|
|
Joined:
November 2002
Posts: 3625
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
----------------
 Ovation Mono VT schematic- smaller image.jpg (68KB - 0 downloads)
Ovation Stereo VT Schematic 2nd copy- smaller image.jpg (77KB - 0 downloads)
| |
| | |
Joined:
June 2012
Posts: 2322
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.
| |
| | |
Joined:
November 2002
Posts: 3625
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! | |
| | |
Joined:
November 2002
Posts: 3625
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. | |
| | |
Joined:
January 2006
Posts: 1122
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
| |
| | |
Joined:
January 2006
Posts: 1122
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
| |
| | |
Joined:
June 2012
Posts: 2322
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. | |
| | |
Joined:
November 2002
Posts: 3625
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. | |
| | |
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. | |
| | |
Joined:
November 2002
Posts: 3625
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. | |
| | |
Joined:
November 2002
Posts: 3625
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" . | |
| | |
Joined:
October 2005
Posts: 4055
Location: Utah | 45 years ago I could have explained the RLC circuit and maths. University was too long ago now. | |
| | |
Joined:
November 2002
Posts: 3625
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! | |
| |
|