DBX 165A help (and LM308 sub)

Hey All,

I've got a pair of 165As that are not quite working right. I've spent all day on one of them, so I'll just go into that one for the moment. Simply put, I'm getting excessive distortion of the signal when under compression in manual mode. Even with attack and release at 12:00 it's pretty noticeable. I don't have a working unit to compare, but my assumption is that I shouldn't be getting a sound like this at moderate compression settings on this unit. The distortion is somewhat light (as far as distortion goes), but still very noticeable on acoustic instrument material. Distortion pretty much goes away with A/R fully CW. Release affects it the most, as it normally would be expected to; with release at its highest, attack makes little or no difference in distortion. Attack and release constants are performing as they ought to on the scope. Sine waves show what might well be some sketchy looking waveforms following the manual A/R circuitry. Things look cool before this point, coming out of the rectification and threshold circuitry.

It seems to be related to the manual time constant circuitry. In auto mode I hear no noticeable distortion, though I didn't try reducing the size of the auto cap to get a lighter time constant. Everything else about the unit seems cool.

Schematic:http://www.dbxpro.com/product_downloads/Schematics/165A%20Schematic.pdf

The only component I have not completely eliminated is the LM308, however, I did swap it out for a TL071 which seemed to give the same result, however, the time constants were off (seemed much shorter), so it wasn't an apples to apples comparison.

Other than this I've swapped all the amps in the SC path, new diodes and transistor in A/R section, and even tested every frickin passive component in the A/R (except the L of the Ls) and following (buffer?) section. I'm curious what purpose this latter section serves, if anyone knows.

I swapped VCA with one sold as working from eBay. I don't have a way to absolutely make sure it's perfect, but it gave no different performance than the original.

If anyone knows off hand of a workable sub for the LM308 I'm all ears. Perhaps lower input current than the TL071, or perhaps even just a readjustment of the release trimmer (R93).

If anyone has any other ideas I'm all ears, cuz I'm starting to run out of them (ideas, not ears).

Thanks!

It seems like there's been similar reports on GS, maybe that was you.  I know there was some discussion, don't recall if there were solutions.  I don't think I've ever seen it brought up here before.  Good luck. 

A buddy of mine has a 165 and 165a. The 165a was making some weird distortion noise and compressing a little funky iirc. Someone eventually pointed him to a diode in the limiter section on the 165a. He clipped the diode off to bypass the limiter and it works perfect now. I can get more info when I see him tomorrow.

Wasn't me posting on GS. I'll do a little search perhaps.

As far as the limiter section goes, I could try disengaging it, but it's a fixed threshold output limiter that isn't affected by the compression settings, so I don't see that it would be playing any role.

Edit:It's not a fixed threshold limiter, but the threshold is set at its max.

As far as the limiter section goes, I could try disengaging it, but it's a fixed threshold output limiter that isn't affected by the compression settings, so I don't see that it would be playing any role.


My 165 has adjustable peak stop you set. If that limiter is coming on and the LED is not showing that its on, it could be the cause of what your hearing.  I have a 165A and the limiter is rude distortion when set to low or if I'm not paying attention to my mic pre level.  Anyway you might want to look at that section.  It sounds logical for your problem.

Thing is, with no compression I can up the output gain and things sound fine. It's only when the compression kicks in, and as mentioned, the compression settings affect the quality and amount of distortion from none/little to very noticeable. I can also easily see the distortion in the compressor SC signal path on the scope.

I appreciate the input, and I will give the limiter section a bit of a look, but I don't see that there's any practical way it's what's causing the problem.

Hey, I've got a 165 and a 165 with the exact same problem!!
Did you manage to fix them in the end?

165s tend to get dirty switches, especially the link pushbutton, so make sure it's really clean. Check the power rails and that the VCA is seated and no cold solder joints. Also, sometimes a dead unit is caused by Q19-Q22 failing. You can pull those FETs out and the unit will still work fine...

If you want to investigate the limiter function, you can lift one end of each diode in the limiter section of the 165A. They are the two diodes next to the silver can transistors. If I remember there are like four or six transistors and the two diodes just to the side of them.

+1 for the link switch - most of the errs I've seen in a 165a were here

As we used these nearly exclusively in mono mode for bass/BD, we ended up hard-bypassing the link switch (it's a 8p2t, 24-pin monster) by soldering pieces of wires across switch contacts on the upper side and removing knob coupling. Easy enough to do, easily reversible if you ever find such a switch nos.

To check if this is your err, simply use a handful of alligator clip leads to lock unit in non-linked mode

The lm108 op amps series is a good old soldier (emphasis on old). With a couple K of resistance between emitters in the input long tail pair it will not be very low noise. There are surely modern replacements that can do a lot better all around.

+1 to the other advice, the audible issue is probably something else.

JR