Preamp for LR Baggs M1 - diyAudio
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Old 2nd July 2011, 12:10 PM   #1
MondyT is offline MondyT  United Kingdom
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Default Preamp for LR Baggs M1

Hi All
I am putting together a belt preamp for a new Takamine Triax pickup (LR Baggs M1 essentially) for my acoustic. This pickup has an output of around 100mV maximum and therefore I need a little over 20dB of gain. I also want a volume control and low output impedance. I have put together the circuit below based on a OPA2134 op-amp (a little over kill for this app, but I need the headroom and very low noise)

Click the image to open in full size.

I wondered if you could advise if it would be better to configure the input op-amp as unity gain and use the op-amp after the volume pot to provide all the gain, or as shown, share the gain between both op-amp sections? I am thinking that if the input op-amp had less output, this may lower any pot noise when adjusting the volume?

Also do you guys and girls think the component values I have in the feedback loops etc will result in good low noise and good fidelity operation?

Any improvement suggestions are very welcome!

Cheers
Ray
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Old 3rd July 2011, 07:39 AM   #2
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I would have thought either way would make no difference at all - as would using a single opamp with a 4.7K pot on the output.
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Old 3rd July 2011, 09:31 AM   #3
MondyT is offline MondyT  United Kingdom
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Hi Nigel

How are you doing and thanks for your reply!

I wanted to put the volume pot in front of the op-amp on the output so that I always have an unaltered high impedance input and low impedance output. With a pot directly across the output, the output impedance will alter depending on the pot setting and generally always be higher than without the pot.

If I use the first op-amp as unity gain (so just a buffer), do you see any noise problems, or other, with running the second op-amp at the required 20dB gain?

Cheers
Ray
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Old 4th July 2011, 12:06 PM   #4
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Quote:
Originally Posted by MondyT View Post
Hi Nigel

How are you doing and thanks for your reply!

I wanted to put the volume pot in front of the op-amp on the output so that I always have an unaltered high impedance input and low impedance output. With a pot directly across the output, the output impedance will alter depending on the pot setting and generally always be higher than without the pot.
That's why I specified a 4.7K pot, that's low enough to fed the 20K input impedance of the following amplifier.

Quote:

If I use the first op-amp as unity gain (so just a buffer), do you see any noise problems, or other, with running the second op-amp at the required 20dB gain?
Like I said, it makes no difference at all - you're only talking about very low gain figures, and a high signal level to begin with.

I doubt you will be able to hear any difference no matter how you do it, not measure any difference either (including the single opamp version with a 4.7K pot).
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