Converting amp to battery power

[Victor Gwee]

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



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...

Hopefully I don't wreck my amplifier! Here goes nothing

- - - Updated - - -

Hunting for a battery [Placeholder]

- - - Updated - - -

Putting it all together + Review [Placeholder]

 

[Victor Gwee]

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

 

[mechanevil]

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.

 

[Victor Gwee]

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!

 

[Victor Gwee]

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:



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

Hopefully it works!

 

[zesn]

Good job, stay innovative

 

[Victor Gwee]

Good job, stay innovative

Thank you!

Batteries have arrived, will update soon

 

[Victor Gwee]

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.



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



6) Flick the switch
and hope nothing explodes
...


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

 

[Victor Gwee]

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!

 

[zesn]

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

 

[Victor Gwee]

awesome! keep it up

Thanks! I've managed to make a circuit to measure the charging voltage and current, as well as have an output to power my EQ and Compressor pedals. Will update with pics soon!

I have also started an Instagram account for my music! Follow me at @busking_gweetarist

 

[Victor Gwee]

The Circuit

View attachment 29205

I decided to use diodes to drop the voltage as I couldn't get my hands on a LM7809, and I was afraid the dropout voltage would be too high (11.5V). I used S2 to activate a ground lift (since ground will be connected via the signal cables).

This is what happens when the amp is connected and these switches are on:

None: +9V out with ground lift
S1: Battery voltage is shown, +9v out with ground lift
S2: +9V out without ground lift

When charging
None: Charger voltage shown
S1: Charging voltage and current is monitored

Putting it together

The hardest part was fitting the circuit inside a suitable enclosure!



And that's a wrap let me know what you think!

 

[Victor Gwee]

Field test 2:

Finally got my busking license and went busking 6 times for a total of 10 hours this week! Sometimes I had to turn the volume up until the amp was distorting slightly as the background noise was too much. But after a week of use, and the battery voltage only dropped from 12.3V to 11.4V! I think that's from 90% to 40% charge. This is way more battery life than I was expecting

Definitely would recommend this setup to other buskers or those who need to play on the move. Some electronics knowledge can save you a lot