traderbam said:
I will try it with 600 ohms tonight (it'll be late so will report back tomorrow!)
As far as I understand - I can't really attenuate the input - from what I can gather piezos are very fussy about this type of thing.
re the output impedance - I genuinely don't know how much I can wander from the 100 ohms figure. Like I say...the unit is drivng a device that the manafacturer states (in its spec) as expecting to be driven by a cct with output impedance of 100 ohm presented as its input.
HankMcSpank said:
piezo transducer impedance is nearly pure capacitance
a capacitor to gnd of the same value as the transducer will attenuate the V by 1/2 - with a flat frequency response
if you have specs for the tranducer we can give you the value that would give a particular attenuation and calculate possible frequency response changes
if the 1 Meg input load of the current circuit is OK then the input C attenuation actually would extend the low frequency response
this really is the simplest approach and should be tried 1st
I have no figures for the piezo's output impedance (the manafacturer doesn't publish it!).....I have reason to believe that the piezo pickup has an output impedance in excess of 5Meg so you can see there's already a fair degree of attenuation at the imput already....I'd rather take the last bit of 'heat' out the signal at the output!!!
Believe me, I'm not being cagey ....it's just I don't have more info!
The piezo in scope is an LR Baggs guitar bridge saddle piezo - I'm kind of working blind here! All I seek is to buffer the piezo & pass it on to the unit that will extract pitch & level info from the signal
I'd read that 1M is as low as you can at the input to the opamp really go...so that's what I went with. As understand it, some attenuation can be applied at the input by placing a capacitor in parallel with the pizo pickup ....but this is muddy waters for me!
The output of the first opamp stage (by & large) is ok, but in my opinion just a tad 'hot' so like I say I'd really like to have attenuated it just a tad more!
The piezo in scope is an LR Baggs guitar bridge saddle piezo - I'm kind of working blind here! All I seek is to buffer the piezo & pass it on to the unit that will extract pitch & level info from the signal
I'd read that 1M is as low as you can at the input to the opamp really go...so that's what I went with. As understand it, some attenuation can be applied at the input by placing a capacitor in parallel with the pizo pickup ....but this is muddy waters for me!
The output of the first opamp stage (by & large) is ok, but in my opinion just a tad 'hot' so like I say I'd really like to have attenuated it just a tad more!
Ok guys, firstly - the attenuation worked a treat (in the sense that the signal reduced as expected).
However, I have to go back to the drawing board though - as it seems the device I'm driving is not happy about detecting the pitch/frequency guitar of the low E string - it's glitching a little...ie when I pluck the string constantly - most plucks it detects as an E, bt it throws a few Gs in there too! (btw it detects pitch on all the other strings fine - & incidentally, there's a separate circuit for each guitar string).
Actually the article posted up by jcx earlier (here... http://www.ozvalveamps.elands.com/pickups.htm#Piezo )
was a revelation! I can see that the frequency response drops off severely when using a 1M input resistor (as I am). The frequency of the low E string on a guitar is 82Hz...looking at the graphs on that page, the low E string going to be somewhat choked using my simple circuit.
looks like using a capacitor in parallel with the piezo is the way to go - this will pad the input & gives a flatter fequency response - I can always add a little gain via the opamp if needs be.
Seemingly, I need to establish what the capaitcance of the piezo pickup is first though...& I need an lCR for that (unless someone can illustrate a simple way of gleaning this info?!)
jcx- great article - you seem well versed in this area?! Incidentally, when you said the following earlier...
"how about C to gnd on the input? - forms a AC divider with the pickup so a small hi quality film or NP0/C0G would work"
excuse me not knowing, but what do you mean by NP0/C0G?
Many thanks for all your help - to paraphrase Arnie ....I'll be back!
However, I have to go back to the drawing board though - as it seems the device I'm driving is not happy about detecting the pitch/frequency guitar of the low E string - it's glitching a little...ie when I pluck the string constantly - most plucks it detects as an E, bt it throws a few Gs in there too! (btw it detects pitch on all the other strings fine - & incidentally, there's a separate circuit for each guitar string).
Actually the article posted up by jcx earlier (here... http://www.ozvalveamps.elands.com/pickups.htm#Piezo )
was a revelation! I can see that the frequency response drops off severely when using a 1M input resistor (as I am). The frequency of the low E string on a guitar is 82Hz...looking at the graphs on that page, the low E string going to be somewhat choked using my simple circuit.
looks like using a capacitor in parallel with the piezo is the way to go - this will pad the input & gives a flatter fequency response - I can always add a little gain via the opamp if needs be.
Seemingly, I need to establish what the capaitcance of the piezo pickup is first though...& I need an lCR for that (unless someone can illustrate a simple way of gleaning this info?!)
jcx- great article - you seem well versed in this area?! Incidentally, when you said the following earlier...
"how about C to gnd on the input? - forms a AC divider with the pickup so a small hi quality film or NP0/C0G would work"
excuse me not knowing, but what do you mean by NP0/C0G?
Many thanks for all your help - to paraphrase Arnie ....I'll be back!
NP0/C0G are low tempco, low dielectric constant ceramic capacitors which measure as good as or better than some film caps and silver mica caps on specs that audiophiles claim important to "capacitor sound"
for highest quality teflon or polystyrene, then polypropylene plastic film caps are considered best for audio
NP0 or C0G type ceramics are an acceptable 2nd for some apps
although cap quality may be secondary with a piezo ceramic source
from the article we can guess that 1-5 nF will likely do the job, just try a few values and look at your output level, should be easy to "tack solder" a trial Cap across the 1 Meg R to check out atten vs value
for highest quality teflon or polystyrene, then polypropylene plastic film caps are considered best for audio
NP0 or C0G type ceramics are an acceptable 2nd for some apps
although cap quality may be secondary with a piezo ceramic source
from the article we can guess that 1-5 nF will likely do the job, just try a few values and look at your output level, should be easy to "tack solder" a trial Cap across the 1 Meg R to check out atten vs value
Just to give some closure here - putting some extra capacitance across the piezo pickup (ie in Parallel with it ) attenuates the input signal nicely & means I don't have to mess about loading the opamp output.
I actually I upped the input resistor R1 to 5Meg which obviously upped the output, but 500pf of capacitance brought it back down enough to get the right output level I sought - a positive by-product of these actions (though I haven't actually proved it) is that the low end frequency response will be better by some margin (a Standard Guitar Low E detuned say a few semitones will be about 60HZ - so I need that the response to embrace that frequency). I was reluctant to use 5M initially due to noise concerns, but apparently the capacitance somewhat negates that worry (I certainly can't hear any noise)
A big hearty thanks to you all - definitely the internet working at its finest!
I actually I upped the input resistor R1 to 5Meg which obviously upped the output, but 500pf of capacitance brought it back down enough to get the right output level I sought - a positive by-product of these actions (though I haven't actually proved it) is that the low end frequency response will be better by some margin (a Standard Guitar Low E detuned say a few semitones will be about 60HZ - so I need that the response to embrace that frequency). I was reluctant to use 5M initially due to noise concerns, but apparently the capacitance somewhat negates that worry (I certainly can't hear any noise)
A big hearty thanks to you all - definitely the internet working at its finest!
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