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Author Topic: LR Baggs Piezo-Pickup preamp  (Read 23970 times)

paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #15 on: June 05, 2016, 03:05:44 AM »
Does anyone knows what kind of fet to use? J201, 2N5457, MPF102 and what for the PNP
Thans

Paosal

phatt

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Re: LR Baggs Piezo-Pickup preamp
« Reply #16 on: June 06, 2016, 09:17:19 AM »
The device is not that important. You work with what is at hand and adjust Drain and gate Resistors to get the best clean signal output.  MPF102 is likely the easiest to come by.

As for the BJT they are just followers so any ~ 25volt pnp transistor will work much the same.
It's a high Z input so watch the grounding as it will pickup noise if not well shielded at the input.

Phil.
 

paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #17 on: June 18, 2016, 02:18:57 PM »
thanks a lot phatt, i'll try with some 5457

paolo

phatt

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Re: LR Baggs Piezo-Pickup preamp
« Reply #18 on: June 19, 2016, 04:35:53 AM »
I've just been running some tests of the LR Baggs circuit as well as something I built a few weeks back to fix a friends guitar which had a dead preamp module.

This Screen shot is an A/B comparison of the 2 circuits output curves. Green is Morris Yellow is LR Baggs

My Preamp circuit is a copy of an old Morris preamp with a couple of slight mods to fix the excess bandwidth which is a common issue for Piezo pickups.

Active Piezo's are a brilliant idea but the outputs are often very different. The output signal strength as well as the response cuves vary a lot. As can be seen here the LR Baggs output is almost half of the Morris and quite different curves.

You may need to experiment with a few circuits to find a happy solution.

I've found that most of the bridge under saddle transducers don't need excessively high Z preamp inputs. Past 2meg is just asking for low freq FB issues, especially on large body guitars. The early bug type transducers did tend to suffer bass loss without really hi Z,, like 10meg.

Regarding low freq FB; I've found that a central post wedged between bridge and back can help in some instruments, A case of suck it and see if it helps. As it's only wedged in place the mod is usually reversible. for those that may think it a rather non professional fix up,, Violin luthiers are known to use a post to reduce resonance between the face and back, moving it around to find the sweet spot. I'd guess guitar luthiers would reject the idea as folly.
Try it if you have bad FB, you might be surprised.
Phil.   


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paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #19 on: July 06, 2016, 01:29:59 AM »
I tried to build a stripboard version but it seems that does not work , very very low signal . I used BC177 and 2N5457. Here is the stripboard version.  Can someone help me?
I've tried a lot of other preamp but no way to have a good bass frequency response like Element LRBAGGS or NaturalI Fishman

phatt

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Re: LR Baggs Piezo-Pickup preamp
« Reply #20 on: July 07, 2016, 09:29:36 AM »
Maybe draw up the schematic of what you have built,, then take some voltage readings on the running circuit.
You need to note down the DC voltage at the drain and source of the Fets and other places if asked.
Then some of the better minds here will likely be able to help you.
This will also help you to work from a schematic rather than a layout.
Sadly layouts are very much harder to fault find and I think most technical people do not have the time to draw out your circuit from that layout.  You will get help a lot faster if you go to the extra effort and draw up a schematic to reference with numbered components and some DC voltages.    :tu:
Phil.

paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #21 on: July 07, 2016, 11:30:45 AM »
Thanks a lot Phatt, in the meantime I've tryed the Phabb fet boost, very nice, warm and big headroom
could be used together with a magnetic p.u. like the lrbaggs? Maybe can I use the same buffer like in lrbaggs schematics?

Thx again

Paolo

phatt

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Re: LR Baggs Piezo-Pickup preamp
« Reply #22 on: July 08, 2016, 03:38:23 AM »
Great! thanks for the feedback :tu: Good to know it's working for others.
I actually built this into my mates acoustic and while testing the idea I found it also worked well as a clean boost on my Strat. In the end I built it into a pedal and very happy with the outcome.

Assuming you are building this with the intention of running both types of PU in the one guitar then the only real issue I can think of is the output of Mag and Piezo will be at different levels so a bit of testing might be in order before you build.
Phil.

paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #23 on: July 08, 2016, 11:36:26 AM »
When I go into the input with the mag it souns good but the piezo disappears. Could work use a simple tillman preamp to gain the mag signal and go straight to the output together?

paolo

phatt

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Re: LR Baggs Piezo-Pickup preamp
« Reply #24 on: July 09, 2016, 07:53:55 AM »
Whoops! Now I see what you mean.

No Magnetic and Piezo don't mix as the Z of each is VASTLY different. :-[

A Mag pu has a much much lower Z (impedance) and will just load down the piezo and it will hardly work.
Yes you will need two separate preamps if you want to use them both at the same time.

Be aware that the frequency response of these two types of PU's will be quite different and two independent sections with separate tone circuits might be worth exploring.
Then blended in a final mix stage.

If recall right,, you might only need to use a unity gain preamp on the piezo. The output is then mixed into the passive Magnetic circuit using a high value pot.
i.e. in the case of a passive wired Strat circuit you would use a 250k pot and join the output of that to the output of the passive pot.

You would need unity gain on the piezo output and even then the signal will likely be louder than the passive magnetic PU.  Long time ago but I think that was how I remember it all.

The Morris preamp idea has +20dB gain so that won't work with the above.

Some help in understanding these piezo elements can be found here;
http://ozvalveamps.org/pickups.htm 
Phil.

paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #25 on: July 09, 2016, 04:15:19 PM »
So, I use a modified tillman preamp to obtain quite the same level. The sounds both really good, the èickup is a Benedetto, so a low output p.u. The mixing of two p.u. is not so udible, you ear it only with both vol opened. Maybe I can post a sample and schematic

paolo

paosal

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Re: LR Baggs Piezo-Pickup preamp
« Reply #26 on: July 12, 2016, 05:01:41 AM »
Hi Phatt, if I hitt hard the strings I have a little saturation, is due to the 20 db gain or is a 2n5457 biasing problem? I put a 50k trimmer instead of the 15k drain resistor to obtain 4.5v
The VR2 is a gain control?

J M Fahey

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Re: LR Baggs Piezo-Pickup preamp
« Reply #27 on: July 13, 2016, 04:15:58 PM »
No 9V powered preamp can have 20dB gain and stay clean, because signal peaks will try to go beyond rails (9V and 0V), thus clipping.
AFAIK the original Tillman had 6 to 10dB tops (3X max.)  which is about all you can get away with.

zedonetx

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Re: LR Baggs Piezo-Pickup preamp
« Reply #28 on: October 24, 2016, 02:24:45 AM »
thanks for reply , and sorry for topic resurrection  xP A time comes and i am so much eager to make my own preamp project for my acoustic nylon string guitar. Then after a time, i forget it. then i remember again and search. and i find my postings here again. :) kind of weird, isnt it?

thank you for your info. i would rather like to make my own preamp since im an electronics engineer and a amateur musician.

And Baggs and others already make a selection of such pickups and preamps.  Unless you are just into making a project, you might consider a commercial product, one that has a pickup designed to work with it.  My friends at Elderly can help you, and other places like Sweetwater as well.

http://www.elderly.com/accessories/cats/PUA.html

http://www.sweetwater.com/c985--Acoustic_Guitar_Pickups

zedonetx

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Re: LR Baggs Piezo-Pickup preamp
« Reply #29 on: October 24, 2016, 02:30:11 AM »
Can i ask some terms in the schematic?

BRIDGE?
RING?
TIP?
Can i learn what they refer to? I am trying to match "piezo", "under-saddle pickup", "jack" and "microphone" terms with them. thank you

My effort of tracing the LR Baggs Piezo-Pickup preamp from a Godin guitar (the guitars belongs to my customer). I triple check the trace.. Hope don't miss something there..  :D







 

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