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dbx 242 mods Dual-Channel Preamps
Old 26th November 2009
  #1
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dbx 242 mods

i've found threads on here referencing dbx 242 mods, but haven't found any info on what they actually are... other than people suggesting sending them to someone else to do the work.

anyone care to share some info on mods to a 242?

thanks!

happy thanksgiving
Old 5th May 2010
  #2
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anyone? modding the dbx 242 eq?

thanks!
Old 6th May 2010
  #3
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mods

Most might start with swapping opamps, but my solution, though not on a dbx 242, was to strap output transformers on the output. I haven't tested tons of examples but most of my information was from the best techs around. I went with a pair of Marinaire 1:1's.
Old 15th November 2010
  #4
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frans's Avatar
more info on the dbx 242

(source: dbx242 eq's arrived...wanna hear? - Topic Powered by Social Strata)

the first thing you will notice about this EQ is that when people look at it and see the DBX Project 1 on it... they will sneer. i think it needs a new face plate. it will sound better.

the shelves are really nice, play around with those. the 6db is nice and subtle, musical. when i track anything synth, drum machiney, or anything line level in general, i use these shelves... i guess the 12db button doesnt give a traditional 12db curve, it curves at that rate where you set the filter, but transitions to a 6db at some point. i dont know if that information is helpful to anyone, i just know it sounds nice.

i think there is a quick mods that you could do as well. orban recommends taking out a cap on the input (you will have to check his post on RAP for specifics). does anyone have one in front of them that they could open up and determine where on the board this cap is?

i'll repost it:

[from the horses mouth]

The design started out as the successor to the Orban 642. The improvement was a complex crosscoupled feedback arrangement that allowed reciprocal
curves to +/-16dB EQ, yet still allowed an infinite notch. The whole
thing required a MAJOR optimization to solve all of the relevant
equations, because, unlike most EQ designs, the properties are not
guaranteed by the topology but rather by correct selection of (if I
recall correctly) six different resistors.

(We have since received a patent on the topology, and, for the
brave-hearted, the optimization procedure and the equations are to be
found there.)

About that time, AKG decided to drop the Orban studio line and just
concentrate on broadcast because they had just purchased the remains of
the dbx Pro operation, and dbx was to become the new "pro" line.

The EQ design was pretty cool, but the powers that be at AKG decided that
we would sell a lot more of them if they were aimed towards the project
studio market. This required a few compromises, like a wallwart power
supply. But the main signal path is 5532s in low-gain inverting mode, and
it's actually pretty clean.

There _is_ an entirely gratuitious electrolytic coupling capacitor at the
input, which I would advise bypassing. At this point, you will have an
admirably clean signal path.

The parametric filters are made with TL074s, and these could certainly
stand an upgrade.

Bob Orban

_______________

... So now ...anybody got a schematic or knowledge which cap to bypass? I opened my 242 yesterday and as I am not the geek I'd like to be I wasn't sure which cap I should take out.
Old 1st December 2010
  #5
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frans's Avatar
Yesterday the head of the german dbx service center told me the following:
"In the signal path of the schematic we have, there is NO electrolytic condenser. The revision number of the schematic is A0, first version. That a condenser would have been inserted in later versions is highly unlikely because the sound would have suffered. How the circuit had been in development/the laboratory is dbx's secret."

So how about that "Bob Orban" post then? It could as well be a misunderstanding or myth. We'd need Bob Orban to clear that.
Old 1st December 2010
  #6
Gear Nut
 

I'm also interested in this. Picked mine up for $15 on craigslist!
Old 12th December 2010
  #7
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bob orban, where are you?

yeah, the shelves are really nice on the 242 as it is.
Old 25th August 2011
  #8
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i love the dbx 242. have been actually passing some mixes through my mono channel (L and then R).

as it is, it has a really, nice tape - warm vibe. the eq is really great for cutting and surgical things.

would be interesting to replace the filters and find that capacitor that orban was talking about though.

for mastering, maybe it'd be better to not have an output transformer, yeah? i have built some cables that "balance" an unbalanced output -- you can get the schematic for them on the jensen website.
Old 28th August 2011
  #9
Gear Head
 

dbx 242 schematic anywhere?

I know it's almost worthless, but I have to ask. I've been searching for it several times for quite a long while with no success.

...or, anyone performed the Bob Orban suggested mods on this unit? I don't know what input cap is the one to bypass. Cannot find it. Maybe there isn't one as Frans pointed out.

I know the TL074 are the ICs to remove, but what replacement ICs should I use? Any direct replacement? Are feedback caps or something like that needed? changing any associated resistors values?

I've found some chats about this mod but never explained, just mentioned.

Any help will be welcome.

BTW, if anyone is interested here's the Bob Orban patent for this circuit. That secret development the dbx support guys talk about... but Bob Orban himself adviced to check the patent to understand the circuit particularities.
Old 28th August 2011
  #10
Gear Head
 

Quote:
Originally Posted by frans View Post
(source: dbx242 eq's arrived...wanna hear? - Topic Powered by Social Strata)

the first thing you will notice about this EQ is that when people look at it and see the DBX Project 1 on it... they will sneer. i think it needs a new face plate. it will sound better.

the shelves are really nice, play around with those. the 6db is nice and subtle, musical. when i track anything synth, drum machiney, or anything line level in general, i use these shelves... i guess the 12db button doesnt give a traditional 12db curve, it curves at that rate where you set the filter, but transitions to a 6db at some point. i dont know if that information is helpful to anyone, i just know it sounds nice.
i am in agreement with my 2004 self.
Old 7th September 2011
  #11
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first off -- i want to reiterate that i love this eq.

today, however -- i tried the cap bypass mod.

with the cap near the input / output being in the circuit... the original source is maybe only slightly muddied, but in a nice sounding way... and the eq is awesome !

however, with the cap shorted out, taken out, bypassed whatever... it really opens the eq up. with the cap bypassed, the circuit, just passing through it, actually cleans up the source material. i can't explain it, but it is clean and beautiful and open sounding. anyone wanting this eq to sound more open and clear should try the mod. i talked to bob orban (who is cool as hell (and a genius designer)) who relayed some thoughts and wisdom.

Quote:
Originally Posted by Snap View Post
dbx 242 schematic anywhere?
Bob said he didn't have the schematic.

Quote:
Originally Posted by Snap View Post
...or, anyone performed the Bob Orban suggested mods on this unit? I don't know what input cap is the one to bypass. Cannot find it. Maybe there isn't one as Frans pointed out.
It is the cap that I'm pointing to. It is the only cap near the input.



Quote:
Originally Posted by Snap View Post
I know the TL074 are the ICs to remove, but what replacement ICs should I use? Any direct replacement? Are feedback caps or something like that needed? changing any associated resistors values?
In response to this (and asking how different IC's would affect the signal and/or color) -- I don't really believe in copying and pasting correspondence, but I think/hope Bob wouldn't mind.

"Anytime you replace opamps, the main thing to concern yourself with is possible parasitic oscillation if the new opamp has a higher bandwidth than the old one. The board layout and/or power supply bypassing may not be there to support the higher-speed part. I don't have any specific recommendations for replacements. However, because the main signal path has 5532s and the TL074s are used only in the parametric filters, the TL074s are less likely to have an significant effect of "color" than they would if they were in the main signal path."

Quote:
Originally Posted by Snap View Post
Any help will be welcome.
Bob Orban recommended soldering a jumper wire across the pads at the bottom to short out that cap. That way, if you decide you want to reverse the mod in the future, it is easy to just unsolder the jumper wire.

I have done much prettier work, but this did the trick:



Good luck ya'll. As Bob said to me after I told him how much I loved the eq and his work... "I am glad you are pleased. Good luck with your future endeavors."

Last edited by Seditionary; 25th July 2015 at 11:52 PM..
Old 7th September 2011
  #12
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frans's Avatar
I wonder what this cap is doing in the circuit... did somebody follow the traces to see what is it's function?
Old 7th September 2011
  #13
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open up your 242, frans.

i just put mine back together !

the eq still functions the same -- it is just... everything is cleaner. every band almost opens up with that cap bypassed.

i think it sounds great either way though.
Old 7th September 2011
  #14
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frans's Avatar
Quote:
Originally Posted by Seditionary View Post
open up your 242.

I already did, a few months ago. But I don't understand electronics thoroughly enough to say exactly what it does. I can guess, but I'm not sure. That's why I have asked. If somebody who knows electronics could have a look at his 242 and tell us....?
Old 7th September 2011
  #15
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Quote:
Originally Posted by Seditionary View Post
open up your 242, frans.

i just put mine back together !

the eq still functions the same -- it is just... everything is cleaner. every band almost opens up with that cap bypassed.

i think it sounds great either way though.
Again, thanks for posting this! Looks like a simple way to improve an already cool piece of gear!

Oh, and thanks to Bob Orban for sharing his insights with you! The 242 is seriously underrated!
Old 9th September 2011
  #16
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yeah man ! serious thanks to bob orban for designing a beautiful sounding circuit and for telling us about that cap. even though it was the only cap on input, i probably wouldn't have shorted it out without some confirmation.

Old 9th September 2011
  #17
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Does removal of the input cap clean the unit up to the point of it losing it's distinct vibe?
Old 11th September 2011
  #18
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no -- it still has the same, awesome vibe. it just opens up the frequencies on the eq. i don't know how to explain it...

i guess the best way is that with the cap, the eq adds a tiny bit of mud to the signal.

without the cap, it still has the same awesome vibe, but it seems to clean up the signal and open up the frequencies on the eq.

solder in a jumper cable as bob suggested -- if you don't dig the mod, you can easily remove it!

i almost guarantee you'll like it though

i wish i could have 4 of these though ! stereo cap and stereo no cap ! ha. but if i had to choose 1, i think it'd be no-cap. i used to mostly use other eqs when tracking as the mud that this eq added always made me reach for something else... but now that that cap is gone, it'll be getting a lot more use!!
Old 11th September 2011
  #19
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Can't wait to get a little time to try this out!
Old 24th September 2011
  #20
Gear Head
 

Thanks so much for sharing, Seditionary.

So it's that one. C29. It was the first cap I've checked but it's not easy to follow those thin copper tracks. I seemed to me like away from the signal path. But I was wrong.

About the TL074, the TL07x family were always cheapo Opamps, noisy and dirty sounding; also not good dealing with HF. They are widely used even in supposedly hi-end gear and hundreds of supposedly high quality mixing consoles for decades. If you check opinions of experts (not me) you will find discrepancies and all kind of opinions about replacing the 5532. But about nobody will recommend you keeping TL07x units in any piece of gear.

I don't have the schem, But as I understand how Eqs work, I really doubt the TL074s are not in the signal path.

Bob Orban designed this unit as the ultimate Eq replacing the already very good 642. Since AKG decided to cut corners in this design and made it part of a dbx budget line "to sell more units" (a real shame). Everything is always on and the TL074s constantly adding noise and crap to the signal. You cannot bypass or engage a single band as was common in any other Orban design, so them all 5 "contribute" all the time. I'm afraid the the four opamps in every TL074 are the ones driving the filters while the dual 5532 are the stages in-between, so signal actually pass through the TLs. I'll try to reverse engineering a single bandpass filter and drawing a schem to figure out how is it actually arranged, but I doubt I'm wrong in this.

Another thing I want to do is adding the (AKG dropped off) on-off switches for every band.

Quote:
I don't understand electronics thoroughly enough to say exactly what it does.
It's a blocking DC cap at the input. It can be bypassed with a small plastic cap to improve the usual losses of Electrolytic caps, or fully bypassed or removed altogether (replaced by a simple wire) if you are sure that your gear preceding this unit won't be sending any DC along with the signal. Which is very likely to be this way. I'll fully remove the cap in mine.
Old 2nd November 2011
  #21
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hereticskeptic's Avatar
Quote:
Originally Posted by Seditionary View Post
no -- it still has the same, awesome vibe. it just opens up the frequencies on the eq. i don't know how to explain it...

i guess the best way is that with the cap, the eq adds a tiny bit of mud to the signal.

without the cap, it still has the same awesome vibe, but it seems to clean up the signal and open up the frequencies on the eq.

solder in a jumper cable as bob suggested -- if you don't dig the mod, you can easily remove it!

i almost guarantee you'll like it though

i wish i could have 4 of these though ! stereo cap and stereo no cap ! ha. but if i had to choose 1, i think it'd be no-cap. i usually use other eqs when tracking as the mud that this eq added always made me reach for something else... but now that that cap is gone, it'll be getting a lot more use!!
Nice! I actually have my two units listed in the classifieds right now, as I think an outboard eq is overkill for my setup and purposes, but it's really hard to imagine letting go of these hard-to-find units (especially since I got them in brand new condition from someone who had them just sitting there all this time, completely unused).

Trying to decide if I'll regret selling them, or if plugin eq will get it done for me. I don't use much eq at all (if any) during mixdown, as I'm exclusively tracking and mixing Hip Hop (for right now). The issue is, if I decide to expand my horizons in the future, will I wish I'd kept this baby? Also, I know there will be a use for the 242s with my Hip Hop mixes, but I haven't so far. Also, I do have the Aurora Audio GTQ2 eq, though with far less frequencies available. Been thinking the GTQ2, along with something like the DMG EQuality plugin, will be more than enough for the limited eq that I tend to use.

Anyway, I'm off-topic now, haha. Thanks for the cap bypass information. If I keep the units, I may look into having this done to mine (absolutely zero soldering skills).

Last edited by hereticskeptic; 2nd November 2011 at 01:00 PM.. Reason: typos
Old 14th November 2011
  #22
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frans's Avatar
I followed the copper traces on the PCB and... I'm not too sure C29 is on the input - it's located close to the in/out but doesn't seem to be in the circuit there, rather belonging to the high shelv band. Could somebody who's good at electronics have a look at it?
Old 14th November 2011
  #23
Those 074's are the filters. Replace them with LME49740 or BB OPA1644. Use good duals for the others like LME49720, OPA1612, OPA1642, etc...

Remove the dipped mylar EQ caps and replace with Wima FKP-2 or MKP-2.

Then check DC offsets off each replaced opamp. If DC is below 3 mv, bypass the following el cap with a wire. Replace the 1000 uf mains caps with 2200 uf Panasonic FR or similar. Replace the secondary psu caps with either 470 or 1000 uf/25v.

Also remember to smile.
Old 3rd June 2012
  #24
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one other thing to note is that the output on this thing is unbalanced. that is the FIRST thing to change in your routing. see this pdf for more information on connecting between unbalanced and balanced equipment -- http://www.jensen-transformers.com/an/an003.pdf

it is a really helpful document from the jensen transformer's white pages... the first thing to do is to build 2.1 -- an adapter cable ! it makes the circuit go from unbalanced to (pseudo) balanced... that will GREATLY improve the sound if you are going into a balanced connection on a converter or going to balanced gear afterwards! i think that these eqs would be worth (and will be worth) more than 100 times more than what they are going for now if people knew how to properly balance their equipment! sticking a balanced line into the unbalanced jack on this will really degrade the signal and make it thin and terrible sounding. going through a balanced adapter cable will keep the signal sounding good, big and fat. i could do some soldering for you if you'd want -- i love building these cables

last -- on both of the dbx 242's i've found (from different parts of the country)... both channels were 3 pin hot ! so when building the adapter cable (or within your patchbay)... switch the cable from 2 to 3 pin hot -- i.e. simply reverse high and low on the male side... this will keep the signal in phase!

so start with that -- if you don't... and you try going through the output jack... you'll get an out of phase, unbalanced signal that will sound bad (but as the circuit is so good, i bet a lot of people are doing this and still swearing by the 242 sound). if you build this adapter and then short out the input cap though -- i think there is a good chance that you'll be super, super happy with this eq.
Old 12th September 2013
  #25
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The electrolytic on the input is C30, not C29. When I probe C29 I'm seeing wet signal (equalizer processed audio, no matter what position the bypass switch is in), but C30 is definitely input signal. C30 is the target. It's the other 47uF 25v cap, up front, near the input gain pot, bypass switch, etc.

And yes, the unit I have in front of me is "pin 3 hot". Or, actually, "ring hot" I guess it would be?

I don't really understand what C29 is doing, it's hard to trace really, but if it's in the signal path (maybe it is?) it too could stand to be bypassed one way or another, but it's certainly not the cap Mr Orban was referring to.
Old 13th September 2013
  #26
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Anyone swapped out the LM741 on the output for a newer opamp? Or the 2n4400 after it? These seem like more "jellybean" silicon that could stand to be upgraded.
Old 13th September 2013
  #27
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i see from the pic a few ways to improve the circuit as far as the cap bypass thing you could put that on a switch and have the best of both the opamps could be upgraded as well as the power supply.
Old 17th December 2013
  #28
mwe
Gear Head
 

I'm sorely tempted to try modding my 242. I'm following most of what Jim mentioned in post #23 but I wouldn't know a dipped mylar capacitor if it was in front of me. C100/C101/C102 in the post #11 pic or C104?
Old 31st December 2013
  #29
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Quote:
Originally Posted by mwe View Post
I'm sorely tempted to try modding my 242. I'm following most of what Jim mentioned in post #23 but I wouldn't know a dipped mylar capacitor if it was in front of me. C100/C101/C102 in the post #11 pic or C104?
big red, like C104, C103, etc. They are mylars, and form a crucial part of each filter section.

the little yellow guys are ceramics and seem to be pulling simple decoupling duty and such.

this a very complicated architecture; most EQs are pretty simple circuits, but i'm having a hell of time understanding this one. but i know what a mylar looks like, and if you want you could just google mylar capacitor and get tons of images of them.
Old 31st December 2013
  #30
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but i urge anyone reading this not to mess with C29 unless you understand what it's doing, because the electrolytic Mr Orban is referring to is almost certainly c30. I had one of my 242s on the scope a few months ago and could see that C30 was clearly on the input; C29 however showed "wet" signal, no matter what position the bypass was in.
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