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Thread: Converting amp to battery power

  1. #1

    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

    - - - Updated - - -

    Hunting for a battery [Placeholder]

    - - - Updated - - -

    Putting it all together + Review [Placeholder]

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  3. #2

    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...)

    Li-ion
    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.

    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.

    Taobao
    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

    User Info Menu

    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

    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!

  6. #5

    Re: Converting amp to battery power

    So while awaiting the arrival of the batteries, I was wondering if I could turn on the amp with the batteries in series, turn it off, and charge the amp with the batteries in parallel — with just one switch.

    I came up with this circuit:

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    Position 1: Power On (Series)
    Neutral Position: Power Off
    Position 2: Charge (Parallel)

    Hopefully it works!

  7. #6

    User Info Menu

    Re: Converting amp to battery power

    Good job, stay innovative
    Your friendly neighbourhood cat

  8. #7

    Re: Converting amp to battery power

    Quote Originally Posted by zesn View Post
    Good job, stay innovative
    Thank you!

    Batteries have arrived, will update soon

  9. #8

    Re: Converting amp to battery power

    Putting it all together

    So once the batteries arrived, I took out my soldering iron and got to work!

    1) Solder the output wires and battery connectors to the DPDT switch
    2) Fit in the DPDT Switch
    3) Remove the two 2200uF capacitors (look at my first post) and solder the output wires accordingly.

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    4) Attach the batteries inside the amplifier housing.
    For this, I used hot glue and velcro tape.
    5) Connect the batteries

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    6) Flick the switch
    and hope nothing explodes
    ...
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    IT WORKS!

    Plugging a guitar in, it sounds just the same as it did before. Maybe a bit less clean headroom as the voltage is lower. But I'm really excited to see how taking this to the streets will work out

  10. #9

    Re: Converting amp to battery power

    Had my first field test last night, and the amplifier performed really well.

    We connected 2 wireless mics to the amp for a dinner event with 140 pax — and the sound could be heard clearly throughout the venue.

    Best of all, the battery only dropped from 12.2V to 12.0V after about two hours of use! Means that I will have plenty of runtime on the street

    Really satisfied with this $130 DIY amp!

  11. #10

    User Info Menu

    Re: Converting amp to battery power

    Quote Originally Posted by Victor Gwee View Post
    Had my first field test last night, and the amplifier performed really well.

    We connected 2 wireless mics to the amp for a dinner event with 140 pax — and the sound could be heard clearly throughout the venue.

    Best of all, the battery only dropped from 12.2V to 12.0V after about two hours of use! Means that I will have plenty of runtime on the street

    Really satisfied with this $130 DIY amp!
    awesome! keep it up
    Your friendly neighbourhood cat

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