Fender FR-12 preamp mod : The Hissterectomy

WKSmith said:
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.
Click to expand...
Just wondering, should I be using +4dB with the FR12? I gigged with it last night and I’m set to +4dB - worked fine but I’m not sure if I should keep it there.
 
Another mod success here!

Howdy folks, I received my order from Mouser last weekend and did the preamp mod too.
I'm by no means a professional solder jockey, but do have years of experience in building fx pedals and a tube amp head.
So, with a little bit of knowledge, experience in soldering and patience, the mod was painless and relatively easy to do.

I used three OPA1604AIDR SMD ICs + adapter as suggested by @Lysander to replace the original TL084s.
(Preparing/soldering the adapters and the SMD ICs was the hardest part to be honest...)
For replacing the RC4558s I had found two older OPA2134s in my parts stash so I socketed and used these:
PXL_20231105_124838874.jpg

It's a bit tight with the adapters over the existing parts, but if you carefully bow the ceramic and film caps down a bit, the adapter pins fit in just snuggly:
PXL_20231105_125412871.jpg

Along with the IC replacements I also did some updates to the input stage as proposed by @WKSmith :

CMF55 metal film resistors:
  • 1K for R2
  • 6.2k for R13

Box film cap changes:
  • C12, C13 use .22uf
  • Output coupling caps (C14, C2) use 1uf

I used Wima MKP 2 caps, which are great, and red, but also a tad bit large... ;-)
I had no problem fitting them on the board though, the spacing is large enough to accomodate the "not so little buggers"...
PXL_20231105_125015288.jpg

So, what's the result of a few hours of work?
The hiss definitely is a lot quieter than before, but it's still there and audible when you are standing close to the amp.
I have no weird phasing or oscillating issues with my replacements too.

One thing I noticed on my amp is that not the loudness, but the "sound" or "tone" of the hiss changes when using the mid and treble controls on the preamp: They both add more pronounced hiss in their respective frequency range, which further goes to show that the hiss definitely is a product of the preamp (design).

I had initially hoped for even more hiss reduction from reading @Lysander 's initial post on the Gearforum, but take a few steps back and you can't really hear it anymore. And it's a total non-issue once you start playing anyways! This thing sounds really great!

Overall I'm happy with the result, so massive thank you m8s to @Lysander and @WKSmith , you guys rock!
I will still follow WKSmith and his preamp update(s) and see what he will eventually come up with to maybe further better this already awesome product!

Cheers folks!
 
quasimono said:
Overall I'm happy with the result, so massive thank you m8s to @Lysander and @WKSmith , you guys rock!
I will still follow WKSmith and his preamp update(s) and see what he will eventually come up with to maybe further better this already awesome product!
Click to expand...

Woot! 🙏 Congratulations, and thanks for the feedback!

quasimono said:
One thing I noticed on my amp is that not the loudness, but the "sound" or "tone" of the hiss changes when using the mid and treble controls on the preamp: They both add more pronounced hiss in their respective frequency range, which further goes to show that the hiss definitely is a product of the preamp (design).
Click to expand...

Yes, the tone stack absolutely colors noise on my FR-12 as well. The rest of the preamp is very quiet, but the EQ section has so many (unnecessary...) gain stages that it's impossible for it to be dead quiet as designed.
 
Edited my original post regarding this, but, for completeness sake: do NOT use OPA4228 as a DIP replacement for TL084s in the FR-10/12. This part is not unity-gain stable, and can cause oscillations - particularly on the very first TL084, which handles the bass/mid/treble controls.

For a replacement in through-hole packaging, OPA4227 should be used instead.

If you already bought 4228's, just leave the leftmost TL084 unmodified and swap the other two. See this post for details.
 
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Lysander said:
Edited my original post regarding this, but, for completeness sake: do NOT use OPA4228 as a DIP replacement for TL084s in the FR-10/12. This part is not unity-gain stable, and can cause oscillations - particularly on the very first TL084, which handles the bass/mid/treble controls.

For a replacement in through-hole packaging, OPA4227 should be used instead.

If you already bought 4228's, just leave the leftmost TL084 unmodified and swap the other two. See this post for details.
Click to expand...
thanks for the heads up! Got a notification from Sweetwater that the FR12 was back in stock so I ordered two yesterday along with 6 4228s from Mouser, and was able to successfully cancel the 4228s today before they shipped and order the 4227s.
 
Lysander said:
Edited my original post regarding this, but, for completeness sake: do NOT use OPA4228 as a DIP replacement for TL084s in the FR-10/12. This part is not unity-gain stable, and can cause oscillations - particularly on the very first TL084, which handles the bass/mid/treble controls.

For a replacement in through-hole packaging, OPA4227 should be used instead.

If you already bought 4228's, just leave the leftmost TL084 unmodified and swap the other two. See this post for details.
Click to expand...
I edited my summary post as well.
 
Obi-Wan Kenobi Not The Droids GIF
 
I received my Fender FR-10 today, quite surprised by the hiss that the amp generates, if the amplification really delivers 1000W....then this can be explained, I believe that some people have expressed some doubts on this subject .
The cut potentiometer allows you to reduce the tweeter hiss to a reasonable level. I plan to adjust it to around 4 or 5. The input has less sensitivity than my GSS amplifier. I will have to raise the level a little. FM9 output level.

I haven't tested it at very high volume, I find the amp generally well balanced, it generates less bass than my CAB F12-X200.
My GSS 100W+ Celestion F12-X200 amp set generates less hiss but the amp only delivers 100W at 8 ohms.
 
Lolo-G said:
I received my Fender FR-10 today, quite surprised by the hiss that the amp generates, if the amplification really delivers 1000W....then this can be explained, I believe that some people have expressed some doubts on this subject .
The cut potentiometer allows you to reduce the tweeter hiss to a reasonable level. I plan to adjust it to around 4 or 5. The input has less sensitivity than my GSS amplifier. I will have to raise the level a little. FM9 output level.

I haven't tested it at very high volume, I find the amp generally well balanced, it generates less bass than my CAB F12-X200.
My GSS 100W+ Celestion F12-X200 amp set generates less hiss but the amp only delivers 100W at 8 ohms.
Click to expand...

Iirc, the fender is 150W into 8Ω per the spec
 
The back of the amp at the power plug shows the max power consumption at 350w.

If they were to posses some alien technology that allowed a full 100% theoretical effiency .. it would only be 350 watts. That technology does not exist, especially not in the $500 range 😉
 
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