Fender FR-10/FR-12 noise reduction mod

[Lysander]

Overall noise floor measured right off the grill dropped 10db compared to stock.

That's awesome!

Then I popped in the 2 OPA2134s (to replace the RC4558s) and the noise floor didn’t change at all.

I'd suggest leaving OPA2134s in, even if you don't notice any noise improvements - it's overall a much better part, specially for something as critical as the differential input. I think i hear some minor tonal improvements on my FR-12 after replacing those, but could just be deluding myself.

 

[Piing]

Any tutorial on YouTube yet? if someone would record the process that would be great for my limited abilities

It all comes down to desoldering and soldering. There are several YouTube tutorials for that. The most delicate part is desoldering the ICs. There are different techniques for that: braid, solder pump, hot air, dedicated desoldering heads for DIL, a solid copper wire along all the pins to melt them simultaneously...

If you have never done it before, I would practice with some discarded electronic device first.

 

[lolainnb]

It all comes down to desoldering and soldering. There are several YouTube tutorials for that. The most delicate part is desoldering the ICs. There are different techniques for that: braid, solder pump, hot air, dedicated desoldering heads for DIL, a solid copper wire along all the pins to melt them simultaneously...

If you have never done it before, I would practice with some discarded electronic device first.

Braid, pump, hot air, dedicated heads? Never heard of those. I think I’ll pass then.

 

[laxu]

Braid, pump, hot air, dedicated heads? Never heard of those. I think I’ll pass then.

A solder pump and braid are tools that anyone doing any soldering should own.

 

[aflynt]

I bought a Yiuha 929D-V desoldering iron from Amazon a couple days ago that made removing all the ICs super easy and quick. Way faster and easier than desoldering braid and pumps. It’s about $40. I had all the old ICs out in about 15 minutes.

-Aaron

 

[aflynt]

A word of warning, though… The pots are kind of flimsy. Try not to bend them back and forth too much as it feels like they’d break off fairly easily.

-Aaron

 

[aflynt]

Here are some general disassembly instructions. Let me know if anyone finds errors here. Just going on memory as it’s all buttoned up already.

1. Remove the 8 black screws from the back of the unit around the back panel, and the two silver screws along the top panel.
2. Remove the chicken head knobs and nuts and washers holding the pots to the chassis.
3. Slide the chassis out of the cabinet by lifting it up slightly and pulling back.
4. Disconnect the connectors/cables attaching the preamp board to the relay board, IO board, and led.
5. Remove the standoff screws from the IO board and move it out of the way to clear room for removing the preamp board.
6. Remove the standoff screws from the preamp board.
7. Lift the preamp board straight away from the front of the chassis being careful not to bend it or the pots at all. (This is where you need to be careful not to stress the pots too much as they can be damaged if bent).

I’m not including detailed instructions for replacing the ICs as it should be fairly obvious how to do this. Just desolder the pins for the 3 14 pin ICs, remove them and replace them with the new parts (being careful to put them back in the same way around).

-Aaron

 

[Baba]

Good work fellas!

 

[aflynt]

That's awesome!



I'd suggest leaving OPA2134s in, even if you don't notice any noise improvements - it's overall a much better part, specially for something as critical as the differential input. I think i notice some minor tonal improvements on my FR-12 after replacing those, but could just be deluding myself.

I went back and forth between the OPA2134s and the RC4558s and decided to leave the RC4558s in. The OPA2134s sounded a bit more high fidelity. Clearer highs and lows. The RC4558 had more pleasant upper harmonics for me though. Maybe just less high end or smeared high end that was less painful if that makes any sense.

-Aaron

 

[aflynt]

Here's a weird one. I was working on some FM3 patches with the FR12 and noticed that there seemed to be more hiss than there had been earlier today. I measured the hiss SPL and it was indeed about 5 or 6 db higher than when I measured it earlier today. Then I started messing around with the tone controls and noticed that certain settings would generate some pretty nasty noises. Could the OPA4228 chips have some weird instability issues with the FR-12 preamp circuit? It seems to happen after the amp has been on for awhile. I stuck the TL084s back in to test it out and they had more hiss in general but didn't have weird issues at different tone control settings.

https://www.aaronflynt.com/wp-content/uploads/2023/11/FR-12_OPA4228_Noise.mp3

-Aaron

 

[Lysander]

I measured the hiss SPL and it was indeed about 5 or 6 db higher than when I measured it earlier today.

Are you using a phone sound meter app by any chance? These will be wildly inconsistent, specially for noises close to ambient level.

I used a proper sound level meter while modding mine, and still had to be careful to get consistent readings out of it (slow mode, exact same position each time, watch temperature/humidity, etc.)

Then I started messing around with the tone controls and noticed that certain settings would generate some pretty nasty noises. Could the OPA4228 chips have some weird instability issues with the FR-12 preamp circuit?

Wow, thats weird. No, they really shouldn't OPA4227 and OPA4228 are internally compensated so it would be very hard, if not impossible, for them to oscillate in this design. Just double checked, and my FR-12 does no behave like that post mod.

Wonder if that's power supply noise you're getting amplified somehow. These opamps have a much higher common-mode rejection ratio than the TL0xx family, meaning they won't be as forgiving regarding input noise.

 

[aflynt]

Are you using a phone sound meter app by any chance? These will be wildly inconsistent, specially for noises close to ambient level.

I used a proper sound level meter while modding mine, and still had to be careful to get consistent readings out of it (slow mode, exact same position each time, watch temperature/humidity, etc.)



Wow, thats weird. No, they really shouldn't OPA4227 and OPA4228 are internally compensated so it would be very hard, if not impossible, for them to oscillate in this design. Just double checked, and my FR-12 does no behave like that post mod.

Wonder if that's power supply noise you're getting amplified somehow. These opamps have a much higher common-mode rejection ratio than the TL0xx family, meaning they won't be as forgiving regarding input noise.

Using a stand alone SPL meter. I have the TL084s back in again now and have been running the amp for a while. No weirdness. Though if it was power supply noise, then it'd be more dependent on noise the AC line in the house then, which may be completely random, right?

-Aaron

 

[Sascha Franck]

Not that I've got any plans to buy one of these so far (unless you folks have a mod for a second, EQ-bypassing input ready to roll, too), but I defenitely bow before you folks and your brave discovering and engineering skills!

 

[Lysander]

Though if it was power supply noise, then it'd be more dependent on noise the AC line in the house then, which may be completely random, right?

No, not really. The preamp is powered by a switching power supply, which is part of the class-D power amp section. There's nothing feeding off AC power directly in this amp.

I'll try digging into the schematic i drew up later tonight, but off the bat i can't imagine other root cause.

 

[aflynt]

No, not really. The preamp is powered by a switching power supply, which is part of the class-D power amp section. There's nothing feeding off AC power directly in this amp.

I'll try digging into the schematic i drew up later tonight, but off the bat i can't imagine other root cause.

Greatly appreciated! I've been playing it with the TL084s for about an hour and had none of the weird noises with different tone control settings.

With the OPA4228s in I'd hit certain ranges on the tone controls and get strange crackling like the mp3 I posted. It was interactive too. If I hit certain noisy ranges on one control other controls could exacerbate or mollify it depending on how THEY were set.

-Aaron

 

[Humberd]

For those of you that are not familiar with the soldering terms in the posts above
The first pic is solder wick or braid
The second one is soldering heads or tips
Third one is a desoldering pump

 

[Alex Kenivel]

Desoldering pumps are also called solder suckers. I have this one per recommendation from @SillyOctpuss and it's great

https://www.amazon.com/gp/aw/d/B002MJMXD4?psc=1&ref=ppx_pop_mob_b_asin_title

 

[SwirlyMaple]

While we're sharing soldering advice, one of the most important for anyone wanting to learn is get a good high-wattage soldering iron with a temperature-regulated tip and your life will be much easier. When you're first learning, it's very common for beginners to spend way too much time heating a solder joint, eventually overheating the stuff around it, and burning or lifting traces right off of PCBs or destroying parts from overheating them.

Even for experienced folks, if the iron is a crappy low-wattage one, the tip will cool when applied to the work and then gradually re-heat, giving too much time for all that heat to soak into the part, the board, and everything around it. You want an iron that reaches a steady temp and can hold that temp so that your solder joints melt quickly without overheating everything else around them.

Also, get a flux pen or some paste flux. It makes melted solder flow and "stick" to things much more easily. Most quality solder has flux in the core, but in many applications you'll benefit from having more that you can apply as needed. (For example, the copper braid used for de-soldering works a hell of a lot better if you soak the braid with flux first!)

Last bit of advice: though flux is super useful, it's also slightly corrosive longterm. So for a truly professional job, you want to get a brush and isopropyl alcohol after you're done, and scrub until all of the flux is removed. An old toothbrush with its bristles cut down a bit shorter (to make it stiffer) works pretty well for this. Most people don't do this step and it's not that big of a deal, but where I work it's S.O.P. for high-reliability over time.

 

[aflynt]

Greatly appreciated! I've been playing it with the TL084s for about an hour and had none of the weird noises with different tone control settings.

With the OPA4228s in I'd hit certain ranges on the tone controls and get strange crackling like the mp3 I posted. It was interactive too. If I hit certain noisy ranges on one control other controls could exacerbate or mollify it depending on how THEY were set.

-Aaron

I figured I'd play around with different combinations of op-amps today to see if I could narrow this issue down. I stopped at my first test since it totally fixed the problem with weird noises at different pot values and still eliminated the hiss too! Basically, I kept the TL084 in the position nearest the input and replaced the other 2 TL084s with the OPA4228s. So all in all it's just a 2 chip replacement for me that seemed to work best.

-Aaron

 

[Lysander]

Basically, I kept the TL084 in the position nearest the input and replaced the other 2 TL084s with the OPA4228s. So all in all it's just a 2 chip replacement for me that seemed to work best.

That's great to know!

PS, i (sort of) managed to reproduce the issue you mention, but on my amp i needed to crank the volume all the way up and set tone knobs to their maximum values; the chirping sound is then faint, but audible. My running theory is this is switching noise from the power supply, somehow getting amplified.

A q&d mitigation would be to change the power-in decoupling caps (100uF 16v) to 470uF and see if it improves things...

...followed by the decoupling caps for the opamps themselves.