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Another approach to amplifying the auxiliary input on R1200RT radio


John Bentall

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John Bentall

Some time ago Rob Syme (username R_Syme) was kind enough to enough to post his satisfyingly successful experiences of amplifying the auxiliary input using a Canakit UK-154 2x 5Watt amplifier. The goal was that when switching sources from radio to auxiliary it was not necessary to dramatically reset the handlebar volume control.

When I showed the kit to a friend in the electronics industry, he maintained that a power current amplifier was an over-kill solution when all that all was needed was a voltage gain. It would require a substantially smaller and simpler circuit generating a lot less heat. We chose a Velleman K2572 Stereo pre-amplifier as a basis (available for $10 through Maplins outlets in the UK), designed to give a voltage gain of 100x. The remainder of the design is based on Rob’s solution – modest power being taken from the rear socket on the CANBUS. The K2572 is a simple kit that has to be soldered together, but that part was no more difficult that attaching the plugs and sockets.

All was not sweetness and light on completion. The volume was much, much louder but not quite loud enough and, even worse, was distorted. Back to see my friendly guru, then. We measured the maximum voltage of the MP3-compatible external CD player at about .25 volts - peaking at .5 volts. Applying a signal generator to the AUX input indicated that we would need the voltage up to 6V with a peak near the 12V maximum capable of being delivered by the bikes battery. We changed the 10K resistors for 47K and removed and bridged the potentiometers (which can be unreliable). This reduced the voltage gain to a more modest 20x and effectively elimiated the audible distortion, though the auxiliary input on the bike does clip at above 6V.

I still have to adjust the volume upwards when switching to Auxiliary, but not by much. The volume and the quality are now perfectly adequate for my untrained ears using “Arizona Al” style monitor earplugs.

My solution sounds like a lot of fuss, but I knew that if I followed by friends hunches, then he would bail me out if it went wrong. So I am another satisfied, but still electronically ignorant, customer.

No pics available before you ask - but my only mod to Rob's design is the PCB - and that is pretty boring to look at.

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Sounds like you found a good alternate solution. I'm using Rob's and it has been working great. My only problem putting everything together was soldering those micro-tiny-tiny wires on to the stereo connectors. Once I got that done, all was sweet. Still, I have ~$100 wrapped up in the solution and that includes $99 worth of material and 20 hours of labor at $.05 per hour!!

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