Presonus ACP88 8-Channel Compressor/Limiter/Gate
Note: I wrote a review for the Digimax LT as well. Since I use this unit in the insert chain of the Digimax LT and treat them as one system, it might be relevant to check out that review also. Some phrases from that review might be familiar to you here.
I'll start by saying I purchased this unit for the purpose of live and studio simultaneous multichannel recording. I usually record bands live and then do individual studio sessions with them later. So I initially chose this unit for the following reasons:
1. I needed an 8-channel dynamics processor to work with my Digimax LT for live recording purposes.
2. The specs and other reviews prompted me to try this unit out instead of two separate 4-channel units. I especially like the ability to gang multiple (e.g. more than 2) tracks together in a group - this is nice when recording live drums in a situation where there is a lot of bleed (I don't always have the option of using excellent mics).
3. For the price, the feature set is excellent.
In use, this unit has handled very well. It sounds great (actually, when I remove it from the signal chain, aside from the obvious effect of there being no dynamics control anymore, its nearly impossible to tell from a noise or coloration perspective that it was in there in the first place). The metering is pretty accurate, and the controls are easy to find and use (I do have thinner fingers than most people, I've heard a few gripes about the little pushbuttons). The knobs are neatly colored according to the section of the channel they belong to (compressor, gate, makeup gain), and the gain reduction LEDs are in the middle of each channel section.
The threshold is adjustable from +20dBu to -40dBu with two LEDs (green denoting under-threshold and red denoting over-threshold condition). The ratio in some published specs is listed as going from 1:1 to 20:1, but its marked on the front panel as 1:1 to infinity:1. I can't say in practice whether or not its one or the other but it does an excellent job (without soft-knee) of brick-wall limiting the signal. The soft-knee feature is very useful for gently approaching the brick-wall effect, but it renders the threshold setting a bit inaccurate. The threshold can be set numerically to a certain value and if soft-knee is in, the output can actually exceed that value regardless of the ratio and attack set. From my experience this is expected behavior for a soft-knee compressor but most of the diagrams you see in literature do not actually illustrate this effect. So don't set your levels without soft-knee and then push it in thinking you still won't go "over" your target setting.
The overall sound is pristine - the unit has a VERY low self-noise (which is excellent when combined with the input/gain stages of the Digimax LT and the fact that I have to use unbalanced connections between them in a very confined environment). I have all 8 channels running to/from the inserts in the Digimax LT on custom 8" insert cables using proper shield configurations and have ZERO noise problems from the mike to my A/D... its an excellent solution. The compression is SMOOOOTH, can be punchy, and is particularly superb on acoustic guitar, female vocals, and drums -- well, heh, all the usual stuff that compression is superb on, I guess. :) But this unit certainly does not disappoint and compared with my friend's DBX and other gear, I feel like I have a piece of gear in a class or two above all that.
Overall I am very happy with this unit. I would change a few things about it however:
1. The pushbuttons (for soft-knee, auto-ratio, gate level, and bypass) are black, tiny, and unlit. Its nearly impossible to tell whether they are pushed in or not unless you click them again to find out.
2. All knobs are "stepped" in minute increments. I personally find this annoying. Plus there is no detent at the "0" or reference level... invariably the detent falls somewhere above or below this value. I don't know if this is a mis-alignment of the knob or simply an arbitrary position for the detents, but if you are borderline obsessive-compulsive like me you will be annoyed by this. I didn't notice any particularly negative sonic or technical effects but it was just a bit odd, and does not seem to be replicated on anything else Presonus makes. Their attention to details like this is actually excellent on almost all of their other products, this one just seemed to get the short end of that.
3. The gate clicks on, off. Does not smoothly open/close, just bang/bang. Some people are used to this (its the behavior of many other gates out there too) but I find it annoying. I haven't used the gates much anyway -- I didn't buy the unit for the gates, just the damn good compression which it has -- but it makes them useless (to me at least). But then gating on the way to disk is not an essential part of what I've needed to do on a regular basis anyway...(for the styles of music I record - rock, punk, indie, folk, and coffeehouse bands). I usually gate (if its needed) in software so I can fine-tune the gate parameters afterwards.
4. The power supply does not use a toroidal transformer, unlike the Digimax. The Digimax can go next to virtually any piece of equipment and not mess it up with its transformer, but the unit in the ACP88 is actually slightly noiser. Its not bad (in fact, most people won't notice it) but I'm extremely sensitive to electronic noise in systems and I have to separate the ACP88, Digimax LT, and my MOTU 828 (also suffers from a noisy transformer) by at least one rack space each to ensure they don't bother each other.
5. It would be very nice to replace the gate section with another "brick-wall" limiter instead, so that musical compression can be done with the compressor and then a hard limit set to protect the A/D converters, but this is a minor point and the CL44 (the 4-channel) version I think does this. Its whether you want 2 4-channel units or 1 8-channel. I chose the 1 for the ease of transporting and because it uses 1 less power strip/cord/transformer.
However, all told, would I buy this unit again? Most likely, unless Presonus comes out with another upgraded version addressing some of the issues I noted above. Then I'd buy that. I love this unit and although its not the most essential or the most heavily used part of my rack, its certainly an elegant, excellent, and functional piece of gear that I would highly recommend to anybody who needs (or might need) multichannel compression, particularly with live work.