The DBX 900 Series

Way back in the stone age, we used some equipment in the studio that was really neat stuff… very high-quality circuitry, mostly discrete, handbuilt equipment that I used to record some pretty good records, even though you’ve never heard them. One of my favorite pieces of outboard gear that we had in our rack was a DBX 900 rack. This was a 19″ wide power supply with spaces for nine 5 1/4″ x 1 1/2″ modules. Two other companies made similar systems- Valley Audio (we had a Valley rack as well, and we liked it better than the DBX) and API. Of the three, only the API Lunchbox has survived, and now there are hundreds of modules manufactured that can fit API racks, and Radial Engineering has just released their own version of a rack cage and power supply for 500 modules. Very nice, but very pricey.

My DBX900 rack, almost identical to the one we had at my studio 30 years ago. It's taken several years to put this together.

In contrast, a DBX 900 rack can be found used as low as $150 on eBay (though I’ve seen some crazy prices lately) and individual modules go for around $100-200 each. One other company besides DBX made modules for the 900 series, and that’s Aphex. These folks make some GREAT compressors, but as far as I know, only the  9721 Dominator and 9651 Expressor is available for the 900 rack, though there may have been others. Because of size constraints, the Dominator doesn’t have all the controls of the 19″ rackmount version, but it’s a very sweet compressor.

The DBX 904 noise gate module for the 900 series rack

Don’t overlook modules that need repairs, either. I was lucky enough to find some 900 units on eBay that were being sold as non-working, parts-or-repair-only. I took a gamble and got these for about $30 each. One was a 905 parametric EQ. This one turned out to be an easy fix… a disc capacitor had blown itself in two.  Thanks to DBX’s unselfish policy of publishing their schematics, I was able to figure out the value and replace it, and it works! Sometimes a blown part is a symptom of other problems, but so far, nothing else has smoked.

Next up was a DBX 903 compressor limiter. This one exhibited some general wierdness in terms of the signal, and the LED meter on the front was out. The 900 series all use the same meter driver board, which helps. I traced the signal at the meter board’s input, and confirmed that it was getting a signal, so I pulled the board and swapped it with another from a working unit… problem fixed. Now we know the problem is somewhere on the meter board. Examining with a microscope showed a resistor with a tiny burn mark around the middle. Replaced the resistor, and presto! The meter board works.

The business end of the 904. The board on top is the meter driver.

The last problem piece is a 904 noise gate, the donor for the working meter board used to correct the compressor. This one was a bit tougher to fix. Reinstalling the now-repaired meter board shows that the unit is detecting properly and gating a signal according to the settings, but there’s no output.  I poked around with a meter, and got about a .4v signal at the input. I started working my way back from the output, and found a non-polarized electrolytic capacitor that wasn’t passing any AC. So to test my theory, I removed C3– a 4.7uF non-polarized electrolytic cap– and replaced it with a pair of 10uF capacitors wired back-to-back, i.e., negative tied together, positive side out… and it works, passing a clean, gated signal!

After a proper 4.7uF NPO cap arrived from Digi-Key, it was soldered in and tested… that fixed it. While I was at it, I ordered enough Nichicon capacitors to re-cap the entire board… fortunately, DBX designed their circuit boards to accommodate either radial or axial-leaded caps, so either type will fit.

The problem cap. Replacing this fixed the unit. Fortunately, the problem wasn't under the big square metal case... that's a DBX discrete voltage-controlled amp (VCA), which is mostly transistors packed very tightly onto another circuit board. Repairing one of these would be really difficult, and replacement would be impossible.

That’s the other big advantage of this old stuff… it’s possible to fix it when it breaks. When parts fail on new gear… likely built overseas, by robots, using surface-mounted components… you’re pretty much done and replacing the entire board is the only option. Sometimes sourcing parts for vintage gear is difficult or sometimes impossible… germanium transistors and diodes come to mind. But vintage gear can almost always be rebuilt, and if the quality is there to begin with, it’s worth the effort.

The 904 fully recapped. The job wasn't that difficult or expensive, but you do need to be careful and take your time.

5 responses to “The DBX 900 Series

  1. I’ve built out a couple of these units for live sound. I like to have a little compression inserted on vocal channels for that I use the Aphex 9651 Expressor. The 9651 is an excellent vocal or solo compressor with attack, release, ratio, threshold and makeup control. One bonus is the 9651 has a real nice tunable high frequency recovery system that works with the compressor. In effect it is a built in de-essor. The DBX 905 Parametric EQ is very helpful for feedback control of monitors and a little eq on the main mix. The 905 is impressive functionally and sonically with three wide bands of EQ that each have frequency, cut/boost and Q. The upper and lower also feature a shelf switch. I LOVE the DBX 160 style 903’s over easy compression on the main mix as well. I’m not a fan of the 160 in recording (except for bass drum and other effects uses) but for live sound, they work very well.

    Some notes: The DBX 900 series modules are all single-ended output. They will drive a balanced load well, but they are only single ended. The Aphex modules are active +/- with a real push/pull operation however they can still drive an unbalanced load. Like Brian noted, some of the units I purchased needed work. On the 905 EQs the biggest problem is the switches. Most of these units have been sitting in production environments for years and have not been adjusted. So switches and pots are questionable and at least need cleaning. I got direct replacements for the switches from DigiKey and simply replaced them all. I also found issues with some signal path caps, but they are easy to locate and replace.

    Also note when purchasing the Aphex 9000 rack, some of the racks only come with power to support Aphex modules, the DBX modules need two +/- DC supplies. Also while all DBX 900 racks hold 9 modules, only the later modules will break out all 9 on the rear terminal strip. The older ones only break out 8.

    I agree with Brian, these are a great value and have that wonderful analog sound that many new systems simply lack. I would love to have Universal Audio compression and API EQ for my live sound, but no way am I going to drag that stuff around when I can build a couple of vintage sounding racks.

    • Peter Mavrogeorgis

      Hmmm, I have done original 903’s with the big gold VCA. One is not working properly. It passes signal and when “in” it does not compress, though it passes audio, the volume knob operates properly, and the “infinity” light lights up when I pass into expansion(though no effect on the signal whatsoever). Any ideas?


    • I hate to say it, but that sounds like an issue at the detector end of the circuit. But I’d replace all the electrolytic caps just as a matter of course, unless you have an ESR meter which can check electrolytic capacitors while they’re still in the circuit. If that doesn’t resolve the issue, then look at the actives… ICs and diodes, mainly. Unfortunately this includes the big gold VCA… the DBX 202. I’ve see three different versions of this circuit… one has the gold cover VCA, another has a bunch of DBX2150 single inline chips on a daughter card where the gold VCA is, and there’s a newer daughter card with regular ICs. The 2150s cost an arm and a leg to replace with new old stock. It might be possible to replace transistors in the DBX 202 VCA. There was someone selling a PC board to replace these opamps, but I can’t remember where I saw it… I’ll do some looking around to see if I can find them, since I should buy a few of these myself. 2150 application note link, thanks to Gyraf. Also, the THAT corporation has some chips that can substitute for the 2150, though this is getting pretty advanced. Look at their 2180 and 2181 chips. There’s a datasheet available about upgrading to these chips here.

      Look really carefully for physical signs of trouble, though… I did find a 903 card with a wierd looking wire on the LED circuit board. On closer inspection, it turned out to be the remains of a ceramic disc cap that had blown apart.. never seen that before or since. A very strong magnifier is a big help here… I have an old stereo dissection microscope that is a huge help.

      Whatever you do, don’t throw it away. Even if it turns out to be too expensive to fix, you can use it as a donor card to fix others. I’ll give you a few bucks for it if you don’t want to mess with it, or you can send it to me and I can TRY to fix it for you, though honestly, it would likely be cheaper to replace… these are usually around a hundred bucks on ebay. A rebuilt 903 with new electrolytics is around $175.

      Good luck, BG

    • Peter Mavrogeorgis

      Thanks for your in-depth and inciteful response. I thought it might be the VCA. I recapped all of my 903’s recently. I had a gold can 903 that wouldn’t pass low end. After the recap it positively killed its neighbors so I ended up recapping all of them. I upped the values quite a bit. I’m thinking of trying Wima polypropylene bypass caps as well. That did wonders for my Amek desk. The Kepexes are being recapped next. Luckily they’re all working!

  2. Glad you found it helpful. I’m not a real expert when it comes to electronics theory, but these things are lots of fun. These old circuits should be preserved now, while we still can… in 20 or 30 years they’ll be gone, and folks will only have Mackie boards to restore! Best, BG

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