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Old 14th June 2009, 11:08 PM   #1
teleman is offline teleman  Norway
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Default Piezo Disc Preamp



Hi All,
I have modified/redesigned this Piezo Disc preamp for acoustic guitars.The original circuit was published in http://www.redcircuits.com on page 16,but it never worked with a p.disc! or may be I (had) overlooked something?

The mods & additions to the original circuit are as follows;
1.A high impedance buffer
2.All IC's changed to TL072s ( works better than TL062)
3.Dual (+/-) power supply...sounds more "spacier"!
4.The o/p capacitor 0.022uF of the 2nd stage from the original was changed to 0.33uF

PIEZO DISC
Diameter - 20mm
Capacitance - 14nF +/-30%
Resonance Freq, 6KHz +/- 0.5%
Resonance Imped; 200? (unit not specified)

The placement of the disc is very critical for optimum sound. It seems that, there's only ONE "sweet spot" on the guitar & then it sounds almost (realistically) acoustic!

Attaching/fitting the disc directly on to the guitar changes the sound drastically,when the volume "pot is turned more than half way up.Placing a small piece of masking tape between the disc/guitar & taping the disc over seems to cure the problem to a great extent at cost of the sound being a bit "thinnish"

Doesn't seem to work well with USTs... at all uhh! (sounds typical piezo!)
However, there are still a few niggles with this circuit. I'm not sure if I have configured the 2nd stage properly or not!

So pointing out any of my oversights or additional improvements to this circuit would be most welcome.

I've no SPICE or any other simulation programs...

Thanks.
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Old 15th June 2009, 01:22 AM   #2
teleman is offline teleman  Norway
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Here's the schematic!
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File Type: jpg austin's piezo pre-1.jpg (31.8 KB, 931 views)
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Old 15th June 2009, 09:30 AM   #3
teleman is offline teleman  Norway
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Can anyone please tell me if there are any components /values which should be changed or added for optimum performance.

1.Is the value 150k resist; correct to the 2nd stage invert.i/p?
2.Should the non-inverting inputs connected by resistors to the
ground? what values should these be?
3.Is the 470k volume p. ok or should i add another low value R in
series?

I AM STILL NOTICING A SLIGHT LOADING ON TURNING THE VOLUME POT ALL THE WAY UP!(THE SOUND CHANGES) This is somewhat reduced by by not placing the piezo directly on the guitar.

The circuit drives the 1Mohm Piezo input of my Roland AC60 quite well.

Added a summing amp..& this drives the Line input as well.

Any suggestions?
Thanks.
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File Type: jpg piezo 3.jpg (41.9 KB, 810 views)
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Old 15th June 2009, 10:13 PM   #4
mjf is offline mjf  Austria
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Location: Austria
hello.
optimum performance........you will hear it (with your instrument).
i think:
the non inv. inputs of the fet -opamp can be connected to ground,some people like to build in a small res (to lower offset or so..)
the input cap 1uf is not essential,perhaps you try it without that
the 470k pot is a high value res that is sensitive to humming and noise........perhaps you can use a lower one (100k) ,and for the 150k use a 33k (and you can increase the 0,33uf to 1uf -filmcap),the voltage amplification is around 3 (=100:33)
do not forget to build in psu decoupling caps nearby the pins of the opamp (e.g. 47.....100nf ,and perhaps an elco 47.....100uf paralell)
try it out step by step with listening tests.............
greetings...........
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Old 16th June 2009, 12:51 AM   #5
teleman is offline teleman  Norway
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Thank you indeed MJF,
As I 've mentioned earlier,it sounds quite pleasent except for the sound,which is a bit thin.But when I turn up the 470k pot 3/4 way up, there is a drastic change..almost like a filtering effect.The sound gets even thinner!

Measured all the IC +/- in & o/p voltages without any signal,& they seem ok..0-3mv???

Wonder,if this could this be anything to do with the resonance impedance of the piezo itself?
I also noticed,that placement of the p.disc & the amount of pressure exerted on it makes a BIG difference.

I will try with 2 piezos in series..

I did try without the 1uF i/p cap..it sounded very harsh & nasty.

I will definitely change both the 470k pot & the 150k res, to the values you've suggested.
What happens when the pot is completely turned off?

It's driven by 2 x 9v battaries (PP3) as i am planning to build this as an onboard preamp..

Yes,I've 0.1uF polester caps on all IC supply pins.

Shall keep you posted.
Cheers
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Old 16th June 2009, 10:02 AM   #6
mjf is offline mjf  Austria
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Join Date: Jan 2009
Location: Austria
hello.
an offset around +-5mv at the output of the opamps is o.k.
voltage gain of the first stage is around 3 (1+(10k:4,7k)),the second stage gives 3.............so it is 9x max. together.and the third stage amplifies too..........when you overdrive it the sound will be thin and hard.
amazing thing that it sounds nasty without input cap (1uf).......
the second opamp stage is like an active volume control (a volume pot with amplification): pot fully turned up :three times.......turned up to 150k (as the input res is) gives 1x.............turned down to zero ohm should give zero volume.
the tone control stage is a standard application similar to national lf 353 opamps or so.............the 470k treble pot is a little bit high,often there is a 50k...........100k pot.will have a look at it.
greetings..............
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Old 18th June 2009, 10:23 PM   #7
teleman is offline teleman  Norway
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Hi,
Thanks again for taking the time & for all the useful info.I've replaced the values to 100K pot & 33k & it works well.

Yes,without the input cap,it sounds just like an UST..

I did try many types/values.Electrolytics give a slight time delay.
especially when you tap on the piezo itself & exhibit rather unmusical sounds!I think it must be due to the p.disc as the source with it's very high capacitance or it's electrical characteristics!!)

Varying the buffer o/p "cap, 0.47uF is a little low on the treble to my ears & 1uF sounds very dull indeed. I do admit,I am little inclined towards more treble on my guitar, the result of using "Teles" all my life, hence,0.33uF sounds just right to me.

I've also connected 22k res. to ground on the + i/puts.

I did have a look at the LF353 application notes & they too have used a 500k pot in one of their tone control circuits.I still h'vnt
tried with a lower value..but do so as soon as I finish my "casing"
for the pre.

Cheers.
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Old 19th June 2009, 07:42 AM   #8
mjf is offline mjf  Austria
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Join Date: Jan 2009
Location: Austria
hello.
i am not a piezo expert,so i cannot give you good hints for the position of this on your guitar and so on...........
sorry,question.......what means the abbreviation ust?
some years ago i built this (hifi) tone control (and simulated). the 500k (or 470k)pot is the treble control and i used a 50k (and 100k) pot in this place and it worked without problems.it is a sensitive tone control,so turn up and down the pot's carefully (i would turn up fully the volume control at the output (10k) and would try to adjust the volume with the second stage pot).
greetings...............
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Old 19th June 2009, 10:00 AM   #9
teleman is offline teleman  Norway
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MJF,
UST's are abb;for UNDER SADDLE TRANSDUCER..the types usually installed under the bridge/saddle of most of the (electric) acoustic guitars. I am sure you would have encountered them some time or other!

Yes,you're spot on about not turning the pre.a pot fully with regard to the main o/p volume pot (the one on the summing/ mixer.

However, p.discs seem very peculier in their behaviour to fre.changes.. & the tonal characterictics seem to corrospond directly...(stating the obvious here)to their diameter...but I am no expert on this subject..it's just my experience trying different discs,but then they sound much more natural than UST's to my ear.

I wish some knowledgeable people out there would enlighten us more on this.

I will give you feedback with different treb. pot values. Here's the PDF file of the LF353 tone circuit.

Cheers.
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File Type: pdf lf353 tone control.pdf (12.0 KB, 188 views)
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Old 22nd June 2009, 06:31 PM   #10
mjf is offline mjf  Austria
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Join Date: Jan 2009
Location: Austria
hello.
..............ust........we use different names here for these things......like piezo pickup.....piezo bridge.....coax piezo (a piece of piezo coax cable).............kontakttonabnehmer (piezo discs? ....to stick on the acoustic guitar nearby the saddle) and so on.
i read in a manual if the tone is not clean with the piezo pickup it has mechanical reasons.......e.g. the pickup doesnot sit tight enough at the corps of the guitar.
greetings............
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