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Post by andy on Oct 12, 2008 11:40:08 GMT -5
andy - the network is shown schematically in the lower left of the screen-shot above. Oh yes, just like you stated in the post itself! This smilley now has two possible meanings... ...just needs the white cane to top it off.
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Post by JohnH on Oct 13, 2008 19:53:18 GMT -5
The preamp is coming along. I mocked one up on a breadboard last night, trying to get away with a single JFET for the piezo stage, and it’s OK, but I think it needs two. Plus another to buffer the mag pups and a fourth to blend them together and send out a low impedance output.
In the meantime, I’ve noticed an annoyance that I need to track down. On the piezo signal, there’s a high pitched metallic ring that gets provoked particularly by the third string around the first or second fret. You can hear it on the clips above. Its definitely resonant, but it cant be suppressed by damping any parts that can be reached. I’m thinking it may be the short lengths of tensioned string inside the trem block, which the piezo is right next to. If so, I can possibly fix it by pushing a short length of insulation sleeve over each string at that location, to act as a damper. But I’ll test it first, if it is that, I think the pitch will change with change of string tensions.
John
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Post by 4real on Oct 14, 2008 3:23:22 GMT -5
That's good thinking john...maybe...though the length of string is so small, you wouldn't think that has so much of an impact. It could be the resonance of the metal bridge or something.
One strategy would be to attach the piezo with some kind of damper...I used doublesided tape but even some thin foam or foam tape or something might be worth a try...
or if there is an air gap...the piezo element itself my have a resonant frequency...
Another test is if it is note specific too...
A down side is that you may have to do a little adjustment to the preamp eq if you make changes to the piezo mounting or location...
keep up the good work...
pete
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Post by JohnH on Oct 18, 2008 7:07:08 GMT -5
I got the pre-amp mixer circuit built on strip board today: It's sized to fit, with a battery, in the trem cavity which is empty. Hence I didn't need to build it to absolutely minimum size - it's much easier to have a bit of space on the board if possible. It's working well with a rough external test wiring. The EQ section has three presets that control the start and end frequencies of the mid scoop, and the overall bass to treble balance The mixes of piezo and mag sounds are especially interesting. But the piezo on its own has its own character. I've found sounds similar to a big old dreadnought, down to a smaller body bluesy kind of sound, by tweaking it The piezo now has a hard rubber bearing pad between it and the trem block, and it seems to be a bit more open sounding that way. Another strange thing I've found, is that by pressing on the bridge, it creates a dc pulse that momentarily overloads the peizo or preamp. Holding the pressure steady, the signal comes back. I have the preamp with a very high input impedance, but I think it may be better with a bit more load, to result in a low frequency roll off below guitar frequencies. I'm now thinking to keep the mag wiring as is, and control the system with a three-way toggle, for mag, both, piezo. There's no volume control for the piezo but in the both setting, the piezo is reduced 6dB, with volume control just for the mag. It gives a simple range of mix options. The circuit could equally be wired with a pot for piezo/mag blending Also, I'm now finding more significance in the phasing ot the mag and piezo. The routes through the preamp for both pickup types are non-inverting , but I think the EQ section causes various phase changes, such that the piezo and neck combo now sounds fuller with the neck reversed in phase. When i get inside to wire it in, I might see if I can flip all the magnets to correct this. John
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Post by JohnH on Oct 19, 2008 1:56:00 GMT -5
It's in and working. In the end, I decided on a blend pot for peizo/mag blending, and so the original blending function between pups in series is handled by the 5-way switch. I reversed all the mag pups, which helped phase combinations of mag and piezo. Here's the installation: and a close up of the piezo 'short stack': There's a single screened multicore cable that makes all the connections to the main cavity, for power control, mixer pot, magnet pups and output signals Tomorrow is band rehearsal - got something new to show you guys..... John
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Post by 4real on Oct 19, 2008 6:38:30 GMT -5
Looking good John...be good to hear it sometime...
That's some substantial cabling.
It's probably a good thing you don't have a tremolo...the piezo could well pickup the squeak of the springs. Also, great idea adding that stopper (and some australian currency I see) as you'll be picking up a combination of the bridge mediated through the rubber (taking out some of the quackiness I suspect) and the body wood behind the block (picking up the woodiness of the guitar)...
pete
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Post by JohnH on Oct 20, 2008 14:53:39 GMT -5
Well it sounded just great through the PA at rehearsal last night, either clean or slightly dirty. The PA speakers brought out the strummy string zing, that was very clear. Its now incorporated into our set on a few tunes and its a nice contrast to my friends Tele. The circuit works fine, but I think I can roll back a bit on the low bass, which I've slightly overcompensated for having started with not enough. I was also pleased not to have any feedback problems at high volume, which is benefit of a solid(ish) body.
I'll make a new clip soon.
John
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zuee
Rookie Solder Flinger
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Post by zuee on Oct 20, 2008 18:56:02 GMT -5
I experimented with my piezo and got very annoyed by all the noise and feedback that I kept getting. Then I got an idea to use my cylinder neodymium magnet and attach the piezo to the metal areas of my guitar with the magnet. This magnet is ridiculously strong, I had a blood blister in my thumb from two of these magnets colliding. The magnet pushes piezo against the metal(s) with a lot of force, reducing the noise and feedback. It gave me a cleaner sound. I attached the magnet with the piezo on different parts of the guitar and all gave different sounds. Placing the magnet behind the neck (the truss rod is the metal) give good sound, too, and the farther down the neck, the deeper the tone. I even placed them on the tone knobs.
I really like your idea with placing the piezo behind the tremelo bridge. I heard your sound clip and it sounds great! Piezos are cheap, yet so wonderful.
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Post by JohnH on Nov 8, 2008 16:42:27 GMT -5
A bit more on this, having had it running for a couple of weeks. It's still sounding good, and with a few recent tweaks, here's the schematic: This is the frequency response when the blender is fully to the piezo: The three presets in the tone network are roughly at the settings shown, giving a dip where I found it to be needed in my case. It still has an element of dobro/banjo type of sound which I'm happy to accept. I estimate that the raw piezo gives a signal about 10dB less than one of my relatively powerful single coils. So the piezo signal gets two gain stages, one to compensate for this lower output, and a second to boost it back up after the tone network. The mag signal is treated neutrally with zero gain. The whole output is then buffered again, and is capable of driving a PA line input or a guitar amp input. The magnetic input is taken after the volume and tone controls - any passive pickup wiring scheme could be used to combine magnetic pickups. The blending is with R32, and its seemless from full mag to full piezo - its really fun to play with and I've found some nice sounds with about 20/80 mag/piezo. One thing that surprised my with the piezo is that I did not need a very high input impedance, which I had read elsewhere was generally required. The piezo seems to act as a capacitor which generates voltages, and the specs say it is equivalent to 20nF. I found that an input impedance (R2) of 150k gave it a full bass sound, but then a roll off at lower frequencies to reduce unwanted sub-bass thumps. The current drain is about 0.35mA, so I'm hopeful of a few hundred hours use out of a battery. I'll be posting more clips, and I want to compare it to real acoustic which I have aquired - watch this space! John
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Post by JohnH on Nov 8, 2008 22:39:14 GMT -5
Now here are some samples, with other guitars for comparison: First some strums: www.soundclick.com/bands/default.cfm?bandID=674084&content=songinfo&songID=7044456This has: 1. An old Yamaha FG100 acoustic, recorded with a dynamic mic. A smallish bodied beginer guitar that I fixed up recently 2. A good Maton dreadought EM325C, recorded with mic as above - recently aquired 3. The Matons own piezo pickup recorded simultaneously with 2. 4. The new Strat piezo system, subject of this thread - full piezo 5. As 4, with a bit of neck magnetic pickup added 6. Strat bridge pickup Secondly, a bit of Tracy Chapman strumming, first with the Maton piezo, then the Strat piezo: www.soundclick.com/bands/default.cfm?bandID=674084&content=songinfo&songID=7044463There you go! I'll take the Maton first, but the Strat system is OK i think! John
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