MXR MicroAmp with XLR balanced output?

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richard44
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Post by richard44 »

Hi everyone. I am hoping you can give me some direction. I want to build an MXR Micro Amp boost circuit which has a balanced XLR output in addition to the 1/4" output we normally use for guitar. So basically, I want a Micro Amp which gives clean boost, but also has the option of converting from an unbalanced to a balanced signal. (The goal is to have the XLR out from the MicroAmp go to my power amp's XLR in when I run directly from my pedal board to the power amp). Does anyone know how I could do this? Perhaps someone has a circuit layout somewhere that converts normal stomp box output to XLR balanced output that I could add on to the Micro Amp circuit readily available on the internet? If not, could you give me any leads or point me somewhere I could do some further research? Thanks.

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apollomusicservice
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Post by apollomusicservice »

Try this on MicroAmp. You need to add only one IC and few resistors.
Attachments
XLR.png
XLR.png (2.56 KiB) Viewed 1612 times

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richard44
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Post by richard44 »

First off, thank you to apollomusicservice for posting that schematic! I'm definitely going to give it a try. I have two quick follow-up questions for you if you don't mind:
1) Where your posted schematic says "to pin 6 TL061 Micro Amp", do mean directly to pin 6 without any filtering caps or resistors in between the LM4558 and the TL061? Pin 6 in the Micro Amp schematic goes through a cap, a resisitor, and a resistor to ground before hitting its (i.e., the MicroAmp's) output. So just to clarify, I could lead into your circuit directly from Pin 6 of the TL061, or from the MicroAmp's output. Just wanted to clarify because I'm somewhat inexperienced and I've ususally seen some type of caps/resistors between op amp stages in other schematics.
2) Can any dual op amp be used (such as an LM833 or OPA2134), or is this LM4558N specific? I don't find the LM4558N readily available though I can find a TJM4558 or NJM4558 - not sure of the exact difference. (While I could study datasheets, I am not so confident that I could understand interactions between all the parameters).
Thanks again!

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Post by apollomusicservice »

1) Yes directly to pin 6 without any caps
2) You can use OPA2134 or some cheaper as TL072
I might add if you want you can add a switch for ground lift on pin 1 of XLR (simple SPST switch). If you want you can add another switch that turned
pins 2 and 3 of the XLR connector (DPST switch). These are all options that you may need when using this supplement.

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richard44
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Post by richard44 »

Hi apollomusicservice,

I am now preparing to build this (getting parts), and have one final question if I could trouble you again. In your extension, is there any Vref or Bias going into the op amps in addition to the full 9V? I understand the full voltage as you clearly marked it. In many circuits I’ve seen (in my limited experience), there is a Vref going into the op amp with the signal, but in your schematic it seems there is none. Just wanted to make sure.

Thanks again for your postings!

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apollomusicservice
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Post by apollomusicservice »

The purpose of this circuit is to change impedance from high to low Z output. Circuit powered with single power supply (that provides isolation with unity gain buffer) do not need biasing with some Vref because op-amp automatically referenced to the center of the supply voltage and output signal swings
between center of supply voltage. Of course there are more complicated circuits that provides balanced outputs but on this way it works very fine. :)

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Post by jonasx26 »

richard44 wrote:is there any Vref or Bias going into the op amps in addition to the full 9V?
R2 and R5 needs to be connected to 4.5V.

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