Piezo wiring w/ tone & volume

[Jack.Straw]

Hello. I'm building an upright bass & plan to put 2 piezo's in series wired to volume & tone pots. Below is the diagram i am using. Does it look correct? Also, what type/Ohm pots would you suggest.







Thanks,
-Jack

[Rob Mercure]

Jack,

Have you tried the two piezos in series first to see what happens? These are high impedance - capacitors for all practical purposes - and putting two in series is just going to double the impedance that you are shooting into your volume control. And a piezo PU wants to see an impedance of around 10 mega ohms - very high input impedance - or your treble will be shorted to ground and the resulting tone will be more "thud" than anything. For a few years before I gave up on piezos - at the best they "quack" - I fought trying to get a good sound without using some sort of buffering preamp and finally gave up (even converted one input of my '67 Vibrolux to a 1 meg series/3.3 meg paralell [grid return/leak] so that I could plug in directly w/o success).

It would be pretty easy to make a preamp that summed the two inputs and buffered the output to a lower inpedance without significantly affecting the piezo tone - and I'm sure there are schematics floating around the net that you could capture (lost mine in a HD crash or I'd just post one for you - but a google search should be fruitful).

Not sure what type of guitar you've got or budget but the piezo that closest approximated the open air, true "acoustic" sound of my Martin is the Baggs "I-Beam" - if you can afford it you might want to try it.

Rob

[Steve Conner]

Piezos are the other way around. Low impedance loading kills the bass, not the treble.

[Ben N]

An interesting review in this month's GP of a Yamaha acoustic-electric sort of describes what sounds like a combination of a conventional under-saddle piezo with a couple of soundboard mounted piezos, one each on the bass and treble sides of the bridge. Instead of conventional eq, it has some kind of mixing circuit. The reviewer was quite impressed, said the net result was no quack. Unfortunately, the review is not online.
The idea is interesting, if not exactly new-I have been hearing about people mixing different placements of piezos for a long time, although I guess a lot of tinkering would be needed to get the balance right.

[Jack.Straw]

I intend to experiment with the placement, starting with soundboard placement, one under each edge of the bridge... as you said. I can experiment with placement (and weather parallel or in series works best for me) to my hearts content, but for now i want to figure out what hardware to buy and get the wiring done right. Do you think 1meg pots would be best for tone & volume?

[Rob Mercure]

Yer right Steve, had the brain on backwards. But Jack I still think you're going to have some "issues" hooking up the two pickups - but like you said you've got the tiime to do it. As to parts you're going to need a signal FET, at least 3 resistors, and one cap - as best I rememember - to build a buffer. But that's probably down the road.

Lotsa luck with the project - looks like your using inexpensive transducers which are a great idea as the sound really good for the price (strip out the speaker element of a telephone - and the mic element gives you a good condensor mic you can use for amplifying the guitar - FYI).

Rob

[cbarrow7625]

Jack,

Use at least a 1M pot for each. Like Rob & Steve said, too low aresistance will reduce the bass output. 1M is really too low but give it a shot, it might work for you.

BTW, putting the piezos in series is going to cut the capacitance in half, again reducing your bass output. Parallel is your best shot here.

What you really need is not just a high input impedance, but also an intergrator circuit. An integrator basicall creates the reverse frequency response of the piezo and offsets if so that the result is essentially a flat frequency response. To work well, it really has to be tailored to the exact frequency response of your piezos. If you are interested just Google "Integrator Schematic", you'll find something. They're pretty simple.

Chris

[David Schwab]

You really need a high impedance buffer/mixer circuit. That way you will get the full tone of each pickup, without them being scratchy and thin, and be able to mix them together.

Passive piezos almost always sound bad.

[Jack.Straw]

wow, thanks for all the great input! I've done some reading, but this will be my first go at this.. maybe i should start a little lower down to get a better understanding of the physics aspect of things. Perhaps a single piezo system on this one as a learner, then maybe add in a 2nd one into the mix later once i get a better understand things like impedance & capacitance.

So, if i've understood correctly, even with a single piezo i'm still going to want 2meg or higher pots to keep the bass signal from being reduced. Is that correct?

I should explain something about the instrument at this point. The strings, at least at first, are nylon. I expect that they will be "thumpier" and quieter than standard strings. The resonator is very small for an upright and will always rely on amplification.

Do you think 1 bridge placed piezo will do the job on such an instrument?

[Steve Conner]

I'd be inclined to leave out the pots from the instrument body and just connect the two piezos in parallel directly to the jack. You can build the pots into a separate box, and replace them with an active preamp later, if you decide you want one. The cable capacitance between the piezos and preamp hardly matters, since the piezos are capacitors already.

The secret to using those piezo discs is to make the vibrations of your instrument bend them. Compression hardly gives any signal. I got great results with one screwed to the body of my electric bass at its edges, with washers to space it away from the wood, and a big bolt epoxied to the middle to act as a seismic mass. I got the idea from piezo accelerometers I used to use at work.

This gave colossal bottom end until the day I dropped the bass and the ghetto pickup broke.

[David Schwab]

So, if i've understood correctly, even with a single piezo i'm still going to want 2meg or higher pots to keep the bass signal from being reduced. Is that correct?

You really should have a preamp. All the commercial EUB with piezos have preamps.

You can get something like this if you want to keep it simple.

http://www.stewmac.com/shop/Electron...ck_Preamp.html

[Jack.Straw]

You really should have a preamp. All the commercial EUB with piezos have preamps.

i think an external preamp would serve me better so i don't have to buy one for every instrument i build. Any recommendations there?

on second thought, i think maybe i'll get an external preamp down the road to use with other projects, but use an internal one for this bass. The volume/tone knobs for this project are an important part of the design. While i generally live by the motto 'function first'... in this case i like the design so much that i'm not willing to remove the volume & tone pots from the body simply due to the part the knobs play in the look of the instrument.

Would i wire piezo->preamp->pots->jack or piezo->pots->preamp->jack?

Also, if there is a common product that has a pre-amp circuit i could strip from?

strip out the speaker element of a telephone - and the mic element gives you a good condensor mic you can use for amplifying the guitar

I'm intrigued by this! That's exactly the sort of thing i like to do..

[Aura]

Hi,
I think there was a little oversight on Rob's part...since 2 piezo's in series will half the value of the capacitive reactance as opposed to resistors,which will give you double the value!

[nbtone]

I'm going through the same thing with my electric cello. It has only one piezo pickup. I don't like the sound of the active pots that came with it, so I tried putting a tone control and volume control copied from a fender precision electric bass. I use a tube preamp which makes the tone better without the active pots, but the volume and tone controls don't work right. I'm going to try some 5 or 10meg pots to see what happens.

[David Schwab]

I'm going through the same thing with my electric cello. It has only one piezo pickup. I don't like the sound of the active pots that came with it, so I tried putting a tone control and volume control copied from a fender precision electric bass. I use a tube preamp which makes the tone better without the active pots, but the volume and tone controls don't work right. I'm going to try some 5 or 10meg pots to see what happens.

I think what you actually don't like is the tone of the pickup. Or maybe the preamp was just cheap. A good piezo preamp can make a big difference, but if the pickup is quacky, you need a different pickup. A god preamp would at least let you EQ the tone better.

The tube preamp is doing the same thing as the old preamp. Did the old internal preamp have tone controls?