Well, the best bang for the buck for a piezo sensor preamp would be the OPA1652(about 1/2 the price of an LT1884)--it's actually cleaner, quieter, and MUCH faster---at the expense of higher current draw---4 ma for the 1652 dual opamp versus 1.7 ma for the 1884. So you'd have to change batteries about twice as often.
BTW, what is the voltage output of your three sensors when strumming, say, an E chord?
BTW, what is the voltage output of your three sensors when strumming, say, an E chord?
I forget the exact source of where I got the idea for this dual JFET configuration, but I remember after testing it, I was immediately impressed, it added a clarity to the overall sound. I have noted in the diagram how I needed to find the IDSS of the individual JFETS and place them in the right order, or the sound was definitely affected in an aesthetically negatively way. So, thus far, I'm pretty happy with the buffer circuit, though I admit I don't have that much reference for comparison. I'm not closed to testing other buffer circuits in the future (in fact, as I show in my new diagram, I'm adding a separate output for the straight piezos so I can easily test entirely new circuits without gutting my guitar), but I thought for the moment that it would be an easier upgrade trying the dual-rails.
Honestly, I'm not worried about the price of the opAmps, since I don't need many of them, I'm much more interested in getting a pleasing and clear sound quality.
I'll try and get this value in the next few days, it's not as simple as it could be finding this out right now since the circuit's carefully installed in the guitar.
The circuit was a bit noisier than I would like before, but I just very recently installed the sensors in the guitar a different way, now they are louder, so I'll have to test and see if the noise is still an issue (for this I still need to update my compression FX settings, I use pretty heavy compression so I can play detailed string harmonics)
So I tried to measure voltage across the passive piezos, but my multimeter can't measure voltage at that low a level. So... in the millivolt range it seems.
From the OPA2134 data sheet it recommends "Connect low-ESR, 10-nF ceramic bypass capacitors between each supply pin and ground, placed as close to the device as possible"
So, I'll add these to the next diagram.
From the OPA2134 data sheet it recommends "Connect low-ESR, 10-nF ceramic bypass capacitors between each supply pin and ground, placed as close to the device as possible"
So, I'll add these to the next diagram.
multi-meter isn't close to the right tool, oscilloscope is better, a cheap Rigol really would be minimum step up if you want to build/debug electronics
your PC soundcard or other Audio ADC may require some tracking down sensitivity number or calibration but you can get the raw digital audio numbers from many free software
your PC soundcard or other Audio ADC may require some tracking down sensitivity number or calibration but you can get the raw digital audio numbers from many free software
OK, so what's the RMS voltage out of the opamp when strumming say, an E chord?
Looks like the opamp gain (with the 56K feedback resistor) would be ~ 15db. Seems low.
Well, I changed this value based on the guitar output signal into my A/D interface... 100K was too hot and I had to use the "pad" function (-20dB). 56K might be a bit low though, and I probably want to maximize the output here, so I will try 75K instead.
One assumption I am working with here is that I want to keep this resistor value as low as possible for the clarity of the signal, is this correct?
So after creating a nice breadboard setup, and testing my circuit with a single buffer and amp, I find I can hear no difference between a single 9volt, two 9volts in series, and two 9volts in a "dual rail" setup. I recorded and listened extensively. I've attached a pdf illustrating how I hooked up the dual rail setup. Hopefully it's correct, but I just took what I could from the opAmp manual and wired it together. Again, I think I should mention that, unlike the diagram, I just had all three piezos going through a single buffer and single amp, so maybe there would be a difference if I used all three buffer circuits and both amps? Might be something I'll try in the future.
Small update: I tried an opAmp buffer (an OPA604 I had on hand) instead of the FET buffer. The two circuits are very similar sounding, but there sounds like slightly more high frequency clarity with the opAmp, which is what I'm going for here... so, probably in the future I will switch to using these instead of FETs.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.