Page 3 of 8 FirstFirst 1 2 3 4 5 ... LastLast
Results 21 to 30 of 77

Thread: DBX 286A Modifications

  1. #21
    Join Date
    Aug 2013
    Posts
    431
    Thanks
    1
    Thanked 30 Times in 25 Posts
    Rep Power
    355323
    Sign in to disable this ad
    Well I'm decided to bail on it guys. The big strikeout for me is that this is a two stage mic preamp. the gain control is on the second stage with a four transistor comparator/Impedance buffer which wouldn't make the best. Second, there is no trim resistors on the power supply, so I would have to sit there and change out regulators. Btw the + rail is .8V off from the - rail and that is why it has so much common mode noise without even a mic connected! I could modify the input board to install a transformer, but its just not worth the trouble.

    If the power supply wasn't so off, I would go ahead and remove the surface mount transistor stage. But since I don't want to correct a brand new one (nor I should have to) , I'm sending this one back....

  2. #22
    Join Date
    Aug 2013
    Posts
    431
    Thanks
    1
    Thanked 30 Times in 25 Posts
    Rep Power
    355323
    Quote Originally Posted by innkeeper View Post
    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.
    here is some pics of the gutz.....20170918_181536-jpg20170918_181523-jpg20170918_181501-jpg

  3. #23
    Join Date
    Jul 2017
    Posts
    19
    Thanks
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0
    Thanks!

  4. #24
    Join Date
    Jul 2017
    Posts
    19
    Thanks
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0
    Quote Originally Posted by drtechno View Post
    Well I'm decided to bail on it guys. The big strikeout for me is that this is a two stage mic preamp. the gain control is on the second stage with a four transistor comparator/Impedance buffer which wouldn't make the best. Second, there is no trim resistors on the power supply, so I would have to sit there and change out regulators. Btw the + rail is .8V off from the - rail and that is why it has so much common mode noise without even a mic connected! I could modify the input board to install a transformer, but its just not worth the trouble.

    If the power supply wasn't so off, I would go ahead and remove the surface mount transistor stage. But since I don't want to correct a brand new one (nor I should have to) , I'm sending this one back....
    Not so fast.....

    i would assume they replaced the single instrumentation amplifier with discreet components. this would have several advantages over using a single chip solution like an ina163 / ssm2017 / ssm 2019 / that1510 etc. namely, you can achieve higher bandwidth, lower noise.

    instrumentation amplifiers are all two stage, that is how they work and provide good CMRR

    on my A model, i had the same issue with the regulators, so i did change those out to make the power fairly symmetrical. also, increased the precision of the resistors on the input of the instrumentation amplifier and the ones in the 48v supply to increase the CMRR.

  5. #25
    Join Date
    Aug 2013
    Posts
    431
    Thanks
    1
    Thanked 30 Times in 25 Posts
    Rep Power
    355323
    Quote Originally Posted by innkeeper View Post
    Not so fast.....

    i would assume they replaced the single instrumentation amplifier with discreet components. this would have several advantages over using a single chip solution like an ina163 / ssm2017 / ssm 2019 / that1510 etc. namely, you can achieve higher bandwidth, lower noise.
    you've got a point. I went and re-flowed the solder on the regulators and its much better

    OK since you put it that way, let's do a facelift on the front end. Say, an API 312 w DI input:
    api-312-di-mod-jpg

    The only thing different with the input circuit I would do, is change it to where the transformer is para-feed and the di input capacitive coupled too.
    Last edited by drtechno; 09-19-2017 at 04:56. Reason: coupling change

  6. #26
    Join Date
    Feb 2009
    Location
    northampton uk home of Dr Who and Blackstar Amps!
    Posts
    10,152
    Thanks
    1
    Thanked 691 Times in 654 Posts
    Rep Power
    9374895
    Quote Originally Posted by innkeeper View Post
    Not so fast.....

    i would assume they replaced the single instrumentation amplifier with discreet components. this would have several advantages over using a single chip solution like an ina163 / ssm2017 / ssm 2019 / that1510 etc. namely, you can achieve higher bandwidth, lower noise.

    instrumentation amplifiers are all two stage, that is how they work and provide good CMRR

    on my A model, i had the same issue with the regulators, so i did change those out to make the power fairly symmetrical. also, increased the precision of the resistors on the input of the instrumentation amplifier and the ones in the 48v supply to increase the CMRR.
    +1 to selected resistors for the spook juice to improve CMMR. Instrumentation amplifiers are not all that useful for audio (ref Self) Their high CMMR is only achieved at very high gains.
    If OP is going for input transformers, the NE5534 works well, e.g. F'rite Red.

    I would be pissed about that crap, clamp style open frame power traff! I would want a toroid at least, preff' a 'universal' SMPSU.

    Dave.

  7. #27
    Join Date
    Jul 2017
    Posts
    19
    Thanks
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0
    For me, id love for the unit to be able to operate at high gain with a low noise floor for use with dynamic mics.
    So far, i've not had my hands on a high gain (50-60db) and low noise preamp that uses an instrumentation amplifier up front.

    not that i am anti instrumentation amps.

    for me, on my dbx 286A anything over about 45db gain the noise starts creeping up rapidly. like -98 dBv at 10db gain, -95 dBv at 30db gain, -90 dBv at 45dB gain, and -77 dBv at 60dB gain. if i boost it by ~30DB with an inline amp (pick your poison for inline amp, fethead, cloudlifer et al.) then i can reach a total of 60DB gain at under -95dBv noise.

    i know my measurements are dBv not dBu so do the conversion if that's important .

    i'm probably fighting an unworthy fight here in tyring to get that in a single unit, and should be happy with the changes i've already made and the results i have. i can always use the inline amp for my dynamic mics for voice over. I could put in a discreet ~30DB amp up front inside the unit, but that would force the lowest gain to about 40DB. kinda makes it a one trick pony. not sure that is a solution.

    perhaps a separate input for the higher gain that switches in when plugged in??

    oh for those who are interested, i did do response curves for the 286A ... and its pretty flat, and the thd is extremely low. I'll post that if there is any interest in actually seeing it.

  8. #28
    Join Date
    Dec 2002
    Location
    Massachusetts
    Age
    71
    Posts
    4,539
    Thanks
    11
    Thanked 235 Times in 216 Posts
    Rep Power
    10181459
    Quote Originally Posted by innkeeper View Post
    perhaps a separate input for the higher gain that switches in when plugged in??
    My separate input for my 286s when I use an SM7b is a Triton Fethead. Pretty quiet with the Fethead and the SM7b takes on a bit more clarity with the Fetheads 22 kohm impedance.
    Mark.......

  9. #29
    Join Date
    Jul 2017
    Posts
    19
    Thanks
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0
    Quote Originally Posted by arcaxis View Post
    My separate input for my 286s when I use an SM7b is a Triton Fethead. Pretty quiet with the Fethead and the SM7b takes on a bit more clarity with the Fetheads 22 kohm impedance.
    Currently using a NOS AUDIO "mic booster microphone activator" yea i know not well known. got one while they were/are blowing them out. internals is basically the cloudlifter circuit, right down to using sk170's, and hence the reason they are discontinuing manufacturing.

    does the job. The dynamic mic i have been using is MXL BCD-1 which has about a -54 dB 1 V/Pa output so jsut a little hotter then the sm7b. rated at 600 ohms. (kinda high) so the higher input impedance of a fethead would suit it better as the cloudlifter is closer to 3k in its design.

    but it gets me by for now while i play.

  10. #30
    Join Date
    Jul 2017
    Posts
    19
    Thanks
    0
    Thanked 0 Times in 0 Posts
    Rep Power
    0
    HA - just scored a DBX286S for 25 bucks - with a non working processor section.
    I should be able to fix that, and i can compare the preamp part even without repairing it.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. HVAC modifications
    By 56BelAir in forum Studio Building & Acoustic Treatment
    Replies: 6
    Last Post: 12-07-2011, 17:39
  2. Modifications for corners?
    By Dani Pace in forum Studio Building & Acoustic Treatment
    Replies: 10
    Last Post: 09-15-2006, 15:47
  3. mackie 32 8 modifications
    By foreverain4 in forum The Rack
    Replies: 5
    Last Post: 03-25-2005, 22:15
  4. mic modifications
    By Andybot in forum Microphones
    Replies: 34
    Last Post: 08-16-2004, 16:19
  5. hardware modifications?!
    By TomK in forum Gear Reviews & Questions
    Replies: 1
    Last Post: 07-14-2000, 23:38

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •