![]() ![]() If you end up with say a standing Id of 0.5mA, a PP3 battery should last about 1200 hours, ie 50 days. 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!ītw, your 220R resistor will give about 30mA with 9v supply, very bright, may well fail quickly. The basic formula for the reactance of a capacitor is:Ĭ is in Farads, with f = 0, Xc is infinite Its value depends on the frequecy you're measuring it at. 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. ![]() Instrument pickups usually, though not always, work best when they're lightly loaded - the thing they're driving is higher z than they are. Its value is not related to that of R3.Ī regular guitar amp will have a high impedance (z) input, the standard till circuit should drive it fine. 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. You can always think about the hot rod version later. It has modest gain, about 6.8/2.2 = about 3x, or 9.5dB. It's not a source follower, but its output impedance is quite low enough for any reasonable cable and instrument amp input. 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: "what sort of amplifier input do you want to drive?" "how much gain, if any, do you need, to drive the amp you're going to be using?" Yes, ideally we'd like that much headroom, but we probably won't get it and might not need it anyway. Without the luxury of selecting devices from multiple batches, specifying a target Vs for a source follower is unwise. I was wrong to have suggested it - fet threshold voltages generally are too variable from batch to batch. My recommendation of trying to get the source up to anything like 4.5v is infeasible for the 2N3819s in my current batch. The 2N 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 I've just breadboarded a couple of 2N3819s to see how they behaved in a source follower like the one I suggested.Ĭhecking the datasheet for 2N3819, the range of Vgs for Id = 200uA is quite broad, -0.5v to -7.5v. 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. There is something of a "how long is a piece of string" to all this. So it's kind of stuck in my mind ever since, even though it was back in the 70's Four of five of the other staff jumped on him, pinned him to the floor, and took the clever off him. At this a chef in a tall white hat went bright red, grabbed a meat clever, and charged across the kitchen towards him. The soup was absolutely beautiful - and Dave (the older of the two disco brothers) said "this must have been a good can". A few minutes later they took us through to the kitchens, where we were given bowls of chicken soup. ![]() They didn't, but asked if we were with the disco, and said they would sort something out. This still sticks in my mind, because it was a disco for my old school leavers party, and while they were eating we asked at the bar if they did bar snacks (seeing as I had come direct from work, no food!). I went there straight from work, and the preamp changed the sound from 'crap' to 'absolutely amazing' I agreed to meet them one night at a disco, and I built a pair of FET source followers in a tobacco can, powered by a PP3 9V battery, and stuck it in my pocket. It shouldn't have any sort of 'tone' (which isn't really an electronic specification anyway), it should pass the piezo signal through unchanged, apart from it's size.īack to the guitar amp thing, years back a couple of friends used to do discos, and they used a guitar amp fed directly from ceramic PU's - it souinded CRAP!!. ![]()
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