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Thread: DBX 286A Modifications

  1. #71
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    i think i figured out whats broken on the 286S unit, and I've got the part on the way to fix it.

    I'm pretty much done with my initial mods and repairs on the 286A unit.
    Seriously someone at harman needs a good talking to. some of the implementation flaws are just horrendous.
    and it isn't a cost thing.

    Sadly some of those flaws carried over to the S model too.

    once i get the S model fixed i'll do some initial side by side testing. hopefully mid week next week.

  2. #72
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    Quote Originally Posted by innkeeper View Post
    i think i figured out whats broken on the 286S unit, and I've got the part on the way to fix it.

    I'm pretty much done with my initial mods and repairs on the 286A unit.
    Seriously someone at harman needs a good talking to. some of the implementation flaws are just horrendous.
    and it isn't a cost thing.

    Sadly some of those flaws carried over to the S model too.

    once i get the S model fixed i'll do some initial side by side testing. hopefully mid week next week.
    There is a few things I would have to say/question about the S model I have.

    1. What is the real purpose of R11 and R12? It seems like they are noting more than rfi filter, but their presence attenuates the signal going in.

    2. the inserts are not ground Isolated. Even though this is a common theme for budget mixers, I find keeping the these inserts isolated lowers the overall noise. Most of the desirable high end mixers used for recording have transformer isolation on its inserts.

    3. The insert return has no metering, nor any trim adjustment. Even though its not a necessity, its nice to be able to control the gain structure going into the compressor after the inserts.

    4. The output is not a balanced connection. No matter how you look at that.

    I still think transformers are the way to fix this design. I would have to compare and contrast the different ways I've seen balanced insert transformer setups ( like the Api set up of a 2623 or 2503 as an insert send and a 2622 on the return vs. a pair of Carnhill vtb9071, like what is installed as the insert transformers on a Neve 8816). I would definitely do some sort of iron like the 2503 on the output.

  3. #73
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    "1. What is the real purpose of R11 and R12? It seems like they are noting more than rfi filter, but their presence attenuates the signal going in."

    They are not resistors, from the schematic they are RF chokes and you remove* them at your peril.

    Certainly a good mic transformer will have an inter-winding shield that will all but kill RF but I would keep the chokes then check the HF response at 20kHz. If it is within a dB or so, keep them. The transformer will likely need a 'Zobel' CR network to dampen the HF resonance so that could be tweaked to get a decently flat response.

    Always be cautious throwing in traffs. HF and LF resonances can occur, hum induced and, unless driven from a very low (best zero) impedance source, distortion rises with level and inversely with frequency.

    *I live within 50clicks of both Rugby and Daventry and grew up with the first transistorised audio gear. I have had a LOT of experience of keeping radio out of audio gear! Valves, especially triodes are fairly immune. 'Bout the only good thing about them really, especially modern ones!

    Dave.

  4. #74
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    Quote Originally Posted by ecc83 View Post
    They are not resistors, from the schematic they are RF chokes and you remove* them at your peril.

    Certainly a good mic transformer will have an inter-winding shield that will all but kill RF but I would keep the chokes then check the HF response at 20kHz. If it is within a dB or so, keep them. The transformer will likely need a 'Zobel' CR network to dampen the HF resonance so that could be tweaked to get a decently flat response.

    Always be cautious throwing in traffs. HF and LF resonances can occur, hum induced and, unless driven from a very low (best zero) impedance source, distortion rises with level and inversely with frequency.

    Dave.
    I wish they included the BOM for the jackboard, because it looks like a #603 surface mount resistor. but comparing this to what is typically there, 8 turns of wire is not going to have 2.5K of resistance.

    I'm not afraid of transformers, and I know how to set them up in the circuit. As far as response, I'll focus on 50Khz and below, even though with injecting signal and the front jack board removed, the mic preamp section is flat to about 130Khz.

    I am going to get a set of the carnhill transformers for the inserts, even though they don't put specifications on them that I look at when I buy transformers (DC resistance and inductance (but will settle for bandwidth at rated source impedance if inductance is not available)) I'm sure they are ok since Rupert Neve used them in his designs. Other ones I know that would work really well, are way out of the budget range ($350/ea) and I'm not trying to take it to the "mastering class" level with it. Just trying to get it into "higher end recording class" instead of "general purpose sound enforcement and recording class".

  5. #75
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    well I got it working really well with the jensen transformer, however, I still had to have one instance of a capacitor in the path to isolate the two bias paths of the differential pair.

    So now I have decided to order a LL1530 because it has two separate windings on the secondary, and I should be able to get dc coupled operation with that transformer. As each secondary winding will be part of the dc bias and signal path.

  6. #76
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    Quote Originally Posted by drtechno View Post
    well I got it working really well with the jensen transformer, however, I still had to have one instance of a capacitor in the path to isolate the two bias paths of the differential pair.

    So now I have decided to order a LL1530 because it has two separate windings on the secondary, and I should be able to get dc coupled operation with that transformer. As each secondary winding will be part of the dc bias and signal path.
    really well? that is a bit ambiguous what difference did it make?
    how did the LL1530 work out?

  7. #77
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    Quote Originally Posted by innkeeper View Post
    really well? that is a bit ambiguous what difference did it make?
    how did the LL1530 work out?
    Indeed! I would like to know as well.

    Dave.

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