Guitar preamp for piezoelectric pickup (JFET input opamp only)

[pEN DoTA]

Sori ngayon ko lang na tingnan to.

Assuming your power supply is 9V, start with the values of R3 = 1K8 and R2 = 560R.
Measure the voltage at the drain and at the source. Vd should be around 5.4V and Vs should be around 1.7V. If not, check that Vsupply - Vd => 3V and Vd - Vs => 3V. The target is to get a +/-3V swing from the drain terminal.

thanks sir TTH
+1 for u.

[n0th1n6]

Nagawa ko na po ito ngayon. Nung battery powered ang linis ng tunog pero nung ginamitan ko na ng
7809 nagkaugong na.

Ito cam phone pic, medyo malabo po
 

[Disabled]

try using cap multiplier.

[n0th1n6]

Baka ibalik ko na lang sa battery powered since mahina naman konsumo nito. Kaya ayaw ko na ng cap multiplier dahil merong mga ilang parts pa na kailangan, ayaw ko na gumawa ng bagong pcb. Kung pwede nga na jumper na lang ang modification ganun na lang

+1 sau sis, kung may time uli pwede ko subukan cap multiplier

[Disabled]

salamat

[n0th1n6]

Noted sis. Pag hinawakan ko ang tab ng regulator, nawawala ang humming. Grounding issue kaya?

[Engr. Dacuycoy]

because of the limited parts, battery talaga ang bagay dyan sis

[efren]

Noted sis. Pag hinawakan ko ang tab ng regulator, nawawala ang humming. Grounding issue kaya?

Ground loop> try this: cut-off negative side of C2 & connect via jumper to negative of 100uF filter cap.
cut-off also after the foil at c6 & connect via jumper @ negative side of 100uF.
solder a .1 at the output pin of 7812 & center pin (Gnd)
now try on ac adapter


you will notice we are making a "star ground" this way

[n0th1n6]

Nagawa ko na yung jumpers. Nabawasan na yung hum. Pero pag around 75% meron pa din. Palagay ko yung 1m kelangan din ilagay sa negative ng 100uF

Salamat sir Efren

[efren]

Nagawa ko na yung jumpers. Nabawasan na yung hum. Pero pag around 75% meron pa din. Palagay ko yung 1m kelangan din ilagay sa negative ng 100uF

Salamat sir Efren

ah yes, i overlooked that 1meg, increase mo rin 100uF to at least 1000uF if possible, that 100uF might have high ESR .....

[n0th1n6]

Ill do that sir. Gumawa na din ako ng new layout at inayos ko na ang ground, kaso wala na akong pre-synthesized board. Antay muna ng 1 week ang new board. Remedyuhan ko muna ito.

[macky]

pa-OT naman sis n0th1n6:

@sir TTH,

parehas lang din po kaya ang principle ng pagcompute ng R2(SOURCE resistor) dito sa attached schema ng "iPad-Guitar Input Buffer"? since walang R3 kasi directly connected na yung DRAIN ng FET sa mic-input ng iPad(which has 2.8Vdc coming from iPad). 2N5457 din po kasi sana ang gagamitin ko.

eto po ang schema:


source: http://www.planetz.com/?p=1531

[tony]

Noli, yung mga jfets mo nasa akin na, pwedeng mapick-up at your convinience...

[n0th1n6]

Sir punta po ako dyan sa inyo. Kaso baka next weekend pa dahil di ako pwede tumakas sa work.

OT:
Meron ka po bang stock pa ng LM3886 , pagbilhan po at a very discounted price

[TinTopHack]

@sir TTH,

parehas lang din po kaya ang principle ng pagcompute ng R2(SOURCE resistor) dito sa attached schema ng "iPad-Guitar Input Buffer"? since walang R3 kasi directly connected na yung DRAIN ng FET sa mic-input ng iPad(which has 2.8Vdc coming from iPad). 2N5457 din po kasi sana ang gagamitin ko.

eto po ang schema:


source: http://www.planetz.com/?p=1531

Pareho lang yan ang pag determine ng R2. From the way the drain is connected directly to the IPad input, it is logical to assume that there is a pull up resistor at the IPad input. This pull up resistor must be pulled up to +5V since IPad uses logic components. You could start with a value of 470R for R2 then adjust it to get 2.5 to 2.8V at the drain.

[macky]

Pareho lang yan ang pag determine ng R2. From the way the drain is connected directly to the IPad input, it is logical to assume that there is a pull up resistor at the IPad input. This pull up resistor must be pulled up to +5V since IPad uses logic components. You could start with a value of 470R for R2 then adjust it to get 2.5 to 2.8V at the drain.

thanks po sir TinTopHack, one question po: tataas po ba ang GAIN if i lower the R2 value? or baligtad? have tried a value of 22K parang overdriven na ang output. thanks po ulet! +1 po sa inyo.

[zrehtiek/keitherz]

maybe the gain is independent on the resistor kasi wala namang feedback loop.
gain will be dictated by the transconductance of the FET. (just a guess)

what we are adjusting with the resistor is the offset voltage for maximum swing.
that overdriven effect is because of the clipping.

[macky]

^so lowering the value of R2 will cause to drop the DRAIN voltage, tama ba?
if tama - what will be its effect? GAIN> or GAIN< ?

[tony]

gain will be dictated by the transconductance of the FET. (just a guess)

correct...the universal gain equation is gm x Rl, where gm is the transconductance of the active device and is proportional to current, Rl is the parallel combination of the internal resistance of the active device, the load resistor and the input resistor of the following stage...

gm is is series with whatever unbypassed resistor present at the source, or emitter or cathode of the active device....and considered in calculating gain.....

by gain we mean voltage gain.....there is also current gain and power gain....but gain alone implies voltage gain...

[zrehtiek/keitherz]

i think the gain is most affected by R3. if you would imagine the equivalent circuit of having a pull up resistor (lets say Rp) from the drain of the jfet to +5V.
then Rp and R3 would form a voltage divider therefore, attenuating the output waveform.

again varying the value of the resistor R2 will only change the offset voltage. we would want this to be equal to half the supply voltage (hence 2.5V because the supply voltage is 5V) at when there is no input so the output could swing from 2.5V to 5V for the high side (positive swing) and 0V to 2.5V for the low side (negative swing).

if the value of R2 is improperly selected and lets say that it made the offset voltage be equal to 3.8V, then we would only have a swing of 3.8V to 5V for the positive swing and 0V to 3.8V for the negative swing. because of this, it would cause distortion and more clipping.

i hope i got things correct and clear.