper") it is. There are several different possible wirings which will give different partly-functional results. $\endgroup$
    – Kevin Reid AG6YO ♦
    Apr 19, 2021 at 2:34
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    $\begingroup$ @Skaiser I came across this n5dux.com/ham/files/pdf/…. Maybe you can confirm if this is the same circuit? At any rate, the circuit I've referenced uses an LM4808 headphone amp (105mW max per channel into a 16 ohm load) and it's likely yours is similar. I'm going to guess your panel speaker is 8 or 4 ohms, which is a much heavier load for your headphone amp. You may need additional amplification to drive your panel speakers (or re-work the audio stage if you're adept). $\endgroup$
    – Buck8pe
    Apr 19, 2021 at 6:43
  • $\begingroup$ Hello and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3 ♦
    Apr 19, 2021 at 15:47

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When testing my speaker and volume knob(with music from my computer via audio cable), I the volume is too quiet.

The line out on your computer is designed to drive the line in on some other equipment, not a speaker. The input impedance on a line input is typically around 10k ohms, and at maximum volume the voltage is around -10 dBV or 316 mV.

This is simply not enough power to be very audible. Furthermore a speaker's impedance is much less than 10k ohms, so you're operating the equipment far out of spec. Fortunately most audio outputs are designed to at least not be damaged even if you connect them to a short.

The radio has a spec that says "headphone output 16ohm min". How do I know what speaker and pot combination would get the best result with this specification for good volume control?

Well, if it says headphone output then you probably won't get very good results with a speaker. Headphones:

  • do not require nearly as much power, since they are right on your ears, and
  • they typically have a higher impedance, some hundreds of ohms, in contrast to speakers which are often less than 10.

I've got 3 ring audio cable that I'm wiring to my small speaker. Do the left and right rings of the cable get wired to the 2 prongs on the speaker?

No. A line output is not designed to be connected to a speaker at all. Instead it should be connected to an amplifier with a high input impedance. The amplifier in turn drives the speakers.

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  • $\begingroup$ @GrahamNye thanks for correction $\endgroup$
    – Phil Frost - W8II
    Apr 20, 2021 at 23:08

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