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Old 8th July 2008, 03:40 PM   #1
teleman is offline teleman  Norway
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Default Acoustic guitar blender preamp



Hi guys,
I just discoverd this "site/forum" to my great delight...well I wasn't living in the jungle all these years!

Well,I am planning on building an acoustic guitar preamp incorporating
1; a Piezo under saddle pu,an AST type ex;B-Band AST1470
or a schertler bluestik under saddle pu.
a Panaonic WM 61A elecret condenser mic (found some on the e-bay) I've a few more to experiment with....

For the tone control..thought of using the TDA 1524A IC since it has stereo in/out!

I 've been looking at some published designs,so would like to combine some of them.

My problem is, although I can read,& construct from schematics
have no knowledge of circuit theory or analysis!

My major expected hurdles will be buffer/driver/output impedence matching,phase switching etc!!

So any/all help and encouragement in this venture would be both welcome & appreciated...i've seen some of you guys are quite amazingly knowledgable!

In my next post I shall give you the basic diagrams etc

Thanks & wish me luck!
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Old 8th July 2008, 07:06 PM   #2
Netlist is offline Netlist  Belgium
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Here you are, and welcome to the forum.

/Hugo
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Old 18th December 2008, 11:27 PM   #3
teleman is offline teleman  Norway
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Default Acousic guitar blender preamp

Hi All!
Here I'm after doing quite a bit of reading/researching on op-amp designs for the past 6 months!
My intention is to build a Piezo + mic blender acoustic g. preamp.

Part1:Piezo Preamp(tone control)
I' lifted this schematic from http://www.next.gr & have already tried this on a proto-board & it worked the first time!
But since I wanted to drive it on +/- 9v supply for better dynamic response,(wishful thinking?) I then removed resistor R12.
It sounded little clearer than when it was run on only + 9v. although still lacking a wee bit in bass.But it sounds rather lifeless,(only 10dB gain)but sounds much better than most piezo preamps I've heard so far!

By the way I'm using a Roland AC60 for testing this.

Also tried as a current follower by increasing R1 from 1M to 4M7 & shunting it to ground, with higher values of C1 but it still sounded very harsh & typical piezo!So have now reverted back to as before.

GAIN SWITCH:
No sound on either 0dB or-10dB settings!Quite adequate sound only on +10dB setting

TONE CONTROLS:
Bass Pot:Hardly any change,only slight boost at max. level.
MID:quite good response
TREB:more response towards max. setting.

Question
Would it be possible/better to run the input/buffer as a comparator instead?
Is it possible get +40dB gain in this configuaration?
What component changes/additions would it involve?
How do I calculate the queisent current?

Can any one out there please give me any suggestions/ideas?
All inputs/ideas are gratefully taken onboard.

Thanks.
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Old 19th December 2008, 12:05 AM   #4
lineup is offline lineup  Sweden
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I did some search as you did not tell what circuit.
You probably refer to this www.RedCircuits.com 'Guitar Control'

Click the image to open in full size.
http://www.redcircuits.com/Page69.htm
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Old 19th December 2008, 12:24 AM   #5
teleman is offline teleman  Norway
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Hi Lineup!

This is the bugger I was ranting about!Thanks a million...I was actually trying to forward it with my query..but for some strange reason it just didn't come up!but then you beat me to it.Well done & thanks again.
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Old 19th December 2008, 09:16 AM   #6
teleman is offline teleman  Norway
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Default Acoustic Guitar pre amp blender

As you can see,it is an inverting amp.If my limited knowledge serves me right I want to change this to a non-inverting config;
for the following reasons.

Looking at the diagram,is it correct that R4/R1 would give only a marginal gain given such low values of these resistors?
Low impedence?

Am I correct in assuming that a non inverting amp would give
1.higher impedence?
2.more gain?

By the way I am using a 20mm ceramic piezo disc for this.
Supply is +/- 9v(two PP3)
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Old 19th December 2008, 09:39 AM   #7
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Piezo discs require very high input impedance. You may take a look at: http://www.carvinmuseum.com/pdf/guit...bassmodule.pdf or http://www.carvinmuseum.com/pdf/guit...iezomodule.pdf
The circuit from RedCircuit may not meet this requirement - it is rather for magnetic pickups.

Mark
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Old 19th December 2008, 03:34 PM   #8
teleman is offline teleman  Norway
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Hello Mark,
Thanks for the Carwin schematic,which I've just had a look at...
as you will agree, this too is only a 2 stage + tone control design apart from the FET in the input buffer section + seperate active/ passive pickup drive circuits.

I am using a 20mm ceramic disc..as described in the original article.& it sounds quite good too!

I have tested " mine" on a Roland AC 60 ac.amp & IT DOES WORK! albeit with rather reduced bass & gain response.However,
it feedsback with all the tone controls turned up full & the AC 60 turned up half +!It also sounds much better so unlike most piezo preamps I've tried,including the Artec Tri-Blender on one of my guitars!

My AC60 has a 1M impedence in the dedicated piezo input,may be
this is too low for an under saddle/Piezo film pu's..but works well with this ceramic disc.

If you have a look at the resistor R1 which sets the impedence,I'd agree that this is quite low compared to the feed-back resistors..hence the low gain?

Any thoughts on that?
Thanks.

I only wanted more gain/head-room for better dynamic response.
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Old 21st December 2008, 01:55 AM   #9
teleman is offline teleman  Norway
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Hi Lineup,
Could you please explain the effect og high gain vs. dynamic range?Am I correct in supposing that increasing th voltage gain would give more dynamic range aswell due o more headroom avilable to the signal?

Is this the reason that studio mixers/ high spec;mic channel preamps have this "large sound"?
I've tried about 20 odd acoustic guitar preampsso far..yet almost all of them sound the same,regardless with either a cermic or a undersaddle pickup!This may not be very critical on solid electric bass,since i presume,the vibrations come mainly from the bridge area & not from the body? But on hollow string intruments,the whole body vibrates,hence more resonance frequencies?
Besides the piezo generates only a very small amount current! ,
which cannot provide adequate signal boost to the input/buffer opamp?

I also gather that API & the Presonus AcouticQ preamps have 60dB & 40dB gain respectively!

1.How can I achieve more gain without loosing higher freq.
2.would it help to run it on +15/-15v instead?
3.could I cascade more stage(s) to achieve this?.

My preamp would hopefully have the folowing:
1 piezo disc input using the JFET IC.
2.A modified Panasonic MW61 capsule driving a AN217 IC mic pre-
amp which is already published in their website & claims to have
a gain of 60dB!!!
3.Include a notch filter with both lo/high controls.
4.An inverter/ phase switch
5.A simple 2 input 2 stage mixer (virtual earth?)

After readig some of your comments on audio amp colums elswhere, I now realize the enormity of my undertaking!

I would only be thankful for your insight /advice or suggestions.



.
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Old 21st December 2008, 09:51 PM   #10
teleman is offline teleman  Norway
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I Found this schematic on http://devices.sapp.org/circuit/piezo, hence thought I could try this with my earlier piezo circuit above to increase the sensivity & gain.

This circuit breaks up the sound!,although I can hear a lot of gain.
Tried removing the diode,but then it sounds like a fuzz box!A lot of distortion!

I've connected it as it is to the previous piezo preamp.(above)

Is there any incompatibility between these two?

By the way I'm running this too on +9/-9 v supply.

Can anyone please tell me why this doesn't work?
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