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Old 7th October 2005, 02:40 PM   #1
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Default Opamp Guitar preamplifier

Hej to all guitar-players.

Im planning to build a Opamp based preamplifier for my guitar(complete SMD). The circuit should act as an impedancetransformer(don't know the right word, sry) with about 10dB gain.

Here's the schematic:
Click the image to open in full size.

Could you check the values which i plan to use?
Which Opamp fits my needs: SingleOpamp, SIL-8, Low power consumption? TL061?
It will be powered with an 9V battery.

Thanks in advantage for your answers.
julian
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Old 8th October 2005, 09:29 PM   #2
wxn is offline wxn  Lithuania
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It seems fine. Though I would change the output resistor to something like 100ohm and use an opamp with lower noise (42nV/vHz is quite a lot for such application)
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Old 9th October 2005, 01:30 AM   #3
Brion55 is offline Brion55  United States
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If you plan on using just one 9V battery, the op amp needs to be referenced to 1/2 the supply voltage in order for it to swing rail to rail. See the drawing I attached. You don't need the volume pot if you don't want it.
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File Type: gif guitar preamp.gif (8.1 KB, 1159 views)
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Old 9th October 2005, 04:40 PM   #4
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Allright, I searched a bit for an good Opamp and found

The OPA234 series low-cost op amps are ideal for single-supply, low-voltage, low-power applications.
So, with this opa, must i use the 1/2 V+ thing?

Changes:

C2-->0,68 to 0,1

C3-->0,68 to 0,1

R4-->1k to 100 Ohm

Click the image to open in full size.
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Old 9th October 2005, 08:23 PM   #5
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Ok, after Reading through the "Op-Amp Design Reference" from TI I came to the decision that I need a split supply(wonder )

So I thought this would be nice:
Click the image to open in full size.

Because I want to use as less space as possible I want to leave out D1/R1(useless) and C1/C2(does that work without?)
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Old 9th October 2005, 08:34 PM   #6
wxn is offline wxn  Lithuania
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D1/R1 can surely be left out. But the caps should stay. Though lower capacitance would do I suppose.
About the opamp, I was thinking about this one:
http://www.analog.com/en/prod/0,,OP27,00.html
Notice how much better all the parameters are. Except for quiescent current... But then again, I'm feeding it from a stationary power supply.
And another, a bit less power hungry:
http://www.analog.com/en/prod/0,,AD8671,00.html
This one looks promising. Even ideal I'd say. And quite cheap
http://www.analog.com/en/prod/0,,OP196,00.html
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Old 9th October 2005, 09:05 PM   #7
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Hm oky, there should be room for some caps.
I've choosen that Opamp because I've have some of them here and also because of the low quiescent current.
The other parameters should also be ok for audio use, but also I don't really understand all of them.
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Old 10th October 2005, 01:27 AM   #8
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If you want something simple and low-power, I'd suggest going with a couple JFET transistors instead of some Op-amp.

I designed one with 4 JFETs, that has up to +20 dB of gain, very low output impedance and draws only about 500 A (max). The plus is that it does have this slightly "tubesque" sound that you can't get with an op-amp. Oh, and after designing your first circuit with JFETs, you start feeling like a grown man. lol.

I'll post the schematics if that ever interests anyone (commercial use forbidden, of course).
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Old 10th October 2005, 02:23 AM   #9
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I personally am after a preamp such as this with about 40dB gain, minimum power consumption..
I'd like to see your jfet design
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Old 10th October 2005, 02:56 AM   #10
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Ok, there you go.

Click the image to open in full size.


Might need some refinement, but it's already nice. I built one prototype so far and installed it in my guitar - it works fine. (The version in my guitar is slightly modified though, I added a "tone control", but the base circuit is the same.)

Should work right "out of the box": nothing to tweak here. The variable resistor allows to set the gain, but any position should yield a relatively low distortion level. As you can see, it was actually meant to be an internal preamp for active pickups - but I ended up using it as a simple on-board preamp on my guitar. (I did make an active bass pickup, but that's another story entirely...)

The JFETs are SST201's here for the SMD version. Use J201's instead if you want to build a through-hole version of this. Other JFETs may work, but I wouldn't count on it without modifying the components values (and the power consumption) a bit...
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