Transformer for 15v bipolar supply

[kvandekrol]

I'm building a preamp that needs a 15v bipolar supply. The catch is that I have never built anything that required a transformer; all of my electronic adventures have been solidly 9 volts with a wall-wart. So I'm looking for some advice!

The preamp will be in a 1U rack case, so the height of the transformer is important.

I will probably be using this as the project for the power supply:
http://www.paia.com/proddetail.asp?prod=K83&cat=50

I did some poking around and found this transformer, which looks like it'll do what I need, for a very reasonable price:

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/164G20/?qs=sGAEpiMZZMtz8P%2feuiupSThvwaRcnmu81WhoAji7k8o%3d

I also found the ST-5-20 on this page, which is very similar in specs:

http://www.signaltransformer.com/content/white-goods-splittran-low-power-transformer

Is one of these what I need, or is there something better I should use? I don't think the preamp will pull more than about 200 mA based on the components, but I went with a 600mA supply just to be safe. They are both PCB-mount, but I figured I could just use a piece of perfboard for that.

[DJPhil]

These are roughly what you're looking for, I believe. I do have some other considerations to add though.

Smaller transformers like this have very poor regulation, meaning that they'll likely only hit their rated voltage very near their max load. For lighter loads the voltage will stray higher. You can calculate the specifics, but often it's worth simply building a mock up so you can test everything at once (like operating temp, etc.).

A 20VCT transformer will give you a 14.14VDC bipolar supply. Given the voltage variability from using a small transformer and the fact that you'll likely need solid rails for a preamp I'd recommend post regulation. This means you'll need to account for the dropout voltage of whatever regulator you use. The 7815 and 7915 require about 2.5V to be comfortable, so you'd need roughly a 25VCT transformer (25 / 2 * 1.414 = 17.675VDC). Complicating this is the poor regulation of small transformers mentioned above, which may give you some breathing room with lighter loads.

Finally, be very careful with mains over pcb traces. I'd recommend not allowing anything near mains traces closer than the distance across the transformer. In such a small case you'll probably need to isolate the area under the mains traces with plastic card or the like to ensure it can't arc to the chassis.

If all else fails you can try to find a brick style power adapter that's already taken all this into account. You can sometimes salvage them from inkjet printers, though they're often marked as +16V/+32V.

Hope that helps some.

[J M Fahey]

Make it easy.
You can just use the 24V CT 6VA (or 12VA if it fits) in:
http://www.signaltransformer.com/content/white-goods-splittran-low-power-transformer
plus a W02 bridge rectifier , 4 x 2200x25V electrolytics and 2 100 ohm, 1/2W resistors..
Transformer -> bridge -> 2 x 2200uF (you get raw +/- 16V DC) and filter each rail with a 100 ohm resistor in series and a 2200uF to ground.
This will power any Op Amp based preamp.

[kvandekrol]

JM, could you draw up a diagram or point me in the direction of something that would help me visualize that? Sounds like it would be loads cheaper and more simple to do it that way - but it's definitely something I want to get right the first time, rather than experimenting and trying to figure it out myself, given the risk involved in the case of failure

Is there a better transformer I could use, like one that mounts to the chassis? I'm not crazy about the PCB mount but was unable to find anything similarly sized that was chassis mounted. Now that I have a better idea of what to search for, I found these:

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/187B24/?qs=sGAEpiMZZMv4oUrzpPKU3J%252b8YG9gjyY6t272u8IOEn0%3d

http://www.mouser.com/ProductDetail/Hammond-Manufacturing/187C24/?qs=sGAEpiMZZMv4oUrzpPKU3Eihc0fH5wJ2%252beffhomZ0Ik%3d

First is 6VA, second is 12VA, and I believe both will fit in a 1U (the 12VA is 1.61in tall, not sure how much clearance is needed in the inside of the enclosure). I prefer the quick connects, as well.

[J M Fahey]

[kvandekrol]

Great! Thank you. Are there any concerns about the power supply being unregulated? And is there a functional difference between the W02 and W04G bridge rectifiers?

[J M Fahey]

No. Opamps work perfectly from +/-18V to +/- 4V5, and have high (to put it mildly) ripple rejection.
Anyway you have very  little ripple there.

[kvandekrol]

Next question, and probably the last until I actually build the thing, but what about a fuse? Is there any need for one, or for any other type of protection against failure or worst-case scenarios?

[J M Fahey]

Use a 100 or 250 mA fuse in series with the primary, not so much as to protect the transformer itself but to avoid it catching fire and burning down your house.
No, I'm not kidding.
That's the main idea behind a fuse, to "pull the plug" when trouble appears.
*If* after that , you can repair a couple parts, change the fuse and everything works, good; if not, your bad luck, but the cost of the entire electronic device is *nothing* against the value of your house and life.
That's why the *worst* idea in the world is to "repair" a fuse with a piece of cigarette paper, using a higher rated one, etc.

[kvandekrol]

In series with the primary - meaning, the fuse should go between the main power cable and the transformer? (or more accurately, between the power switch and the transformer)

[joecool85]

In series with the primary - meaning, the fuse should go between the main power cable and the transformer? (or more accurately, between the power switch and the transformer)

I would put right when the power cable first comes into the chassis, before any switches or circuits of any kind.

[J M Fahey]

Agree, and add: the hot wire (blue in Argentina) should go to the fuse, through it to the switch, through it to one of the transformer primary leads.
The other transformer primary lead should be connected to the neutral wire (brown in Argentina) .
This way either open fuse or open switch disconnect the most dangerous wire first.
Both live and neutral should be very well insulated from chassis and any other part of the amplifier.
The ground wire (green/yellow) goes to the chassis, very safely bolted to it, and at least one inch longer than both hot and neutral, so if somebofy yanks the power cable, it will be the last one to be cut.
I gave you the Argentine code colors, I think in Australia and South Africa they are the exact opposit, USA uses other colours (black and white?) , *always* check your local norms.

[joecool85]

I gave you the Argentine code colors, I think in Australia and South Africa they are the exact opposit, USA uses other colours (black and white?) , *always* check your local norms.

In the USA it is black for hot, white for neutral and green for ground.  You can also have red as a "switched hot" on a 3 way circuit, but that doesn't pertain to guitar amps what so ever.

[kvandekrol]

I wondering if I'm making this too complicated... I noticed that Jack Orman has a pedalboard power supply project here where he gets multiple +9v, a +12v, and a -9V from a single 18v DC supply.

Would it be possible, using a charge pump like a MAX1044 along with regulators, to get +/- 15V from an 18v wall wart?

[DJPhil]

Would it be possible, using a charge pump like a MAX1044 along with regulators, to get +/- 15V from an 18v wall wart?

Possible yes, but charge pump regulators can't handle much current draw. Usually the output is limited to 10mA or so. I'm not extremely familiar with the current demands of the average preamp, but this seems low. The positive supply should be fine, however.

I think the linear supply already described would be simplest. It seems involved, but that's really the nature of split supplies. They're not as common as they once were. I think the only thing you could do to make things more simple would be to buy a premade 'brick' external power supply, but that may be more expensive in the long run. They tend to be switchmode as well, which may require a bit of filtering for sensitive preamp stages depending on the circuit and how noisy it is.

Just my two cents, hope that helps some.