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Old 5th August 2008, 03:56 AM   #1
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Post Guitar pickup balanced preamp

I've done some experiments on a preamp design that is fed by a weak signal such as the signal from a guitar pickup.

I've tried this approach, http://sound.westhost.com/p87-f1.gif but what I really would like is a modification with ultra high input impedance. One could think of omitting the 10k input resistors but that wrecks its operation since the inputs of the op's need some bias current.

My version has an extra inverter on the output in case one want balanced output as well. the R7 resistor is replaced by a pot and allows me to adjust the gain.

Many thanks in advance.
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Old 5th August 2008, 07:10 AM   #2
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Default Simulation says ....

That circuit has pretty good CMRR. Unmatched 1% resistors, shown below, give about 36dB CMRR. Matching to 0.1% gives 56dB CMRR.

I don't know too much about guitar amps, most of the circuits I saw so far had 100k or so ZIN.

Resistors R1 and R2 can be virtually any value, as they do not affect the gain. You could increase them to 100k and beyond. This might make the system noisier, though due to the resistor noise. Give it a try.
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Old 5th August 2008, 12:53 PM   #3
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I've successfully tried to omit R1 completely and this appears to work for some reason. The advantage with this is that the source doesn't see and load. This should have a bad impact on the CMRR figure shouldn't it at least theoretically? I've found out that I could choose R2 pretty big to diminish the sacrifice of symmetry. Is this a good idea?

I'm soon about to design a low-profile layout on this. Would be nice to find a device with extremely low input noise. Not so many dual opamps have extremely low noise figures like the single INA217 for example. With popular op's such as the TL072 the circuit behaves good but with a low noise supersonic device such as THS6062 there's a lot of hiss coming from the setup. I want this preamp to be premium audiophile quality.
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Old 5th August 2008, 03:45 PM   #4
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Just use a JFET input opamp and forget about that TL072. You'll get the nice high input impedance you want.
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Old 6th August 2008, 03:21 AM   #5
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Actually I'm not too concerned about the op amps internal input impedance but rather the circuit I'm using. I guess they all need some amount of bias current into the input either through a resistor or through source.

I've come this far with the design, http://www.carmi.se/misterstarshine/...eamp_schem.png

I wonder if the THS6062's are a good alternative under the condition that one manage to make them work in the layout. They are high-bandwidth devices but the noise figure and THD are superior. I haven't managed to make the design work well with the THS6062's on the breadboard though.

I'll soon post a layout proposal for the design. I'm pretty happy with the design tbh.

I want to point out that the circuit is pretty common and I've nicked it and modified it.
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Old 6th August 2008, 03:22 PM   #6
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Quote:
Originally posted by MisterStarshine
Actually I'm not too concerned about the op amps internal input impedance but rather the circuit I'm using.
This makes no sense to me. What is the point of playing around with resistor values if that effect is completely negated by the opamp's internals?

IMHO, omitting R1 is a big no no. One reason it is there is to terminate the input so that there is no oscillation when the input is unplugged.
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Old 6th August 2008, 04:04 PM   #7
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Quote:
Originally posted by MisterStarshine
I want to point out that the circuit is pretty common and I've nicked it and modified it.
I don't want to make it sound like I'm trying to pick on you, after all you should and will wind up doing whatever you want, but I have to say that your circuit is not common at all in any way for a guitar preamp. You mentioned Rod Elliot, so look at how he builds a guitar preamp:


Click the image to open in full size.
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Old 6th August 2008, 08:22 PM   #8
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Could be my mistake but I thought most op amps had several mega ohms input impedance when run in non-inverting mode.

Anyway, it's most likely to be a good idea to keep them. I hope I'll be able to use this design with high bandwidth op amps. This design could very well be used in any application.

Here are my schematic and board:

http://www.carmi.se/misterstarshine/...eamp_schem.png
http://www.carmi.se/misterstarshine/...preamp_brd.png

The board is slightly bigger than a square inch. I'm about to attempt to make some prototypes. This design should be totally perfect for all high-end audio purposes. If it will prove necessary I will have to adjust it in case it would prove not to work with 100MHz op amps.
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Old 6th August 2008, 08:22 PM   #9
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Maybe I should've pointed out that "my" design is optimized for use with a computer. It's supposed to have pretty low impedance output and high impedance input.

What's fancy with my design is that it incorporates the concept of balanced audio which has proven to have big impact on hum rejection in my environment.

I never meant to imply that my circuit would be common in guitar contexts. This is in fact an experiment and what I've also done without being aware of it is that I've taken a mic preamp design and made it high impedance input to suit any signal source.
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Old 6th August 2008, 09:04 PM   #10
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Quote:
Originally posted by MisterStarshine
Could be my mistake but I thought most op amps had several mega ohms input impedance when run in non-inverting mode.
Actually, that's not true at all, but I found something funny when looking at the TL072 datasheet....it is a JFET input opamp. Well that explains why Rod Elliot uses them for guitar preamps!

So I looked up the spec for the NE5532 that you are using. Datasheet says input resistance is 300K typical but only 30K minimum. Not good.
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