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I am making a hybrid tube preamplifier (one vacuum tube and one op-amp). Both use very different voltages - the tube is getting somewhere about ~150V DC and the op-amp gets ~6V DC. The differences are huge.

Can I connect the ground from 150v DC to the ground of 6V DC? Both are 'made' from a transformer and a diode bridge, filtered with capacitors. I assume they both should be at 0V. Based on that it seems to me that it should be somewhat safe.

Am I wrong? Can it be done? And can it be safe? Or is it really stupid and should never be even considered? I am just wondering, didn't actually try it - now I am asking.

enter image description here

*First stage of the preamplifier: (~ 150V DC) enter image description here

Second stage of the preamplifier: (6v DC)

enter image description here

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    \$\begingroup\$ If the two AC inputs are isolated, and the two DC sides are currently not connected to each other in any other way, then it should be fine, and a reasonable thing to do. \$\endgroup\$
    – user253751
    Jun 28, 2016 at 10:49
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    \$\begingroup\$ One is direct from the outlet (220V AC) but I 'named' it as 150V DC, as it later is dropped by a resistor to the tube... Let's say that it's 150V DC for now.. It goes through a diode bridge and is filtered by ~300uF to the ground. The other is coming from a transformer. Goes through a diode bridge and is filtered by 1000uF capacitor. Basically, they come from different sources (different transfomers) \$\endgroup\$
    – Giedrius
    Jun 28, 2016 at 10:49
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    \$\begingroup\$ No, I am not running it through a transformer. I run 220v AC to the diode bridge and then filter it with capacitors. I get ~220v DC which goes through a resistor to the anode of the vacuum tube. Is this wrong that I am not using a transformer? \$\endgroup\$
    – Giedrius
    Jun 28, 2016 at 11:46
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    \$\begingroup\$ I think you need to edit that information into your question. \$\endgroup\$
    – nekomatic
    Jun 28, 2016 at 12:07
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    \$\begingroup\$ The transformer is also part of the circuit ! It should have been included in the first schematic. Then it would be clear that the two AC inputs are isolated and that you can connect the grounds together. \$\endgroup\$ Jun 28, 2016 at 12:44

2 Answers 2

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Yes, you can do that, even need to do that if each of the AC feeds in your schematic come from separate transformer windings. Put another way, the two AC feeds must be floating with respect to each other.

With the AC feeds floating, the DC voltages derived from them will also float. If you want the circuitry powered by one to drive a signal to the other, you also need to connect one other point between the two sections that will be the reference voltage for this signal. The negative of the DC supply is a obvious choice, and it simplifies thinking about the circuit if you consider that to be the ground for both sections.

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    \$\begingroup\$ Olin, your answer assumes the questioner has both power supplied isolated from incoming mains voltage by transformers. His comments indicate that this is not the plan. As I commented above, even the 150V supply on its own is dangerous without this isolation. \$\endgroup\$ Jun 28, 2016 at 16:58
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    \$\begingroup\$ @Mike: First, I clearly said both AC feeds need to isolated. Second, that's exactly what they are. See paragraph 2 sentence 2 of the question: "Both are 'made' from a transformer and a diode bridge". \$\endgroup\$ Jun 28, 2016 at 19:26
  • \$\begingroup\$ I'm sorry if what I wrote came across as critical of you. That wasn't what I intended. I felt a bit scared the OP was about to kill himself, since his comments below the question indicated that what you or I might mean be isolated by a transformer (in the appliance) is not what the OP meant: he is imagining the power company's transformer will do the business for him. Sorry again for the offense. \$\endgroup\$ Jun 28, 2016 at 21:41
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    \$\begingroup\$ @Mike: If he meant the power company transformer, then that's bad. If this is really true, then he should add that to the question. Pertinent information should never be hidden in comments. No, I'm not going to read a lengthy comment chain. \$\endgroup\$ Jun 28, 2016 at 23:04
  • \$\begingroup\$ Olin, this was not the first question/problem you helped me to solve. Thank you! \$\endgroup\$
    – Giedrius
    Jul 1, 2016 at 13:14
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Assuming you aren't regarding the whole circuit as potentially at mains voltage wrt ground and appropriately protected, then you need two isolating transformers, as noted.

This reminds me of old valve televisions which often had a hot chassis. They were (very nominally) safe because the entire system (including the speakers) was inside an insulated enclosure (having said that, there were numerous accidents where somebody managed to contact the chassis regardless). I doubt they'd be to code today.

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