Modifying DBX 163X

Sean Sullivan · Post by **Sean Sullivan** » Tue Nov 17, 2009 12:51 pm

I found an old post by Jim Williams on how he modifies the dbx 163X. Some stuff is fairly simple, like replacing all the electrolytic caps, removing the HF roll-offs, replacing op amps and the VCA, and replacing the main caps.

However, there are some things that are over my head because I only have a very basic understanding of electronics.

For example:
"The power supply is rebuilt with large caps and a polyprop film bypass. Audio cap gets a polyprop bypass"

I'm not sure what caps are part of the power supply because it doesn't have it's own circuit. I understand what a capacitor bypass is, but in an audio circuit is it important to use several capacitors in parallel?

"The input impedance is lowered to 100k ohms."

How is this done?

"Phase compensation added."

Don't know about this either.

The Scum · Post by **The Scum** » Tue Nov 17, 2009 3:26 pm

Any chance you've got a link to a schematic handy?

It's easier to discuss this stuff if we can reference part numbers and schem locations.

Sean Sullivan · Post by **Sean Sullivan** » Tue Nov 17, 2009 4:38 pm

http://www.dbxpro.com/product_downloads ... Manual.pdf

The Scum · Post by **The Scum** » Tue Nov 17, 2009 5:01 pm

Bingo.

The power supply is the top right corner of the last page...caps 13, 14, 18, 19, 20, 21.

but in an audio circuit is it important to use several capacitors in parallel?

Bypassing 'lytics with small film caps is commonplace - it supposedly alleviates some of the distortion caused by the 'lytics. It's supposed to help with transient response because the film caps have lower series resistance, thus having current available sooner.

There's no actual requirement to put caps in parallel...sometimes it's just dictated by the fact that we need to squeeze lots of capacitance into a 1 RU chassis, and big single caps won't fit.

"The input impedance is lowered to 100k ohms."

Switch R4 to 100K.

"Phase compensation added."

Find all of the unity gain opamp stages, where faster opamp stages might oscillate. Put a small cap (~100pf) across the feedback resistor...if you can reverse engineer the circuit far enough, you can turn those opamp stages into ~100kHz lowpass filters - plenty hifi, but not RF generators. Probably best done empirically with an oscilloscope, because when they oscillate, it's at crazy high frequencies, that you'll never hear.

Sean Sullivan · Post by **Sean Sullivan** » Tue Nov 17, 2009 7:24 pm

Thanks Scum! Most of the power supply caps are ceramic disc, is there an alternative type of cap that works better/last longer?

Also, I don't know how to read value of the caps:

10SE1
1M50E1
470M35E
.1MD3

Nate Dort · Post by **Nate Dort** » Wed Nov 18, 2009 5:39 am

minorleagues wrote:Thanks Scum! Most of the power supply caps are ceramic disc, is there an alternative type of cap that works better/last longer?

Also, I don't know how to read value of the caps:

10SE1
1M50E1
470M35E
.1MD3

10SE1 = 10uF (25V would be fine here)
1M50E1 = 1uF 50V
470M35E = 470uF 35V (you might want to go up to 1000uF 35V if they'll fit)
.1MD3 = .1uF (use polypropylene here and at all the other .1uF caps listed in that section below the power supply schematic. Those are the supply bypass caps for all the opamps. I prefer Wimas, but that's just me. You probably won't find any with a voltage rating below 50V. That's fine.)

Double check all the actual caps in the box to see what form factor they are (axial vs radial, lead spacing, etc).

Sean Sullivan · Post by **Sean Sullivan** » Wed Nov 18, 2009 7:00 am

Thanks Nate. Is anything not bypassed that needs to be?

I hope these things end up sounding pretty good, I've dumped about $40 each into them for a new VCA, op amps, and caps.

So, in the shopping cart I've got replacement bypass caps (WIMAs), power supply caps (Nichicon HE series, a tantalum substitute), and Nichicon VZ for electrolytic replacements.

Nate Dort · Post by **Nate Dort** » Wed Nov 18, 2009 10:18 am

minorleagues wrote:Thanks Nate. Is anything not bypassed that needs to be?

None of the electrolytics in the power supply are bypassed according to the schematic. Apparently Jim suggests that you add them. Get some more .1uF Wimas to put across all those electrolyics.

Brian · Post by **Brian** » Wed Nov 18, 2009 1:48 pm

FYI, that schematic is way off as far as assignments go, there are no ceramic power supply caps, the power supply caps are electrolytics and they are bad, I'm going off memory here, but, they should start in the area of 26 as labeled ON THE BOARD ITSELF.
The unit has HF suppression all over it on every IC in there, you'll need to remove some but, do it "insitu" so you can hear the difference.
Double the value of the electros,
I think C!, C3 & 4 can all be films, the ones that are already ceramics probably oughtta stay ceramics. some of them need to stay the same size too. There are a couple of crucial areas in the C1-7 range that you have to be careful or the unit will not sound right.
I did mine and i just used it this morning, A/B'd it against a Joe meek and it ate it's lunch, smooth on the top, clear as a bell, and that was on the supersquash all the way over to the edge "way more" setting. Ha ha.
It really did sound great. I was surprised how good the top end sounded. It had sparkle, it had meat, and the noise floor went way down.WAY down. AND it had a very controlled sound too.

Sean Sullivan · Post by **Sean Sullivan** » Wed Nov 18, 2009 4:54 pm

Well, shit the link from DBX is wrong, that's the manual for a 163A

Here is the DBX 163X schematics:

http://www.dbxpro.com/product_downloads ... ematic.pdf

Nate Dort · Post by **Nate Dort** » Wed Nov 18, 2009 5:19 pm

Power supply is pretty much the same. All the bypass caps are .01uF however.

Sean Sullivan · Post by **Sean Sullivan** » Wed Nov 18, 2009 5:23 pm

There is a .01uf caps coming off the VCA, and 1 or 2 coming off each op amp. Should I replace those with WIMA, as well as put WIMAs across all the electrolytics in the power supply? Should I bump them all up to .1uf instead of .01uf?

From my understanding of the schematic, C4, C8, C9, C19, C11, C10, C18, C19, C26, and C27.

Is the resistor to change the impedance still the same? It's actually a trimmer resistor and I don't know what power rating is appropriate. I'm guessing the hi-z is if I wanted to plug an instrument directly into the compressor, correct? If that's the case I'm not going to worry about it, I don't plan on doing that.

Sean Sullivan · Post by **Sean Sullivan** » Wed Nov 18, 2009 9:19 pm

Well, I ordered 32 WIMA MKP-2 .1uf polyprop caps to replace all the bypass caps and bypass the electrolytic (someone might have to help me with this when it comes time) and Nichicon HE series electrolytic caps. Hopefully everything fits!

I wasn't sure which resistor to replace for the input impedance since R4 on the dbx 163X is a trimmer resistor...and I really don't understand phase compensation enough to mess with that.

Nate Dort · Post by **Nate Dort** » Thu Nov 19, 2009 4:09 am

I was going to reply to your email, but I figured posting here might help other people down the road.

Those .1uF will probably be alright as bypass caps, though it's going to be hard to hear the difference that switching from ceramic to film makes when you're changing values as well.

As for bypassing the electrolytics, you're just putting those .1uF caps in parallel with them. The electrolytics will have two leads that get soldered to the board (+ and -). After you solder them in, just take your Wimas and solder them to the bottom of the board at those same two points. It's the same as if you had a resistor on a circuit board and you wanted to "bypass" it. You would just solder a piece of wire across its terminals. Film caps aren't polarized, so don't worry about which leg goes to + and which to -.

Input impedance is set by R4 in the 163A schematic. It's a resistor that connects from the input signal to ground before it goes through the first opamp. It's in the same place and it's the same value in the 163X schematic, though it's called R2 there.

Are you replacing the opamps? The only place I can see where you would need to do any phase compensation is at OA2 (labeled "output buffer"), but it looks like there's already some 100pF caps there in the appropriate places.

Brian · Post by **Brian** » Thu Nov 19, 2009 5:12 am

Don't bother with the impedance switch, there is a separate high impedance input jack, so, you don't need to bother. That thing may only affect that jack.
For the bypass caps, I used polyester, you can hear the difference better with them compared to polypro if you replaced ceramics. Polypro's are better used in the audio path.
Bypass with polyester, they're cheaper, smaller, and you'll hear the difference better for bypass. The HF supression needs to stay ceramic, it's more effective for that, BUT, you can remove or lower the amount of HF suppression that's going on in there, and you won't need it unless you swap the chips, which from my experience is not necessary.

Message Board

Modifying DBX 163X

Modifying DBX 163X

Who is online