Help modifying DBX 160XT

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user 40373

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Nov 22, 2009
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I bought a pair of DBX 160XT's and want to recap and do some same modifications to it. However, the schematic from DBX is hard to read and I'm having a hard time reading it. Below I attached a link, it's 7 pages  :eek:

I'd like to replace audio capacitors, replace the coupling capacitor, increase the size of the power supply capacitors, change the VCA with a THAT 2180A, and replace input and output op-amps. I'll also add power supply bypass and any phase compensation caps that are needed.

From what I can tell, the power supply capacitors are C3-C6, and the VCA is the DBX 1252 (U11). I'm pretty sure the LM353N (U1) is the input op-amp but I'm not sure what the output op-amp is (U4?). I'm not sure where the coupling capacitor is.

Here is a link to schematics from DBX:

http://www.dbxpro.com/vintage_download.php?product=160XT
 
sws2h said:
I'd like to replace audio capacitors, replace the coupling capacitor, increase the size of the power supply capacitors, change the VCA with a THAT 2180A, and replace input and output op-amps. I'll also add power supply bypass and any phase compensation caps that are needed.

From what I can tell, the power supply capacitors are C3-C6, and the VCA is the DBX 1252 (U11). I'm pretty sure the LM353N (U1) is the input op-amp but I'm not sure what the output op-amp is (U4?).
There are several output opamps; U2 (with associated Q1 & Q2) for the unbal out, and U3+ U4 for the balanced out. Don't forget U1 is actually in the output circuitry. Improving on U3/4 is gonna be tough; 5534's are still almost unbeatable today, when properly implemented and spec'd within reason (I mean they wouldn't be right for a design that calls for 10MHz bandwidth). You could improve on the 351 and 353 but it may not be a simple swap; you may face oscillation problems. Very likely, you would have to work on the compensation scheme. For what improvement? I don't know...
I'm not sure where the coupling capacitor is.
C14. Don't expect much improvement there except if the cap is way out of specs.
 
Thanks for the help. In ever compressor I've opened up, the op-amps are usually 5532, 5534, 301's or LM353. I've had luck using LM6172 in place of 5532's, and LT1357 or LME49710 in place of 5534's. I'll probably leave the LM351's and LM353's.

If someone can help me located the any capacitors in the audio path, and input op-amp and I'll be on my way!
 
here a link where you ll find some infos.
http://recforums.prosoundweb.com/index.php/t/16187/0
jensenmann post is the best.
he s a member here .so his comments will be great...

audioforge
 
be aware that the output section of a 160xt is different to a 160x (IIRC). So some of my comments might be wrong. But I recommend you to first know what you want to achieve before you start modding the hell out of this comp without any particular direction. As said, recapping is a good starting point. It will make a difference and is worth every penny.
 
Thanks guys, I'm just trying to figure out how to do what Jim suggest. The reason he does things it to raise slew rate and bandwidth and lower distortion, which are all things I like  ;D

But, I think I got it figured out:

LM353 to LM6172
5534's to LT1357
VCA to THAT 2180
Replace coupling capacitor (C14? I'm 95% sure) with Wima 4.7uF MKS-2, bypass with MIT .01uF PPMFX
Increase size of power supply capacitors, bypass with WIMA MKS-2 .01uF 250V

With the new op-amps I'm sure I'll have to do some power supply decoupling caps for the op-amps (it looks like they are already installed by DBX) and a small cap across the feedback resistor of the opamps.

If anyone wants to explain changing the diodes to LEDs I'll give that a try.

Here's what I got from Jim from various old threads:

When I do the mod on them the VCA is replaced with a that 2180A, the opamp is changed to a National LM6172, the trim circuits are removed from the vca, the electrolytic coupling cap is replaced with a Wima 4.7 uf mks-2, bypassed with a MIT .01 PPMFX. Power supply cans are replaced and enlarged and some phase compensation and power supply bypass caps are added. Bandwidth is raise up one octave. Slew rate is increased. Distortion is lowered. Fog is removed. All is well.

All can be improved by replacing the VCA with a that 2180A. Remove the 51 or 47 ohm resistor from the trim to pin 4 and remove the trim circuit feed 100k resistor as these are pre-trimmed at the factory. On an A it's R110 and R111.
 
Oh, and Jim also suggest this:

"These circuits can be inproved by replacing the cross coupled 1N4148 diodes with green LED's"

 
So, I finally have some extra money and I was going to go forward and order some parts for my DBX 160XT's

However, I don't think C14 is the coupling capacitor, as it is a .1uF electrolytic.

C45 is a .047uF "green chicklet" film capacitor...honestly, I'm not really sure where to look! There isn't any cap rated a 4.7uF.
 
sws2h said:
So, I finally have some extra money and I was going to go forward and order some parts for my DBX 160XT's

However, I don't think C14 is the coupling capacitor, as it is a .1uF electrolytic.

C45 is a .047uF "green chicklet" film capacitor...honestly, I'm not really sure where to look! There isn't any cap rated a 4.7uF.

Are you sure you're reading the code on the capacitor properly?

C14 appears to be a 1uF film cap, located directly after the first LF353 (U1, pin 7) and before R8 (24K9).

Jim suggests increasing this to 4.7uf with a bypass cap. 4.7uf is most certainly overkill in this spot, but what the hell.

Mark
 
I guess the schematic is wrong, C16 there is a 1uF film cap and C14 is a 1uF/50V electrolytic (same as the ones used all over the unit). The trace off of pin 7 goes to C16, as well as R6 and C15.
 
sws2h said:
I guess the schematic is wrong, C16 there is a 1uF film cap and C14 is a 1uF/50V electrolytic (same as the ones used all over the unit). The trace off of pin 7 goes to C16, as well as R6 and C15.

It sounds like the correct cap.

The part designation is smudged in that spot on the schematic so it could easily be C16.

Does it connect to R8 on the other end?

If so, BINGO!

edit: I see that C14 on the schematic is in fact a 1uF/50v lytic coming off pin 4 on the same LF353.

Mark
 
I do have one final question. According to Jim:

"These circuits can be inproved by replacing the cross coupled 1N4148 diodes with green LED's"

"A couple of 1N4148 diodes are used. Replacing them with 3mm green led's helps as it places the diode conductance at over 2 volts, it's under 2 volts with the silicon diodes. Remember to use a Sharpie to black out the LED's as they can conduct leakage current from external light sources."

There are 18 1N4148 total in the circuit. Are the two in the power supply (CR18 and CR19) the pair he is talking about? I'm guessing I don't want to replace them all!
 
These are the two diodes CR8 & CR9, in parallels with a 8M2 res, around the LF353 (U9 I think, my copy is not very good).
Replacing these with LED's will drastically change the behaviour of the non-linear time-constant.
You really have to understand the operation of this circuit.
In a steady-state condition, the timing capacitor is a combination of the physical cap C41 (10uF) that goes from pin 6 of the RMS to pin 6 of the LF353 (inverting input) and the virtual capacitance resulting from the active capacitance multiplier Ceq = C41/C45 . C47 = 10u/47n x 220n = 47u. Resulting in a total of 57uF.
Under large signal variations, the output of U9 swings and the diodes dynamic resistance is very low, reducing U9's gain to 0. The only capacitor left is C41 + C47 = 10.22uF. That means that the circuit is 6 times faster under large variations than at steady-state. The difference of threshold between LED's and signal diodes makes the detector stay in its slower behaviour for much longer, making it less responsive to attacks. Is that really what you want? That's why dbx, in their great wisdom, have modified ALL the components around the circuit when they went for LED's instead of Si diodes. Perhaps Jim Williams is not telling you the whole truth. He makes a living out of modding gear, I understand he may not want to disclose all of his tricks.
 
Thanks for the detailed explanation!

Jim usually recommends high speed replacement op-amps, so perhaps in conjunction with a high speed op-amp the LED/Diode swap would make more sense. (pure speculation on my part)

I found the thread where Jim talks about these mods. :)

http://www.gearslutz.com/board/geekslutz-forum/53182-dbx-160xt-mods-schematic.html

abbey road d enfer said:
Perhaps Jim Williams is not telling you the whole truth. He makes a living out of modding gear, I understand he may not want to disclose all of his tricks.

Mark
 
That is my thread, Jim doesn't comment on here but he often does on Gearslutz.

Thanks for the explanation abbey road. If I can't get a clear answer, maybe the next time I send him some gear I'll ask. I have one of his modified C414B-ULS and 3 C460B's, and I need to get one more of each. I use them on everything. Unless I need some extra mics, everything else (including some KM84s and a U87) sit on the shelf.
 
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