S2400 Analog Filtered Outputs sound very different from SP1200

I’m unsure if my particular unit is faulty or if all units have this problem:

The filtered outputs on the S2400 sound nothing like the SP1200 - they sound much darker on the S2400 and the envelopes of the dynamic filters are different.

I’ve attached a zipped folder containing a raw audio file of a vintage Prophet 5 Rev 3.3 playing a chord with its Curtis filters wide open. Then, I sampled that into the Isla S2400 and my Rossum SP1200, carefully gain-matching along the way. From there I triggered the same sample from the S2400 and the SP1200 and captured the sound from the various outputs (unfiltered and filtered). You can see/hear very easily that the S2400 sounds far darker and muffled compared to the SP1200 with all things equal when comparing the corresponding outputs. You’ll also hear that the S2400 dynamic filter has a strange envelope shape that doesn’t sound much like the SP1200’s more natural envelope shape for the corresponding dynamic filtered output. All audio is mono since the Prophet 5 is mono.

Can anyone confirm that this is expected or not? Here’s a dropbox link with the audio samples: Dropbox - S2400 Filtered Output Test

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got a question for you. similar conversations have been had around the S2400 over the past few years and the one prevailing truth that has been present through all of that is that the S2400 was never built or intended to replicate or replace the SP1200. it’s always been viewed as a spiritual successor with modern utility and its own sound + character. even though it’s using the SSI2144 chip (Rossum lists theirs as the SSI2144 IC, don’t know if there is a difference) it seems kinda off to label a machine as being faulty for not sounding like the Rossum if it came out before the Rossum, no? not trying start an argument, just genuinely curious if this is a “problem” or a comparison that comes up short in your eyes?


In this video, Brad states that the analog circuitry for the inputs and outputs are identical to the sp1200: Isla Instruments SP-2400 Filters Explained - YouTube

I’m not at all trying to spark unnecessary criticism or anything like that. I want to love the S2400 esp for all its modern features like stereo and workflow stuff.

If you listen to the sound samples I provided, my unit sounds like it is not working correctly. For example the difference between analog output filters 3-6 are barely noticeable and very very dark (greatly reducing high frequencies of the samples) making them basically unusable.

One of the reasons I purchased the unit is for those filters as they are integral to the rich sound of classic samplers (I own plenty of digital analog hybrid gear from the great synth and sampler designers of the 20th century, and they all attest to the importance of running digital signals through analog filters for a rich sound, ie see Dave Smith discussing how “magic” happens when digital signals are properly run through analog filters with any of his hybrid designs, or of course Rossum himself discussing this over the years as well). Most importantly, I use my ears to judge, and I simply want the filters to be useable if they are a feature and circuit I’ve paid for. I just need to confirm as I will return the unit if it is faulty since I have a limited return window.

Can anyone confirm similar results or am I dealing with a faulty unit or if I’m just missing something in the settings?


Listening to ur samples, and comparing the dynamic filt outs from both machines you provided with the original sample, I can hear that on the S2400, the envelope seems to only tentatively “bite” for a short segment after the initial attack. It could maybe be a gain staging thing, where the S2400 is not getting enough input gain into the dynamic filter to do it’s thing? Could try upping the track gain on the S2400 to make the filter engage more, since numbers are arbitratry anyways it might not be possible to numerically match them, but could still be to do it by ear.



I’ve also just had a chance to sit and listen to the sound samples, as well. Ever since the new update, I’ve actually noticed that same kind of annoying “bite” at the beginning of sounds that I have sampled in, and it has happened when the samples are gain staged low and high. I’m inclined to think that this is a bug and not due to a faulty unit @noiselabrob. I know that your original post centered around the actual sound of the filters but I can’t help but wonder if that “bite” is part of the issue. @Mickey have you heard anything about that?

It’d be helpful for me and perhaps many of us if someone else could post audio examples of mono and stereo samples assigned and triggered through the analog filtered outputs. Please post the raw, unfiltered sound file as well. I could then check and compare the frequencies with a spectrum analyzer to get a sense of where the analog filters are rolling off.

I’d want to know if my unit is too dark (too rolled off) versus other owners here or if this is just how they are designed. Ultimately I’d want the filters to be a useful feature as it is part of the circuitry you pay for (not just as they are on the sp1200 esp as they are integral to its magical sound, even though I was led to believe they were identical in that regard, but even just useable in their own right even if they are calibrated differently)

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The analog filters on my unit also sound way to low-passed to be useable for anything other than only keeping the bass frequencies. Even the filter on channel 6, which is supposed to be the brightest one barely leaves any high frequencies.

They’re not useable for just filtering out some of the aliasing, as I understand is what the filters on the sp1200 were used for. Because of this, I hardly use the filters which is a shame. How is this for others?

i use filters 1/2 for stereo samples (classic engine, 12b26k) and filter 6 for mono samples as kicks, bass, sometimes snares and hats. i think it glues the samples together nicely.

it‘s also a matter of preference and what you are looking for. in case of that „oldschool“ boom bap sound, i really like what the filters do. and with the option to turn them off or on, they are enough flexibel for my workflow.

that extra filter card, will may be what you are looking for. lets see…


Same here, I only use 1&2 on stereo samples.
If I want more control, I use the digital filters which sound good to me.
An extra filter board doesn’t look useful to me unless it improves the existing design (low output volume, the way main/and separate outs work).


Btw, I m probably wrong but there might be a way to tweak the analog filters. I may have read that on the forum a couple of years ago or heard that on a YT video. Sorry very vague memory.
You probably should ask if it’s possible to do it via mail or DM to official Isla people :slight_smile:

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Same for me, and I’m perfectly happy with the digital filters. I was just wondering whether I was missing something with the analog filters, but I guess they have their uses!

Yeah I was going to reply the same but I couldn’t remember if the pots were for all the filters or just the dynamic. In the old you tube video on the filter design and stuff I wanna say he only mentions the pots in reference to the decay of the dynamic filters on 1 and 2 but I could be wrong!

How is anyone using the 1/2 filters for stereo samples, when they are the dynamic envelope filters? Unless you’re just talking stereo one shot drums and not longer melodic samples which will just turn to mud after the first half a second or whatever.

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Having owned the S2400 and the SP-1200 for some time now, I can confirm, that they sound very different (as opposed to what Brad mentioned in some of the videos discussing the input/output circuitry being identical) and I’m glad I kept the SP-1200 and probably will never sell it. It makes samples sound amazing (esp. drums) by adding a certain fatness and weight coupled with a tightness that is not present in the S2400.

I probably should have been more skeptical before buying the Isla S2400… seeing how exacting Dave Rossum was in his design and engineering process and the pains he went through to get identical (or as-close-to) circuitry for his current SP-1200’s (this page says it all: FAQ – Rossum Electro-Music). A job like this is not easily replicated and there’s a reason why Rossum is considered a genius and a legend - being thorough is part of that.

I’m bummed to find out that the implementation of the Isla S2400 analog filters feel like they’re an afterthought, sonically speaking. They’re a core part of the original SP1200 sound and even if the same chips were used, if they’re not calibrated properly, they can result in very different sound, which is the case here. There maybe some use cases for them, but they sound nothing like the SP1200 with which all the analog filters are useful and help the machine have that legendary/magical sound we all know and love. I know everyone says stop comparing the two machines, but Brad himself said the filter circuitry was identical in multiple videos.

That said, I see the S2400 as covering duties in my production workflow in the territory of “general stereo vintage sampler” along the lines of an MPC or equivalent so it has its uses (even if I felt a bit misled in the marketing/explainer videos from Isla about how close it would sound/circuitry to an SP-1200). The modern workflow in it is great, and by no means does it sound bad - it sounds good and shouldn’t be compared sonically to an SP1200 though it originally was drawing those comparisons by Isla instruments themselves. That said, I’m currently on the fence about whether to keep the S2400 or just replace it with a classic MPC. If I keep it, its mostly for its modern workflow over the classic MPC workflow/slower OS,etc.

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I’ve used the analog filters on outputs 3,4,5,6 for cutting out aliasing artifacts after resampling 33@45 and pitching down. Isn’t that their only reason for existing?

The dynamic filters on 1,2 I’ve only used for single hit drums and percussion when I’ve felt it sounds better. Not compared to original so have nothing to complain about.

Same findings here…bought and sold the Isla and am sticking with the SP1200. I found the analogue filters so rolled off as to be unusable. I’m curious to know if there’s been a hardware revision as it appears some users have no issue with them? I don’t have my serial number to hand (bought second hand early '22 in the UK) but on mine, even outputs 5 and 6 would roll off so much from a snare that all the crack and fizz was completely cloaked.

As the OP says, the output filters on SP1200 are a big part of the sound. I get it that the 2400 is not a SP1200 clone, and if the output filters sounded a bit different…fair enough, but on my unit, they were completely unusable…also interested to know if this was just my unit or otherwise.


This is all interesting. I recently picked up a Tube Opto8 to experiment with the filters in the hopes I could dial in my sound a bit better. It’s a bit disheartening TBH, but we’ll see how they can be used. I only got to play around with the SP a little bit in my youth, but I’m still chasing that vibe.

I did it for a lot of tracks and I liked how it sounded. (analog filters 1&2 on long and stereo melodic samples).
Probably because the other outputs sounded too dull too me though.

If you want an Sp1200 then buy one. The output filters on my S2400 behave exactly as the specs state, so if yours don’t then either your unit is faulty (and you could raise a support ticket) or you have such perfect hearing and unreasonable expectations that nothing will ever satisfy you.