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Old 22nd May 2008, 03:10 AM   #1
RonNovy is offline RonNovy  United States
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Default Balanced+Phantom powered Guitar Pre-amp

Ok... I'm new guitar preamps and to this forum and forgive me if this has been discussed before, but here I go...

I'm looking to build a Phantom powered Balanced output guitar pre-amp for direct connection to a tube pre-amp or recorder... I've done many circuit simulations and analyzed the frequency response, phase, S/N ratio of many different circuits, but I'm still not sure if any of them will work as I can't afford to build any right now. My goal is basically to get a flat frequency response with as little noise and as little parts as possible. I've found that simple FET preamps can color the sound a bit when gain is applied to the circuit so I'm trying to use low noise op-amps. I know the sound of op-amps is said to be dull, but some EQ and a Re-amp device can fix that right?

Anyway, I'd like to know what people think of this design using an INA2137 (or two INA137s). It's simple and uses about 5.5ma which is ok for a phantom powered circuit (max 10ma). I'm not a true engineer so suggestions are welcome...

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Old 22nd May 2008, 06:25 PM   #2
Minion is offline Minion  Canada
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While I"m no expert I do see a Whole bunch of problems with your design....

1) choice of Opamps ,The INA137 is NOT suitable for a Guitar preamp ,a Guitar Preamp needs a Very High Input impedance and useing a Line Reciever opamp just doesn"t have the Input impedance that you need , Pluss you haven"t even added the Input resistor that sets the input impedance....Pluss your tone controll is in the wrong spot ,The tone coltroll will only work properly when fed by a low impedance source so it should be after the opamp ,and because this type of tone controll only cuts frequencies it might be better to have a another gain stage for Makeup gain....

2) power supply, +48v will fry most opamps ,so it needs to be reduced useing a Resistor, or a Regulator or a Zenner diode....
Another problem is you can not run these opamps off of a single supply the way you have them set up...You need to have your Virtual ground connected After the Cap not before, the Bias Voltage will not enter the opamp the way you have it set up....

So all in all I would suggest that you make a proper Guitar preamp useing a High input impedance Fet Opamp with the Tone controlls after the Opamp with another Makeup gain stage....
You should also fix your Virtual earth Biasing configuration....A Fet opamp shouldn"t add any Color to the signal at low Gains but are pretty much manditory for Guitar inputs....

Also if you want a Ballanced output from the Preamp stage you should be useing a Ballanced Line Driver not a reciever (DRV134?), You can use the INA137 in the Mixer section which needs a Line reciever but even a simple opamp can be used a a Line reciever (and 2 can be used as Line drivers).....


This is all that I can see right not at a quick glance but I"m sure there are quite a few other improvements you can make, Like I said I"m no expert but I do know my way arround opamps for the most Part......

Good luck
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Old 22nd May 2008, 08:32 PM   #3
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Quote:
Originally posted by Minion
1) Pluss you haven"t even added the Input resistor that sets the input impedance....
I can clearly see a 1M resistor.

Quote:
Originally posted by Minion
Pluss your tone controll is in the wrong spot ,The tone coltroll will only work properly when fed by a low impedance source so it should be after the opamp ,and because this type of tone controll only cuts frequencies it might be better to have a another gain stage for Makeup gain....
I think you'll find the volume and tone controls just model what's normally built into the guitar...

Other than that, your points apply. Input impedance make INA137 useless for the application.
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