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Thread: DBX 286A Modifications

  1. #11
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    Quote Originally Posted by ecc83 View Post
    http://www.analog.com/media/en/techn...016SSM2017.pdf

    Those ^ are in the posted circuit Mark. Nice 1nV/rtHz jobbies. Maybe the legend has been overprinted to confuse Behringers!

    D
    I think the 'ZNF115' chips may be 339 quad comparators used for the LED metering. Seems the copper traces from those are headed in that direction and there is a 339 on the chip.
    Mark.......

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    Quote Originally Posted by arcaxis View Post
    Have fun Doc!!!! . I popped the 'bonnet'(British speak Dave might like) on my DBX286s and found pretty much what I expected. Personally I avoid/hate working on surface mount components, but if it's something that makes you feel warm and fuzzy, certainly have a go at it. I don't find the 286s to be bad by any means with the variety of mics I've used with it. Usually an RE20 or 320 straight in or an SM7b with a Triton Fethead(22kohms of the Fethead brightens up the SM7b). If I were you and didn't like the High-Z input of the 286s, just sell and get something more to your liking.

    I didn't look very far, but the IC's marked 'ZFN115' didn't get any Google hits. The 4580 is pretty common.
    Looks really different from the schematic. It looks like they went to a transistor based amp like what is in the Behringer ADAT and others. I'm personally not a big fan of surface mount and audio because its so easy to go cheap too far and the product suffers. The polymer electrolytic caps they are using for audio coupling might not be the greatest but there hasn't been enough data out there to just trust that they are ok either.

    I think these newer generation of transformer-less mic preamps are not really engineered that good or there is something lost in sourcing the parts since its surface mount.

    Btw I didn't realize this thing went surface mount. But that doesn't deter me. Its more of a challenge of a different flavor sourcing good surface mounts though...

    But it does explain why no one as really attempt modifying them.

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    Quote Originally Posted by ecc83 View Post
    I don't know where you are getting these ideas from friend? You seem to be some 50 years behind modern practice, nobody 'power matches' audio signals any more (no, not even valve amps and speakers!) .
    .
    I think there is flaws in modern practices. Mainly because there is no real electronic law it is truly fallowing.

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    Quote Originally Posted by drtechno View Post
    I think there is flaws in modern practices. Mainly because there is no real electronic law it is truly fallowing.
    I don't really understand that. No matter. ALL electronic circuits have to follow the laws of Mr Ohm and Co. It might SEEM at times that something 'magical' is going on but if you have the equipment and chops you can always find a rational reason for circuit behaviour, at least for things as simple as mic pre amps!

    I shall commend to you Small Signal Audio Design by Douglas Self. (V good section about electrolytic and other capacitors)

    Dave.

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    Quote Originally Posted by ecc83 View Post
    I don't really understand that. No matter. ALL electronic circuits have to follow the laws of Mr Ohm and Co. It might SEEM at times that something 'magical' is going on but if you have the equipment and chops you can always find a rational reason for circuit behaviour, at least for things as simple as mic pre amps!

    I shall commend to you Small Signal Audio Design by Douglas Self. (V good section about electrolytic and other capacitors)

    Dave.
    I do agree that all electronics circuits have to follow Ohm's and Kirchoff's law. I really think that the newer generation of E.E.s that are designing in the field don't necessarily look what the operator's needs are in pro audio. So I think they look for "it works" scenarios instead of "quality of design". A lot of E.E.s know how to build a preamp. But very few can do it on the same level as Mr. Neve and Mr. Putnam (rip).


    I'll look at that book. I'll see what his view is on caps. One of the things I have noticed over the years that the construction materials and design of caps has changed. So one manufacturer seem to be better than another on one application, but doesn't work in a different circuit. A year or so ago I changed out a orange drop in a multi channel unit that the replacement orange drop made that channel sound different. I knew it was the part because swapped it onto a different channel and the problem followed the part.
    Last edited by drtechno; 09-18-2017 at 12:59. Reason: added the paragraph about the book and the caps

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    BTW I was hopefully going to see an ssm2017 or an ina217 installed as the mic preamp instead of four SMT transistors. Its not big of a deal as I've heard an API DOA and it was really good. But something tells me that the transistors and resistors are not going to be matched in the DBX like the guy who built the DOA.

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    ya, you take a look and see if it's worth the effort. Like a caboose on my new Behringer mixer to house the discrete op-amps

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    A little mod for better performance is sometimes OK. Just depends how much room you've got to stuff the improvements into.
    Attached Thumbnails Attached Thumbnails vw-jpg  
    Mark.......

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    Quote Originally Posted by arcaxis View Post
    I didn't look very far, but the IC's marked 'ZFN115' didn't get any Google hits. The 4580 is pretty common.
    Those are lm339's or ST's equivalent.
    i noticed the changed to using that 4805's for the vca / rms cips.
    also changed to a jrc4580

    interesting changes.

    I would be curious what the rest of the board uses. if you pop the hood again id love to see some more pics as i cant see the chip numbers for the glare... and even be interested in what they did on the "S" version to the power supply and transformer too.

    it does looks like there is not enough chips there, id bet they have surface mount chips on the underside.

  11. #20
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    Popped the bonnet again.....

    Full sized images....
    IMG_1202.JPG - Google Drive
    IMG_1201.JPG - Google Drive
    IMG_1203.JPG - Google Drive
    IMG_1204.JPG - Google Drive
    IMG_1205.JPG - Google Drive
    https://drive.google.com/file/d/0ByS...ew?usp=sharing
    https://drive.google.com/file/d/0ByS...ew?usp=sharing
    https://drive.google.com/file/d/0ByS...ew?usp=sharing

    There are a couple 4305 IC's that seem to be doing the VCA stuff > http://www.thatcorp.com/4305_Low-Cos...ngine_IC.shtml
    339's for level comparators to drive the LED metering.
    There are several transistors that appear possibly to be the front end of the preamp which then feed the 4580(?).
    A few TL074C JFET OPAMPS.
    LM3406M towards the output end of the board > http://www.ti.com/product/LM3406 (???)

    Some similarities to its predecessor, but more different.
    Mark.......

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