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The Dynamic Duo

Alesis 3630 Compressor/Limiter & Alesis RA100 Reference Amplifier

Alesis made their name on high-quality, low-cost digital effects units, and their latest products offer the same affordable quality on two basic analogue units — a compressor and an amplifier. Dave Lockwood conducts the field test.

Although Alesis will probably forever be best known as the company that brought affordable digital reverb to the mass market, their product range has become far more diverse in recent years. Recent additions to their Studio Series include the RA100 power amplifier and the primary subject of this review, a two-channel compressor/limiter. Designated Model 3630, it features a comprehensive array of controls and an unusually wide choice of operating modes, making it a very versatile unit indeed — in addition to offering true stereo or dual mono operation, for example, gates are featured in both channels.

The 3630 is housed in an exceptionally shallow 1U rack case, finished finished entirely in matt black. Like most Alesis products, the unit uses an external 9v AC adapter, although the power switch is front mounted. Sadly, there is no fail-safe bypass condition, so a failure of the power supply will prevent the unit passing any signal, regardless of the in/out status of the channels — an important consideration in any semi-permanent installation.

All audio connections are via unbalanced 1/4" jacks; facilities include a sidechain send/return access point, via a stereo 1/4" jack, for patching in external keying signals or an EQ for frequency-dependent gain reduction. Inside, although the connectors are soldered directly to the primary circuit-board, they are also supported by the rear panel and therefore do not stress the connection to the board every time they are used. The unit is so small and light that there is effectively no mechanical strain involved in racking it via the integral lightweight rack-ears. It also runs very cool, as you might expect with an external supply, but more surprisingly in view of the lightweight nature of the construction, it displays good immunity to induced noise when racked close to other devices with substantial PSUs.


The control line-up is conventional and easily understood. Coarse system level matching is achieved by means of rear panel switches that select +4dBu or -10dBV operation. There are switches for each channel, which adds to the flexibility of the unit in smaller systems where signals will not necessarily be sent via a patchbay with all circuits at the same level. The Threshold setting that determines the onset of processing is variable about its nominal 0dB point over a 60dB range, +20 to -40dB. Most normal operation should take place within the central 20dB or so, and if extreme settings of the threshold are found necessary, then it is probably best to have another look at interface/source levels.

The 3630 offers the user a choice of characteristic around the threshold point, operating in either hard-knee (standard threshold) mode or soft-knee mode, where compression is applied progressively until it ultimately reaches the specified ratio. This produces a more gradual effect, with a less discernible change in sound than when the full amount of gain-reduction is applied the moment the threshold is crossed. In this mode, the effective threshold level is usually somewhat lower than the control's apparent position, in order to cater for the progressive onset of the soft-knee characteristic. A hard-knee action is obviously better in applications where peak level limiting is the primary function, for it will not only apply the full ratio at once, but it will also allow any signal below the threshold to pass through entirely unaffected.

Ratio (the relationship between input and output level for signals above the threshold) is continuously variable from 1:1 to infinity:1. The front panel is actually marked with positions for a number of specific ratios; I don't know if these are accurately calibrated, but in practice I don't think it really matters for this parameter.


The 3630 offers a very welcome choice of either program-dependent or user-determined attack and release characteristics, designated Peak and RMS modes. This significantly enhances its range of applications compared to many other devices which tend to offer only one of these primary operating modes. The 3630's front-panel attack and release controls are disabled in program-dependent RMS mode, becoming active only when switched to Peak mode.

A program-dependent setting, where the detector signal is based on the average (RMS) level of the input, is invariably preferable for sources where you want the compressor action to be as transparent as possible, particularly a complex signal such as a complete mix. However, automatic setting of the envelope controls in this way prevents deliberate, creative use of compressor side-effects (such as using an over-fast release for a powerful 'pumping' effect, or using a delayed attack to achieve an emphasised transient).

Attack time (the time taken for gain reduction at the specified ratio to be applied) can be varied over a particularly wide range from 0.1 ms to 200ms, which should certainly be enough to cover most applications. Release (the time taken to restore normal gain) is generously variable from 50ms up to 3s. As is often the case with modern 'fast' compressors, you can set up an envelope that will allow a compression cycle to occur on individual waveforms of low frequency signals, causing very audible distortion. It is left to the user to apply common sense when setting parameters — a situation I prefer to having the versatility of the unit compromised in order to protect the user from the potential consequences of his or her actions.

The Output control, where 'make-up gain' is applied, to restore level after gain reduction, is variable from -20 to +20dB with 0dB at the centre representing unity gain. System headroom is specified at a respectable 18dBu and I encountered no level-related problems during testing at either nominal interface level.

The 3630 offers two 12-element horizontal LED column meters, displaying either input or output level on one and gain reduction on the other. I find being able to see input/output level and gain-reduction simultaneously preferable to having a single meter which must be switched between functions.

The input/output meter displays the range -30 to +6dB, with a colour change from green to yellow at -4dB, moving to red at +2dB and beyond. The gain reduction display (all red), which increments from right to left as normal, is calibrated from -1dB to -30dB, with single dB steps down to -6dB. Gain-reduction however, unlike input/output level, is only indicated when the compressor action is actually switched in. Whilst this does prevent any possibility of confusion as to whether the device is active or not, I think on the whole I still prefer being able to set up compression characteristics visually, so that the device can be dropped into a circuit, if necessary in midtake, with its parameters somewhere near to being correct — perhaps this is a legacy of a number of years spent in live location recording where unforeseen events could sometimes require drastic corrective action.


Any sidechain processing required will have to be external, for no on-board facilities are provided for modifying the control signal action apart from the basic soft-knee/hard-knee characteristic selection. A simple fixed high-pass filter, to reduce the sensitivity of the detector circuit to low frequency anomalies such as microphone proximity effect or floor rumble, would have been useful. It seems unfair however, to criticise the 3630 for lack of anything, for it is far more comprehensively equipped than anyone has a right to expect at this price.

The sidechain insert returns can be used to facilitate external control over the gain-reduction characteristic of the 3630, such as would be used in a 'ducking' configuration. Program level through the compressor will then undergo gain reduction in response to the level of the external signal.

A Stereo Link switch is provided to offer normal stereo operation of the 3630, with gain reduction in the two channels tracking one another accurately — without this characteristic, signals that should be centrally placed would constantly be on the move towards which ever side had the least amount of gain reduction. In Stereo mode the left channel's controls determine the compression parameters for both sides, obviating the need for tedious control matching. The compressor action, as one would expect, is still derived from a combination of both signals. On audio performance alone, it is difficult to find fault with the 3630, and in terms of operational flexibility, I am not aware of a comparable unit within this price range.


As is becoming increasingly popular, the 3630 offers on-board gating to combat 'compressor-enhanced noise' — if make-up gain is applied, every dB of compression is a dB lost on the signal-to-noise ratio. Although only a simple two-control affair, the gate actually works rather well, exhibiting very good resistance to 'chatter' (indecisiveness around the threshold point). No separate gate in/out switching is provided, merely a nominal off position at full counter-clockwise rotation of the threshold control. Gate closure time, designated Rate, is the only other control, variable from 20ms up to 2s. This range offers a fair amount of flexibility; the severity of the gating action when a high threshold is required can be to some extent disguised by a slow onset.

Setting-up the gate is facilitated by a pair of LEDs which display green for gate open and red for closed. Gating is still available even if the compressor channel is bypassed, but both gates are tied when the Stereo Link facility is selected. Gate threshold is also independent of the compressor stage output — an operational advantage over an external gate patched after a compressor.


The main strength of Alesis' new compressor is that it offers a quality of sound that will greatly surprise the sceptical — even the prejudiced. In fact there is no reason why it shouldn't, using as it does high quality dbx VCAs and a featuring a very high standard of construction on the electronic side. Not so good, however, is the fact that some of the controls on the review model were so stiff as to be difficult to turn at all, never mind adjust precisely. The situation is further exacerbated by the control knobs having rather smooth surfaces, which means there is nothing to get a decent grip on. A much more heavily ribbed surface is required to prevent controls of this size and proximity becoming a liability. However, the clear white pointer on each knob, which extends right down the side to the base, is a welcome feature.

If this was a device aimed unequivocally at the professional market, I would feel compelled to complain about the size and location of the switches, which again are rather small and fiddly — this is not a unit I would like to work with under adverse lighting conditions. The bypass switch in particular should be more readily identifiable, either by position or colour (black on black was perhaps not a good choice of colour scheme). But then if it was a pro device, it would have balanced connections, a proper power supply, and probably cost twice as much. The semi-pro and private studio markets will not worry about such things to the same extent. To them the 3630 is a high audio quality compressor with an attractive array of really quite sophisticated facilities, and one which represents exceptional value for money.

On audio performance alone, it is difficult to find fault with the 3630, and in terms of operational flexibility, I am not aware of a comparable unit within this price range. In RMS mode, using the soft-knee characteristic, it is possible to achieve a worthwhile degree of gain-reduction (in excess of 6dB) on a complete mix without any undesirable compression effects, provided the ratio is kept to something sensible — 4:1 is about the practical limit if you want the mix to retain any sense of dynamics. Even under quite heavy limiting however, very little compressor 'dulling' occurs, helping to minimise the impression of a 'squashed' signal.

In Peak mode, where the operator has a bit more work to do, everything is pleasingly progressive and predictable; during testing it proved very easy to achieve a range of controlled effects associated with 'abusing' signal dynamics, with more than enough range in attack times to achieve the optimum retarded-attack characteristic for a range of instruments. The very fastest, 0.1 ms setting is really only useful in protective applications, such as 'end-stop' limiting, where the next device in the audio chain has an absolute ceiling.

The Release setting is the control that most inexperienced operators have trouble with. Release time is critical in determining how much of an impression of the original signal's dynamics will be retained — although the fastest release is the one that ensures the maximum possible constant level, which is often the primary aim of using compression, the side-effects can sometimes render this setting unacceptable. There are no hard and fast rules, for suitable settings are so dependent on the context. In a powerful rock track it can be perfectly acceptable to let even the lead vocal pump and surge a bit; indeed this can push through detail that would certainly otherwise be masked. More naturally dynamic material, however, would show this up for the totally unnatural effect that it is, and soon have you screaming for a bit of light and shade. Choosing a release setting thus always involves a balancing act between the desire for maximised level and the risk of totally re-enveloping the signal.

There are of course times when re-enveloping is precisely what you want, and the 3630 proved rather good at 'pumping' bass guitar (attack about 50ms, fast release according to track tempo) and 'ultra-squeezed' clean DI guitar (minimum release time, with attack at about 10ms, depending on the desired amount of pick effect). Given the all-important flexibility to always adopt the optimum operating mode, provided by the Peak/RMS and hard-knee/soft-knee switching options, there really wasn't any type of source material for which I couldn't find a setting that I would be happy to use in a professional situation — I am really most surprised to be able to say that about what must be regarded as a budget processor.


I feel I can thoroughly recommend the Alesis 3630 compressor, both for its sonic integrity and operational versatility (it is all the more regrettable therefore, that a small but basic error in ergonomics should have been made, compromising its ease of use). For a first attempt at one of the major studio processing tools, as opposed to effects devices and excluding the Micro series which don't really count in this context, it really is most impressive, auguring well for future developments from Alesis. Noise performance is well up to professional standards, and hopefully the absence of balanced ins and outs will not prevent the unit from receiving the serious evaluation it deserves in all sectors of the market.

Alesis have always had the happy knack of making the right compromises; of making products that actually work, at a price the potential purchaser is willing to pay.

Advanced manufacturing techniques and clever design can go a long way, but the concept has to be right in the first place. The 3630 dual-channel compressor/limiter has all the characteristics to make it look another clear winner from Alesis.


Alesis 3630 £260 inc VAT.
Alesis RA100 £310 inc VAT.

Sound Technology plc, (Contact Details).


Rather grandly entitled 'Reference Amplifier', the RA100 represents Alesis' first move into serious power amplifiers. It is a 19" 2U rackmounting stereo unit, delivering 100W RMS into a 4 Ohm load (75W into 8 Ohms), with front panel controls consisting simply of a centrally mounted, recessed power switch, plus level controls and clip indicators for the two channels.

Finished entirely in matt black, the RA100 looks very stylish, with the sides of the chassis forming both the substantial rack-ears and two large heat-sinks for the output devices. Without making the amp unnecessarily large, this provides an extensive area for heat dissipation, allowing the design to dispense with any form of forced-cooling. The resultant absence of a fan will be most welcome to those with small control rooms where such noises can become most intrusive over a prolonged period. Unfortunately, I can't really tell you anything more about the output transistors other than that they are not MOSFETS, are very large, and have device numbers that do not appear in any of the standard UK reference texts. These are the essence of the RA100, and they look rather special and interesting, but further information from the US was not immediately available.

The clip LEDs appear to use multi-point monitoring, unusually early in the circuit, and they are tied-in with the protection circuitry. Referred to as a Fast-Mute circuit, this also implements the anti-thump turn-on delay, without using the usual relay.

All connections are made to the rear panel, with a standard IEC mains input (fused) and, unusually, 1/4" jacks for both input and output. I know some users will find this convenient, but I remain unconvinced of the suitability of this type of connector in any semi-permanent connection, such as a studio monitoring amp. An alternative is provided in the form of spring-clip terminals, although these are probably no better than the jacks; screw-down binding posts, or even XLRs, would have been a better option for studio monitoring use, I feel.

Inside, the review model looked rather like the pre-production example that it apparently was, and I would certainly expect to see better looming of the inter-board wiring, with separation of signal and power lines, in production versions. In spite of this however, there was practically no residual hum in the output (i/p shorted, gain Max) — the RA100 really is a very clean and quiet power amp. Good stability is claimed under reactive loading and I can confirm that the unit sounded comfortable with each of the variety of systems used for testing, including studio monitors (nearfield and main), a bin and horn PA system (discernibly sweeter on the HF circuit than the amp it was substituting for) and some 'difficult' hi-fi speakers.

Power delivery is quite sufficient for small studio main monitor system usage, where the low distortion and noise performance will really be appreciated. Input sensitivity is sufficiently high to give full output on the end of a -10dBV nominal level system — an important consideration sometimes overlooked by purchasers of large 'pro' power amps for home studio monitoring. This is a good solid power amp with no unnecessary frills — why pay for stepped attenuators if you are always going to turn the amp to flat-out?

The RA100 will not be without a host of competitors, but the Alesis name will certainly ensure both that it is taken seriously, and that it gets a fair hearing. Good performance, reasonable price; should do well.


Dynamic Range: 118dB ('A' weighted)
Signal/Noise Ratio: >100dB
Headroom: +18dBu ('A'weighted)
Frequency Response: 10Hz to 30kHz (+0, -0.5dB)
Crosstalk: <-85dB@ 10kHz
Output Impedance: 470 Ohms (Unbalanced)
Distortion: <0.05% THD (@ +4dBu, 'A' weighted, 6dB compression, any switch setting, nominal attack and release).
Nominal level: +4dBu or 10dBV (switchable)
9vAC external transformer


Output Power: 100W into 4 Ohms, 75W into 8 Ohms
Distortion: 0.05% THD @1kHz (8 Ohms)
0.2% THD 20Hz-20kHz (4 Ohms)
Frequency Response: +/-1dB (20Hz-20kHz)
Noise: >100dB below rated output
Damping Factor: 200 @ 8 Ohms
Input Sensitivity: 0.5VRMS for full output
Input Impedance: 7kOhms, unbalanced

Previous Article in this issue

The Art Of Recording Electronic Instruments

Next article in this issue

Analogue Synths

Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Sound On Sound - Aug 1991

Gear in this article:

Studio FX > Alesis > 3630

Amplifier > Alesis > RA100

Gear Tags:


Review by Dave Lockwood

Previous article in this issue:

> The Art Of Recording Electro...

Next article in this issue:

> Analogue Synths

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