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?