Help with audio amplification

You can probably get away with a series resistor and a shunt capacitor. This forms a low-pass filter. By tweaking the capacitor and resistor values, you can probably reduce the pop to an almost inaudible level without attenuating the voice too much.

You want your cutoff frequency to be around 5 kHz, I guess. That should let the voice through. You have a 10 k pot, so you want the R to be around

C = 1/(2 * pi * 5kHz * 100 Ohms) = 318 nF

So use the next lowest value, 220 nF. This will move the cutoff up from 5 kHz, but the exact value isn't critical, so this should be OK.

I am infamous for making arithmetic mistakes in this forum, so double check my arithmetic. Hopefully I got equation right. ;-)

Oh, since you say you don't really know what you are doing, I'll draw an ASCII art schematic (view with courier or other fixed-width font):

Heh. I just finished telling him to try a low-pass filter. Would that not work?

--Mac

So I have now gathered. For some reason I thought the pop was mostly high frequency.

Oh well.

Nice thing about usenet is that bad information will usually get corrected by followups.

--Mac

Probably you will have more success if you put a capacitor in series with the audio input, between the GPS and the pot. This would form a high-pass filter. You would still hear a click, but with less bass. You could adjust the cutoff frequency, fc=1/(6.28 * R * C) where R is roughly the 10k of your pot, and C is the capacitor you put in series. Try maybe 100nF or

Do you hear the click when the earphone is connected instead? If there is no click with the earphone, then you might have one of those capacitorless headphone amplifiers which are so trendy these days. They work by biasing the "ground" of the headphone jack up to some DC voltage so that the other terminals of the headphone socket can swing above or below the "ground" of the headphone socket, without the expense of a 5 cent capacitor. If it is one of these chips then you could use an audio isolation transformer. (should cost $1 or so as a component, or more with connectors already attached, but you might be able to salvage one from an old transistor radio or similar) You could also use a differential amplifier but that would be more work.

Chris

I read in sci.electronics.design that Mac wrote (in ) about 'Help with audio amplification', on Sat, 5 Nov 2005:

Not quite.

You have proposed a low-pass filter. Ideal for getting rid of the signal and leaving the pop.

He needs a high pass filter.

Having looked at the product description though, I wonder if the amp isn't simply defective. I'd expect the amp module to be already ac coupled.

Graham

A HIGH PASS filter might change the pop into a click. If you want to get rid of the click too , you'll need something more complicated, like a gated attenuator.

Mark

Terminate the GPS output with a resistor equal to the normal headphone load.

Thanks very much for all the replies. If nothing else I have a lot of ideas to keep me busy with!

It was interesting to see the dissent regarding high/low pass filters. Having a physics background, I had assumed that the click was essentially a square pulse reflecting the bias of the GPS output. Thinking of the corresponding Fourier series, I would have thought that blocking high frequencies would remove the sharp corner of the square pulse and round it out more.

Thanks once again,

Jim

Actually, that would turn a 'click' into a 'thump'. What you need to do is provide a leakage path from the amplifier side of the coupling capacitor, to drain off the DC bias from the cap. Or mute it completely, which is another project. ;-)

Good Luck! Rich