I've been on the fence for buying a desktop acid machine but I want to consider my options for what I can throw together in a rack first. This is my first time putting a rack together so please be easy on me, any advice is much appreciated. I'm just looking to make some deep growling acid. I've considered other sequencers but I really enjoy stepping out of typical 8 step acid sequences.

(UPDATED RACK)ModularGrid Rack

@troux will probably have some good advice for you here. I've tossed around the idea of adding the Erica Bassline to my rack. It sounds really good. As it is though, I already have the inexpensive Behringer TD3 (16 step sequencer) and haven't been able to justify the cost of the Bassline just yet.
Have fun and good luck.

as farkas said - @troux is likely to have some good advice...

I'm no expert on acid but I think you would benefit from a slew for portamento which is one of the main characteristics of acid (the slowed rise and drops to the notes) as far as I am aware

general issues that I can see though are:

the sequencer is single channel and you do not have a buffered mult to accurately make a copy of the v/oct signal to both the bassline and the pico voice - this would also benefit from a sample and hold or a second sequencer (and a precision adder - to accurately add the 2 v/oct signals together) - so you can derive a second sequence from the single channel - otherwise you will only be able to play an interval - which would be dependent on how you tune the voice modules

the sequencer has a quantizer built in so the 2hp tune module is redundant - redundancy is often good in modular - nay desirable, but not in this case at your stage of the journey

you may have confused the tune with a tuner, it's not (see above) - get a pedal and the appropriate cable or use your phone - I use a korg pitch black I had lieing around

both the bassline and the pico voice are voices as opposed to vcos - this implies that they have filters and vcas built in (the bassline definitely does - I have the DIY version) so the filter is kind of redundant and probably not in an overly desirable way either

you don't have any envelope generators - whilst you could use the gate from the sequencer to trigger both the voices (the passive mult will work perfectly for this) it is an on off switch - sending this first to an envelope generator is a good idea - even better sending it to 2 different envelope generators - with one channel going to a clock divider or some other method of altering it (a gate delay for example) would massively help in terms of making 2 melodic parts, one for each voice

no modulation source - you really want one of these - at least an lfo of some sort preferably 2, one that can sync to the clock and one that can run free at the same time - consider Maths - for the simple reason that it is a great learning module and incredibly versatile - see the illustrated manual for further details

what's the use case for the input module?

you don't have a mixer - most people would want to mix the 2 mono voices (and potentially your external sound source) before sending to the output module - if you desperately want stereo get a mixer that will pan mono signals to stereo - if your use case for the output module is headphones then get a mixer with headphone outs too - I started with (and still have a rebel technology mix02 which does both these things)

you may not need the output module - where are you geographically? US and japan apparently often need output modules because the power is unbalanced - in which case a balanced output is probably a better idea - otherwise they are often just a 'nice to have' module

small modules such as the mult and the pico modules are very awkward to deal with as they are so small (you have seen a module in real life haven't you?? some people are shocked by the size, even after watching weeks of video) you will want to space them out between bigger modules

it might be an idea to have a read through of these - the sticky threads in the eurorack sub-forum of muffwiggler, sound on sound magazine synth secrets series (available as a pdf online I believe) and there's a couple of older articles on starting modular synthesis that are good (if a bit dated), the muffwigglers book of bad ideas, the maths illustrated manual and read a load of other newbie threads - even if you think they are not relevant to you, because of genre - a lot of the advice will be generic

even if right now all you want to do is make a squelchy acid bassline - do the background research, it will serve you in good stead for the future - there's a reason it's often called eurocrack!

I've updated the rack to have just one voice; added in some effects, slew and an output module. Let me know your thoughts!

Check out Bloom as a sequencer. It's a love it or hate it module, but can work really well for unorthodox acidy bassline sequencing.

I've been spending a ton of time working on acid racks so I spent a few minutes on this and came away with the below:

ModularGrid Rack

Some notes:
1) I extended to 104HP because that extra 20HP can go a long way for us here as you'll see.
2) The biggest and probably most important addition from my POV is the Make Noise Pressure Points. This lets you easily create 4 part song structures, with let's say two of the inputs going into the meloDICER and a third controlling the Bassline filter cutoff... all of a sudden you can easily change chords and dynamics across multiple parameters with one press. Not to mention it's playable, you can use pressure to drive CV changes through the top outputs, meaning you have structured control of 3 parameters and expressive control of a fourth, lots of fun.
3) I added a WMD Time Warp for gateable slew and as another envelope generator.
4) I added a Bin Seq and a Tree to get weird gate patterns going. This isn't something I've tried personally so you could take another route and add a temps_utile or an Intellijel Steppy and get good and perhaps more grokable results that way too.
5) Pip Slope as a backup envelope generator and also as an LFO.
6) FX Aid because it's got huge range and is superior to the Delay and Verb at their own game from what I can tell.
7) A Disting MK4 to fill in any number of possibilities, wavefolder, recorder, tuner, more effects, etc.
8) I left 4HP open for you to fill in with a distortion module of your choice, I couldn't pick one (though I love the Manhattan Analog DTA) but ultimately the Erica Synths one felt like it took up too much space so I cut it.

Some alternate possibilities worth considering:
1) With Metropolix coming out Metropolises will be going on sale used at a discount I bet, probably worth comparing to the melodicer, in which case you could cut the Time Warp.
2) I recently got an M303 and love it, compare it sound wise to the Bassline and throw the X0X Heart in there too to see all the voice options out there.

Hope this is helpful, and let us know what you end up with!

Hi, you may want to investigate adding a Phaser for great acid sounds. I saw a long time ago a great video by Aiyn Zahev (Sami Rabea) about getting acid sounds using a phaser. Check it out here at about 1min45sec

The core idea is having a phaser with rate=0 (no phasing sweep) and the resonant bump from the phaser gives a lot of the "juicy / rubbery" acid sound. I'm not expert on this SO you may want to investigate it more BUT I have gotten good results from this technique using vsts; I imagine it likely works in hardware too!

Other comments:
-- I agree with above posts that portmento (slew) and a style-fitting sequencer will help get an acid sound
-- there's a ton of software that can do acid well and videos online; if you haven't already, I would say view some more videos and use some software trials to see what software setups produce acid results you like. Those will give you some ideas and validation for what you might want to achieve with a hardware setup.

Good luck!

The thing about the TB-303 is that the key to its sound isn't necessarily found in the synth part of the device. The SEQUENCER is the important part; the way it handles legato, the way the "glide" function works, etc etc...all of that is where we get the "acid house" sound. It's worth noting that when users were modding their 303s back in the 1990s during the ACIIIEEED craze, those mods generally were done to expand the synthesizer, and the sequencer would be left alone for the most part.

However, I should note that I'm very much a TB-303 hater...not because I haven't used them (I have, quite some time ago), but because I could NOT see the point of dropping up to $3k for a plastic box that really only did about 7-8 things well. That, and the fact that that sound (as well as the infamous "Juno Hoover" as well as the M1 House Piano) was super-played-out unless the machine(s) were in the hands of someone who could wring something amazing out of them through adding effects, modding the synth (such as the famous "Devilfish" mod), and doing something other than yanking the batteries out to generate sequences. So I'd suggest following nickgreenberg's lead here and going with a software version...by using a VSTi version, you open up the 303 to control capabilities that the hardware sequencer in it could NEVER do, plus the sound is super-easy for a VSTi to replicate without massively upping your CPU load.

I hear all that @Lugia, but as an acid fanatic I'll chime in to say that modular does open up some really interesting room to explore the acid sound, and folks are looking at the challenge you've raised re: the sequencer thoughtfully, for example the m303 has a Slide input that aims to mimic the TB-303 sound. It never hurts to go the VST route (and certainly helps the budget) but as someone who started his Eurorack journey to make ambient/drone/noise I couldn't resist making squelches once I got a rack in my hands... there's something there there.

Oh, no doubt...but the key is to get a sequencer that properly emulates the TB-303's behavior, something like a Stepper Acid or similar. Beyond that, the sky's the limit as far as sound generation...and that factor is the big thing here. You can still have 303 patterns, but that 303 sequencer getting harnessed to some raw sonic power...THAT'S the point. Even if you do this in a DAW, it's still the "right thing to do" to use a 303-style sequencer to drive a very complex VSTi synth these days.

Ahhh, if only Aphex hadn't released the Most Expensive Acid Track Ever ("180db" on "Syro", which uses not one, not TWO, but THREE Korg PS3300s controlled by a 303), this would be SO much simpler...