08 June 2015

DBX 266A Mod (in lieu of, and inspired by, the 3630 mod)

I know, I promised myself I wouldn't do any more modding until I got some other stuff done first. But, I have one of these on the way, and well.... Sheesh, I still haven't finished the PRR Variable-mu. But I've been looking at that schematic over and over again, as well as some in a couple of ARRL handbooks, and I think I more or less get the idea, gain section, tap off the gain, rectify that, send part of that to the gain section (VCA, tube, etc) to reduce  gain section (side chain) to reduce gain, then another gain section to make up gain, to bring the loud down and keep it all even, with variations on that theme.  Am I right? I think so, and thanks to PRR's variable mu and the ARRL for helping me (I hope) basically getting the concept. Also, thanks to some other folks on GroupDIY who e.g. commented on the 6BA6 concept.

I've always been fascinated by the Alesis 3630 mod. Reading through some of those threads, there was a reference to the 3630 being "basically a dbx compressor" I couldn't find the Alesis schematic (until later, I did find a redraw by the person who did the mod... on GroupDIY), but the schematic for the 266A is on the dbx website. Comparing the two, well, they do look **very** similar, and I'm not smart enough to know what the subtle differences do. The 266A is here: dbx 266A schematic and the 3630 is here: 3630 schematic

Well, why not do the "basically a dbx compressor" mod on a dbx compressor? The 3630 mod basically replaces the VCA and cuts out or switches out the gate, and the PSU is upgraded. The 266A has on board PSU rather than a 9v wall wart so I am going to assume that the PSU is already a plus up on the Alesis. It also uses the same VCA as the Alesis. A THAT replacement is readily available, 2180, and is pre-trimmed. The 2180A is available from Mouser.

An upgraded RMS detector is still available as NOS from SmallBear and replacing that is the other Alesis mod. There are some other more extensive(expensive)  mods suggested on other sites too: replacing opamps, resistors, and some of the capacitors with more expensive and exotic ones, etc. (well also upgrading the PSU capacitors too)

The resistors in the signal path are already metal oxide and the opamps already 5532, so that can wait...

BTW The dbx 1066 uses some special patented new vca chip. Mmmh, maybe a THAT2180 would be a nice new alternative to the original VCA in the Alesis/dbx 266A. OK, what about the RMS detector? There is a THAT upgrade as well. So, here we are so far with about US$30 worth of parts (that 2180A are about US$10 a piece at Mouser) and about a few hours time.

Oh, and I just got one of those 4301 boards from Russia. ungifted's 4301 (Another VCA compressor, so that will mean I have four total...)

This project is simply a take out four parts, put in four parts. (I also have to replace a broken switch) So this one should be quick and easy. Now, if I would just stop and actually use the gear I have. . .

Hey you, drop the soldering iron and step away from the bench! I mean it.

9 July update

Got the new VCAs from Mouser. The RMS detectors on the way from SmallBear. Note, the 2180As are pretrimmed, which eliminates a couple of resistors in the circuit. The RMS detector still needs to be trimmed. I need to look up how to do that, in the meantime, will likely leave that part alone for now. Forgot to buy the inline sockets, so back to Mouser... or Small Bear.

8 September update

Pouring over schematics, and wondering if using a 2181 vice 2180 would be better. Not sure I have the test equip to properly trim it though.

Maybe leave socket points for removed parts ?


See the Prague Spring blog for an update.  www.praguespring.org


1 comment:

  1. Problem with the level on channel 1, have to compensate by increasing gain slightly. Been over and over it, can't figure out where the issue may be. There was a cut trace somewhere, and I had to jumper power to one of the opamps. When I got it the +4/-10dB switch was busted. Not sure if this issue is related. All the voltages seem correct at and around the VCA, so the problem seems to be in the output opamp gain. At this point it's too cumbersome to trouble shoot further. It works OK, but won't track properly if stereo strapped.

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