Guitar pickup balanced preamp

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MisterStarshine

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Joined 2008
Yes I noticed that the improvement was not very big to single ended. At very high overdrive saturation gain it can be heard a difference thought. Probably because since this pickup is shielded. One obtains an advantage by having the shield separated from the audio signal everywhere.

Now it's time to try to take care of the overall SNR of the circuit. Some guy told me that one can calculate that by making a model of the circuit by drawing equivalent noise generator blocks. According to them the design itself is more critical than the choice of op amp device.
 
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MisterStarshine

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Joined 2008
I haven't made any scientifically measurements on this design but I must say I'm not entirely happy with it.

Maybe I'm poor at reading datasheets but to me it appears as if the THS6062 should have equally good performance as the popular OPA2134 for example. At least when it comes to device noise performance. I guess this should apply to high impedance sources as well?? :confused:

Have I misinterpreted the datasheets??

The obtain was to improve the SNR for the high frequencies primarily. But as it is now it appears as if I could just as well crank up treble boost in software at destination and obtain just as poor SNR as I do with the preamp. :eek:

Now this clearly makes the preamp redundant. :(

Would need to talk about this with someone with insight in these things. Here is a photo of the board slightly before it was finished,

http://www.carmi.se/misterstarshine/img/bal_preamp_photo1.jpg

Ignore the text on the op amps. I've soldered two THS6062's onto it now.
 
leadbelly

leadbelly

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Joined 2002
Enzo said:
In the couple inches between the pickup and your little preamp circuit, balanced or not is going to make very little difference. We use balanced lines for things like mic cords so we can run the tiny mic signal across the stage without picking up tons of noise. But inside the little innards hatch of a guitar, which OUGHT to be shielded anyway, I don't see the advantage.
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No, you need to re-read the Naiant material. What they are trying to accomplish is to have a balanced guitar input. So the internals of the guitar are wired as balanced, AND a stereo guitar cord is used so that noise pickup can be common mode on the 2 conductors inside the cable. A good balanced preamp can then reject the common mode noise. That's the goal, how well it can be made the work depends purely on execution.
 
leadbelly

leadbelly

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Joined 2002
MisterStarshine said:
Would need to talk about this with someone with insight in these things.
Click to expand...

Thanks. I feel like slamming my head against a wall. Forget troubleshooting circuit details, HOW HAVE YOU WIRED UP YOUR GUITAR AND CABLE? Again, if you are using a regular guitar cord, more hum is exactly what is supposed to happen. The noise picked up by the single conductor and the shield inside the cable is not common mode, so the balanced input cannot reject it, and more hum is the result. Forget quibbling about opamp brands!
 
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MisterStarshine

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Joined 2008
Actually I found some information about the noise from preamp designs, http://www.beis.de/Elektronik/LNPreAmp/LNPreAmp.html

It turns out to be seriously complicated. Just to look for the "equivalent input noise" figure in the datasheet doesn't seem to give a clue how the end system will perform.

The hunt for the extremely low noise preamp continues. The battle against interference is already won.
 
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