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 Forum index » DIY Hardware and Software » Musical Interfaces
Will this phantom power circuit work?
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chulian



Joined: Apr 13, 2017
Posts: 1
Location: japan

PostPosted: Fri Apr 14, 2017 1:37 am    Post subject: Will this phantom power circuit work? Reply with quote  Mark this post and the followings unread

I'm designing a musical instrument in which two oscillators (one based on a Quad Nand gate, the other on a 555) are controlled by a signal from a condenser microphone. At the moment the mic is being phantom powered via my computer audio interface, then coming from an output of the interface and entering the circuit at line level.

I now need to add a 48v phantom power circuit so that the mic can be powered directly from the instrument. I've purchased a 12v-48v DC-DC converter which seems to work fine. My question is about how to incorporate this without damaging the subsequent circuitry.

This is essentially my proposed circuit. Phantom power works by putting both signal pins of the XLR at 48v with respect to ground; the mic signal is amplified by a LM386N-4 power amp and this signal controls the oscillators via an LED-LDR arrangement. I haven't included the rest of the circuit, since I'm mainly worried about whether the phantom power will damage the LM386N-4. (I'm also not too bothered about audio fidelity, as the mic signal is only used to control the oscillators, and won't be directly audible - it just needs to be high enough going into the rest of the circuit):
Posted Image, might have been reduced in size. Click Image to view fullscreen.
What I'm particularly worried about is blowing up the LM386N, perhaps by it internally referencing either of its inputs to ground (and hence seeing 48v across some of its internal structure). Will this circuit work, in the sense that it will amplify the mic signal without damaging anything?

Incidentally, the phantom power circuit proposed here suggests putting a pair of zener diodes between each signal line and ground, as shown below - would using these in my proposed circuit prevent damage to the LM386N-4, or any subsequent circuitry?
Posted Image, might have been reduced in size. Click Image to view fullscreen.[/img]
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gdavis



Joined: Feb 27, 2013
Posts: 359
Location: San Diego
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PostPosted: Fri Apr 14, 2017 1:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

I don't mean to sound too harsh, but your circuit is seriously flawed.

The phantom power and 6.8k resistor connections are completely wrong and will prevent the circuit from working at all. Look again at the phantom power circuit you posted below your circuit.

The choice of the LM386 is inappropriate for this application. It is a power amplifier for driving things like speakers, you just want a general purpose audio opamp like the TL07x or NE5534.

You'll also want to take the gain into consideration and may want to add a level control so that you can adjust the sensitivity of the mic.

Figure 23 at http://sound.whsites.net/articles/dwopa3.htm is a simple balanced input that I think will be appropriate for your needs (again, some gain adjustment may be necessary). Add phantom power as shown in the second circuit you posted (pay close attention this time!).

The diodes are there to suppress large spikes that can occur when the mic is connected. I wouldn't leave them out.

The 22uF caps are blocking the 48V from getting to the opamp. As long as they don't short (and there aren't any other shorts) it will be fine. The diodes would provide protection from the caps shorting out, as long as the 6.8k resistors are intact.

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