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Old 11th April 2012, 05:28 PM   #1
JGAN is offline JGAN  United States
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Default Are piezo elements line-level or instrument-level signals?

Hi guys,
This is probably a stupid question, but I was wondering whether piezo elements used in instrument pickups output a line-level or instrument-level signal? I would like to know so I can get an appropriate preamp with the correct amplification for a pickup.

Thanks,
JGAN
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Old 12th April 2012, 04:17 PM   #2
kevinkr is offline kevinkr  United States
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This seems to be musical instrument related and should therefore be in I&A where I will move it.
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Old 12th April 2012, 04:19 PM   #3
kevinkr is offline kevinkr  United States
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Should really be looking for a pre-amp specifically designed for use with piezo elements.. Is this a bridge pick up, under saddle type pick ups or something else?
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Old 13th April 2012, 12:52 AM   #4
Simon B is offline Simon B  United Kingdom
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The key thing with piezo elements is that they must work into very high impedances.

Line implies not just signal strength of the order of one volt, but medium to low impedances.

Instrument level seems to mean different things to different people, but usually higher impedances than line. You might have some luck with some high-z guitar amp inputs, though not all.

Often, piezos have a preamp as close to the pickup as possible, to avoid capacitive losses of high frequencies in cabling, and interference pickup.

A simple fet source follower would be a good starting point.

As Kevin asks, what's it for?

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Old 13th April 2012, 02:52 AM   #5
sreten is online now sreten  United Kingdom
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Hi,

Typical piezo elements are instrument level and can work directly into high
impedance instrument inputs, as on guitar amp, but don't like long leads.

Battery powered preamps built into guitars allow EQ etc and the ability to
drive long leads but don't change the level, its still suitable for a amp input.

Best arrangement I found years ago was a shortish lead to an acoustic
guitar multi effects pedal and then on to the amplifier. Undersaddle pickup
with the socket in the endpin the best way of converting an acoustic.

rgds, sreten.
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Old 13th April 2012, 07:40 AM   #6
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Quote:
Originally Posted by sreten View Post
Hi,

Typical piezo elements are instrument level and can work directly into high
impedance instrument inputs, as on guitar amp, but don't like long leads.
Except high impedance instrument inputs aren't really anywhere near high enough (only a few 100K usually), raw piezos need multi-megaohm inputs.

A simple FET buffer is all that's required, with perhaps a gain stage as well, if you need to bring the piezo up to a similar level as other instruments?.
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Old 13th April 2012, 10:29 AM   #7
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1M is a common input impedance for (passive) instruments, but yeah, ideally they're looking at several megaohms of impedance, because you loose low-end otherwise.
The output of a piezo element can be pretty large, depending on how hard and how you compress/bend it (and the type of piezo element of course). You can expect several volts of output, so you need a preamp which has some overhead.
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Old 13th April 2012, 04:18 PM   #8
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Quote:
Originally Posted by Jarno View Post
1M is a common input impedance for (passive) instruments.
Most are less than 1M though.
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Old 14th April 2012, 02:59 AM   #9
JGAN is offline JGAN  United States
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This is a pickup in the bridge of a cello. I'm aware of the fact that the preamp should be close to the pickup, along with the problems with interference and signal degrade over long cables.

For preamps, I was looking at a modification of the Tillman preamp, which would offer about 14dB gain, or the Ritter preamp, which gives about 30dB gain. However, I'm not sure what sort of tone the Ritter preamp offers.
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Old 14th April 2012, 09:00 AM   #10
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Quote:
Originally Posted by JGAN View Post
This is a pickup in the bridge of a cello. I'm aware of the fact that the preamp should be close to the pickup, along with the problems with interference and signal degrade over long cables.

For preamps, I was looking at a modification of the Tillman preamp, which would offer about 14dB gain, or the Ritter preamp, which gives about 30dB gain. However, I'm not sure what sort of tone the Ritter preamp offers.
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.

Back 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!!. 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. I went there straight from work, and the preamp changed the sound from 'crap' to 'absolutely amazing'

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!). They didn't, but asked if we were with the disco, and said they would sort something out. A few minutes later they took us through to the kitchens, where we were given bowls of chicken soup. The soup was absolutely beautiful - and Dave (the older of the two disco brothers) said "this must have been a good can". At this a chef in a tall white hat went bright red, grabbed a meat clever, and charged across the kitchen towards him. Four of five of the other staff jumped on him, pinned him to the floor, and took the clever off him.

So it's kind of stuck in my mind ever since, even though it was back in the 70's
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