Designing a "hybrid" in-guitar pickup - Page 1
Forum section: electronics / projects

author: Moondeck
author_rank: Regular Contributor
timestamp: June 08, 2017, 12:21:15 pm
content: Hi,So, i have a Washburn semi-acoustic guitar that is missing the internal pickup. I am planning to re-purpose the pots inside for my own circuit, and i had this idea to make it "hybrid", as in combine both a piezoelectric pickup and a microphone, and so i have made this circuit (the op-amp is LM386):https://goo.gl/photos/4KZHWScDCqvFvVt59(sorry for google pictures)Now, 2 questions about it:Can i regulate the volume like that? Or will the inputs get lost in the noise somehow? What about mixing the inputs?Can i wire the input elements like that?

author: Sferix
author_rank: Contributor
timestamp: June 10, 2017, 02:01:49 pm
content: There are several issues with your circuit, and insufficient detail. The issues are:Typically for an electro acoustic guitar the piezoelectric pickup will be a piezo bar mounted under the bridge. It's source impedance will be dominated by its series capacitance, and it will need to be connected to a high impedance input (perhaps 5Mohm) to ensure good bass response. Your amplifier won't achieve this.Typically the microphone will be an electret type. These require a dc supply, bias resistor and coupling capacitor that you don't have. It's source impedance will be lowish (a few kohm), and it will need to be connected to an input with perhaps 50kohm impedance. An example electret is here.http://industrial.panasonic.com/cdbs/www-data/pdf/ABA5000/ABA5000CE22.pdfThe input mix control will not function well. It will not suit the very different characteristics of the input piezo and microphone. Adjusting the input potentiometers will change the impedance at the ends of the mix potentiometer, and hence affect the mix ratio. In any case why do you need independent level controls and an input mix control. Perhaps a better way would be to have a mix control and one master volume.The choice of an LM386 for your circuit is inappropriate. The LM386 is a power amplifier designed to drive an 8ohm speaker directly, it has a low input impedance (50k) and is not ideal for low noise applications. You would do better using a standard low noise op amp, e.g. TL082 or better.There are no supply decoupling capacitors on your circuit.You should also consider some additional decoupling (bandwidth reduction) to help prevent external interference being picked up.Given the issues I'd suggest you redesign your circuit from scratch, or google search for an existing design that meets your needs.An approach you could take is to use a low noise op-amp on each input. This would buffer each input suitably for the microphone and piezo characteristics, allow you to normalise the input levels and produce a low impedance output for mixing. Implement your mix control at the op-amp outputs. Follow the mix control with another op amp and master volume control.Another thing you may want to incorporate is a stage to be able to switch the phase of the microphone (0/180) relative to the piezo, to avoid unwanted cancellation.I hope that is of some help, and wish you luck.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 10, 2017, 03:21:45 pm
content: Holy shit, I saw this thread and was prepared to talk about pickup impedance, preamps, and mixers only to discover probably the most comprehensive reply ever.I wish I could actually add more to that, but I can't. Just posting to tell the OP that Sferix is spot on.

author: Moondeck
author_rank: Regular Contributor
timestamp: June 11, 2017, 11:35:43 am
content: Thanks a lot. I have skipped the powering stage on purpose. So, what you mean is having a separate opamp for each pickup? What about the input impedance? Isnt it theoretically infinite for opamps?

author: Audioguru
author_rank: Super Contributor
timestamp: June 11, 2017, 02:47:52 pm
content: The LM386 power amplifier IC will not work with a + and - power supply because it is already biased to use a single positive supply. Its datasheet shows a capacitor at its output in series with a resistor to ground to prevent oscillation. it shows a capacitor in series with the resistor or pot between its pin 1 and pin 8.Audio opamps are available with Jfet very high impedance inputs. The microphone will have a low output level so its preamp opamp will need fairly high gain. The piezo pickup will have a fairly high output level so its preamp opamp or Jfet instead will need fairly low gain. The volume controls should be at the outputs of the preamps and switch should be used to select which volume control feeds the power amplifier or recorder.

author: Moondeck
author_rank: Regular Contributor
timestamp: June 11, 2017, 02:49:44 pm
content: I want it to be "mixable", not switchable. How much gain should i have then?

author: Sferix
author_rank: Contributor
timestamp: June 11, 2017, 06:19:24 pm
content: I think you should reckon on doing some work yourself. It is difficult for anyone here to comment on 'how much gain you should have' without knowing, what specific sensors (microphone & piezo) you are going to use, and what the acoustic coupling will be. So do some comparative level measurements on the sensor outputs, and then you can work out the gains required.

author: Moondeck
author_rank: Regular Contributor
timestamp: June 11, 2017, 06:42:30 pm
content: Alright, i understand. What about the voltage i will feed into the actual guitar amp box? What is the peak voltage i should feed into it?

author: P90
author_rank: Frequent Contributor
timestamp: June 11, 2017, 08:31:10 pm
content: Another option... if you google piezoelectric saddle pickup, you can find EQ/pickup kits for this very purpose... just a thought...

author: Moondeck
author_rank: Regular Contributor
timestamp: June 11, 2017, 08:32:08 pm
content: Quote from: P90 on June 11, 2017, 08:31:10 pmThats the easy way

author: P90
author_rank: Frequent Contributor
timestamp: June 11, 2017, 08:33:20 pm
content: Quote from: Moondeck on June 11, 2017, 08:32:08 pmLOL. I'm lazy...

author: Audioguru
author_rank: Super Contributor
timestamp: June 12, 2017, 12:28:14 am
content: If you mix an acoustical microphone signal with a piezo vibration sensor then the frequency response of the guitar will have severe peaks and nulls caused by their differences.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 12, 2017, 08:52:09 pm
content: Quote from: Audioguru on June 12, 2017, 12:28:14 amNot to derail the thread by talking of 'how to mic up an acoustic' but a couple of decades ago, this was all the rage - the 'quackiness' of the piezo saddle pickup, mixed with the boomy bass of an internal mic. It must have been all marketing because the systems I heard were dreadful (IMO of course), often taking the worst of both worlds. It is why even today, live acoustic stuff is generally saddle pickup with the same brittle 'quackiness' as they have always had, and players just deal with it.With that said, having access to both signals allows one to record both (with a stereo jack and stereo preamp) and one can post-process that to your hearts content. With a mix knob as you have mentioned, combined with an on/off switch for both you have all the possible combinations.For DIY, it would be prudent to add an impedance buffer for the piezo pickup (gain of 1, just a very high input impedance, and plenty of headroom) and play around with mic placement that has its own little preamp. Keep the signals separate for now so you can check on an oscilloscope (or if you dont' have one, your soundcard) for their levels. This is long before you start thinking about mixing. Piezo pickups can have quite a high output (<2V), but with such little energy behind it (thus the high impedance) it can appear to be very low amplitude in <1Meg inputs. Electret mic preamps vary in their gain, but 20dB is standard. For being placed inside the guitar (boomy) you would certainly have to reduce the gain to maybe 6dB or lower. The crappy generic electret mics I have used for mic'ing inside an acoustic all clipped when playing hard and sounded dreadful.

author: Sferix
author_rank: Contributor
timestamp: June 13, 2017, 07:14:35 am
content: On voltage output to the amplifier. With the volume at max and tone controls flat, my electro acoustic guitar produces over 0.5volt peak to peak when I play an open E chord hard (all 6 strings). I have gigged it a lot, plugged into guitar amplifiers, and directly into PA systems (unbalanced line input), and it has always been enough output to drive the amplifier.