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Old 14th April 2012, 03:28 PM   #11
Simon B is offline Simon B  United Kingdom
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You need to find out what signal level you're going to be getting out of your pickup.

It needs current gain, yes, so it can drive amp inputs and cables ok.

If you've got a scope and a 10x probe, attach that, play, discover output.

Without a scope and probe, try this circuit, three resistors, two caps, one n-channel j-fet.

preamp2.gif

It's a simple source follower. Vth of fets varies, so for max headroom select the source resistor R2 to put the source roughly half way between the power rails - ie about 4.5v. I suggest you start with 47k. If the voltage is lower than this, increase the resistance, if higher, decrease it, no lower than say 3k3.

Play it, see how loud it will go. If it isn't loud enough, then you need some voltage gain.
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Old 14th April 2012, 08:40 PM   #12
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Quote:
Originally Posted by Simon B View Post
I suggest you start with 47k. If the voltage is lower than this, increase the resistance, if higher, decrease it, no lower than say 3k3.
That's essentially the source follower I was building back in the 70's, 47K would be FAR too high, I can't remember exactly what value I used, but it was only in the low single kilo-ohms, probably 1K or 2.2K

But as you say, select it for about 4.5V on the source - but no need to get too carried away, or you'd have to make it adjustable - just get a reasonable value (for a specific bunch of FET's), then stick to it.
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Old 14th April 2012, 09:08 PM   #13
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Definetelly is in the 1-3 kohm range.
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Old 14th April 2012, 09:33 PM   #14
JGAN is offline JGAN  United States
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The only reason I am using the modified Tillman preamp is because I was talking to someone who said that the j201 JFET should be biased at .1mA, not the .5mA Don recommends. Also, R4 should be 10x the value of R3. Could anybody clarify this?
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Old 14th April 2012, 11:57 PM   #15
Simon B is offline Simon B  United Kingdom
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There is something of a "how long is a piece of string" to all this.

I am surprised at the low values being suggested here for Rs - my schematic was for a source follower, with the assumption that it would be used to drive a hi-z instrument amp input - I wonder if there might perhaps be bit of confusion about exactly what type of circuit needs what value of Rs? What purpose is served by using a value as low as 1k or 2k here? The higher the standing current the shorter the battery life.

I've just breadboarded a couple of 2N3819s to see how they behaved in a source follower like the one I suggested.


Testing the first two out of the bag:
with Rs = 2k2, Vs = 1.21v & 1.33v
with Rs = 47k, Vs = 1.59v & 1.73v

Checking the datasheet for 2N3819, the range of Vgs for Id = 200uA is quite broad, -0.5v to -7.5v. The 2N5487/8/9 appears just as variable as the 2N3819, and my datasheet for the J201 again doesn't give Vgs range for a useful standing Ids, but it does give cut-off Vgs in the range -0.8v to -4.0v, with Idss in the range 0.2mA to 1.0mA

My recommendation of trying to get the source up to anything like 4.5v is infeasible for the 2N3819s in my current batch. I was wrong to have suggested it - fet threshold voltages generally are too variable from batch to batch. Without the luxury of selecting devices from multiple batches, specifying a target Vs for a source follower is unwise. Yes, ideally we'd like that much headroom, but we probably won't get it and might not need it anyway.

Two questions thus far unanswered are

"how much gain, if any, do you need, to drive the amp you're going to be using?"

and more fundamentally

"what sort of amplifier input do you want to drive?"

If you just want to cut to the chase, and assuming you want to drive a high z instrument amp input, which seems the most likely case, I'd recommend you go for the standard Till type preamp:

Discrete FET Guitar Preamp

It's not a source follower, but its output impedance is quite low enough for any reasonable cable and instrument amp input. It has modest gain, about 6.8/2.2 = about 3x, or 9.5dB. You can always think about the hot rod version later.

R4 is simply a bleed resistor to keep the output side plate of C1 at ground potential, so that plugging into an amp doesn't produce a nasty crack. 51k is fine, as is 220k. Its value is not related to that of R3.

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Old 15th April 2012, 03:23 AM   #16
JGAN is offline JGAN  United States
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Thanks Simon. I will take your advice and just build the Tillman preamp for now. I do have about 20 j201 JFETs to choose from for the correct threshold value, but I do realize that the tolerances for JFETs are generally very high.
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Old 15th April 2012, 03:49 AM   #17
Simon B is offline Simon B  United Kingdom
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Hi JGAN, no worries.

I've had vague ideas about using piezo pickups on various musical oddities for ages now, and your thread gave me a push to actually do a little bit of thinking, as opposed to day dreaming, so thank you.

I think in future I'm going to add a couple of jfets every time I order components, so as to try to get a selection from different batches, though I bet I'll end up with loads from the ends of the Vth spread and none from the middle...

3M3 will be easier to get hold of than the 3M0 specified and should be fine. If the bass frequencies show some drop off, bump that up to 4M7, it'll depend on the capacitance of the piezo element.

If you do select devices for the standard circuit, try to end up with roughly, half the supply voltage dropped across the fet.

If you get stuck or want to ask about something, feel free to do so, here, pm, or email. Be interested to hear how you get on.



Last edited by Simon B; 15th April 2012 at 04:00 AM.
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Old 15th April 2012, 04:10 AM   #18
JGAN is offline JGAN  United States
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No problem Simon,
I actually forgot to answer your questions posted earlier Now, to preface this, I'm not a rock/electric musician of any sort, so this is my first project with pickups, preamps, mixers, amps, etc. I'm just going to put it in layman's terms, because that's about as good as I can get...

Quote:
Originally Posted by Simon B View Post
"how much gain, if any, do you need, to drive the amp you're going to be using?"


I'm not driving a power amp, just one of those regular guitar amps.

Quote:
Originally Posted by Simon B View Post
"what sort of amplifier input do you want to drive?"


Not exactly sure what that means, but 1/4" TS? If you're talking impedances, then I have no clue.

For impedances, I read that ideally the preamp input impedance should be 10x the pickup impedance. So if I do the Tillman preamp, could I use a multimeter to measure the impedance of the pickup, multiply by ten, round up, and then change R1 to that value?

And one last question

I would like to add a power-on LED + appropriate (220 ohm) resistor to the circuit. Should I put these between the +9 volts and ground, parallel to the C2 cap?

Last edited by JGAN; 15th April 2012 at 04:16 AM.
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Old 15th April 2012, 04:56 AM   #19
Simon B is offline Simon B  United Kingdom
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A regular guitar amp will have a high impedance (z) input, the standard till circuit should drive it fine.

Instrument pickups usually, though not always, work best when they're lightly loaded - the thing they're driving is higher z than they are.

Your piezo element should give a reading of infinity when checked at dc with a meter - simplifying a bit, its impedance is like a capacitor - it's a complex impedance - not just R, but C too. Its value depends on the frequecy you're measuring it at. The basic formula for the reactance of a capacitor is:

Xc = 1 / (2 x pi x f x C)

C is in Farads, with f = 0, Xc is infinite

You could add a power on led, but I really really wouldn't! Why? Because a reasonable drive level for an led is about 20mA, ie at least 40 times the amount of current the preamp will draw!

Btw, your 220R resistor will give about 30mA with 9v supply, very bright, may well fail quickly.

If you end up with say a standing Id of 0.5mA, a PP3 battery should last about 1200 hours, ie 50 days. Put and led in at 20mA and you're down to 30 hours....up to you though!
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Old 15th April 2012, 05:17 AM   #20
JGAN is offline JGAN  United States
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How do I calculate C then? Or should I just stick with 3M ohms? I feel like I should use an oscilloscope for this... haha

I understand that using a LED will greatly diminish battery life, but tbh I don't really mind. And the bulb does have a current of about 30mA, so yes it will drain even faster (12 hour lifespan).

Last edited by JGAN; 15th April 2012 at 05:31 AM.
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