Alesis 3630 Compressor Mods July 26, 2011 11:55 6 Comments

Touted officially by Alesis as the "most popular dynamics processor ever made," the 3630 is a bargain bin compressor that can be polished to resemble a professional one. The circuit employs some solid design fundamentals, but by all accounts suffers from inferior components and design choices. Various mods have been proposed over the years to considerably expand the sweet spot the 3630, lower its noise floor, and remove the snaps, crackles, and pops the stock unit is prone to. Collected here are those mods that have been deemed the most successful by the people who have tried them. Many thanks go to the originators of these mods who have shared their research with the DIY community.

"The 3630 is basically a dbx compressor. It uses dbx-designed RMS level sensors and VCA gain-change chips. Problem is, most folks have not taken the time to learn how to use this box, find the sweet spots for level setting, gain reduction, gain make up and attack / release times." -Michael Joly via Gearslutz

"I'm not saying that if you follow these steps that you'll have an 1176 or an LA3-A, but the quality of sound exiting the box will be vastly improved merely by swapping out a few components... No smoke? Then the chances are that you've just made your 3630 into something resembling what it should have been in the first place." -Peter Purpose via his GroupDIY writeup

"We have a really small compressor, the Alesis 3630, which is $300. That's the main one we used on Homework and Discovery." -Daft Punk via Mix Magazine

"This upgrade of the Alesis brings the VCA section up to the same spec as a DBX 1066." -"Icarus" via Sound on Sound

Please keep in mind attempting any of the following modifications will void your warranty with Alesis. 

Get a 3630

If you don't have one already, these comps can now be found for a pittance on eBay. With a little patience, you should be able to get one for less than $100. 

1. Beef Up the Power Supply

Make sure you're modded unit gets plenty of current by switching to a more robust AC adapter. This wall adapter from Line6 provides a hefty 2 amps. While you're in there, consider making these component upgrades to the power section:
Qty New Component Type Position Part #
2 220uf/35v Capacitor Power C2,C3
2 2200uf/25v Capacitor Power C4,C5
4 UF4001 fast recovery Diode Power D1,D2,D3,D4

2. Upgrade the VCAs and Opamps

The VCA (voltage controlled amplifier) is responsible for controlling the compressor's gain and as such has a significant effect on the its sound and performance. The THAT2180 series of VCAs represent a high-quality alternative to the stock DBX2150s. Before you remove the stock VCAs, take note of their positioning so that you are sure to install the new ones with the correct orientation. The LF347N quad opamps can be replaced with a number of chips, the LME49740, MC33079P, LT1359. Make sure that the chip you order comes in the DIP-14 format. These upgrades were originally suggested by Peter Purpose in his GroupDIY article.

Qty New Component Type Position Part #
2 LT1359, LME49740, or MC33079P DIP-14 Quad Opamp Input U2,U6
2 2180BL08-U VCA IC VCA U3,U7

Alesis 3630 modded

Peter Purpose's modded unit. Note that the Burr Brown Quad opamps are now unavailable and must be substituted for one of those listed above.

3. Connect the Grounds

In his article on the 3630, Peter Purpose suggests connecting all of the input and output grounds via heavy gauge copper wire.

4. Replace and/or Upgrade Passive Components

Many of the passive components, resistors and capacitors, should be improved or changed to different values. The chart below was compiled by Sound On Sound forum member "Icarus" in this post.

Qty New Component Type Position Part #
2 100k metal film Resistor Input R12,R54
2 1.69k metal film Resistor Input R14,R56
4 2.2k metal film Resistor Input/VCA R13,R42,R55,R72
4 6.2k metal film Resistor Input/VCA R15,R41,R57,R73
4 240k metal film Resistor VCA R27,R208,R69,R211
4 20k metal film Resistor VCA R22,R23,R64,R65
2 470 metal film Resistor VCA R26,R68
2 100 metal film Resistor VCA R29,R71
4 33 metal film Resistor VCA R209,R79,R212,R102
2 5.1k metal film Resistor VCA R25,R67
2 5.6k metal film Resistor Knee R83,R106
2 6.8k metal film Resistor Knee R17,R59
2 2.2M metal film Resistor Level R10,R52
2 10k metal film Resistor Level R8,R50
2 22uf audio grade electrolytic Capacitor Level C11,C29
6 10uf audio grade electrolytic Capacitor VCA/Level/Power C16,C43,C6,C7,C33,C55
2 22pf metal film Capacitor VCA C15,C32
2 150pf silver mica Capacitor Input C12,C30
Remove -- Resistor VCA R28,R70
Remove -- Capacitor Level C42,C56

5. Disable the Gate

If you don't find yourself using the gate function on your 3630, you can send it and its nasty artifacts packing by simply snipping the two jumpers shown in the picture.

Alesis 3630 disable gate

5 Ways to DIY With Discrete Opamps June 17, 2011 11:09 5 Comments

glk522 opamp

This week, I was lucky enough to receive a pair of GKL255 discrete opamps in the mail from Bearcat Audio, the newest vendor on the DIY Recording Market. (Thanks guys!) Even as I opened the parcel from Montreal, my mind was already thumbing through the projects in my DIY to do/wish list for a special place to socket these new DOAs. But first, a little overview of discrete opamps for the beginner:

An operational amplifier (opamp) is a single-ended differential amplifier. This means it takes two inputs, inverted (-) and non-inverted (+), and produces a single output which is many orders of magnitude larger than the difference between the inputs. Usually opamps are manufactured as monolithic integrated circuits (ICs); however, a few intrepid designers have created their own using discrete transistors, resistors, capicitors, etc. There are three classic DOAs that most DIYers have tried to clone, imitate, or "improve": the Melcor 1731, API 2520, and Jensen 990. Each classic opamp has it's own sound which, without getting into too much audiophoolishness, can summed up thusly:
  • Melcor 1731 (1960's): Smooth highs and lows, beefy harmonics
  • API 2520 (1970's-): Aggressive mids, solid lows, classic 70's coloration
  • Jensen 990 (1980-): Low-distortion, fast transient response, wide open sound

1. Build an API or Jensen style mic preamp

The 2520 discrete opamp is at the heart of the "API Sound." And these days, there are a cornucopia of projects for the API-style preamp builder, including:

  • Access 312 and Bicycle 312 are PCBs for 312-type preamps
  • FiveFish X12, Seventh Circle A12, and Sound Skulptor MP12 are complete kits for building preamps in rack-mount enclosures
  • Bearcat 412 a complete kit for building four 312-inspired pre's in a 1RU chassis 
  • ClassicAPI's VP25, VP26, 312, and 312DI are 500-series replicas of API circuits

The 990 DOA was used in several very successful preamps, including Jensen's twin servo design, which utilized two Jensen opamps and transformers. Seventh Circle Audio's J99 kit is a version of the twin servo with SCA's own 990-type opamp and Lundahl and Cinemag transformers. It's also possible to create a Jensesque preamp with JLM Audio's Baby Animal Dual kits, using any 990-type DOA and Jensen transformers.

2. Concoct your own preamp design

One of the distinct pleasures of building with discrete opamps is that they have a common footprint and can often simply be swapped. This makes the basic transformer/single opamp preamp design a great platform for experimenting with new component combinations.

  • JLM's Baby Animal kits provide such a platform with a pre-fab power supply and chassis so you can focus on transformer and opamp choices.
  • The Bicycle 312 Hybrid PCB is also a great place to start if you want to build your units from the ground up.

3. Build a ClassicAPI "Missing Link" Console Channel

One theory about the lack of "mojo" or "warmth" in modern recordings is that we're simply not running audio through as many transformers and gain stages as they did in the days of the large-format console. Preamps, in other words, were only part of what made those consoles sound so good. ClassicAPI provides some consolation for the console-less in the form of the VC528 ST+ console channel for the 500-series/51x Alliance. The VC528 "completes" a preamp with a fader, filters, and an output amp, running the signal through two transformers and discrete opamps of your choice along the way.

5. Make a unique hybrid 1176 compressor

Igor's 1176-style compressor for the 51x Alliance/500-series, the F76 FET, provides the option to replace the original amplifier stages with DOAs. Why not be the only engineer in town--heck in the county, heck in the tri-county area--with an "API 1176"!? 

6. Upgrade the DIP8 opamps in your mixer, compressor, equalizer, converters, etc.

Until yesterday, June 16th, 2011, the sacrifice you made for using high-quality discrete opamps was not being able to use them in modern DIP8 format gear. AC Sound has begun briding this gap with a series of discrete opamps in the DIP8 format. The opamps will come in "tall" and "wide" formats and M, A, and J models (can you guess what the letters stand for?) AC Sound listed the first set of model A's on the market yesterday and plans take pre-orders the "J" and "M" models soon. Your cheap monolithic opamps had better watch their backs!

How to Perform the MCM DAC Mod October 26, 2010 07:10 13 Comments

Jim Williams of Audio Upgrades first tipped recording folks off to the MCM 50-7774, a "hidden treasure" D/A converter with a stellar DAC chip and a very low price tag. The design is an early version of the audiophile-endorsed Beresford TC-7510 that was licensed to OEM manufacturers. And, while it is marketed as a home hi-fi DAC, the 50-7774 has gained popularity among engineers as a professional mixing and mastering converter. The stock unit is impressive in itself (various Gearslutz members have said they prefer it to RME ADI8-Pro, Presonus Central Station, and MOTU 828 mkIII converters), but a couple of mods proposed by Williams and others can put it on par with really high-end DACs. The mods can be done in a couple of hours, and the whole project costs about $135 including the stock unit and premium replacement components.