Can I replace this pot with just a resistor?

[OasisMcFly]

I am working on building the acoustic preamp from IvIark's blog found here:

http://tagboardeffects.blogspot.com/2014/04/acoustic-piezo-preamp.html

What I am hoping to do is to just set this to always be max volume and take the pot entirely. Based on my still relatively inexperienced DIY knowledge, my conclusion is that I should put a 10k resistor coming off the wire labeled Volume 3 and then the other end of that to output and then put a jumper from Volume 1 to Volume 3, ignoring Volume 2 completely.

Having said that, I am seriously doubting myself. Would someone be kind enough to point me in the right direction?

[samhay]

Connect a 10k resistor (or 100k, doesn't matter) between Volume 1 and Volume 3. Take your output from Volume 3.
Having said that, it's not a great design and you could do better for minimal extra parts.

[OasisMcFly]

Thanks! I've got it on my bread board at home and I'll give that a go tonight. I am curious though, since I am still really quite a green thumb with this stuff, what makes this a bad design, and could you point me towards a better design? I'd love to learn more about it. And since this is only breadboarded at this point, I could scrap it, and go straight to a better design.

For clarity sake, I was considering this design to help with our band. Its a country band and the main singer likes to play mandolin on a few songs. Her mandolin has a passive 1/4" out on it, but the volume coming to the mixer is so low that we end up having to turn the gain and volume all the way up on its channel and even then, it barely competes with the rest of the band, so we crank the mains until the mandolin is at a good level and then bring all the other instruments down to volume match. I was hoping that a simple preamp like this could boost the mandolin signal enough to compensate.

[samhay]

Sorry - meant to reply, then slipped out of sight.

The posted design (below) has a fixed gain of about 3. To have some versatility, this should be variable from 1 to quite a large number.

The input impedance is 1.1M. This is just big enough for most piezo's and I would aim for 1M and 10M switchable. The biasing could also be done better - google 'noiseless biasing'.

There are a few other things missing that, while not critical, make for good design practice. The output should have some series resistance, reverse polarity protection might be a good idea, etc.



(from: http://www.eleccircuit.com/wp-content/uploads/2013/06/acoustic-guitar-pickup-circuit-using-tl0711-600x482.jpg )

[PRR]

> good design practice

This scribble includes many of the things Sam is talking about.

http://oi62.tinypic.com/vd1jjo.jpg

Since you don't actually *know* how much gain you need, I kept a pot. If you detest knobs, get it working (on stage in live situation) with the pot, take pot out and measure.

All part values +/-30% as convenient. ("5Meg" will probably be 4.7Meg, etc.)

Caps (0.01uFd, 10uFd, 100uFd) must be 10V or higher rating (16V and 25V are common). Caps with "+" marks will be electrolytic, observe polarity.

Opamp can be TL071. (Or TL072, but battery drain increases.) I leave it to you to find pin-outs.

For debugging: the chip should show +8.9V at + power pin, zero at the - power pin, and +4.5V (+/-10%) at the Output and - INput pins. Voltage at + INput pin will not read correctly due to meter loading; 1V to 4V might be reasonable and not real significant.

Cross-arrow on 50K pot points to the "louder" end of the pot.

Battery might have a switch, or use the common guitar-pedal jack-switching. Battery life will be over 100 hours, or 2 cents an hour, 2 months of 3hrs/night 3nights/week gigging. A 6-pack of C-cells will run non-stop for 2 years.

Input jack should probably be "shorting" when not-plugged because the high input impedance will suck-up a lot of buzz even in a metal box.