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Old 27th December 2006, 11:23 PM   #1
Minion is offline Minion  Canada
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Default Grounding with Multiple Power supplies question??

Hi Folks, I am building a Small Preamp For my LM3886 guitar amp useing a couple 6418 Sub Mini Tubes, I am getting the +25v Plate voltage from an Unregulated supply that is also powering the LM3886 but for the 1.5v Filament Voltage I am useing a 3v DC adapter and a Voltage divider to divide the Voltage between 2 Tubes....

My question is do I just connect the 0V/Ground wire from the 3v Dc adapter to the 0v/Ground wire from the +/-25v supply and use a common ground for all circuits or do I have so Isolate the Dc adapters ground from the power supply grounds??

I think that I would just connect the DC adapters 0v/Ground to the Same ground as the +/-25v Power supply 0v/ground but I have never used 2 power sources in a Project before so i wanted to be sure....

Thanx
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Old 28th December 2006, 01:00 AM   #2
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You may let the DC3V "floating",need not to ground.
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Old 28th December 2006, 02:16 AM   #3
Minion is offline Minion  Canada
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So are you saying that the DC adapter will use the Ground from the +/-25v Power supply to create the circuit?? So in essance Any 0v will work to create a Circuit, it doesn"t have to be the 0v from the adapter??


Thanx


PS: Would it hurt to connect the Adapters ground to the Power supply ground??
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Old 28th December 2006, 04:39 AM   #4
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Quote:
Originally posted by Minion
PS: Would it hurt to connect the Adapters ground to the Power supply ground??
I'd say you should connect the grounds of both supplies. I found from experience that hum is minimized that way. There is capacitive coupling betweent the heater and the cathode. If the heater supply is left floating it can allow intereference into the circuit.
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Old 28th December 2006, 05:26 PM   #5
Minion is offline Minion  Canada
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Thanx for the tip....


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Old 28th December 2006, 09:41 PM   #6
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My suggestion is to regulate to make the 3volt supply. I wouldn't rely on a wallwart to produce anything near the intended voltage. They don't.

I'd also suggest regulating your B+ voltage.

There are two big advantages for you.
1) you can use input voltages that are somewhat higher than the target voltages (3 & 25 for you) without any concern.
2) The voltages will come out exactly where they are supposed to be
and
3) There will be far less hum and noise on the power supply lines.

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Old 28th December 2006, 10:19 PM   #7
Minion is offline Minion  Canada
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Well I thought of Regulateing but I don"t know of any regulators that will regulate down to 3v...I do Have one LM317 left but I don"t think I can regulate down to 3v with it pluss it needs a couple Volts of overhead which I don"t think the 3v adapter will produce....

Actually I just tested the 3v DC adapter and it puts out a steady 5.35v DC so I guess I could use a LM317 regulator Or I could just base my Voltage divider on 5.35v, The Fillament can take between 1v and 1.5v (1.25v rateing with +/-20%)....

Is there a way I can use a 1.5v LED as a Voltage Regulator?? I have heard that this is totally possible but I just don"t know how to implement it....Anyone??

I am trying to do this the easiest way possible as I am putting this mini tube circuit into an existing preamp circuit....


Cheers
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Old 29th December 2006, 10:55 AM   #8
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Yes you can use a LED, they are a bit noisy though.
Supply the longer of the two LED legs with a current.
For a 12v supply approx 330 ohms will do. At the point
where the resistor meets the LED longer leg you will find
1.7v for a Red and about 2.1v for green LED's if the
LED shorter leg is grounded

If you need higher currents than the current drop across a
330 ohm resistor, you could use a transistor
as a pass element , choose a NPN for a positive supply
feed its collector with the raw incoming DC join a
resistor of 330 ohms from the collector to the base
and stack the LED underneath - its longer leg to the base
of the transistor and its shorter leg to ground.
The emitter the circuits output should be fairly close to
1.5 volts. If you need three volts stack another LED
underneath in series with the first LED its shorter leg
connecting to GND .

Hope this helps
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Old 29th December 2006, 05:01 PM   #9
Minion is offline Minion  Canada
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Thanx for the Tip.....So I would use a LED pretty much the same way I would use a Zener as a Regulator?

I actually think I have a few Low voltage Zeners so I might try a Zener as a regulator and a Voltage divider after that to get my 1v to 1.5v filament voltage......


Thanx
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Old 29th December 2006, 05:15 PM   #10
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You should also verify that the wallwart can supply the current for those tube filaments. Do you know if it can?

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