Preamp adventures (In need of a bipolar supply solution)

#1

Hello. First, I'm sorry if this is not the right place to post this, I truly am. I'm a newbie to the Axoloti.

Everything has been very smooth sailing, well, at least since I understood that the numbers that seem to go from -64 to +64 are really treated as if they went from -1 to +1. This days I've found the first obstacle in my path: it turns out that, to really take advantage of my dynamic microphone, I need a preamp.

For now I've made do with a small mixer connected before the Axoloti, but I'd like to make a more practical solution. If there was a way to integrate a preamp to it, my Axoloti experience and live audio endeavours would become very stream lined. With the mixer, it seems I just want a banana and I'm getting the orangutan and the jungle with it. I've found the famous $5 preamp and many variants of it. Making a preamp that's good enough seems easy enough.

Except that they need a bipolar supply, and it can't be a noisy one. It is suggested that a +/-15 v or +/-18 supply should be used, but I guess that, for the levels that the Axoloti manages, a +/-12 supply should provide enough headroom. Most of the designs I've found for such bipolar supplies are made to convert 110v ac to the bipolar dc supply; that doesn't feel right to me, because I'd like to avoid having two separate power adaptors to connect to my Axoloti enclosure.

Ideally, I'd like to take the dc input for the Axoloti and use it to feed the preamp too. But I get lost in the datasheets of charge pumps and switching regulators; I don't really get the physics for them. Some of them seem to be affected a lot by the way the components are arranged and the size of the traces. Some other solutions seem to provide very little current and I worry if it would be enough. And some of them are said to be too noisy.

Has someone one here encountered this kind of problem with a diy preamp or with the need of a bipolar supply? Can you offer some advise? I'd really appreciate it. Thank you very much. Also, thank you for reading all this; I realise is kind of weird when all newbies do is ask for help and I genuinely want to contribute all I can to this nice community as soon as I get a little more familiar with the apparatus.

#2

There is no lack of suitable DC/DC voltage converters - for small currents (2x25mA) I've used the TMA 0512D for a +/-12V supply out of one compact IC, the TMA 0515D would be the 15V equivalent.

#3

Thank you very much sevo. Do you think it would be noiseless enough? or shall I build some kind of noise suppressor circuit? Also, I'm really sorry, I know this most be kind of a stupid question, but do you think it would be enough current?

Thanks again.

#4

It really depends on the whole circuit. Personally I'd insert a capacity just in case, but if the preamp already is stabilised, it might be redundant. As far as power consumption goes, what does the spec sheet say?

#5

Thank you for responding. Ahm the spec sheet of the INA217, which is the standard chip for the famous $5 preamp, says that it draws a typical 10ma and a max 12ma quiescent current. There's another chip, the 1510 from the That corporation, that can be used instead of the INA217 and it says it draws at most 7.9mA when there is no signal. I can't find anywhere on their sheets an idea of how much current they consume when there is signal; I guess someone with more knowledge could deduce it, but, alas, that's not me.

Also, I guess this this is going to be another stupid question, sorry: in which of the three lines of the resulting bipolar supply would you insert that capacitor filter? Thank you very much, again.

#6

What is the peak current? It presumably is more than the empty current...

As far as capacitors are concerned, you'd put them between whatever lines you load - if you only use the 24V between - and +, one would be sufficient, if you use all lines (+12, -12 to ground, and 24V across), you'd need three capacities, between each pair.