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DBX386

Discussion in 'Electronic Repair Modifications DIY' started by archieaa1, Aug 7, 2013.

  1. archieaa1

    archieaa1 Active Member

    Hello All

    A friend gave me a broken 386 and after going though it and getting it working, I am wondering if it might be wise to add a transformer on the input. I haven't traced it out myself to confirm Bushy's trace of the circuit but it looks like its using a 4580 op amp for the first stage of amplification. This would seem to give us questionable CMRR. I have a bunch of 1:3 ratio mic level Tamura transformers. I was thinking of adding one to the input to eliminate the DC blocking Caps and increase the CMRR. Any thoughts about the wisdom of this?

    When I first opened the thing up I saw one distended capacitor so I went trough the circuit with an ESR meter and just about ALL the electrolytic caps were reading quite high. Some were so high as to exceed the range of the meter. Given that the unit was made in late 2000 it is odd to see so many failed caps. This suggests that either it has cooling problems, they used garbage capacitors or both. I keep my parts cabinet filled with Nichicon Muse series caps. Hopefully these will last a little longer and sound a little better. I was also thinking about adding a small fan to help keep things a little cooler.

    Any comments or thoughts would be greatly appreciated.

    Archie
     
  2. Boswell

    Boswell Moderator Distinguished Member

    Adding a mic-level transformer at the inputs to get around the need for the phantom-power isolating capacitors is certainly something that a lot of the older-style pre-amps used to do, but 1:3 turns ratio (giving a 1:9 impedance ratio) is probably too high for this particular circuit. It may be worth experimenting with a couple of the transformers in inverse-series (1:3:1) to get back to the same nominal voltage and impedance levels while still giving you d.c. isolation.

    Keep us updated on how you get on with the re-capping and the transformers.
     
  3. archieaa1

    archieaa1 Active Member

    Well I replaced all the electrolytic caps with the exception of the 5-10uf 450vdc caps and 3-10uf 16vdc caps. It's working rather nicely. Its a little noisy. The gain staging really makes a difference when using the analog outs. I have yet to play with the digital side of things. I hope to this weekend. I'm thinking about trying one side with a transformer and one side with out and comparing the two.

    I'll let you know what I find.

    Archie
     

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