Fender FR-12 preamp mod : The Hissterectomy

marsonic said:
Well, based on this....



and some assumptions, if you did want to mod it and if WKSmith actually produces boards, it looks like it could just be a couple plug-in connectors and un-attaching the pots from the chassis....which isn't too much for a lot of people and would be easy for an amp tech.

But maybe I'm making incorrect assumptions.

@WKSmith the scoop that someone (maybe you) measured....do you know if that's from the EQ not being flat with the knobs set neutral or from the EQ actually being neutral and the scoop coming from the speakers?
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It would be simple to drop in a replacement preamp board. Unplug 4 cables, unscrew 5 pots and pull out the little xlr input board.

The scoop I measured below is from the crossover tuning and not the eq, based on my spice sims. That notch is common in a lot of biamped two way systems.

Screen Shot 2023-10-17 at 12.50.22 PM.jpeg
 
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WKSmith said:
It would be simple to drop in a replacement preamp board. Unplug 4 cables, unscrew 5 pots and pull out the little xlr input board.

The scoop I measured below is from the crossover tuning and not the eq, based on my spice sims. That notch is common in a lot of biamped two way systems.

View attachment 128567
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Right, I remember that. I meant the scoop in the pink/red line that I think is centered at ~600Hz and the boost at 100Hz....assuming that those bolder lines are decades (adjust my numbers accordingly if they're not).
 
IMG_1102.jpeg
That is the freq curve the amp has with the knobs on 5 and the cut at 0 .

I need to measure some more to know how much of it in the eq, and how much is the cab, ports, and speakers.

That is what my 4th filter will be tuned to do without a knob, but probably with a bypass switch to make it flat response.

But that curve is the mojo. That is the real amp in the room magic.

We want flat response to create presets and monitor with surgical precision. But most of us humans like a curve in live music.
 
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Image 10-25-23 at 1.37 PM.jpeg

Here is the eq on in blue
And bypassed in white.

For the blue plot.. all eq on 5 cut on 0

Shot at 90dB reference level.

The mojo is mostly in the cab, ports, amps, and speakers. Not the eq.

That slight difference in plots is a big difference in sound though.
 
WKSmith said:
The mojo is mostly in the cab, ports, amps, and speakers. Not the eq.
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Awesome. That basically answers my question.

WKSmith said:
That slight difference in plots is a big difference in sound though.
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Yeah, it looks like it....~5dB swings are bigger than a lot of people realize. The difference is not much of a change for the core guitar tone but easily enough on the edges to be responsible for that "elusive" cab-in-the-room thing.

Especially if you get the hiss under control, this thing is awesome.
 
marsonic said:
Awesome. That basically answers my question.



Yeah, it looks like it....~5dB swings are bigger than a lot of people realize. The difference is not much of a change for the core guitar tone but easily enough on the edges to be responsible for that "elusive" cab-in-the-room thing.

Especially if you get the hiss under control, this thing is awesome.
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It looks to me like 5 on the bass knob is actually boosting a little bit, and 0 on the treble knob is actually cutting a little bit.

That is easy to do on an eq by adding pot taper resistors. Log taper on one, anti-log taper on the other. Leave the mid as a linear taper.
 
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fsharpminor said:
Likewise...very interested....
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Same.

Honestly, the EQ Bypass switch is probably the most intriguing option for me. I think I said it on TGF, but....Fractal already has like a dozen completely noise-free EQs I can apply to the signal going to the cab, and it's not particularly hard to figure out routing so it doesn't affect FOH or recording unless you want it to...having it there is just more knobs to get bumped, screw up your sound, and perhaps not notice. And, at the same time, a little 3-band and a cut isn't really enough to actually tune it to a room if you want to do that. So...I don't see that much value in it. Plus, when I played it, I liked how it sounded when set "flat" anyway.

But...that's just me. I'm sure other people disagree, but I'm excited either way.

I'm pretty sure I'm in for a WKSmith board or at least plans whenever they get made. It's kinda between that and actually getting a clean power amp and a cab for a good bit more money...not sure which is going to win.
 
If Fender and Hanser aren't interested in my design, I will quickly release instructions for bypassing the eq and the 2 filters to adjust in Fractal global eq to make the bypass exactly the same response.

I believe they would want to license this eq and possibly some other tweaks, as I don't want a ridiculous amount money to license the design. And this eq design saves them a lot of money in components and fixes the hiss problem.

I don't want to build new preamp boards myself, but if I do... I may just build a similar headrush 108 replacement preamp board also while I'm at it. 😉
 
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WKSmith said:
If Fender and Hanser aren't interested in my design, I will quickly release instructions for bypassing the eq and the 2 filters to adjust in Fractal global eq to make the bypass exactly the same response.

I believe they would want to liscense this eq and possibly some other tweaks, as I don't want a ridiculous money for the design. And this design saves them a lot of money in components and fixes the hiss problem.

I don't want to build new preamp boards myself, but if I do... I may just build a similar headrush 108 replacement preamp board also while I'm at it. 😉
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Wait...are they analog? I kind of assumed they were DSP....
 
WKSmith said:
No they are old school analog! PTH full size components... like back when stuff sounded great. Way back before surface mount toaster oven tech made the components cheaper than a grain of rice in china.

This may be the last piece of vintage Audio gear mass produced the old way.

God bless 'em
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That really surprises me for Headrush (sorry if it wasn't clear which one I was talking about). But, that's kinda cool too. That, and a board that fixes some of the issues people have reported, makes them a much more attractive tiny/super-portable option.
 
marsonic said:
That really surprises me for Headrush (sorry if it wasn't clear which one I was talking about). But, that's kinda cool too. That, and a board that fixes some of the issues people have reported, makes them a much more attractive tiny/super-portable option.
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The headrush preamp is SMT toaster oven Chinese cheap trash. (But analog)

But what if It had a goregeous analog front end ?

How many of those cheap headrushes are out there?
 
Here is some really nerdy math about the gain staging in amp stage 2. That I will fix today.

The bi-polar power supply in the FR-12 is showing +/- 18.55v at the farthest IC
(the balanced input and the 2nd gain stage) That is a 37.1v delta between Vcc+ and Vcc- (the power supply for the ICs)

The chips are rated at +/- 18v for a delta of only 36v

Bad design !!! Chip failure possible in their future.

Also: the gain is fixed in stage 2 at exactly
X 7.67

+4dBu is exactly + 4.77 dBv
4.77 volts X 7.67 gain is 36.59V which is inside the over-voltage designed rails but...

The first OpAmp (nasty4558) is only rated at +/- 30 v voltage swings. That extra 6.59 V is clipping!

Analog clipping sounds a lot better than digital clipping. ... but don't run the volume on your Fractal up to 10 when set to +4dB out levels!

So gain dropping resistor and switch is inbound today ...to match our Fractal +4 out to the same level that -10 hits inside the FR-12. With a serious reduction is the small (gain stage) hiss.

* I also need to test any voltage drop caused by removing the extra redundant quad OpAmps and jfet. It may raise the voltage even higher by reducing the current draw.
 
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