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Phantom Power and Preamps

Discussion in 'Amps and Cabs [BG]' started by ESP-LTD, Jun 30, 2005.

  1. ESP-LTD


    Sep 9, 2001
    I recall a thread recently that suggested that some preamps might take damage from phantom power (48vdc) from a mixing board. I have an old Yamaha PB1 that I'd hate to fry.

    Does anyone know if it has a transformer to block the DC ? Or are concerns about phantom power 'groundless' ?
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    If in doubt insert 1 mfd 25v tantalum capacitors on the output legs (pins 2&3) of the pre's XLR out. If the XLR is soldered to a board you can fit a pair of caps inside an XLR connector to make a dedicated cable.
  3. ESP-LTD


    Sep 9, 2001
    That sounds like an excellent solution; thanx!
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis

    If the phantom power is 48V, the 25V tantalum isn't going to do much good...... Capacitors rated 63V will, however.

    Also, you will need to 'point" the "+" end of the capacitors towards the board (source of +48 voltage).

    If I were "designing" a solution, and couldn't use a transformer isolation, I'd also put a "leak-off" resistor to ground on the circuit side of the blocking cap, , and likely protective diodes to the local power supply for prevention of excessive surges back into the circuit when the 48V is applied/removed. Yes, phantom power is supposed to have substantial series resistance, but.............
  5. ESP-LTD


    Sep 9, 2001
    I opened the case today, and found no transformers near the XLR out, so I guess it's time to start soldering some caps.

    Thanx to all for the help.
  6. ESP-LTD


    Sep 9, 2001
    I did that last week and then used it for a show on Saturday night with a commercial sound crew- the final result sounded good in the board. I ended up with using 35v caps in series to handle the 48v. Thanx to all.
  7. pedroferreira


    Feb 10, 2003
    I did this experiment and made some tests.
    Using the caps in series get you a loss of +/- 3dB in the signal, and it acts as a high pass filter (removes lows in the signal).
    I compared this by recording the signal using a normal cable at first, and then using the same cable + the cable with the caps.
    I won't be using this option :rollno:

    Best Regards,

  8. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    Sorry Bill, !µF is simply too small. 22µF or even 100µF would be better. The ideal value depends on the input impedance of the mixer.

    To solve this I used a good quality mic input transformer (Peerless made for Altec). Amp to low impedance primary and ran the high impedance secondary to the mixer balanced via an XLR cable. Another way is to use an XLR cable that has a transformer in a 1/4" plug. These will isolate the 48V entirely.


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