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Ovation SK-M Preamp Recreation

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seesquare
Posted 2025-02-10 6:47 PM (#560743)
Subject: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		Okay, both. I will be engaging in some fun while I attempt to build a stacked-knob preamplifier. Since I can't find a specific SKM schematic, I was curious if I used one half of the Ovation Stereo Preamp to construct this device. Here are the images I think are relevant to the matter. I have added the tone control components to the Mono schematic, as the same structure on the Stereo schematic. Yes, I will have to figure out the plumbing diagram for the concentric potentiometer, too.
So, is this kosher? Does this qualify for BFLG activity? Are they giants, or just windmills? Steve McCormick at RenOvation was kind enough to provide the Stereo schematic and I accessed the Mono VT circuit from Jerome's OvationTribute website. There is also the Ovation Volume-Only preamp schematic, but it appears identical to the Mono VT circuit, anyway.
BTW, on the Stereo diagram, what are all those numbered "TP" sites identifying? And, what's the Reader's Digest explanation of what those 2 capacitors in parallel with the 100 Mh inductors in the JFET Source section do to the sound quality?
Thanks for your bemused attention and any reality-testing comments. It may be a pipe dream that goes up in smoke, eh?

Edited by seesquare 2025-02-10 7:14 PM




(Ovation Mono VT schematic- smaller image.jpg)



(Ovation Stereo VT Schematic 2nd copy- smaller image.jpg)



Attachments
----------------
Attachments Ovation Mono VT schematic- smaller image.jpg (68KB - 0 downloads)
Attachments Ovation Stereo VT Schematic 2nd copy- smaller image.jpg (77KB - 0 downloads)
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DanSavage
Posted 2025-02-15 2:14 PM (#560766 - in reply to #560743)
Subject: RE: Ovation SK-M Preamp Recreation



Joined:
June 2012
Posts: 2323

Location: Pueblo West, CO

Personally, I think this is perfect BFLG material.

I'm having a bit of a pre-amp problem, myself. A few years ago, for whatever reason, I removed the SKM pre-amp from my 1624. Since the battery was hard-wired, I de-soldered the wires. Now, I don't recall where they were wired to the CB. I'm pretty sure I've identified the hot location, but am still a little fuzzy on the ground. Alas, that is a discussion for another day.

Doing a quick search I found a schematic for the VT Mono in Jerome's site on the page labeled, Parts Catalog.

See: Ovation Schematics Circuits Diagrams

Here's the schematic:

Looking closely at this schematic reveals that the only difference between it and the VTM schematic you posted is that it has a single 22K resister in place of the 15K resister and 50K pot.



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seesquare
Posted 2025-02-15 4:54 PM (#560768 - in reply to #560766)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		AFAIK, the ground for the 9V is the ring contact on the stereo output jack. So, there is no variable gain control in your circuit drawing, whereas there is in the Mono VT schematic? I still am wondering what those coupled caps and inductors are doing in the circuit. It would appear yours is the SKM circuit, so that answers that part of my conundrum. I guess I will just have to wire this thing up and run it up the flagpole, eh? Thanks for your input, Dan. Vive la BFLG!
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seesquare
Posted 2025-02-15 6:19 PM (#560769 - in reply to #560768)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		Okay, some more info about the circuits. Probably above my paygrade, but the speculation is the inductors coupled to the capacitors control and stabilize the tonal frequencies at 2 levels (maybe).
"An inductor is used in an amplifier circuit to prevent oscillation when driving difficult loads with high capacitance1. It can also be used to isolate the internal feedback loop from external RF signals4. In a circuit simulation, an inductor with a nonlinear magnetic core can be modeled using a distributed current technique2. The impedance of the inductor at a specific frequency can be calculated using the formula Xl = 2piF*L5."
That last sentence, I think, is the crux of the application issue. Piezoelectric sensors usually present with relatively high impedances.
And, I may be all wet, too.
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numbfingers
Posted 2025-02-17 10:55 AM (#560774 - in reply to #560743)
Subject: RE: Ovation SK-M Preamp Recreation


Joined:
January 2006
Posts: 1123

Location: NW Washington State		I have a stacked-knob 1624 but don't want to take the strings off right now.

It's possible that the output jack could have a switch built in instead of using the plug to connect the battery.

The parallel inductors/capacitors look like LC bandpass filters. They're probably doing some tone shaping, but why are there two filters in series, and how do they work connected to the FET source and ground in parallel with the resistor? Too much thinking for me, discuss amongst yourselves.

-Steve W.

Edited by numbfingers 2025-02-17 11:08 AM
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numbfingers
Posted 2025-02-17 11:07 AM (#560775 - in reply to #560743)
Subject: RE: Ovation SK-M Preamp Recreation


Joined:
January 2006
Posts: 1123

Location: NW Washington State		CC, here are your reading assignments, written by real old-school electronics guys, not random "experts" on the web.

Rufus P. Turner on FETs and LC Circuits - I paid too much for these books and now they're on the web

https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Sams-Book...

https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Author-Gr...

https://www.worldradiohistory.com/BOOKSHELF-ARH/Technology/Author-Gr...

Ray Marston's 4-part series on FETs. These are also out there in .PDF format.

https://www.nutsvolts.com/magazine/article/fet_principles_and_circui...
https://www.nutsvolts.com/magazine/article/fet_principles_and_circui...
https://www.nutsvolts.com/magazine/article/fet_principles_and_circui...
https://www.nutsvolts.com/magazine/article/fet_principles_and_circui...

Unknown Somebody's JFET Amplifer and Filter Project that might be related.

https://www.diyaudio.com/archive/blogs/synctronx/1424-jfet-amplifer-...

-Steve W.



Edited by numbfingers 2025-02-17 11:09 AM
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DanSavage
Posted 2025-02-17 3:22 PM (#560776 - in reply to #560774)
Subject: RE: Ovation SK-M Preamp Recreation



Joined:
June 2012
Posts: 2323

Location: Pueblo West, CO
numbfingers - 2025-02-17 8:55 AM

I have a stacked-knob 1624 but don't want to take the strings off right now.

It's possible that the output jack could have a switch built in instead of using the plug to connect the battery.


I removed the internal battery box on my 1655 and replaced it with a battery drawer because I didn't want to have to slacken 12 strings to replace the battery.
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seesquare
Posted 2025-02-18 9:18 AM (#560777 - in reply to #560776)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		Okay SW, 2 responses here.
1. "CC, here are your reading assignments, written by real old-school electronics guys, not random "experts" on the web." Thanks, I will muddle through, but my absorption rate may be slow.
2. "Too much thinking for me, discuss amongst yourselves." That's what we have YOU around for!
So, theoretical conjecture aside, I am gathering the necessary electronic gizmoes and will attempt to construct the SK-M circuit. Steve McCormick, at RenOvation, was bemoaning his lack of a 100K/250K concentric potentiometer for repair of a SK-M he presently has on his bench. So, what happens to the sound quality if you would employ, say, a 100K/100K versus a 250K/250K pot? I think I could use a reduction resistor on the volume side, if I was using the 250/250. Maybe?
I will keep you posted. Now, wading through my assigned readings may consume some time. There ain't no free lunch, I guess.
In the meantime, I am building an onboard 6418 tube preamp to try out with a piezo pickup. Crossing my fingers, anyway. Will see how it handles all the vibration inside the bowl.
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Mr. Ovation
Posted 2025-02-18 2:47 PM (#560778 - in reply to #560743)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
December 2001
Posts: 7225

Location: The Great Pacific Northwest		Those inductor or filter components could be there for any of a variety of reasons. It's been a long time since I cared about components so you would have to do the maths, but they could be anything from tone shaping, to simple filtering to ensure outside influences of nearby electronics stay out.

FWIW, I would get a push-pull booster and eq (two separate items) from Seymour Duncan or EMG and I'm sure there are others, and re-wire with a stacked pot running the appropriate pot to replace the appropriate pot on each board. They are small, and someone else already did all the maths.
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seesquare
Posted 2025-02-18 5:27 PM (#560779 - in reply to #560778)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		Thanks for the input, Miles. Very valuable information and germane to my project. I will check out the stuff at Seymour-Duncan, certainly.
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seesquare
Posted 2025-02-20 8:44 AM (#560780 - in reply to #560779)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		On an allied note, I just did some minor surgery on an OCP pickup that came with an Ultra 1528 from a Goodwill auction in Miami (long FedEx trip). No cable on the device (drat!), and no other electronics, for that matter, so the pickup was carefully disassembled and the end wires exposed for reattachment of a replacement cable. Delicate technique would be the apt term. So, it appears to be functioning, but how well will await the reinstallation and restringing phase. I might need to make an extension cord for the short cable, though. Probably won't reach to the intended, newly-fabricated preamp can on the bowl edge. Worse things could happen than that, assuredly. IKYP (new text-speak for "I'll keep you posted".
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FlySig
Posted 2025-02-22 8:07 AM (#560782 - in reply to #560743)
Subject: Re: Ovation SK-M Preamp Recreation



Joined:
October 2005
Posts: 4056

Location: Utah		45 years ago I could have explained the RLC circuit and maths. University was too long ago now.
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seesquare
Posted 2025-02-22 9:01 AM (#560783 - in reply to #560782)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		Pardon the hubris, but I'm of a mind, "Rage against the dying of the light". This type of inquiry and mental gymnastics keeps me busy and productive. Tailored to what I believe is comprehensible and feasible, to be sure. So, sure, some of the stuff is above my paygrade, but even a small success is great fun. Now, I wish my guitarplaying reflected that, also!
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seesquare
Posted 2025-03-02 5:17 PM (#560790 - in reply to #560743)
Subject: RE: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		I have a question about the choice of components & application. I have sourced 2 concentric potentiometers, one is a 100K/100K type, the other is a 250K/250K type. So, if the schematic requires a 100K volume and 250K tone pot, I see 2 options. One is a series 150K resistor on the 100K tone control to make it function as a 250K. The other option is a parallel 167K resistor on the 250K volume control making it function at 100K. The issue is which option will (likely) result in better overall performance? I will probably breadboard the whole circuitry mess and substitute in either pot, anyway, but thought I would get some feedback in the meantime. Here's some visuals for the right hemisphere:



(Ovation SK-M preamp pot options 1- smaller image.jpg)



Attachments
----------------
Attachments Ovation SK-M preamp pot options 1- smaller image.jpg (59KB - 0 downloads)
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numbfingers
Posted 2025-03-03 2:16 PM (#560791 - in reply to #560790)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
January 2006
Posts: 1123

Location: NW Washington State		Good idea to breadboard it first. You might start with two regular pots instead of stacked, maybe get both values in both audio and linear taper to experiment. When you find the best type, you could take some stacked pots apart and swap the conductive tracks to get the right mix- if you're more patient than I am.

Another idea for the breadboard would be to make it easy to try different FETS, maybe a little socket if they still make those. I've read that FETs aren't very consistent, so you might have to try several and/or different part numbers to get it working.

-Steve W.

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DanSavage
Posted 2025-03-03 7:18 PM (#560792 - in reply to #560791)
Subject: Re: Ovation SK-M Preamp Recreation



Joined:
June 2012
Posts: 2323

Location: Pueblo West, CO		I'm with numbfingers. There's no reason not to have two separate knobs, volume and tone.

If it's good enough for Adamas, it's good enough in this application.

Once you've got the breadboard built, you can experiment with concentric pots and different resistors.
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seesquare
Posted 2025-03-03 9:36 PM (#560793 - in reply to #560792)
Subject: Re: Ovation SK-M Preamp Recreation


Joined:
November 2002
Posts: 3627

Location: Pacific Northwest Inland Empire		Thanks for the input, guys. I was pursuing the stacked pot design cuz that's what was in the guitar, originally. I am attempting a reasonably faithful refurbishment of the instrument, so wanted to replicate the SKM preamp with the single hole in the bowl. Yes, I know I have to attach the battery to the other, smaller hole. And, as for eviscerating two perfectly good potentiometers, then reanimation of the swapped parts to obtain the correct combo of 100K volume & 250K tone, well........thanks for the supposition that might be in my wheelhouse, so-to-speak. I think the breadboard is going to get a real workout in the next few weeks. And yes, I have a socket strip I can wire into the circuit to audition different FETs, though I only have about 2-or-3 types in the inventory, at present. I will keep you posted. I have all the parts now, at least.
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