Problems with EM87 stereo level indicator for computer line out - diyAudio
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Old 23rd April 2010, 07:26 AM   #1
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Angry Problems with EM87 stereo level indicator for computer line out

Hi all,

I am trying to build a stereo level indicator using EM87 magic eyes I got off ebay.
I have a simple transistor circuit to give me a negative voltage for the grid, which is driven from either the headphone of lineout on the soundcard.
The 6.3V for the heater I am just regulating from the 12V rail on the computer PSU.

Below is a schematic of my circuits for the PSU and the amplifier for the grid.

Click the image to open in full size.

The tubes light up correctly and I measure the correct voltages on them.

The problem I am having is when I connect the ground from my amplifier for the grid voltage going to PIN1 to the ground of my rectified AC voltage it blows the tracks off the board.

It also tends to destroy the diode bridge causing a short circuit between the AC inputs and the negative DC terminal.

Also it is just mono at the moment and I have been testing it with just a function generator.

I am not sure what the problem is, and as you may have gathered I am new to the tubes...

Thanks in advance for any advise!
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Old 23rd April 2010, 11:42 AM   #2
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rectified AC voltage
You need isolation!
With directly rectified mains voltage the 'ground' isn't really ground,it's basically at mains potential.(!)
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Old 23rd April 2010, 02:58 PM   #3
pjp is offline pjp  India
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Congratulations on still being alive!

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Old 23rd April 2010, 03:16 PM   #4
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Buy a valve power supply transformer. It will (A) MAKE THE CIRCUIT SAFE, (B) provide the necessary 250VAC and 6.3VAC for the EM87s
Steerpike's Toybox
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Old 23rd April 2010, 04:08 PM   #5
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Ahh I thought it could be something like that.
But I think I still sit with a problem. I must make the approx 1.2Vrms from the soundcard into about 0V to about -6V or so.
So I still need 12V for the transistor circuit.

Now if I obtain one of these valve power supply transformers can I connect my grounds from my computer PSU and soundcard one of the 0V outputs on the transformer?

Can the 250VAC be directly coupled to the tubes, obviously through the correct resistors some of the pins?

Steerpike, I see you are from South Africa, I stay in Cape Town. Any idea where I can get one of these transformers?
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Old 23rd April 2010, 04:47 PM   #6
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Nothing but a transformer should be used if you want to live.

Buy a tube transformer with a 12.6V winding (or two 6.3V windings would be better) so you have all the different voltages you need.

If required, just add a small transformer - but you should be able to find one with all the voltages you need.

As mentioned above, congratulations on being alive, AC mains will usually find a path to ground (unlike a transformer output, which is isolated from ground) and if you're heart is near that path you die.
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Old 23rd April 2010, 05:00 PM   #7
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Sorry, what do you mean by this?
Originally Posted by Globulator View Post
Nothing but a transformer should be used if you want to live.
As far as I understand the grid voltage of 0V to -10V must be referenced to the same ground as what is on the tubes pin 3?

Also is there an online store that stocks such transformers or is it easier to get one custom wound?

Thanks again

Last edited by reflexsa; 23rd April 2010 at 05:05 PM.
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Old 23rd April 2010, 05:27 PM   #8
TheGimp is offline TheGimp  United States
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The reference is to you directly applying line voltage 230VAC 50Hz to a bridge rectifier with one side to ground, without using an isolation transformer or tube supply transformer.

(1) You have applied line voltage thorugh a diode to ground which is what is causing your traces to blow.
(2) you have created a potentially lethal circuit which could electrocute a person who touches it.

You should use a transformer with a 230V and 6.3 v outputs to isolate your circuit from mains AC for safety purposes.
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Old 23rd April 2010, 05:32 PM   #9
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Ok, I understand that I should now use a transformer, but with using one how will I make sure my other circuits share the same ground?
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Old 23rd April 2010, 06:44 PM   #10
Arnulf is offline Arnulf  Europe
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Transformer floats the entire circuit it supplies. Any solid connection (such as ground to ground at the input, but could just as well be any other voltage potential to any other voltage potential on the other component) equalizes potentials connected together and all voltages within certain component are referenced to this common potential as far as you're concerned. If your supply isn't floating (= the way as shown in your initital schematic), something will go bang if there is low enough an impedance path between two different potentials once another common reference is established.

Note that there is really no such thing as "ground" in your circuit. It would mostlikely measure ungodly high number of ohms to real ground (the one you stand on).This is just a reference point that is commonly used as it connects together both input source, amplifier and output (speaker).
mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."
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