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Alesis 3630 Compressor Mod

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Alesis 3630 Compressor Mod

Postby Icarus » Wed Jul 23, 2008 3:57 am

Got interested in modifying/upgrading an Alesis 3630 compressor a couple of months ago. After many hours tracking down internet posts and testing suggestions this is the substitution list that worked.
This is a component list and substitution guide to alter an Alesis 3630 to the current generation of THAT Corporation 2181 VCA chips. (see their technical data sheets on website.) Paste it into a table using the ‘–‘ character as a separator for easier reading.
The entries in this list are in the order of the following column titles separated by a ‘-‘ character.

Qty-New Components-Circuit Location-3630 PCB Ref Master Side-3630 PCB Ref Slave Channel-Replaces Old Component Value
2-Quad op amps, 14 pin PDIP LT 1359 (Linear Technology)-Input-U2-U6-TL084 or LF347
2-VCA IC’s
THAT 2181LB-VCA-U3- U7-THAT 2150
4-Fast Recovery Diodes UF4001-Power-D1, D2, D3, D4--1N400x
2-100k metal film resistors-Input-R12-R54-100k
2-1.69k metal film resistors-Input-R14-R56-1k
4-2.2k metal film resistors-Input/VCA-R13,R42-R55,R72-2.2k
4-6.2k metal film resistors-Input/VCA-R15,R41-R57,R73-6.2k
4-240k metal film resistors-VCA-R27,R208-R69,R211-150k
4-20k metal film resistors-VCA-R22,R23-R64,R65-22k
2-470 Ohm metal film resistors-VCA-R26-R68-470
2-100 Ohm metal film resistors-VCA-R29- R71-100
4-33 Ohm metal film resistors-VCA-R209,R79-R212,R102*-33
2-5.1k metal film resistors-VCA-R25-R67-5.1k
2-5.6k resistors-Knee-R83-R106-1k
2-6.8k resistors-Knee-R17-R59-3k
2-2.2M Ohm metal film resistors-Level-R10-R52-1M
2-10k metal film resistors-Level-R8-R50-10k
2-22uf audio grade electrolytic capacitors-Level-C11-C29-22uf
6-10uf audio grade electrolytic capacitors-VCA/Level/ Power-C16,C43,C6 & C7-C33,C55-10uf,.22uf, 10uf
2-22pf metal film capacitors-VCA-C15-C32-50pf
2-150pf silver mica caps-Input-C12-C30-150pf
Remove these components and see notes below.-----
* In the REV D PCB the resistor R102 (to match R79) is positioned in association with the Stereo/mono switch so that R79 provides the path to earth when the side chain of the Master Channel is controlling the Slave VCA.

Additional Items
Trimming the VCA
Check position of trim pots R24 and R66 (VCA circuit). THAT Corp 2181 chip technical data available from web site outlines tuning procedure. See the instructions in the THAT 2181 data sheet.
Trimpots R24 and R66 are usually set around half way.
There are three quality levels of the THAT 2181 available. This mod is based on the 2181B. If either A or C are used the resistor values need to change – see THAT tech data available from their web site. The value of the trim resistors increases from the C variant to the A variant. In practice this means that the variable range of this trim adjustment decreases from C to A chips as expected – the A chips being closer to ideal tolerance.
What all this means is – in the absence of the appropriate test gear – just set the trim pot exactly half way and the results will be close to ideal (at least for B and A 2181s). C variant chips potentially need considerable changes to the trim to produce their best performance – that is, their ideal trim may be well away from the midway position.

Other issues
When R28 & 211 are removed are these paths left open? YES – the 2181 has an internal resistor performing this function.
Remember that the Peak switch is not operational due to the removal of C42/56 and will break the sidechain path. Connect + side of C43 & C55 to the Peak/RMS switch pad of C42 & C56 to prevent this. Do this by threading the + leg of the capacitor (C43/55) through the board and looping it over to the switch pad. Be careful not to let it touch the track that it crosses – or leave the loop so high that it touches the aluminium extrusion when the case is reassembled.
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Re: Alesis 3630 Compressor Mod

Postby IvanSC » Wed Jul 23, 2008 2:16 pm

Having worn out a lot of shoe leather, dialling finger skin and presumably polluted the atmosphere with more solder and resin fumes, is this a cost effective mod? I have a 3630 somewhere in A1 condition that pretty much never gets used, so doing this might keep me occupied during my upcoming 3 weeks summer hols.
So tell us - wossit sound like?
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Re: Alesis 3630 Compressor Mod

Postby Icarus » Thu Jul 24, 2008 2:01 am

I did this to learn more about compressors - how they function and what the meaningful differences are between them. I use compressors only for live performance in our pop/rock cover band. I use them as inserts on every channel to control dynamic range because I do the sound and play in the band at the same time. So in general I'm not chasing exotic colouration of the sound, I'm looking for clean reliable dynamic control within the normal usage range of live compression - say 2:1 up to 8:1 with gain reduction set between -3dB and -10dB depending on the instrument.

This upgrade of the Alesis brings the VCA section up to the same spec as a DBX 1066. So in Australia in pure dollar terms it was worth doing. $A165 each for three second hand 3630s, $A100 worth of bits so in the end each modded 3630 cost say $A210 (compared with $A850 for a 1066). If I factor in the time I spent researching what to do, then doing it, then fixing stuff I got wrong the first time, I might as well have paid full RRP for three more 1066s.

In the Alesis 3630 the side chain path (the stuff that tells the VCA what to do and when)and the range of user controls is not as sophisticated as in the 1066, and you still don't have a separate limiter control.

However using current integrated chip VCA compressors live within the range outlined above I'd challenge anyone to identify one from another wearing a blindfold.

Gates are another matter altogether and for me are the feature that decides which of my compressors I apply to which instrument. But even here it is not about sound properties it is about range of control (whether the gate is fixed or variable depth, range of threshold control, release control etc.) None of the changes in the first post make the gate control in the 3630 better. (This is my next learning project.) So in general I'll be using these 3630s for inserts on instruments that don't need good gate control - keyboards, guitar cab mic, maybe DI'd bass.
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Re: Alesis 3630 Compressor Mod

Postby steveman » Tue Aug 05, 2008 1:45 am

I think the 1st mod usually listed for the 3630 is to take the gate out of circuit completely...
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Re: Alesis 3630 Compressor Mod

Postby Icarus » Tue Aug 05, 2008 9:28 am

I can't see the point in this. When the gate threshold is set to its minimum value it has no effect on the VCA circuit path.
The 3630 gate only has threshold and release control. What I want to add is variable depth - possibly in place of the rate control.
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Re: Alesis 3630 Compressor Mod

Postby PaulMc » Sun Oct 26, 2008 11:29 am

The 3630 mods are well documented on the prodgy-pro forum, as is the issue with the Gate:

I wouldn't mind picking up a cheap 3630 myself just to do these mods!
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Re: Alesis 3630 Compressor Mod

Postby Icarus » Mon Oct 27, 2008 1:50 am

Yes - I went through all these posts at the time that I did this. There was so much confusing, conflicting and just plain wrong information that once I sorted it out I thought it would be helpful to post a summary with only the correct changes included.

Just paste the part list above into a table. It gives you the component number on the Alesis PCB so all you have to do is a one-for-one substitution.
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Re: Alesis 3630 Compressor Mod

Postby Karg » Fri Nov 13, 2015 6:30 pm

Thanks for the mod!

As a note: I have tweaked my Alesis 3630 mainly using these suggestions and found it hard to identify the corresponding components on the board. Therefore I have made a visual overview of the relevant parts and posted it on my blog:
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Re: Alesis 3630 Compressor Mod

Postby dmills » Fri Nov 13, 2015 9:57 pm

Figure I could probably BUILD a compressor from scratch with that lot!

Regards, Dan.
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Re: Alesis 3630 Compressor Mod

Postby Karg » Sat Nov 14, 2015 8:20 pm

Lol, yes, it looks a lot, but it was actually an afternoon project. Let's say a full day if you include parts sourcing, etc.

However, I am still eager myself to find out about the full benefits. Especially if it about the sound of something, it gets esotheric quite quickly in my opinion. So I temporarily got a second unit now, to compare (1) dry signal, (2) original compressor, (3) modified with MC33079P op-amps, (4) modified with LME49740 op-amps.

Now I need various and divers stuff to run through them. If anyone is interested in the comparison, send me a message and a singe track, a short loop, whatever, and I'll post the results... To not oversee your message, it is best to use email. Contact data is on my blog:
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