Fractal X-Load Modification

[James Freeman]

I thought I should start a thread about my modifications to the Fractal X-Load LB-2.
The goal was to fix the resonance frequency and Q by changing the resonance inductor and caps.

The stock X-Load has two 330uF in series which sums to 160uF in UK mode and the switch adds the other two 330uF in parallel for a total of 330uF in US mode, the inductor is about 18mH.
The values above form two center frequencies of 95Hz in UK mode and 65Hz in US mode, neither offers the "correct" 110Hz - 120Hz resonance for a typical Marshall 4x12.

Stock gut shot and response:

I bought a few 220uF and 100uF 100V bi-polar capacitors and a big 9mH inductor from Aliexpress for a total of 60$ to bring the X-Load more in spec with Suhr and a 4x12 cab.

Desoldered the stock inductor and caps out.

Soldered in the new caps.
The upper position is permanently wired (UK) and the bottom cap is switched in in parallel (US), so either 220uF or 330uF.

The fun part was to unwind the inductor form 9mH until the resonant circuit measured exactly 115Hz in REW impedance measurement tool.
9mH + 220uF gave me around 107Hz, unwinding the inductor raises the resonance frequency.
Unwind 10-15 turns, strip the tip, touch the black wire, measure, repeat until exactly 115Hz.

All done.
I also unsoldered one end of the 20RJ resistor closer to 220uF cap to disable the resonance limiting which measures more like a cab, Suher RL doesn't have limiting either.

Impedance measurement in REW with the final result, 116Hz for UK mode and 96Hz for US mode, both are useful now.
I'm happy with the results.
Yes, I have cranked the Plexi and JCM800 into it, no fireworks or magic smoke so far, the inductor does not make more noise than the stock potted one, it's tight.

Measured amp response.

 

[MirrorProfiles]

nicely done! Looks quite similar to what I’d expect of a Suhr load.

How easy is it to tighten the Q of the bass resonance?

 

[James Freeman]

How easy is it to tighten the Q of the bass resonance?

You have to replace both the cap(s) and inductor.
To narrow the Q you have to raise the Capacitance and lower the Inductance.

Suhr has 240uF+8mH
Stock Fracal has 160uF+18mH

For this mod mod I went with standard cap value of 220uF and about 8.5mH for the inductor.

LT Spice simulation of Suhr and modified Fractal, basically the same Q now.
Yellow - Suhr.
Blue - Fractal modded.

 

[jazzarian]

You certainly can tune an inductor, I suspect you excel at tuning guitars as well.

I'm a retired engineer who spent the last half coding unfortunately. I still enjoy a bout of Norton and Thevenin now and then, and I'm quite jealous of your storage 'scope.

Tell me, did you hear much of a difference after all your fine work? Did the original "sound" of the Fractal need improvement from your vantage point? I ask because I might buy one soon, or the Suhr Reactive Load (not IR).

 

[James Freeman]

My modified Fractal LB-2 vs Suhr RL vs 1960BV.
Each load is running from the corresponding output impedance tap (8->8, 16->16).

Fractal LB-2 needs to be modded to have a resonance response of a 4x12 otherwise it's quite far off, the Suhr RL does a much better job stock.
Seems like both load boxes are a touch (1-2dB) brighter than my 4x12 cabs but pretty damn close.

 

[Humbug]

I wonder if Fractal is using the two toroidals to do the rise portion of the curve the same way I'm doing it or if there's something else going on there. The midrange portion does seem a bit "meatier" than the Suhr's... Have you traced out the circuit?

 

[James Freeman]

Have you traced out the circuit?

I can but will not out of respect for Fractal.
BTW, the measurement above is with the resonance limiting resistor lifted, without doing so the peak amplitude is lower than a real cab or Suhr.

 

[Humbug]

I can but will not out of respect for Fractal.
BTW, the measurement above is with the resonance limiting resistor lifted, without doing so the peak amplitude is lower than a real cab or Suhr.

No problem :-) I was really just curious if Fractal were as wonderfully brilliant as me.

 

[James Freeman]

Lacquer potting the inductor.

 

[Humbug]

Lacquer potting the inductor.

I like the vacuum setup :) Did you notice a difference in noise and/or response?

 

[James Freeman]

Still in the oven.

 

[Humbug]

Still in the oven.

View attachment 34595

Tell me you live by yourself without telling me you live by yourself (or you have a spare oven, which is not nearly as funny).

 

[James Freeman]

Tell me you live by yourself without telling me you live by yourself (or you have a spare oven, which is not nearly as funny).

It's the family oven, I took a day off (for other things), it will be my little secret.

 

[2dor]

Still in the oven.

View attachment 34595

Awwww...the traditional winter season inductor roast is almost ready

 

[JiveTurkey]

Awwww...the traditional winter season inductor roast is almost ready

I was going to go with "My food tastes like amp?!?!" but yours is better

 

[James Freeman]

SUCCESS!

All the buzz and rattling noises form the inductor when playing notes around the resonance frequency of 115Hz (G# to B) are gone!
There is a faint low rumble (purr) from the inductor when the 100W Marshall is cranked but no buzz whatsoever, the amps own transformers noise/buzz is way louder now.
So there we have it, Transformer/Inductor potting is important.

In this clip my phone leaning against the open box, you can hear the guitar strings in the background.
Just a faint purr on the resonant frequency notes, no buzz.

 

[LeftyLoungeLizard]

95Hz in UK mode and 65Hz in US mode

WTF were they thinking? And 65Hz? That's bass guitar range. You definitely did the right thing.

 

[James Freeman]

WTF were they thinking? And 65Hz? That's bass guitar range. You definitely did the right thing.

Lower resonance frequency is typical for an open back 1x12 cab, that's what they were going with the US setting, I don't need that, I need proper closed back 4x12 with a 110-120Hz resonance frequency and slightly lower resonance for oversized 4x12 cabs.

 

[LeftyLoungeLizard]

Lower resonance frequency is typical for an open back 1x12 cab, that's what they were going with the US setting, I don't need that, I need proper closed back 4x12 with a 110-120Hz resonance frequency and slightly lower resonance for oversized 4x12 cabs.

You'd think modern times here (since it is a modern product) that "US" would be more modern type Mesa Boogie half-stacks (4x12), not old school type Fender combos (1x12, 2x12).

But then again... who knows.

And yeah, if I need more "oomph" on my guitars (more often than not Marshall flavored) I typically raise 125Hz.

 

[Humbug]

WTF were they thinking? And 65Hz? That's bass guitar range. You definitely did the right thing.

65 Hz or thereabouts is pretty common for open back cabinets where the resonant frequency of the speaker isn't affected by the box. The amplitude of the peak is also considerably higher in such cabs, though, and to replicate this in a reactive load you'd have to switch in more inductance rather than capacitance. Which I suppose would be expensive and, possibly, have other challenges.

I think Fractal built a box that was Close Enough(tm) that would work with their software (and hardware, I guess) and not be stupid expensive to manufacture.