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Old 5th November 2007, 12:58 AM   #1
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Default Preliminary 12AX7 guitar preamp

This is preliminary, but it's what I drew up in a few minutes for a simple guitar preamp. I'm no expert so if anyone has questions as to why I did something whatever way..
Click the image to open in full size.

I haven't shown any of the power supply but I plan on designing an SMPS for the plate, opamp, and heater and putting it all on one board.
Martin K
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Old 5th November 2007, 12:59 AM   #2
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I forgot to say that I haven't settled on values for any R/C components yet.
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Old 5th November 2007, 03:00 AM   #3
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You must use a resistor between pin 7 of V1B and ground...

1 nF for C8, is to great a value.

R5 must be smaller , with 100 K you only get a overall gain of 2 ...

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Old 5th November 2007, 03:56 PM   #4
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More or less everything tube_dude said above plus:

R5 needs to be reduced, gain is really about 10dB. (3X) Also running the op-amp open loop for ac is probably not the road to good sound, and may result in HF oscillation. Set gain around 20dB or so and the tube gain stage will provide anything required beyond that.

Watch out for the dc time constant in your op-amp dc feedback loop.
This might or might not be ok depending on other constants inside the loop. (I don't like this feature, I'd get rid of it - see above.)

C8 is likely to cause stability problems

Unless you have a very low output pickup there are situations where those diodes across the input are going to forward bias and clip your pickup signal with really nasty results. Reference them to the supply rails (reverse bias of course) or use back to back zeners in series. (Note that this solution may not be optimum from a sound standpoint. Use low capacitance diodes.)

Given that you apply feedback from the tube output stage back to the input of the op-amp there is a possibility of overdriving the inverting input under transient conditions. It would be a good idea to clamp this input to the op-amp supply rails to prevent damage to the op-amp if this happens.

Most mag pickups and all piezo like to be terminated into a combination of parallel resistance and capacitance. Finding the sonically optimum values I'll leave to you. Most electric guitars like 500K to 1M..

You can do all of this with a couple of tubes and no op-amps, particularly if you are not using a piezo pickup. (And with piezos with some care.)
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Old 6th November 2007, 09:24 PM   #5
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you might also want to think about using just tubes for two other reasons:

1> you won't need a bipolar low voltage supply for the op amp. it's just another unneccesary thing to add, unless you are using one for other devices in the preamp.

2> you get a better "tube sound" effect if the input "hits vacuum" first. you can get more gain with silicon later if you want. if you're using tubes to get the characteristic harmonic balance of tubes, you'll get a better result with the tubes first, especially if you want a tube "overdrive" sound. the input impedance of a tube stage is "just right" for guitar pickups. that's why companies that make SS amps with tube pre's put the tube first.

also, add input and output level controls for more flexibility. you have a lot of guitars out there now with active preamps, and they tend to have rather hot output levels.
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