Friday, October 23, 2015

Alesis 3630 Compressor Mod

I have been a bit quiet lately due to travels and too many things going on. Being back home and having some projects in a publishable state, here is the next part of my 19" rack series...

While searching for an affordable all-purpose hardware compressor, I naturally came accross the Alesis 3630. Naturally, since, according to Alesis, this "has become the most popular dynamic processor ever made". Similarly, according to Daft Punk, this little box gave the signature sound to their Homework and Discovery albums. After continueing to read about it, it quickly became clear that, together with its price, these are almost the only positive things that can be found about this compressor. Most people seem to agree that it's best usage is as a doorstop, and even the one Daft Punk used repeatedly was not the original, but a modified version. So let's modify it...

The modifications implemented here summarise the ones suggested on Icarus' post on Sound on Sound and Smallbutfine on groupdiy, which seem to replicate the original Buta mods, but also include the opinions and suggestions of several other people. Thanks to all of you!

Location of all relevant components on a Rev. D board (Click to enlarge)

Opening the Alesis 3630 is straight forward, and although the potentiometer knobs sit tightly on their shafts, they can be removed with a bit of gently controlled pressure. After disassembling the unit, the two pcbs unfold and can be separate for easier handling. Since the component's labels are printed underneath them on the one-sided boards, I made a visual overview of all changes on my Rev. D board. If your board has a different revision number, just slightly bend the components and try too peak underneath to identify the labels. Good light helps a lot on this!

Among the first things suggested for upgrading, is beefing up the components of the power supply section to give the unit a bit more headroom power-whise. For this, we replace the four big capacitors with better components and swap the four diodes with fast switching alternatives:

- Power -

Quantity New component Position Old component
2 220µF/35V Electrolytic cap C2, C3 ?
2 2200µF/25V Electrolytic cap C4, C5 ?
2 10 µF Audio grade electrolytic cap C6, C7 10 µF
4 UF4001 Diode D1, D2, D3, D4 1N400x

Next, we turn our attention to the op-amps and VCAs (Voltage Controlled Amplifier) that are responsible for the compressor's gain and thus play an important role Alesis' sound character. Several OP-Amps work equally fine in technical terms, but make a significant difference from an audible point of view. Judging which one is best depends on personal taste and the purpose of your compressor and trying to solve that question by simply reading about the different sound characteristics gets you very quickly into highly subjective, sometimes esoteric, terrain. However, for my trials I got a pair of MC33079P, which are supposed to sound very surgically clean, as well as some LME49740, which have a more musical character. Other OP-Amps that work well, but I have not tried myself, include the LT1359 (not sure about the sound), TL074 or TL084 (supposed to be darker, grittier), OPA4227 (smooth and musical) and OPA404 (very clean and quiet).

- Input -

Quantity New component Position Old component
2 MC33079P DIP-14 Quad OP-Amp U2, U6 TL084 or LF347
2 100 kOhm metal film resistor R12, R54 100 kOhm
2 1.69 kOhm metal film resistor R14, R56 1 kOhm
2 2.2 kOhm metal film resistor R13, R55 2.2 kOhm
2 6.2 kOhm metal film resistor R15, R57 6.2 kOhm
2 150 pF silver mica C12, C30 150 pF

All changes to the VCA (voltage controlled amplifier), level and knee sections consist of replacing passive parts only, so here we go:

- VCA -

Quantity New component Position Old component
2 2180BL08-U VCA IC U3, U7 2150
2 2.2 kOhm metal film resistor R42, R72 2.2 kOhm
2 6.2 kOhm metal film resistor R41, R73 6.2 kOhm
4 240 kOhm metal film resistor R27, R69, R208, R211 150 kOhm
4 20 kOhm metal film resistor R22, R23, R64 ,R65 22 kOhm
2 470 Ohm metal film resistor R26, R68 470 Ohm
2 100 Ohm metal film resistor R29, R71 100 Ohm
4 33 Ohm metal film resistor R79, R102, R209, R212 33 Ohm
2 5.1 kOhm metal film resistor R25, R67 5.1 kOhm
2 10 µF audio grade elko C43, C55 10 µF
2 22 pF metal film capacitor C15, C32 50 pF
Remove - resistor R28, R70 ?

- Level -

Quantity New component Position Old component
2 2.2 mOhm metal film resistor R10, R52 1 mOhm
2 10 kOhm metal film resistor R8, R50 10 kOhm
2 22 µF audio grade elko C11, C29 22 µF
2 10 µF audio grade elko C16, C33 0.22 µF
Remove - capacitor C42, C56 ?

- Knee -

Quantity New component Position Old component
2 5.6 kOhm metal film resistor R83, R106 1 kOhm
2 6.8 kOhm metal film resistor R17, R59 3 kOhm

Many people complain about how the mere presents of the gate circuits negatively affect the 3630's sound. The easy solution is to disconnect the gate by cutting the bridges marked in the picture with yellow circles. However, in general, I am not a friend of ultimate loss-of-function approaches, so I decided to replace the bridges with switches. Since I the front plate of the compressor is rather populated, and its rear is little accessible in a rack, I opted to replace the threshold potentiometers with switchable ones. Measuring the original potentiometers, they are 10k with their taper range somewhere between a linear and a log scale. Seeing it as an advantage to have finer control on the lower end of the scale, I ended up opting for logarithmic potentiometers as replacements. The integrated switch makes their body larger and I had to desolder and bend two capacitors on the LED board (marked with a pink circle) to gain the necessary space in the re-assembled box. Also, the pins of my replacement ones did not fit into the pcb holes, so I had to used small wires as adapters.

As a last upgrade, it is suggested to connect all input/output grounds with heavy gauge copper wire. These (thick, brown), as well as some of the wires used to connect the switchable potentiometers (thinner, orange), are well visible on my finished unit:

So how does it sound now? While it would be nice to have an original model at hand to compare it to, the 3630 has a nice musical character now. My first impression is very good and the mods seem very worth doing...

Some people also suggest to replace the side chain op-amp. I did not implement this, since I am fine with the side chain as it is. However, if anybody has experience with this modification please say a few words in the comment section about your experience and which replacement op-amp you have used.

- Side chain -

QuantityNew componentPurposePosition
1?Quad OP-AmpSide chainUx

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