Surf Pi modifications (clarification needed)

[bklein]

Why do you wish to use low impedance headphones vs higher impedance ones?
My Koss Pro4AA’s are pretty low impedance and stop the CTX WM10 from working. I have read several cases where low (how low?) impedance phones have damaged detector output circuits. If you go a low impedance audio route with added amplifier you increase battery drain and likelyhood of noise into detection circuits. Put an amp in the headset or inline. Try earbuds.

Oops, my Koss model is Pro-4A which is 8 ohms impedance. The Pro-4AA's are 250 ohms.
Earbuds are around 40 ohms.
So like said earlier, if series resistor there you don't blow anything.
I guess now a-days 8 ohm headphones are pretty rare....

[kt315]

much detectors use series connection of earbuds, not stereo. it gives 16ohms for 8+8.
ground/general wire is connected nowhere.

[bklein]

Yeah but the WM-10 wants >=100 ohms.
Pro-4A has to stay with old Fisher tube stereo where they sounded ok.
I sent them to Koss for a refurb but one channel is different audio than the other. Ebay time. Pro-4AA's would be ok though.

[Carl-NC]

ulix, most detectors use a pretty simplistic audio drive circuit similar to the Surf PI, with a transistor that basically switches the speaker across a supply voltage. Usually a series resistance is added to limit the current, and the value depends on what speaker is being driven. On detectors with both a built-in speaker and a headphone jack, the headphones often get an additional resistance so the volume doesn't blow out your ears.

It's not too hard to set all this up to drive both a modest impedance speaker and a piezo off the same circuit. I've done this, but don't recall the exact setup. It's one of those things you have to play around with to figure out.

BTW, a TO-92 can easily be used as a speaker drive. Besides the current limiting resistor, the chopper duty cycle can be used to limit current. In cases where a very low impedance headphone caused the detector to shut down, it may have exceeded the power supply drive and dragged it down. Or, it may be that the jack wiring wasn't compatible. Some detectors in the past have used funky headphone jacks that get dead-shorted by certain headphones.

KT, the condescending and disparaging responses are not the least bit helpful. If you can't be polite in your responses, then don't respond.

- Carl

[ulix]

Thanks Carl!

Now only a short question: In the schematics there is a diode and capacitor d1 and c5 drawn (audio stage)
They are marked for the Clicker Pro. Would it do any harm to add them to the Surf? The diode is only for security?