DIY Universal Powerbank for 200W amps

[Victor Gwee]

So I previously converted my 15W acoustic amp to battery power here, but found that the volume was slightly lacking in open areas. Another minor problem was that I accidentally fried the speaker!

Seeking to upgrade my amp and create an efficient, portable power supply that I could power different amps, I created this:

Powerbank construction:
What it does is take 2 11.6V Li-Ion batteries and convert that into a split supply of ±12 to 35 V, with a max power of about 200W. This could power just about any split-rail amp!
The key component: DC-DC Boost Converters! I found these cheap on Lazada. It's rated for 150W but I'd derate this tiny thing to 100W. As advised online, I replaced the insulating mica with thermal paste. Adding extra electrolytic and ceramic capacitors reduced the output ripple.
Here's the schematic for the powerbank:

I used a 230V socket to connect the amp to the powerbank. This allows me to power other amps in future as well.

Amplifier conversion:
My amp is a 200W Lunchbox Acoustic. Measuring the voltage rails gives ±25V. I adjusted the voltage supply to ±20V for lower power dissipation at the cost of some headroom. I drilled a hole for the 230V cable and soldered it to the voltage rails.

I'll keep you guys posted on the results of this experiment!

 

[Victor Gwee]

I've tested my amp in the field! Here's my results:

Noise:
When powered on, there is a bassy pop from the amp. No audible hissing at all! I was surprised at how quiet this runs. Powering down gives a short crackle as well. Connecting a pedal using the integrated 9v output gives quite a lot of high-frequency hiss... But it's dead quiet when the ground lift is activated.

Power:
The Lunchbox is based on a bridged LM4780 into an 8 ohm load. At ±20V, this should give 70W RMS output with 40W dissipated as heat.

Performance
I've taken this outside to busk in 3 places. The volume has been very satisfactory but I ran into some EQ-ing problems.

First, I placed the amp on the floor near a pillar. It turned out to be very bassy and muffled and I had to do some extreme EQ-ing.

Second place I put it was near another pillar, on my IT-fair trolley. This turned out to give a well-balanced sound and many people stopped to listen and take videos!

Third place was on a trolley in an open-air setting. Somehow the EQ-ing seemed really off here. Wasn't satisfied after any amount of tweaking.

Conclusion
I'm extremely satisfied with the low noise from the powerbank-equipped amplifier. I'm not sure whether the extra capacitance on the rails resulted in extreme bass response. Still need to do more field testing and tweaking my amp placement and preamp settings to optimise the EQ-ing. Will keep you guys posted!

 

[Victor Gwee]

Field testing this week yielded 5h 20 min of runtime from 12.3V to 10.2V. Pretty good if I say so myself

 

[Whammy]

This is really impressive!

 

[Victor Gwee]

This is really impressive!

Thanks for the encouragement

I just realised that the DC-DC step up converters cut off at about 10.2V (because it runs on an 78L10 into a UC3843) so that prevents you from accidentally over-discharging your batteries This cut-off sees one of the rails go from 20V to 10.2V so the audio is cut off but your speakers don't go up in smoke with 20V DC running through them. Useful feature.