1977 Ovation Preamp Schematic

Pickups, wiring schemes, switch techniques and onboard active electronics for guitars and basses
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5thumbs
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Post by 5thumbs »

I recently had the pleasure of repairing the preamp on a 1977 Ovation Country Artist 1624-4. The primary complaint was that the guitar did not work when plugged in. To make a long story short, after audio probing through the circuit, I first determined the electrolytic caps were dead, followed by determination that the JFET was dead as well. After replacing the electros (C3 & C4) and tranny (didn't have a 2N5459 handy, but used a 2N5458 with great results), it sounded very nice.

It's cool getting to repair an instrument that hasn't made a sound (electronically) for over a decade. It's also cool to get to trace the circuit and post it here for future benefit. (Hopefully, someone will find this useful someday. :) )

I want to give special thanks to Analogguru for his 'Ovation Preamp' schematic because without it, I'd never have figured that the empty pads in parallel with R1 were intended for a high-pass capacitor on 12-string models.

Please click here to download the schem as a PDF file.

And here's the schem as an Inline Image:
Image
Modding a DS-1? Please read 'Build Your Own DS-1 Distortion'.

"Other than a good first-in-line buffer/booster, using other FX to improve bad clean tone is like gift-wrapping garbage."

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Post by SPeter »

Thanks a lot!
:applause: :D

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Post by RnFR »

cool, inductors!
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Barata
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Post by Barata »

Do you think that preamp would work fine amplifying a set of graph tech saddles? Would it be as loud as a "normal" (magnetic) pickup?
I'm tempted in trying this circuit in my next frankenstain project!

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Post by 5thumbs »

Barata wrote:Do you think that preamp would work fine amplifying a set of graph tech saddles? Would it be as loud as a "normal" (magnetic) pickup?
I'm tempted in trying this circuit in my next frankenstain project!
I've never worked with the Graphtech piezo saddles before (don't know why...just haven't got there yet), but as long as their output is summed and can be fed into the preamp as a single input, I don't see why not. It's important to present a very high input impedance to piezo transducers, otherwise they squeal like poor old Ned Beatty. So once you do that, the output through this preamp should be comparable to other piezo pickups on the market. (I've never A/B'd piezo and magnetic pickups, so I can't answer your question directly.)

It's actually a very simple JFET buffer at the heart. To me, the only interesting value-add is the tone-shaping network of caps and inductors.
Modding a DS-1? Please read 'Build Your Own DS-1 Distortion'.

"Other than a good first-in-line buffer/booster, using other FX to improve bad clean tone is like gift-wrapping garbage."

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Post by ansil »

just a question has anyone really looked at piezo's alot. paul marossy and love kraft and a few others were talking about this over at project guitar. there have been tests showing that piezos have quite an output swing {one test was 18v p p although personally i have only seen 10v on my old strat }but that its output level will change based around freq. what i can tell you is that i have 6 guitars in house right now that all have piezo in them and all of them have seperate preamps outside the guitars {i prefer to build my own} but everyone of them is as loud output wise as my electric or magnetic pickups. i currently play a brian moore guitar for the past three years and was wondering why my piezo's sounded so good and the battery never died on them. well it wasn't till i changed out the jack {btw bmoore guitars use a cheap ass plastic stereo switching jack i highly recomend the switchcraft version if you can get ahold of it.}i had to use whats on hand so i opted to use alternate switching for the power and found out that there is no preamp for my guitar and they are very very amazing sounding. just wondering. most people dont' realize the preamp is really more for tone shaping than volume. you can do wonders with a simple 7 band eq and no onboard preamp. sorry for the long rant but i have seen so many posts about bringing the piezo's signal up to the magnetics and such it just really kind of makes me want to eat a bullet ;)

good job on the schematic i had to repair one of these as well its real similar to another one they put in real good trace of it kudos

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Post by Barata »

Thank you ansil for sharing your experience with us! I never thought that just the saddles by themselves were that loud! I'm truly surprised
Since I'll mount the piezo in a DIY guitar i'm try the many different options!
One question how would wire it up if you had a HSS guitar, a 5-way switch and 3 pots?

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Post by ansil »

i didn't either till i got into it a little check out my clips these are my piezo's in my guitar no preamp

https://www.freestompboxes.org/viewtopic.php?f=8&t=7462

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Post by karul »

Attachments
Ovation_EA_Preamp_Schematics.jpg

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Post by ppluis0 »

Interesting equalizer network in paralell with source resistor.
Perhaps a forum member can run a simulation of this schematic to see freq response ?

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Post by Ichabod_Crane »

There is a resistor before the Volume control: 5600k?

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Post by karul »

Ichabod_Crane wrote: 24 Feb 2022, 09:19 There is a resistor before the Volume control: 5600k?
5.6k
http://www.ovationtribute.com/Ovation%2 ... reamp.html

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Post by karul »

The image above was part of the owner's manual.
http://ovationtribute.com/Catalogues/Ov ... anual.html
Attachments
Ovation Acoustic Owner Manual 11.jpg

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