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PostPosted: 19 Feb 2013, 23:57 
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Joined: 06 Jun 2008, 18:23
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Location: Australia
I built a single jFET preamp for my mate's double bass which is fitted with a piezo pickup under the bridge. I couldn't make it work properly and therefore this OPAMP based attempt. Mustard caps are all the rage with pedal makers and I just happened to have about 6 from my early electronic days mostly 0.1uf. Mustard cap on the input and polypropylene on the output. The paint job is a poor attempt at hammertone. Right colour no hammer?? The OPAMP is my fav. OPA2134 and is running at 18V. Input impedance is a ridiculous 10M. There appears to me good gain and the pot seems to enhance treble as it is turned up. From the images you can see I work both sides of the proto-board and even cut a track for compactness.
Attachment:
PeizoPickupPre-amp.jpg
Attachment:
PiezoOPAMP.jpg
Attachment:
PiezoOPAMPI.jpg
Attachment:
PiezoOPAMPII.jpg
Attachment:
PiezoOPAMPIII.jpg


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PostPosted: 20 Feb 2013, 23:46 
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I have a TT which will play 78s. I have been given a piezo pickup and have been told there is no equalisation required for a piezo pickup when playing 78s. I have been touting this preamp as a musical instrument piezo preamp. I really think it maybe a phono piezo pickup. But it will probably work for both. I will build another for my 78 TT.

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PostPosted: 23 Feb 2013, 06:49 
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I have started another piezo pickup preamp. Once again I have sprayed the cast Al case hammertone. Once again the nice "Blue Ocean" colour but not a lot of hammertone. This time I did manage a much better paint finish. After a lot of sanding with a fine paper I acid etch primed, waited a day and top coated. Looks good nice a and even thought the can went spack a couple of times.

The cct. Brd. Is nearly finished. Not my neatest work but very compact. Once more I'm using a mustard cap on the input and a poly on the output. Also I will install an extra switch to switch between 10M and 4.7M input impedance. I have a new head shell for the piezo cartridge and now are getting closer to playing my first 78.

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PostPosted: 26 Feb 2013, 03:55 
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Got the piezo preamp finally finished. Gain is a little down but I think I can fix that. I have a dutch peizo pickup - Ronette. Does anyone know anything about the cartridge? It has a 78 stylus one side and flips over to the 33-45 stylus the other. I set the tracking force to 3.5g. I think it should be higher but the 78 played well and tracked perfectly.

I didn't know that 78 were generally were cut for piezo pick-ups, no equalization, just buffering and pre-amping required. Listening to the balance of bass (not a lot) and treble from the 78 proved this. It was OK. I played the only 78 record I have. I was impressed how well it played. I expected a lot worse but I was quite surprised. It was even quiet. Tomorrow I hit the local secondhand stores for the best 78s I can find.

The knob is a tone control from bassy to very bright. I found the best setting about half way. The front blue RCA is a battery test point. On the back there is one input (red) and the two outputs are both black. The two outputs are wired together (internally) and plug into both the inputs of your preamp. The switch on the back allows me to switch between 10M and 4.7M loading for the cartridge.
Attachment:
PiezoPupI.jpg
Attachment:
PiezoPupII.jpg
Attachment:
Pickup.jpg


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PostPosted: 27 Feb 2013, 02:18 
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I found that not using a 25K pot with the piezo pickup is very much the wrong thing. Also putting a 47K resistor across a 50K pot works. It's nice to be able to raise and lower the treble of what you are listening to. I still have a gain issues but so far a pleasing result. Bought a stack of 78 today, yet to find a good one but lots to wade through.

Here is an image of the inside of the second of the piezo preamps and an image withe Mongrel Dog Audio decal. Not that neat but compact. Once more another mustard cap on the input and metal film output cap. I though the paint job good. Even my wife was surprised at the finished.
Attachment:
InsideThe Puppy.jpg
Attachment:
PuppyDecal.jpg


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PostPosted: 02 Mar 2013, 23:00 
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Hi MWHouston,
I worked on Piezo pickup development for acoustic instruments for a while, input impedance was always a hotly debated issue. (No experience here with stylus/78s)
10M input Impedance is the value I settled on when using a custom variation based on the FRAP.
I got the best gain structure out of that, feeding into EQs and mixing desk, to PA.
Mind you, I use a hybrid signal of magnetic pickup at about 20% and a 3D Piezo at 80%. (glued to soundboard, not under saddle)
Has your friend with the Double Bass tried a MiSi pre?
All mounted in one unit, in the input jack, charges externally, very accurate.
I only say this, because I mucked around with preamps for piezos for ages, with some success with LR Baggs and a few of my own, then I tried this "MiSi" unit. Pretty clear sound and the convenience outweighed any external box type units I could come up with for a live sound environment. Less potential for AC hum, interference from stage lighting, feedback etc.
http://mi-si.com/wp-content/uploads/201 ... manual.pdf
Sorry to be an advert here, but I looked for years before I came across this, it helped immensely.

Cheers, Sat.


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PostPosted: 04 Mar 2013, 00:59 
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Joined: 27 Feb 2013, 14:19
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By the way, nice website mate! :up:
I only just realised, it was because of your posts that I became aware of the Oatley Electronics preamps. I just built a 240V mains powered version of the stereo line buffer, with the 6418 tubes as my first tube based beginners project. It has some gain issues, but I suspect a few tweaks will fix that.
Later, I will look into a pre (possibly based around Oatley's) for Piezo and magnetic hybrid signal.
I know very little about tubes, but I have been collecting tubes and old amps for 20 years, in anticipation of a resource like this very site. I have some weird ones tucked away, raised a few eyebrows amongst a few old timers who I have occasionally let into my shed.
I have been pointed in the direction of a tube bank, or sort of exchange program here in Australia too, called EVATCO, the Electronic Valve And Tube Company: http://www.evatco.com.au/.
They buy and sell tubes and they helped a buddy of mine build an absolute monster guitar amp years ago. Just thought I would throw that info out there, in case you guys had not heard of 'em. ;)
Cool design ideas, I like the homes you give your projects, very cool indeed. :up:

Cheers, Sat.


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PostPosted: 04 Mar 2013, 13:39 
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I have been dealing with EVATCO for many years now. They started in Vic. I even wouldn't mind their business if they decide to sell. Seem nice folk. I didn't know they had a tube exchange. Oatley are developing an MC stage for phono and I would like to see one in 6418 tubes.

Funny with the piezo stage I built the one which my friend has for his slap bass has too much gain and the one I built for myself has not enough gain but they are exactly the same build??

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PostPosted: 05 Mar 2013, 23:16 
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Joined: 27 Feb 2013, 14:19
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Check out these pickups (Which are not FRAPs), Teddy really has it going on mate.
They hook up and respond well to 10M preamps.
With such high signal output, you can then have room to, and can afford to turn down the gain a little, which is always good for noise/gain issues associated with piezo technology.
Best piezo out there for the money and the after sales service is second to none.
http://www.dazzopickups.com/
He's picking up some serious players - he just got some Dazzos into Ben Harper's latest lap steel.
I have an early prototype, it "listens" like no other piezo I have heard, the detail is incredible.
Most people swear I am using a Mic, when I am not.
I think it picks up on 3 axes, so you get a much more accurate representation of what your soundboard is doing than with under-saddle elements (Which I do not like at all).
Placement is the key, as it also listens to what is in front of the pickup.
Good for Double Bass I hear too.
Sorry if this is another ad, just trying to throw options out there for my fellow musos.

Sat.


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