Thie Cave

Last Wednesday (16/4), I finally received all quotes from several music retailers regarding a PA package, particularly, YAMAHA EMX62M power mixer with a pair of AS108 speakers. The retailer that offers lowest price is Allans music direct for only $1095 (incl. GST). However, I was surpised the price offered by Allans Sydney branch is $100 more expensive. Ordering the PA direct from Internet only incurs freight cost of $25.

After making the order, I found out that they guaranteed for lower price within 30 days. That means if I can find any lower price, they are willing to refund the difference. *DOH* If I knew this earlier, I should have ordered it first and then searched for any lower price from other retailers.

The next item that I’m pursuing now is multicore cable. The best offer so far is from Pro Audio, 12 channels 25m for $350 plus $10 shipping. It is not exactly European/USA branded product but that is the quality that we could afford.

Keeping in mind that the PA package weighs at least 40kg, I cannot undermine the need for a medium-duty trolley. My option for now is to check prices in hardware shops. Hopefully, I can find trolley with load capacity of 60kg for under $20.

The final item that is nice and handy to have is a pair of speaker stands. Venue offers a good deal – $159 for a pair (church price). Obviously, this exceeds the budget but I am hoping we could stretch the budget a little bit.

The long awaited use of lavalier (lapel) microphone could finally go on air. This owes much to a recently built general-purpose stereo pre-amplifier, capable of supporting mini electret microphone. We needed this pre-amp for two reasons. Firstly, the mic gain is rather low and the pre-amp provides 10x gain (20dB). Secondly, the mic operates on an AA battery and the quality degrades with battery life. The pre-amp which is powered by an AC plugpack provides continuous DC voltage biasing to the mic, removing the need for battery.

We tested the pre-amp and lapel microphone during the Church Camp. It went well for the entire 4 talks. From this Sunday on, we no longer need to use the theatre’s lapel microphone.

I will prepare the pre-amp specifications some time in near future. This includes circuit schematic, wiring diagram, components arrangement on prototype PCB and casing design.

The FOCUS church retreat is approaching. Around the same time last year, few weeks before the church retreat, Felix, Charles and I tried to get some books from Koorong to sell during the retreat. On one weekday, we drove to Koorong and enquired about “opening Koorong store at our church”. It was easy apparently. We just need one person who has an account with Koorong and so can get books on credit. Charles had an account there, few hundred dollars. Then, we started collecting books from varous categories. Eventually, we had baskets of books and we made final selection and decides the quantity of each book. It was an extremely difficult task, especially to cater for over 180 people with such limited budget. I said to myself, “We need a machine to which I can specify all these books and then it will give me a list of books that I should get plus how many I should get for each book while satisfying my dollar budget.”

For many engineers and especially mathematicians, the problem we are facing is in fact a classic constrained optimization problem. In mathematics world, this is often called “Operation Research.” We try to come up with an optimal quantity selection of books that will maximize a certain measure of “usefulness” subject to a constraint that the total cost is no greater than the available credit (budget).

You may choose to continue reading for the formulation of the above optimization problem.
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