Hi All, I'm new to the forum, so please forgive me if this isn't posted correctly.
I'm a total electronics newbie, so you guys may laugh at my thought process here....
I have a 15w guitar practice amplifier, to which I would like to add a battery powered option, just so that I can annoy my neighbours outdoors too.
I have a few 18v power tool batteries from the same series, so the goal is to power it from one of these. The internal mains transformer has two outputs; one at 30v and one at 7v, with the 30v coil having a centre tap. I was planning to use a boost converter to create the 30v supply and a buck unit for the 7v (I've bought adjustable versions of both from Amazon, as they were cheap).
I'm working on the assumption that the existing rectifying circuit won't mind the dc from the buck/boost boards, but I've come to understand that the centre tap is being used as a 0v?
I'm sure there are other holes in my plan here, which I'm currently ignorant of, but my question is; how do I create the 0v supply, or is it even required as I'm not altering the existing electronics?
Grateful in advance for any advice/enlightenment offered.

 

Welcome to AAC!

First and foremost, do tell us the make and model of the amp.

 

You are better off using two 18V batteries for +/- and see if a 5 to 7V LDO is all you need at low power off +18 to run the low power tasks.

 

30VAC center-tapped produces +19VDC and -19VDC. The total DC voltage of 38V is probably reduced to about 33Vp-p with low distortion at the output of the amplifier producing 17W into an 8 ohms speaker.

 

So two 18V batteries sounds good to me since @AG verified it.

 

Before anything else you need to know the power supply voltages that the amplifier system uses, which will certainly NOT be the transformer voltages. Those will be the voltages measured on the filter capacitors with the amplifier working. When those voltages are known we can provide some advice on battery operation.

 

Before anything else you need to know the power supply voltages that the amplifier system uses, which will certainly NOT be the transformer voltages. Those will be the voltages measured on the filter capacitors with the amplifier working. When those voltages are known we can provide some advice on battery operation.

I think Audioguru's analysis is correct for no load and rated load, as well as my recommendations. But it doesn't hurt to verify and tolerate 10% variation at nominal Vac input.