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Thread: Converting amp to battery power

  1. #1

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    Converting amp to battery power

    This started because I decided that I would want to try busking! But I realised I only have an amp that plugs in to the mains... And all busking amps need to be battery-powered.

    So I decided to put my sec-school physics to the test and disassemble my 15W acoustic amp

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    After opening up my amp and removing the transformer, I saw that it gave an output of 14VAC*2 at 0.7A. This means this put out 28VAC with a centre tap (black lead), and I could get decent battery life out of it (pretty low current draw).

    I found a 4-legged chip on the circuit board directly connected to the transformer. My suspicion was confirmed when I googled the part number — it was a bridge rectifier! There were also two large capacitors nearby rated for 25v each. Using a multimeter to verify the connections, I deduced it was a split-rail 13v DC supply (accounting for voltage drop across diodes). That's tricky... I'd need at least two 12v batteries.

    Another important component was the power amp — the IC that directly drives the speaker. Wasn't hard to find... The LM1875T was attached directly to the case. Googling the datasheet, I found that this operates on a split-rail power supply as well, confirming my deduction.

    So this was what I came up with... Name:  1500213432338-793010271.jpg
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    Hopefully I don't wreck my amplifier! Here goes nothing

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    Hunting for a battery [Placeholder]

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    Putting it all together + Review [Placeholder]

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  3. #2

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    Re: Converting amp to battery power

    Hunting for a battery

    What type of battery?

    I looked at two types of batteries:

    Lead Acid (Deep Cycle)
    Pros: Cheaper, High Peak Current
    Cons: Heavy (Not really what I want for busking...)

    Pros: Higher Capacity, Lightweight, Maintenance Free
    Cons: Expensive, fire/explosion hazard (if mishandled)

    Decision: Li-Ion

    Purchasing the Battery
    As mentioned, I needed 2 12v batteries. The amp draws 0.7A maximum, so I wanted about a 7Ah (7000mAh) battery for decent runtime. I know that some ebikes use these batteries, so I first looked at Carousell.

    Wasn't keen on buying used batteries as they could have deteriorated. One listing caught my eye — a 9800mAh battery for $35! This looked like an imported China battery though, so I went to Taobao to check it out.

    True enough, it was selling for $20 (without shipping). Curiously though, the listed weight was 350g. Doing some math, 9.8Ah×11.6V/0.35kg = 325 Wh/kg! But the energy density for consumer Li-ion is around 100-265Wh/kg... I smelt a rat. Further research showed that dishonest practices like overstating capacity, re-packaging used cells etc. were common in the Chinese Li-ion industry . Buyer beware!

    Browsing around, I saw a listing for a 7800mAh battery at $24. This was 510g, so the energy density was a far more believable 177Wh/kg. A protection chip was also included, which ameliorated fears of my busking performance becoming an impromptu fireworks display. 1.2k positive reviews couldn't be wrong either.

    Purchasing them through ezbuy, the 2 batteries cost about $55 including shipping. Not too bad!

    Stay tuned for the next update on how I put them all together

  4. #3

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    Re: Converting amp to battery power

    Very interesting! I look forward to seeing what you come up with. Just be careful playing around with all that stuff, wouldn't want to destroy your amp and get a nasty shock in the bargain.

  5. #4

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    Re: Converting amp to battery power

    Thanks for the encouragement and caution My dad taught me these things when I was younger, or else I wouldn't have the confidence to even open up the amp!

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