would this AC adapter be suitable for a mic preamp power supply?

[JamesHE]

https://www.mouser.com/datasheet/2/410/media-1068277.pdf

18VDC 1A. 


I think I purchased this 10 years ago during my first foray into DIY. Was trying to make a little preamp + eq for acoustic guitar pickups - that never panned out and all this stuff just sat in a box since then.

Pres will be INA217 based. I'm looking for + /- 18V rails and +48V for phantom power, of course. I've looked at several schematics and it just haven't clicked with me yet what I need to do. The schematics I've come across mostly seem to use a transformer with a center tap (if I'm understanding right) Or even an AC input.  Can anyone point me to a schematic that uses a similar type of transformer as this adapter?

 

[JohnRoberts]

If you were stranded on a desert island and had to make that 18v DC supply work, there is a charge pump technology than can make a mirror -V supply.

Likewise the +48 could be made from a several step charge pump up from the +18v. 

Most of the dedicated charge pump ICs seem aimed at lower (logic) voltage applications. Of course you could make a DC to DC switching supply that runs from +18V to make anything you want.

Or you could get an AC wall wart and save a lot of drama.

JR

 

[JamesHE]

Or you could get an AC wall wart and save a lot of drama.

JR

Could you elaborate on why using this would be so problematic?

This adapter just seems to be a transformer in a box with a diode bridge included to convert to DC. The power supply schematics I've seen all pretty much start with a Diode bridge just after the connection with the transformer, like this one from JML. http://www.jlmaudio.com/JLM%20ACDC%20Schematic.pdf

Would following a similar  schematic and just omitting the first diode bridge not work? The rail for the 48V has it's own diode bridge and is tapped from the AC, so I guess this is the issue then? But there has to be some somewhat straightforward way of doing this - just trying to learn and understand here.

 

[JohnRoberts]

If you only have +18V available it takes DC-DC switching technology, or perhaps a charge pump to generate a -18V from that. Likewise the +48 will need to be stepped up from the +18V requiring some kind of switcher/charge pump.

With an appropriate AC supply you can generate +/- 18V directly with rectification and reservoir caps. +48 can be generated with a voltage doubler/tripler relatively simply and easily.

If you have trouble grasping these concepts, you will have even more trouble designing a switching power supply.

JR 

 

[JamesHE]

If you only have +18V available it takes DC-DC switching technology, or perhaps a charge pump to generate a -18V from that. Likewise the +48 will need to be stepped up from the +18V requiring some kind of switcher/charge pump.

With an appropriate AC supply you can generate +/- 18V directly with rectification and reservoir caps. +48 can be generated with a voltage doubler/tripler relatively simply and easily.

If you have trouble grasping these concepts, you will have even more trouble designing a switching power supply.

JR

My understanding of this topic is obviously rudimentary at best - eventually I will learn how to ask the right questions.

the schematic as drawn in the link from JML that I posted above (http://www.jlmaudio.com/JLM%20ACDC%20Schematic.pdf)  seems to be able to take a transformer without the center tap.  How does this get the negative voltage in this setup?

 

[JohnRoberts]

My understanding of this topic is obviously rudimentary at best - eventually I will learn how to ask the right questions.

the schematic as drawn in the link from JML that I posted above (http://www.jlmaudio.com/JLM%20ACDC%20Schematic.pdf)  seems to be able to take a transformer without the center tap.  How does this get the negative voltage in this setup?

It has a center tap that is connected to the ground symbol.

JR

 

[PRR]

> seems to be able to take a transformer without the center tap.  How does this get the negative voltage

It runs with a *different* rectifier configuration as a Voltage Doubler. (The drawing is a bit unclear on that, why JR missed it.)

Your sealed wart does not allow access to change the rectifier.

(And at the present point of your studies, it may be unwise/unsafe for you to bust the wart open and tamper the insides.)

Pre-made bipolar supplies are readily available. Tri-rail with 48V can be bought. Until you have more circuit experience, don't mess with wall-power, just buy a supply.

 

[JamesHE]

Pre-made bipolar supplies are readily available. Tri-rail with 48V can be bought. Until you have more circuit experience, don't mess with wall-power, just buy a supply.

Thanks. exactly why I was looking at the JLM site anyways.

Hopefully I can find some use for this huge, heavy brick of wart at some point.

 

[buildafriend]

Thanks. exactly why I was looking at the JLM site anyways.

Hopefully I can find some use for this huge, heavy brick of wart at some point.

The JLM stuff is usually good.

If you do want to move beyond pre made supplies I would suggest looking at the 317, 337, and 783 regulators. They are very common for audio bi polar supply rails and phantom power.

Always keep one hand in your pocket when working on power circuits, that way you can't make a loop through your heart by having both hands shorting something out.