Fender FR-12 preamp mod : The Hissterectomy

There's something really cool about seeing someone engineer a product that they're passionate about. I always find it fascinating. I definitely enjoy medicine, but in another life...

Any updates on this, @WKSmith ? I'm strongly considering a FR12 and could maybe figure out a few of the mods myself with <25% chance of electrocution... but I would much prefer a mod board done by a pro.
 
After tuning and/or gutting every aspect of this preamp and deleting some really dumb stuff.. I have decided the most effective way to release a fix, is to completely rebuild the preamp board so that it can easily be plugged in without any soldering. This would eliminate customer service troubleshooting emails and potentially boinking the stock pre. And it can be returned to stock very easily.

I have most of the schematic drawn up and a rough layout. I was pretty happy to see that I already have almost all of the passives (resistors, caps, and nice pots) here in the shop.

I am socketing the ICs so they can easily be swapped for Burr Browns or your favorite PDip sized opamps. I added a clip light that will probably light up a clear volume knob (no drilling for the LED). And I am working on using an optocoupler for audio delay and soft amp turn on instead of the current ancient 80's transistor gimmick in the stock pre.

The differential inputs, 2nd gain stage, 4 eq filters, are all tuned up and ready. I still need to lay out the tuned Xover, amp turn on, and the TDA2050 pwr amp for the horn.. and figure out final cabling and sockets.


Here is the current drawing of the work in progress.

v2ish.png
 
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WKSmith said:
After retouching and/or gutting every aspect of this preamp.. I have decided the most effective way to release a fix, is to completely rebuild the preamp board so that it can easily be plugged in without any soldering. This would eliminate customer service troubleshooting emails and potentially boinking the stock pre.

I have most of the schematic drawn up and a rough layout. I was pretty happy to see that I already have almost all of the passives (resistors, caps, and nice pots) here in the shop.

Here is the current drawing of the work in progress.
View attachment 131164
Click to expand...
Amazing. I think I'll order a FR12. If this product materializes, you'll have a customer.
 
There is one downside I found after replacing the TL084’s with OPA4227PA’s in my two FR-12’s:

The FR-12’s are so quiet now that I sometimes forget to turn them off and end up leaving them on overnight :p

You’d think I’d at least notice the red lights but I sometimes don’t notice because they are tilted back.
 
I took the preamp out to look it over. For some reason I forgot to take pics at each stage. So now I can't remember/figure out which wire goes where. It's just the two wires, one black from the switch, one white from the IEC input. Any help?

IMG_3876.jpeg
 
WKSmith said:
After tuning and/or gutting every aspect of this preamp and deleting some really dumb stuff.. I have decided the most effective way to release a fix, is to completely rebuild the preamp board so that it can easily be plugged in without any soldering. This would eliminate customer service troubleshooting emails and potentially boinking the stock pre. And it can be returned to stock very easily.

I have most of the schematic drawn up and a rough layout. I was pretty happy to see that I already have almost all of the passives (resistors, caps, and nice pots) here in the shop.

I am socketing the ICs so they can easily be swapped for Burr Browns or your favorite PDip sized opamps. I added a clip light that will probably light up a clear volume knob (no drilling for the LED). And I am working on using an optocoupler for audio delay and soft amp turn on instead of the current ancient 80's transistor gimmick in the stock pre.

The differential inputs, 2nd gain stage, 4 eq filters, are all tuned up and ready. I still need to lay out the tuned Xover, amp turn on, and the TDA2050 pwr amp for the horn.. and figure out final cabling and sockets.


Here is the current drawing of the work in progress.

View attachment 131164
Click to expand...
If this turns into an actual product I’m buying an FR-12 and this board. Done deal.
 
Found an open box deal + a coupon and ordered a nicely-discounted FR12 from the Guitar Center. @WKSmith, did you notice (or would you expect) any significant sonic improvement after substituting the op amp alone? That sounds like an easy mod as we wait for your mod board release!
 
Rockuronium said:
Found an open box deal + a coupon and ordered a nicely-discounted FR12 from the Guitar Center. @WKSmith, did you notice (or would you expect) any significant sonic improvement after substituting the op amp alone? That sounds like an easy mod as we wait for your mod board release!
Click to expand...
Glad you got one! I am happy to see open box deals are starting to show up.

I love the opa2134 in the first and last stages. It does sound better to swap them, but it is subtle.

@Lysander has done an awesome job swapping and testing different opamps. The 3 big quad amps in the eq make the most difference in hiss from what I am hearing from folks.

My new schematic sounds great .. even with cheap TL072 amps. And there are 8 less opamp stages in the design.

If I get some time soon, I will release the instructions on bypassing the eq with just one resistor and one wire. Then you can use an eq block to dial it in on the Fractal like @Deadpool_25 recommended. And even replicate the FR eq in a PEQ block if you like.

The solution he has for the hiss is to boost the highs before the amp and use the cut knob to roll the highs back and help tame the hiss. This idea is a lot like Fractal's secret sauce. And is also pretty brilliant.
 
WKSmith said:
Glad you got one! I am happy to see open box deals are starting to show up.

I love the opa2134 in the first and last stages. It does sound better to swap them, but it is subtle.

@Lysander has done an awesome job swapping and testing different opamps. The 3 big quad amps in the eq make the most difference in hiss from what I am hearing from folks.

My new schematic sounds great .. even with cheap TL072 amps. And there are 8 less opamp stages in the design.

If I get some time soon, I will release the instructions on bypassing the eq with just one resistor and one wire. Then you can use an eq block to dial it in on the Fractal like @Deadpool_25 recommended. And even replicate the FR eq in a PEQ block if you like.

The solution he has for the hiss is to boost the highs before the amp and use the cut knob to roll the highs back and help tame the hiss. This idea is a lot like Fractal's secret sauce. And is also pretty brilliant.
Click to expand...

The inconvenience of this method is that it inutilises the DI output of the FR12, since the signal it receives is EQd in a way that would also require to be corrected in the FOH mix table, which adds another variable to live shows. Or I would have to route two different outputs in my modeler, one including the EQ block just for the FR12, and another output to FOH without that EQ block, which is perfectly doable, but can be a PITA sometimes...

IMHO, the ideal solution would be to fix the FR12 in the first place, and that's why I'm following so closely your amazing work at this thread.
 
davi_ssilva said:
The inconvenience of this method is that it inutilises the DI output of the FR12, since the signal it receives is EQd in a way that would also require to be corrected in the FOH mix table, which adds another variable to live shows. Or I would have to route two different outputs in my modeler, one including the EQ block just for the FR12, and another output to FOH without that EQ block, which is perfectly doable, but can be a PITA sometimes...

IMHO, the ideal solution would be to fix the FR12 in the first place, and that's why I'm following so closely your amazing work at this thread.
Click to expand...
Thank you. ..and I agree
This should have been fixed before it was released. And I want the eq on the FR ..not something else that needs to be programmed.

But an EQ block it is a solution that only costs a few cents and some solder and may be exactly what some folks need so I'm going to share that fix too.

You make a good point about the xlr out being affected by eq upstream. I can run out of outputs quickly on my FM9 when I add a loop and monitor sends. And even quicker on the FM3 with it's 2 outs.
 
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