Fender FR-12 preamp mod : The Hissterectomy

WKSmith

WKSmith

Power User
Edit 3/20/24:

The 1200+ following posts hacking the FR series preamps turned into a plug and play replacement preamp for the FR-10/12 that solves the hiss problem.. and has several other major design improvements... named the Archangel V2.

If you are just getting here and trying to get on the waitlist, click on my forum name and send me a PM with your email address (important!)

Here are some bookmarks to help navigate this 66 page thread.

The V2 online owners manual.
V2 Features
V2 Boards
V2 Clip Light
V2 Pricing and IC upgrade options, and more waitlist details
V2 Review #1 and Review #2 from my awesome beta testers
V2 Crossover tuning and charts
V2 EQ spice Modeling vs stock eq graphs.
V2 balanced differential input tuning vs stock with Graphs
Gutshots of the original Stock FR preamp and F-corp's "factory hiss fix" secret IC Swap
V2 vs FR-10 vs FR-12 freq Charts
FR dis-assembly instructions from another forum member
Instructions to Bypass the stock EQ with one piece of wire.
V2 tuning vs stock tuning comparison graph with all eq knobs centered, cut knobs on 0
V2 Flat(ish) eq Settings
The Hiss that started all of this.



And Here is where this rabbit hole started:

I hacked the 1st 2 amp stages chasing the hiss. The hiss is still apparent and is probably in the eq. More on that later. I have a much cleaner eq design for this thing.

The FR12 has a true balanced differential front end and can can almost take +4dB line levels. The first 2 stages are using a nasty 4558 (tube screamer) op amp. The first stage is the differential input and averager. Into the volume pot then into the second amp half for gain. The stock gain is set at 1+ 10k/ 1.5k so 1+ 6.66 for 7.66X gain.
Which is too high for even rail-to-rail op amps, and way too high for the 4558.

Edit: gain is still too high, even with first stage signal losses of -12dB from bad design in the stock preamp.

I replaced the poly caps with metal film box caps, replaced 10k feedback resistor with a metal one, Swapped out the 1.5k gain resistor for a 10k trimmer, swapped the electro cap for a metal film 1uf, and socketed a Burr Brown OPA2134.

The hiss is only moderately better. The tone though ..is very nice. It was nice before, now it sounds a lot more refined in the top end , much faster slew rate for the bottom. It's a very musical sounding op amp.

First pic is stock, 2nd pic is modded,
IMG_1072.jpegIMG_1073.jpegIMG_1067.jpeg
 
Last edited:
Just a little better.
It hisses quite a bit to me in a small room. No more than a tube head though, but it's more hiss than the headrush.

Since the hiss changes with eq knobs, I'm pretty sure the circuit is a gained up baxandall eq. I will dig into it more later.
 
IAmThePepperoniKing said:
Jesus, people are too damn smart. This is amazing!
Click to expand...

The thing is Analog. It's not that complex inside.

It is kinda cool, in that everything "fancy" is on the same board, not using tiny surface-mount components, and that it's bi-amped after a line-level XO. So...really about as simple as it can get. Which...very much was the "right" way to do it. Very low development costs, easy to produce with ~80 years of experience building halfway-similar preamp circuits, and apparently relatively easy to mod if you can solder on a PCB.

I wonder if bypassing the second gain stage would fix the noise issue. It looks like that would take off ~18dB...but I doubt many people would need that "at home" where the noise is a problem. I'd imagine it could be put on a switch (and labeled either "11" or "turbo") to be able to do both well.....right, @WKSmith ?
 
marsonic said:
The thing is Analog. It's not that complex inside.

It is kinda cool, in that everything "fancy" is on the same board, not using tiny surface-mount components, and that it's bi-amped after a line-level XO. So...really about as simple as it can get. Which...very much was the "right" way to do it. Very low development costs, easy to produce with ~80 years of experience building halfway-similar preamp circuits, and apparently relatively easy to mod if you can solder on a PCB.

I wonder if bypassing the second gain stage would fix the noise issue. It looks like that would take off ~18dB...but I doubt many people would need that "at home" where the noise is a problem. I'd imagine it could be put on a switch (and labeled either "11" or "turbo") to be able to do both well.....right, @WKSmith ?
Click to expand...
I think just by bypassing the eq and its make up gain stages ..would probably clean her right up.

I put the trimmer in to dial the gain from stock to none on the second stage. Most of the hiss is past that stage.

There is one surface mount mosfet hiding on the back. But everything else is indeed old school analog.
 
It’s very cool that the board is so serviceable. I’d hate to lose the eq, though. That’s one of the nicest things about the cab for me. I wonder if there’s a few components in the EQ section that could be swapped for lower noise ones that would knock down the bulk of the hiss.

-Aaron
 
I can tune another eq design to those same bands that will not hiss. ..and using the same pots and less gain stages. I have one that is pretty amazing.

I built the cleanest Acoustic Preamps in the world until Parkinson's and life set in. My eq is freaking clean ! Several CandyRat monster fingerstyle players toured and recorded with them.

Here is what they sound like:


This is a prototype that I built for Antoine Dufour.

IMG_0670.jpeg
 
WKSmith said:
I think just by bypassing the eq and its make up gain stages ..would probably clean her right up.

I put the trimmer in to dial the gain from stock to none on the second stage. Most of the hiss is past that stage.

There is one surface mount mosfet hiding on the back. But everything else is indeed old school analog.
Click to expand...
Ahh, I misread or mis-remembered what you said on one of the forums. Sorry.

How complex would your EQ mod be?
 
WKSmith said:
I hacked the 1st 2 amp stages chasing the hiss.

The hiss is only moderately better. The tone though ..is very nice. It was nice before, now it sounds a lot more refined in the top end , much faster slew rate for the bottom. It's a very musical sounding op amp.
Click to expand...
I'm intrigued. Burr-Brown stuff was in my old car audio head unit and that was phenomenal (the system was built to compete in sound quality competition but I never competed lol).

Looks like a really simple DIY mod if you can solder.

If you make the amp section better, I have to ponder if a tweeter replacement might also be a good idea. Hell, even a woofer replacement could be cool assuming you can find something with similar specs that works with the cab.
 
marsonic said:
It is kinda cool, in that everything "fancy" is on the same board, not using tiny surface-mount components, and that it's bi-amped after a line-level XO. So...really about as simple as it can get. Which...very much was the "right" way to do it. Very low development costs, easy to produce with ~80 years of experience building halfway-similar preamp circuits, and apparently relatively easy to mod if you can solder on a PCB.
Click to expand...
I think “repairable” might be a good word to use too.

I cringe when someone starts telling me how wonderful their new modeling amplifier is, “and it was so cheap!”, knowing that the “cheap” amp won’t be that cheap if/when the board has to be replaced.
 
Greg Ferguson said:
I think “repairable” might be a good word to use too.

I cringe when someone starts telling me how wonderful their new modeling amplifier is, “and it was so cheap!”, knowing that the “cheap” amp won’t be that cheap if/when the board has to be replaced.
Click to expand...

Yeah...I mean....that is the way a lot of technology is going. Even the Fractals kind of count (I know some individual components can be replaced). Apple computers absolutely do. And all TVs. And almost all consumer-level stereos. Most game systems. Most audio interfaces. Really....a lot of stuff.

But, an FRFR that's user serviceable with readily available parts - serious win compared to the closed source DSP-driven wedges a lot of people like. Especially if the noise is solvable and/or the thing is that easily modable...it seems like there's a good chance they would at least potentially outlast this generation of modelers. I like the direction.
 
Lysander said:
Really appreciate those backlit PCB photos, @WKSmith ! I've been considering tracing the preamp out and see if i could come up with some mods to lower the amp's noise floor, but i was too lazy to get going :D
Click to expand...
Here are the first amp 2 stages 🙂👍

The Mod: use box caps (WIMA or similar)
C12, C13 use .22uf
Both output coupling caps use 1uf
R13 use a metal 10k resistor
R2 use a 10k trimmer
Swap 4558 for OPA2134

JRC4580D , NJR4580D is also a nice budget op amp choice for these 2 stages.

It sounds stunning, If I say so myself. You should try it!

IMG_1074.jpeg
 
Last edited:
You must log in or register to reply here.
Back
Top Bottom