Piezo buffer - Page 1
Forum section: electronics / beginners

author: mrkva
author_rank: Unknown
timestamp: January 27, 2015, 01:36:10 am
content: Hello everyone!I am working on a design of a piezo buffer for audio use. By piezo I mean something likes this:Here are few requirement- it should be powered via phantom power (48V, 10mA)- it should have as little noise as possibleSo far I have come up with this:My question is - what do you think can improve the design? It is quite difficult to work with it on breadboard - it catches FM radio even! Plan is to fit in shielded XLR shell.Thanks for any suggestions and help!Jonas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2015, 04:36:57 am
content: BF862? Hell, you're probably getting not only FM radio, but everything from DC to light through that sucker! Putting capacitance on the gate is, ironically, only making it worse: between the two bypass capacitors, you get something like a Colpitts oscillator at some obscene frequency (500MHz?).Simplest, most likely remedy: add a series resistor at the gate, say 47 or 100 ohms. Won't have a damn bit of change on your application (it could probably be up to 100k and still work), but should stop RF in its tracks.What are you really doing, on a system level? I don't get the impression piezo transducers are all that weak; is it because you're driving a long transmission line with it? Do you need voltage gain as well?Tim

author: mrkva
author_rank: Unknown
timestamp: January 27, 2015, 09:20:58 am
content: Quote from: T3sl4co1l on January 27, 2015, 04:36:57 amAha! But, for some reason, the capacitor there seems to help with the FM-catching. I have to mention that the radio seems to be related to the shielding somehow. When I touch the recorder (metallic case - direct connection to the "ground" in that circuit), I can hear little clicks & pops. Or when I get my hand close to the exposed part of the cable on breadboard.Quote from: T3sl4co1l on January 27, 2015, 04:36:57 amThanks, will try that! But, won't the resistor insert some noise to the system?Quote from: T3sl4co1l on January 27, 2015, 04:36:57 amFrom my basic understanding, the problem is impedance and capacitance. When I plug a piezo directly to my recorder, the sound is very "high-passed" and I need to crank the gain way too much for my taste to get proper sound.My recorded btw (so you have an idea of the input spec):http://www.sounddevices.com/products/702/specs/Thanks again for the info! Jonas

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2015, 11:59:38 am
content: Mind that it will sound "high pass" naturally, because a piezo disc (if that's really what you're using) has a relatively high mechanical resonant frequency that filters (band pass, actually) what it's exposed to. Correcting this is a rather aggressive and unrewarding task; you're better off with a wideband transducer, for whatever it is you're working with.Tim

author: mrkva
author_rank: Unknown
timestamp: January 27, 2015, 12:04:23 pm
content: Quote from: T3sl4co1l on January 27, 2015, 11:59:38 amThat is not what I mean - as you say, that is resonant bandpass. But the signal is missing lower part of the spectrum, which is actually there when the piezo is properly buffered.http://f.cl.ly/items/3v2Z242z3H1b0V2x0s0G/contact_mic_test.mp3Here is a little recording for you with a variation of the circuit above (actually using J201). You can hear it sounds way more natural then just pure piezo recording. The problem is noise floor though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2015, 02:25:28 pm
content: Right, of course... the element itself is a capacitor, and working against load resistance. Simple high pass.I don't see a spec on what that resistance is, unless it's normally very high, to the point where they don't bother to specify; in that case, I guess the phantom power supply resistor would be dominant.To reduce the noise floor, you need low noise and high gain; low noise requires matched source and load impedances, which doesn't make much sense against a capacitor, so, you're kind of stuck there. A JFET is the way to go, having an input resistance about as high as possible. You're also stuck on the output match -- 6.8k or whatever the load is. The remaining matters are gain, and selecting a device which matches best to its surroundings.Really, the only thing missing in your circuit, I would guess: reduce or eliminate the source resistor, to maximize gain.Tim