Replies to This Discussion

Permalink Reply by Don Thompson on October 20, 2010 at 11:31am

I have never heard a difference in ohm ratings for a piezo volume control. Doesn't really matter to me.Audio or linear taper yes.
Don

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Permalink Reply by Mark on October 20, 2010 at 1:02pm

The diameter of a piezo disk is not necessarily the best indicator of its sound characteristics. An interesting thing to look at is the quoted resonant frequencies. I have 27mm diameter piezos with resonant frequencies varying from 1.8kHz to 4.2 kHz. I also have a couple of 41mm diameter piezos with a resonant frequency of 2kHz.

The question about multiple piezos is also a little more complicated than it might look at first. With piezos we're talking about impedance rather than resistance. Although both are expressed in ohms, impedance is not the same thing as resistance unless you're dealing purely with direct current - which we're not. In the case of an alternating current situation (which is what you have with audio) then impedance can be considered as having components derived from a capacitance effect and an inductance effect as well as a resistance effect.

While resistances add in series, capacitances add in parallel. So adding impedance is not simple like adding pure resistances or pure capacitances. To work it out properly you need complex numbers and you need to take account of frequency. Frankly that's way too complicated to worry about when you're building CBGs.

I know opinions vary, but the ones I've found most credible all tend towards wiring piezos in parallel and that's worked well for me. It doesn't seem to give any noticeable increase in volume but (to my ear) a combination of two different piezos seems to give a more balanced sound.




carverman said:

MichaelS said:

As far as 2 in parrallel that is how I use them and to me they don't sound louder, just better to my ears and they reduce the feedback from the amp, I don't know how, it just works. I use 2 of CB Gittys 20mm piezos.

Well if the soundboard vibrates and creates pressure on one, and you get a electrical
a/c signal from that which represents those vibrations, then
two in phase... should add more electrical signal. The impedance lowering is
another thing that
I can't quite understand how it works on piezos. It' certainly works that way on
copper wire pickups.
Ohms law I suppose..if you take two resistors of equal value (10Kohm + 10kohm)
the net result
measured with an ohmmeter is 5kohms.
The current splits down both paths and half goes through each resistor but the
voltage drop across both is the same. Piezos transducers are probably another math equation..but
I would think they should work in a similar fashion.

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Permalink Reply by Don Thompson on October 20, 2010 at 1:03pm

Correct, if I asked which taper to use for a volume control I would recommend audio taper. Linear for tone, which in this instance does not apply.
Don



Turkish said:

Don Thompson said:

Audio or linear taper yes.

Audio taper is recommended for a volume pot, as the human ear does not respond linearly to loudness.

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Permalink Reply by carverman on October 20, 2010 at 6:41pm

C. B. Gitty (Ben) said:

Carverman - the opinions on this stuff vary widely depending on who you ask, and I don't claim to be the grand oracle on any of it. I can just tell you what I have perceived to be the "average" take. Some guys say that the larger piezos have better volume and bass response, other people say there's no difference. Personally, I think there may be a bit of difference but that your placement and mounting method are MUCH more important than the size of the piezo. I cover all of what I consider to be "piezo gospel" in a two-part article on my site: http://www.cbgitty.com/content/view/34/43/
/blockquote>
Ok..I've looked at the different sizes of piezos and their resonant freq response. Now whether
that makes any difference in tone, I don't know..but after dealing with a very thin sounding
Piezo t-o-m (on one of my previous projects) and having to fine tune it with a tone control,
I'm leaning more now towards
just getting the EQ-7545R preamp, even if it takes a battery and you need a large
opening (3.75 x 1 9/16) in the top side. I still need space for the volume/tone for
the P-90, and a selector switch, so it may be a tight squeeze, but I could always
put the volume/tone selector on the bottom side with only 1 grommet and the
preamp and a grommet on the top side for balance. Does this sound like it's
feasible?

Oh and..how does the rod piezo mount? lengthwise underneath the bridge?

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Permalink Reply by carverman on October 21, 2010 at 2:20am

TO CB GITTY:

It appears that my response to your response got buried in the response exchange,

MY SPECIFIC QUESTION TO YOU is in the form of this comment. (See below)

Ok..I've looked at the different sizes of piezos and their resonant freq response.

Now whether
that makes any difference in tone, I don't know..but after dealing with a very thin sounding
Piezo t-o-m (on one of my previous projects) and having to fine tune it with a tone control,
I'm leaning more now towards
just getting the EQ-7545R preamp,
even if it takes a battery and you need a large
opening (3.75 x 1 9/16) in the top side.


I still need space for the volume/tone for
the P-90 pickup, and a mini-selector switch, so it may be a tight squeeze, but I could always
put the volume/tone selector on the bottom side with only 1 grommet

and the
preamp and another grommet on the top side for balance.
Does this sound like it's feasible?
Oh and..how does the rod piezo mount? lengthwise underneath the bridge?

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Permalink Reply by Ben "C. B. Gitty" Baker on October 21, 2010 at 2:55am

The rod piezo can mount under the bridge, you can make a slot in the bridge itself and glue it in, you can glue it to the inside of the box, pretty much anywhere you could mount a disk.

As for the pre-amp and feasibility... I can't guarantee that it will give you the exact sound you are looking for, but it should sound pretty good! When it comes to stuff like that, about the best advice I can give is to try it and see!

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Permalink Reply by David L. on October 21, 2010 at 4:00am

Hi Caveman,

Regarding -
" still need space for the volume/tone for the P-90 pickup, and a mini-selector switch"

It depends upon if you want separate controls for the P-90. If not you could run the piezo and P-90 via a 3 way switch and the output from this to your preamp.
I did this for a CBG with 2 minihumbuckers and disc piezos using a 5 way switch.

Video link is here http://www.cigarboxnation.com/video/comparing-cbg-pickups

When I get around to it I still plan to add a preamp bypass so I can use it if the battery goes flat.

If you prefer piezo discs rather than a rod you can get the 2.5mm connectors off ebay which plug into these preamps (if it is the same as the ones I have come across) and wire the disc to that.

Regards,
David

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Permalink Reply by carverman on October 21, 2010 at 7:27am

Roy Alderman said:

Haha - I guess the jokes on me. I thought it was hard spacing 4 tuners on a 5 inch headstock!Quite a wide variety of skill levels here.

For a 3 + 3 tuner arrangement, you need a minimum of 6.5 inches of headstock from the nut.
If you want some kind of emblem or decal to personalize it, sometimes it's better to have
an extra 1/2 inch of headstock as well, but most often the emblem will fit between the
rows of tuners.

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Permalink Reply by carverman on October 21, 2010 at 7:55am

C. B. Gitty (Ben) said:

The rod piezo can mount under the bridge, you can make a slot in the bridge itself and glue it in, you can glue it to the inside of the box, pretty much anywhere you could mount a disk.

As for the pre-amp and feasibility... I can't guarantee that it will give you the exact sound you are looking for, but it should sound pretty good! When it comes to stuff like that, about the best advice I can give is to try it and see!

Thanks CB...I think I will go that route and order one through your shop along with some
other things. The preamp is priced right and until I get it in and working, I really won't
know for sure..BUT..and here's the beauty of getting CBG supplies..you are not paying
hundreds for Fishman or Schatten or LR Baggs piezo transducer/preamps.

I went that route one of my LP customs and the T-o-M bridge (from Stew-Mac)
sounded very thin to me, because the individual saddles were the peizos.
I had to add a tone control/tone caps and a preamp to tame the sibulence.
It took some experimentation..as it general does with
these kinds of mods..but I'm going to go for one of your preamps shortly.

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Permalink Reply by carverman on October 22, 2010 at 12:33am

Don Thompson said:

I have never heard a difference in ohm ratings for a piezo volume control. Doesn't really matter to me.Audio or linear taper yes.
Don

So what value of volume pots have you used on your builds?
I know from past experience with the piezo tuna-matic bridge, that the piezos seem
to be sensitive to pot values. LR Baggs supplied a 5 megohm vol pot with their piezo t-o-m
bridge and I thought that was rather high..but after installing it, I found out that the pot
would work up to a point,d then cut out completely about 1/4 of way down to zero.

Now, it may have something to do with the o/ps of the individual saddles which were
then summed together on a printed circuit summing board.


It just wasn't as smooth an audio taper or even a linear taper, as I had expected,
so I had to add a 100k resistor to prevent the pot from going to "zero" accidently.

Does the disc style piezo(s) and the 500K pot have a nice smooth tapered volume response?

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Permalink Reply by carverman on October 22, 2010 at 12:38am

Turkish said:

Don Thompson said:

Audio or linear taper yes.

Audio taper is recommended for a volume pot, as the human ear does not respond linearly to loudness.

What you say is true..but Gibson, and other guitar makers, have traditionaly used LINEAR resistive element taper
in vol pots and Audio (logrithmic) taper in the tone pots. It may have something to do
with the pickup combining switch and the way the controls were wired up, though.

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Permalink Reply by Darren Gosling on February 22, 2011 at 12:02pm

The piezo electric effect can be summed up in a nut shell. The voltage output is determined by the pressure applied (sound waves) on its surface area with a fiddle factor of the piezo electric effect.

So, as the most piezos are made of the same material the fiddle factor is constant. Also the sound waves pressure are constant across choice of piezos its the size or surface area that will affect the choice. 20 and 27mm wouldn't really make much difference in the scheme of things. The 41mm piezo should be louder but there is the volume pot and the impedance matching between guitar and amp. The resonant frequency will change over the sizes but I find all piezos to be biased to the treble end compare to magnetic pick ups.

When dealing with audio there is never a linear response. Often there is an exponential response when falling in impedance, 1M to 500K appears to be 'half' the actual response may only 10% different.Whereas 100k to 1K may be as much of 60% difference in volume. Actual responses may be difference but the principal is as you approach a low impedance the effect is greater.

When dealing with parallel pick ups/piezos, I fear of going in to Kirchhoff, Thevenin and ac theorys and deep mathematics to resolve the ultimate sweet spot for whats sounds good and gives best signal transmission from ac sources (pick up/piezo) to amplifiers, effects pedals or mixing desks etc.

I think the best way to look at this, transducers in parallel will not give a louder signal than a single one if you got a volume pot over a high impedance say 250K. However that said there are reactance of capactive and inductive elements (including wires and leads) which will change over the frequency range of said tranducers.

By adding parallel transducers and the positions of the transducers you introduce phase shifts and even more possibilities for tuning to specific frequency response curves. This starts getting into mega-deep calculus and material worthy of a good Phd.

What I use is a simple rule of thumb. I never go below 47K pots for volume because the impedance mismatch of guitar output to amplifier input starts to affect the signal strength and you start getting a quieter guitar.

At the end of the day I turn my volume pot to a point that sounds good for what I want at the time and if the tone is a or volume is a bit low I'll use the controls on my amp.

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