Building a SLA power supply

[TheSloth]

I've decided to build a SLA power supply for my HeadRoom amp.

SLA is an excellent candidate due to the natural voltage of 12V, which with two SLA's can easly be wired for +/-12V, which can be run without regulation into the amp.

I'm planning on using two of these: http://www.batterymart.com/battery.mv?p=SLA-12V10-F2.

Components I will need:

- An enclosure big enough to fit the two batteries and associated wiring.
- One chassis mount DC entry connector for the SLA charger.
- 2 2-way chassis mount switches.
- 2 SLA 12V 10Ah batteries.
- 1 12V SLA charger.

Notes: The two 2 way switches will have one side each wired in parallel to the DC input jack, allowing both batteries to be charged at 12V from a single charger. The two batteries will be wired with the opposite polarity to generate the + and - voltages, and the return leads will be wired as one (?).

I don't know how to draw a circuit diagram on my computer, but from the intended function I would hope that the circuit I have in mind is clear.

Have I missed something obvious here? Any suggestions for the enclosure and swithces? It seems like a simple solution for a very high quality power supply.

 

[Rescue Toaster]

You CAN charge batteries in paralell but it's generally not preferrable.

I'm not familiar with the schematic for the headroom, does it have Class A biasing or anything else that would cause one battery to drain faster than the other? If so, you should consider charging the two batteries separately. If it's really just a couple opamps, the current should be so close that it'll probably be OK to charge in parallel.

It's easy to charge them both. I had a schematic & PCB layout at some point for a dual charger where each charger floated (dual secondary transformer, no common ground) so they could charge series batteries in place without lots of relays/switches. IIRC it used the L200 chip.

 

[TheSloth]

Quote:

Originally Posted by Rescue Toaster You CAN charge batteries in paralell but it's generally not preferrable. I'm not familiar with the schematic for the headroom, does it have Class A biasing or anything else that would cause one battery to drain faster than the other? If so, you should consider charging the two batteries separately. If it's really just a couple opamps, the current should be so close that it'll probably be OK to charge in parallel. It's easy to charge them both. I had a schematic & PCB layout at some point for a dual charger where each charger floated (dual secondary transformer, no common ground) so they could charge series batteries in place without lots of relays/switches. IIRC it used the L200 chip.

I thought that the current draw on the + and - rails was about even, but you are suggesting that class A biasing would likely draw from one and not the other? Because in that case there would be an imbalance and two chargers would probably be preferable. I was just trying to keep the cost down by doing it in parallel.

 

[Rescue Toaster]

I think you'll find that with a dual-secondary transformer, a second bridge diode, two (maybe three) caps, and the L200 chip will only add like $15 to the cost of the charger really. And it'll keep both batteries happy for years even with class-A biasing and such going on.

I used the L200 design from vt52.com: http://www.vt52.com/diy/myprojects/o...er/charger.htm (scroll to bottom). That 1N4001 diode can be omitted if you use some relays to only connect the charger to the load when the charger has power. (Which I recommend.)

 

[TheSloth]

Quote:

Originally Posted by Rescue Toaster I think you'll find that with a dual-secondary transformer, a second bridge diode, two (maybe three) caps, and the L200 chip will only add like $15 to the cost of the charger really. And it'll keep both batteries happy for years even with class-A biasing and such going on. I used the L200 design from vt52.com: http://www.vt52.com/diy/myprojects/o...er/charger.htm (scroll to bottom). That 1N4001 diode can be omitted if you use some relays to only connect the charger to the load when the charger has power. (Which I recommend.)

The problem with that is it goes beyond my abilities. I can wire things together, but that's about it. That's why I liked the idea of this project, because it could be made out of preassembled parts, with my job simply being to put them together in the right order!

 

[Rescue Toaster]

Do you have one of the "Home" or "Max" Headroom amps? They're the only ones with class A biasing afaik. For the rest of thier amps, what you propose should work. My recommendation though, when the amp is off, even if not charging, have the batteries wired in parallel as if you were charging.

 

[mono]

Preface: Don't take what I write below to mean it wouldn't work or be "bad" per se, only my thoughts on why I wouldn't do it.

Quote:

Originally Posted by TheSloth I've decided to build a SLA power supply for my HeadRoom amp. SLA is an excellent candidate due to the natural voltage of 12V, which with two SLA's can easly be wired for +/-12V, which can be run without regulation into the amp.

SLA is a poor choice because SLA has the least recharge cycles of most modern battery technologies like Li-Ion, NiMH or NiCad. It's core benefit is high amperage which you don't need for this amp. The detractions include outgassing (per type), size, weight, and cost for a charger that (given same cost and minimal addt'l complexity) could just be the entire supply instead, without, the SLA cells. This assumes you don't need to take the amp on a camping trip where days of runtime away from a wall outlet matter.

Quote:

I'm planning on using two of these: http://www.batterymart.com/battery.mv?p=SLA-12V10-F2.

This is going to weigh, probably over 20 lbs. when all is said and done, and take up a lot of space.

Quote:

Components I will need: - An enclosure big enough to fit the two batteries and associated wiring. - One chassis mount DC entry connector for the SLA charger. - 2 2-way chassis mount switches. - 2 SLA 12V 10Ah batteries. - 1 12V SLA charger.

Ideally you'd have a 24V SLA charger, as you don't want to charge in parallel, any minimal differences in battery voltage will be a large current surge from one cell to the other when they're paralleled for charging, unless there is isolation which is a further voltage loss from the charger's >12V float voltage. It could be considered into the design but IMO, an unnecessary thing to deal with instead of > 24V charger.

Quote:

It seems like a simple solution for a very high quality power supply.

Not really, there's no benefit to SLA for a headamp. SLA is a high current, low frequency supply. It's the opposite of what's optimal for a headamp. Granted, putting a capacitor on the output, or those in the amp, will offset this, but a basic RC filter on any non-SLA would do it as well without the inherant cost, size, weight, etc.

So it could work, but there isn't any point to it. Remember that we are insane audiophiles, if SLA was the answer we'd all be doing it for headamps without a second thought.

 

[sheya]

It wouldn't cost you much to try it without the case or the switches, just the batteries wired together, some lamp cord and a plug. I bought four 12 volt sla's to power a gainclone that I haven't finished, and I've been able to use them for all sorts of things. Right now one of them is powering my PINT. I haven't gotten around to getting rechargeables for the PINT, and it sounds great with the sla, maybe better than with alkalines.

I tried it with two 12 volt sla's wired together, +12/-12, and I think it sounded better, but the opamps (8397) were getting very hot so I went down to 12 volts. When I get some rechargeables, I'll try the 24 volt SLA supply out again to charge the 9 volts, and to run the amp, since the charging might draw enough to lower the voltage a little and not overheat the opamps.

I don't see how trying out a couple of SLA's could hurt. As long as you don't mind spending a little money to experiment.

-Aaron.