Let's say I have an electronic device, inside which is a power transformer that outputs a known DC voltage. Can I bypass the transformer and wire in a jack to accept the correct DC voltage from an external source instead?

Here's the context: I have a nearly 50 year old hifi spring reverb unit (Sansui RA-500) that has a 60 Hz hum problem even after I replaced its capacitors. I believe the problem may be that the power transformer is inside the unit right next to the spring reverb tank, which picks up its magnetic fluctuations. In guitar amps, one solution to this problem is to move the tank outside the amp to get it away from the transformer. While I don't want to move the tank in this case, it would be great if I could "move" the power transformer outside the unit to achieve the same thing. If I could use a wall wart or laptop power supply, that would be ideal. I assume there would be additional safety benefits to this approach as well.

In case it's relevant, here's the schematic. Page 10 of the PDF, top left, shows the power transformer outputting 21V, so if I can do this, I would try applying 21V to the circuit at that point. But a more general answer would of course be useful as well.

  • \$\begingroup\$ If your power supply is moved from the chassis to outside the amp, you should add a filter or at least a significant decoupling capacitance near where the power is used. Otherwise, you may be replacing one problem for another. Since your power supply is DC, you might consider using a common mode choke on power cable to the amp, as well as the decoupling capacitor. However, at audio frequencies, I'm not sure whether that would provide much benefit. \$\endgroup\$ Jan 24, 2021 at 7:54
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    \$\begingroup\$ Also, be aware that when a transformer secondary is given as 21V, that is 21 rms volts. It's peak will be 1.414 times that, and it's average after rectification will depend upon both the bulk capacitor and the load current. \$\endgroup\$ Jan 24, 2021 at 7:57
  • \$\begingroup\$ The circuit looks slightly more complicated than a rectifier as well. You could spend $20 on a crappy battery powered oscilloscope and see what and how clean a voltage is being provided to the circuits. \$\endgroup\$
    – K H
    Jan 24, 2021 at 8:15
  • \$\begingroup\$ The transformer is not an issue, here You may keep it. \$\endgroup\$ Jan 24, 2021 at 9:08

2 Answers 2


Apart from the capacitors, the problem could also be caused by a faulty diode D 503 or faulty transistor TR 511.

enter image description here

Since you have already replaced the capacitors, you may go ahead and check the diodes and the transistor.

The transistor is part of a 'capacitance multiplier' circuit. It multiplies the capacitance value of C 526 by the factor 'β' (current gain of the transistor).

It would be worth your while, should you be able to maintain the originality of the unit.

  • \$\begingroup\$ Thanks, I appreciate you looking into it. However I checked both of those components just now and they tested as they should. \$\endgroup\$
    – Anthony
    Jan 24, 2021 at 19:34
  • \$\begingroup\$ Anytime, Anthony! Have you connected an earth wire to the ground terminal on the rear? Connecting the ground terminals of other interconnected units to the ground terminal on the reverb unit may also help. \$\endgroup\$
    – vu2nan
    Jan 25, 2021 at 4:21

I'm going to go ahead and answer my own question a few months later to say that yes you can, although my question was flawed. When I asked the question, I thought that the transformer output DC power, which of course it does not. The transformer outputs AC, which then gets rectified by some diodes immediately after. Without getting into too much detail, basically I removed the transformer from the circuit and then inserted DC power from a laptop charger where the diodes previously output DC power. It works now, and with less hum.


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